A weighting scale, dBA The unit of sound intensity expressed as a logarithmic scale, related to a reference level of 10"12 W nr2. The A weighting scale is the most com­monly used scale, as it reduces the re­sponse of sound meters to very high and low frequencies and emphasize those within the range audible by the human ear.

Abatement The reduction in air or water pollution from all sources by preventive measures.

Abrasive blasting The process of prod­uct cleaning by the use of an abrasive material.

Abrasive dusts Dusts used for abrasive blasting, grinding, polishing, and buffing.

Absolute filters Strictly dry filters used in installations that require a very clean environment, classified according to efficiency.

Absolute humidity The mass of water va­por present in a unit mass of dry air.

Absolute pressure The pressure recorded relative to the absolute zero value of pressure.

Absolute radiant heat flow The radiant energy emitted per unit area in one di­rection.

Absolute roughness The roughness of a pipe or duct wall, normally expressed as a dimensionless ratio of the linear mea­sure of the internal roughness K< di­vided by the diameter.

Absolute total pressure Sometimes called stagnation pressure, the algebraic sum of the velocity and static pressure at a given location in a moving system, in Pa.

Absolute ventilation efficiency A value that provides a means of determining the actual ability of a ventilating system to reduce the concentration of a pollut­ant, compared to that theoretically possible.

Absolute zero The temperature at which a perfect gas kept at constant volume exerts no pressure; it is equal to -273.16 °C (OK).

Absorber The material that Is used in an Absorption process.


1. The process of diffusion in which molecules are transferred from the gas phase into a solid or liquid sorption medium.

2. The taking up of radiant energy by A Material as it encounters the body, or as it passes through. A physical change or chemical change or both may accompany it.

Absorption coefficient Measure of the amount of sound or heat absorption provided by a material.

Absorption spectroscopy Analytical tech­nique involving measuring the amount of energy absorbed by a compound.

Absorptivity The fraction of the energy incident on a body that is absorbed by that body (absorbance). Relating to thermal radiation and acoustics.

Acceleration loss The energy input neces­sary to accelerate air to a higher velocity.

Acceptable air quality When workspace air has no known harmful contami­nants present which may influence the immediate or future health of the occupants.

Access door An opening providing ac­cess to some plant item or ductwork to allow for inspection, cleaning, or maintenance.

Accessibility Ease of access to a plant item.

Acclimatization The state of the human body becoming accustomed to a given thermal environment, either heat or cold.

Accuracy The degree of closeness of a measurement to the true reading of the value being measured.

Acid A substance that liberates hydrogen ions in solution.

Acid cleaning The use of acids to clean scale or other deposits from pipes or tanks.

Acid dew point The temperature at which a vapor containing an acid appears as condensate on a cool surface, causing corrosion.

Acid fumes Particles in the air generated by the condensation of acid vapors. They may also arise from sublimation, conden­sation, or chemical reaction.

Acid mists Mists resulting from a process in which an acid is produced or used.

Acidic smuts Solid and liquid conglom­erates formed by the condensation of water vapor and sulfur trioxide on a cold surface. A typical case is combus­tion products in a flue, which come into contact with surfaces at tempera­tures below the flue gas dew point tem­perature. These products contain metallic sulfate and carbon aqueous particles approximately 1-3 mm in size.

Acoustics The science and understanding of the behavior of sound in the environ­ment.

Acoustic environment The level of sound in an internal or external environment, related to a standard prescribed level.

Acoustic force An acoustic field used to enhance the evaporation, coagulation, or condensation of particulate matter.

Acoustic insulation A material that has the ability to absorb sound energy.

Acoustic leak detection A technique used to detect cracks in a building structure through which infiltration or exfiltra­tion is taking place. A constant source of high frequency sound is produced within the building and a detector mi­crophone positioned outside the build­ing detects the weak spots by recording a volume increase.

Acoustic muff (muffler) Sound-absorbing material that is placed around a noisy item in a plant.

Acoustic pod Sound-absorbing material inserted in ductwork to absorb sound.

Action level A term describing the air­borne concentration that triggers certain provisions of a regulation; generally, but not always, it is 50% of the PEL value.

Activated alumina Hydrated aluminum oxide, a granular desiccant activated at high temperature that absorbs moisture and gases.

Activated carbon or activated charcoal Carbon in the form of charcoal gran­ules, which has an affinity to adsorb many gases and vapors and, in so doing, removes odors. It is manufactured by exposing coal, coconut shells, or peat to steam at 800 to 900 °C.

Activated carbon filter A canister filter containing activated carbon.

Active site The position on an adsorbate surface where adsorbate molecules are trapped.

Activity The nature of the work carried out by a person, measured in Met units. Also, the decay rate of radioactive parti­cles.

Actuator An automatic device providing valve or damper control of fluid flow by means of an electrical, hydraulic, or pneumatic motor.

Acute The immediate influence of given concentrations of an air pollutant on the health of a person.

Acute effects Symptoms of injury or other physical manifestations that fol­low an acute exposure.

Acute exposure Exposure to a high level or concentration of a pollutant for a rel­atively short time.

Adhesion The phenomenon of particles sticking to the fiber surfaces in a filter.

Adhesion of particles Small particles ex­perience adhesion forces, allowing them to attach to surfaces. These forces may be made up from surface tension of liq­uid films, or London (Van der Waals) forces.

Adiabatic A process that takes place without loss or gain of heat, e. g., when air rises it expands adiabatically in the atmosphere.

Adiabatic lapse rate The adiabatic tem­perature change that takes place with height of a rising (or falling) parcel of air, approximately -1 °C/100 m.

Adiabatic mixing A mixing process that takes place without the removal or addi­tion of heat.

Adiabatic saturation temperature The tem­perature attained after an adiabatic pro­cess.

Adjustable flow rate A controlled state of flow that is achieved by means of a damper or valve.

Adjustable grill See grill.

Adjustable pattern air diffuser An air dif­fuser incorporating a device that allows the direction of the leaving air to be ad­justed.

Adjustable pitch fan A fan in which the pitch angle can be set to provide the re­quired airflow rate. The pitch angle may be preset or controlled with the fan running.

Administrative controls The working method that allows workers to be exposed to set exposures of contaminants in the work­place. Lower exposure levels are achieved by the use of work assignment, job rotation, with set periods working away from the hazard zone.

Adsorbate The contaminant collected by an adsorber.

Adsorbent A medium that traps vapor or gaseous contaminants on its surface by chemical and physical properties.

Adsorbent, regenerable An adsorber, which is treated when fully contaminated in order to restore its original collection properties.

Adsorber An adsorbent material used in the adsorption process.

Adsorption A physical process in which a molecule of a vapor or gas (adsorbate) is condensed on and taken up by the sur­face of a porous material (adsorbent) such as silica gel or activated carbon.

Adventitious opening Any unintended open­ings in a building structure, such as cracks through which infiltration or ex­filtration can occur. The terms uninten­tional opening, fortuitous leakage, and crackage are also used.

Aerobic microbes Microbes used in a biofiltration process in which gaseous pollutants are removed from a process gas stream by aerobic digestion.

Aerodynamic diameter The diameter of a unit-density sphere that has the same settling velocity in air as the particle in question.

Aerosol A special class of particulate con­sisting of colloidal suspensions larger than molecular size, but not large enough to settle under gravity.

Aerosol, monodisperse An aerosol with a size-distribution function described by a geometrical standard deviation less than 1.15. If the deviation is between 1.15 and 1.5, it is classified as a Quasi-mono­disperse aerosol.

Aerosol, polydisperse An aerosol with a geometric standard of deviation of size- distribution greater than 1.5.

Aerosolize Mixing a gas and a liquid to form microscopic (0.5 to 10 ^.m) air­borne droplets.

After filter A filter located in the flow stream after another filter to remove any particulate matter that may have passed the upstream unit.

After heater An air or water heater posi­tioned in a long duct or pipe work run to remedy distribution heat losses.

Afterburner A unit installed after the main combustion zone in a process to further provide combustion to reduce the emission of certain pollutants. It may be

1. A direct-flame afterburner, in which an auxiliary fuel burner provides all the heat from a flame, or

2. A catalytic afterburner, in which the surface action of catalysts allows incineration to take place at a temperature lower than a direct flame, reducing the auxiliary heat required, or

3. A recuperative afterburner, a heat exchanger combined with a direct flame unit, that preheats the combustion gases.

Age of air A statistical measure of the air

In a space.

Agglomeration The process of the clus­tering or adhering together of a number of small particles.

Aging Any chemical process that reduces the effectiveness of the properties of a material.

Agitator A device used to stir or agitate a fluid, reducing stagnation or stratification.


Air The composition of gases that make up the earth’s atmosphere, approxi­mately 79% nitrogen and 21% oxy­gen. “Pure air” has no definite meaning regarding the proportion of these gases; this term is used to imply the absence of industrial particulate matter.

Air after-treatment Treatment of the sup­ply air after the main treatment.

Air barrier A device that provides a jet barrier between two zones in a building. See Air curtain.

Air bleed-in Openings at the end of a branch duct that provide increased flow rates for the transportation of heavy particulate matter.

Air change coefficient The ratio of air volume flow rate and the volume of the space.

Air change efficiency The ratio of the nominal time constant and the time taken to change the air within a space.

Air change rate The ratio of the volume of air supplied or extracted to the vol­ume of the space usually measured in air changes per hour (ach) and normally re­lated to the fresh air change rate.

Air cleaner A device that removes air­borne contaminants from air.

Air cleaning equipment Equipment that removes airborne contaminants, either

Equipment to clean the ambient air, normally classified as air filters, or

Equipment that will collect large concentrations of particulate matter from industrial processes, called dust collectors.

Air conditioning The process of air treat­ment in which the temperature, mois­ture content, purity, and distribution is controlled to set conditions. The result­ing conditions may be chosen for human thermal comfort or to meet the require­ments of a manufacturing process

Air conditioning installation Any plant assembly which can achieve the condi­tions outlined above.

Air conditioning, partial The process of air conditioning which is lacking one or more of the ideals, i. e., no moisture con­trol, no heating, or no cooling.

Air contaminants Aerosols, gases, va­pors or dusts which may cause adverse effects if discharged into the indoor or outdoor atmosphere.

Air core area The gross area of the open­ings of an air terminal device (ATD).

Air core velocity The air flow volume di­vided by the core area of an air terminal device (ATD).

Air current Air movement in a space pro­duced by either thermal or mechanical means.

Air curtain A high-velocity air jet that provides an air barrier between two dif­ferent building or work zones.

Air diffuser A circular, linear, rectangu­lar or square device from which supply air is discharged in a controlled direc­tion.

Air-diffusing ceiling The process of air distribution into a given space through a perforated ceiling.

Air diffusion The process of air distribu­tion into a space by means of an air ter­minal unit. The components of air diffusion are

Air terminal devices (ATD), components of the installation

Which are designed for the purpose of achieving the predetermined movement of air into or from the treated space, e. g., grills, diffusers, etc.,

Complementary accessories for ATD components of the installation which are used in conjunction with, and in several cases form an integral part of, the air terminal device for the purpose of achieving the predetermined profile or rate of flow into or from the air terminal device (e. g., air flow controllers, dampers, flow equalizers, baffles, etc.), and

Fixed accessories for ATD, components of the installation which assist the fitting and fixing into place and/or maintenance of the air terminal devices and their complementary accessories (e. g., plaster frames, snap-in fasteners, etc.).

Air discharge velocity The average veloc­ity of the air discharged from the open­ing of an ATD, Vc/(C^ATf).

Air distribution The mechanical or natu­ral delivery of air into a space in a de­signed manner, or the transport of air in ductwork. The components of air distri­bution are

Elements of air distribution, the

Components involved in ensuring air distribution from the plant room to the space, e. g., ductwork, damper, etc.,

Air terminal units (ATUs), item at the end of a duct run to control velocity, pressure, flow rate, and/or temperature,

Accessories of distribution, any components to keep the unit in place, or allow maintenance to be carried out.

Air douches Versions of fresh air jets in which air is discharged from a wide noz­zle or plenum at a relatively low velocity close to the worker.

Air, dry A mixture of air with no mois­ture content present.

Air duct An enclosure that conveys air from one location to another.

Air ejectors (jet pumps) A device inject­ing high-velocity (pressurized) primary air into the secondary air. It allows air to be moved without passing through a fan, it may be a simple or Venturi-type ejector.

Air envelope The surface of a jet pro­duced by an ATD with the same veloc-

. ’ty’

Air excess Air provided to a combustion or ventilating process in excess of that theoretically required for the combus­tion process.

Air exfiltration The uncontrolled rate of air interchange from a space to outdoors due to density variations, or by space pressurization by means of a fan.

Air exhaust The volume or mass flow of air that is mechanically rejected to out­doors due to its unsuitability for further use within the space. See also Extract air (EA) classification.

Air extraction The removal of contami­nated air from a space, either directly to outdoors or recirculated back to the space after suitable treatment. See also Extract air (ETA) classification.

Air extraction cooker hood A range hood positioned above kitchen cooking equipment, designed to collect without spillage the plume generated at the range.

Air filters

Air filter A device that removes particu­late matter from a gas flowing through it. These are classified as

Absolute A high-efficiency particulate air filter that is at least 99.79% efficient in the removal of thermally generated monodisperse dioctylphthalate smoke particles with a diameter of 0.3 fim, also known as a HEPA filter.

Activated carbon An adsorption filter that makes use of an activated carbon bed to remove odors and various gases from a ventilating or gas cleaning system,

After A filter that is positioned after a coarse filter, to ensure that the air entering a space is at the required degree of cleanliness.

Automatic roll A motor-driven continuous roll of material, either viscous or fiber, which ensures a set pressure drop in the airflow. Its operation depends on sensors that record the pressure differential on either side of the filter.

Blow-off A filter, such as a bag filter, that has the dust burden (cake) on the surface blown off by strong blasts of compressed air.

Cleanable A filter that can be cleaned by manual or automatic washing.

Coarse A filter fitted before a HEPA filter to remove the larger particulate matter to ensure the HEPA filter has a longer life without clogging up.

Dry A filter manufactured from cotton wool, glass-fiber fabric, pleated paper, foamed polyurethane, cellular polythene, or other suitable materials. As the name suggests, the filter is dry in nature and has no oil coating.

Electrostatic A filter in which a high voltage is used to collect the particulate matter onto earthed


Fabric A dry throwaway filter in panel or wedge form mounted in supporting frames.

Grease A filter used in kitchen air extraction systems to prevent the contamination of the ductwork system with grease, which would be a fire hazard.

HEPA See Absolute.

Impingement The process by which particulate matter is stopped in its path by colliding with A Surface, normally a surface coated with oil.

Panel Any type of filter material mounted in a rigid frame. The frame must allow only the absolute minimum of particulates to pass between the frame and the sides of the filter material.

Pre A prefilter is placed before the main filter to extend the life of the main filter,

Rotary viscous A continuously rotating oil-coated roll of material. The filter resistance is kept low by a motor drive that ensures a clean surface is always presented to the airflow.

Self-cleaning A filter similar to the rotary viscous in which the filter plates are passed through an oil bath to remove the collected dust.

Terminal The ultimate filter at the end of a run, which ensures that the airflow entering the room is at the required design purity.

Throwaway A filter which is thrown away after collecting its dust burden, no attempt being made to clean it.

Viscous As the rotary viscous or fixed-cell type, in which the particulate matter sticks to the oil — coated material.

Wet The various forms of washers and scrubbers.

Air filter cell The material that forms the active part of a filter.

Air filter dust-holding capacity The amount of dust, by weight, retained by a filter under a standard test such as EN779.

Air Filter Efficiency The ability of a filter to remove dust from the air, expressed in terms of the contaminant concentrations upstream and downstream of the filter. It may be obtained by

A. Weight test (gravimetric),

B. Dust spot test,

C. Arrestance tests, or

D. Particle counting.

Air filter medium The material type from which the filter is constructed.

Air filter performance An overall assess­ment of The Collection efficiency, pres­sure drop, flow rate, fire rating, health aspects, and behavior of a fil­Ter In the environment in which it is used.

Air filter resistance The pressure drop created between the upstream and downstream faces of a filter, increases as the dust burden.

Air filter resistance, final The maximum pressure drop allowed across a filter to ensure that the design airflow rate is achieved.

Air filter test Any test to determine filter efficiency, flow rates, or other character­istics by some preset method.

Air free area (Ap) The sum of the smallest areas of the cross section of the opening of an AI D.

Air free area ratio (ARF) The ratio of free area to the core area.

Air grill An entry or exit in a duct consist­ing of a mesh or lattice opening; it may be fixed (unadjustable) or adjustable.

Air grill, adjustable An air grille with manual or automatic adjustable louvers.

Air handling function The ability of an air handling plant to perform a given type of air treatment.

Air handling unit A self-contained unit for the introduction or removal of air from a space. It consists of one or more of the following plant items: heating or cooling coils, filters, fans, damper provi­sion for mixing and recirculation, hu- inidification controls, and acoustic treatment.

Air heating and cooling coil As Tube or Plate heat exchanger in which Air, The secondary fluid, is either Heated or Cooled with a primary medium.

Air humidity The moisture content of air. See Absolute humidity and Relative humidity.

Air infiltration The uncontrolled air in­terchange through structural imperfec­tions and other openings into a space, due to natural convection, rising cur­rents, or wind forces over a building.

Air infiltration, balanced Infiltration that takes place at a constant or balanced rate.

Air infiltration, unbalanced Infiltration that takes place at an unbalanced or variable rate.

Air inlet Any opening that provides air for any purpose, either from outside or inside the building.

Air leakage See Air infiltration.

Air leakage rate The leakage of air through an enclosure such as a building or ductwork, expressed as air loss in L/s per m run, or as a percentage loss of the total volume.

Air, make-up The air quantity necessary to replace air extracted from the space, this may be required for combustion or process work and may be obtained by mechanical or natural means.

Air mass flow rate The mass flow of air or any other fluid, expressed in kg s_1.

Air monitoring The process of continu­ous sampling and measuring of the quantity of pollutants present in indoor or outdoor air.

Air movement The direction and veloc­ity of air within an enclosure.

Air pollutant Any undesirable element present in indoor or outdoor air, such as air contaminants, moisture, etc.

Air pollution The presence of foreign matter (gaseous or particulate or combi­nations of both), bacteria, sound, or other undesirable elements in air, which is detrimental to the health or welfare of man, animals, plants, or materials.

Air pre-treatment See air treatment.

Air pressure Atmospheric air pressure or static, velocity or total pressure in a ven­tilating system, Pa.

Air, primary The actual quantity of air injected into a space before secondary air induction occurs.

Air-purifying respirator A respirator that removes airborne contaminants, such as particulates, gases, vapors and fumes, from ambient air through filtration, ab­sorption, adsorption, or chemical reac­tions on the media contained in the cartridge or filter.

Air quality The concentration of one or more pollutants in the air, measured in either ppm or tig m-3. It may be related to pollution sources inside an enclosure or outdoor air. Temperature and mois­ture conditions may also be considered.

Air quality standard (AQS) A standard providing a level beyond which air pol­lutants in the atmosphere can cause damage to plants, animals, or materials. The concentration and time factors have to be considered.

Air, recirculated Any air that is mechani­cally extracted from a space that, after suitable treatment, is returned to the space.

Air recirculation The process of returning exhaust air to the air-treatment plant and after treatment returning it to the space.

Air relief The release of air from a space, either by pressure difference or mechan­ical extraction.

Air resource management The enforce­ment of set standards to reduce contami­nation supported by control regulations, planning, and quality testing facilities.

Air, return The air returned from a space for processing (also called recirculated air).

Air, sample T he air obtained for analyti­cal testing.

Air sampling The process of collection of an air sample to determine its contami­nation level and dust concentration.

Air, secondary Air entrained in a primary airflow, or the additional air supplied to a combustion process.

Air sedimentation A measuring technique for particulate matter using a microme — orograph, to determine the Stokes num­ber of fall.

Air space The gap between two surfaces containing either still or moving air.

Air spread The maximum horizontal dis­tance between two vertical or horizontal planes of equal velocity in a jet, tangen­tial to the supply envelope from the ATD.

Air stratification The layering of air within a space due to density differ­ences.

Air stream A defined air current within an enclosure, resulting from natural thermal air movement or from mechani­cal air movement produced by a jet.

Air supply The quantity and condition of treated or untreated air discharged into a space.

Air terminal device (ATD) A specially de­signed component in an installation, which provides a predetermined air movement into or from a treated space. They are classified as

Automatically controlled, if they have moving parts to change the local conditions of temperature, humidity, pressure difference, airflow rate, and levels of pollution,

Fixed, if they are preset to achieve the desired results and have no moving parts, or

Manually adjusted, if adjustments are made manually by the service engineer or the occupants.

Air terminal unit Air distribution equip­ment that provides set conditions by the mixing of primary and secondary air. The device may be fixed, having no control or means of manual adjustment, automatically controlled or manually controlled. If automatically controlled, a sensor is used to indicate any required

Change in temperature, humidity, air­flow rate, pressure, or C02 entering the room.

Air terminal unit assembly The air termi­nal unit, consisting of casing, mixing section, flow control, heating, cooling, filters, fans, sound attenuators, etc.

Air throw The maximum distance from the outlet of an ATD to a plane tangen­tial to the jet envelope and perpendicu­lar to the initial jet section where the velocity is reduced to a predetermined level.

Air transfer Air that is allowed to pass ei­ther mechanically or naturally from one zone to another. In the mechanical case, the transfer volume is controlled by means of suitable control valves.

Air transfer device A mechanical device that allows controlled flow of air from one room or zone to another.

Air treatment Any technique used to control the temperature, moisture con — rent, or levels of dusts, gases, vapors, pollens, bacteria or viruses in air.

Air treatment, thermodynamic Relating to the various thermodynamic changes that occur in the specific volume, en­thalpy, and wet and dry bulb tempera­tures of treated air.

Air turning vane A vane fitted in a duct­work bend to reduce pressure loss.

Air type Classification of air at a spe­cific point in its passage through an air conditioning or ventilation sys­tem, either in the duct or the space, e. g., outdoor air, supply air, treated air, recirculated air, extract air, etc.

Air, vitiated Contaminated or polluted air unsuitable for a given application.

Air volume flow rate (qv) The volume of air or gas moved per unit of time.

Air washer One of the following devices that adds water to an airstream to in­crease the moisture content of the leav­ing air.

Capillary cell An arrangement of porous pads sprayed with water,

Through which the air to be treated is passed.

Saturation efficiency Relating to air washers, the percentage of water added to the flowing air supply compared to that which must be added for the air to leave fully saturated.

Spinning disc A rotating disc onto which a fine jet of water impinges, resulting in fine droplets that add moisture to the air supply.

Spray Banks of water sprays that inject atomized water particles into the airstream.

Airflow Steady or unsteady air mass or volume movement past a fixed point, ei­ther in a duct or in a free space.

Airflow model A mathematical or com­puter model of the airflow path within an enclosure.

Airflow pattern The air distribution within a space resulting from discharges from a fan or ductwork.

Airflow rate The speed of volume or mass airflow that takes place in A Duct or space.

Airflow rate controller Any mechanical, pneumatic, or electrical device that con­trols the air flow rate into or out of a space by increasing or decreasing the flow resistance. See Dampers and Valve.

Airflow straighteners Vanes in a duct work section positioned to reduce tur­bulence after a change in section.

Airlock A two — or three-door enclosure providing access to a clean room that re­duces the air leakage of externa! pol­luted air into the clean room.

Airlock, active An airlock connected to an air treatment device.

Airlock, passive An airlock that is not connected to the air conditioning system.

Airtightness ductwork class A description of the quality of a ductwork system and its ability to contain air with the mini­mum of air or gas leakage, classified based on air leakage factor F, expressed in L s"1 nr2 or m3 s’1 nr2, a function of pres­sure P in Pa

Airtightness class

F (maximum)


0.027 p;us


0.009 p<>-65


0.00.3 P°-65


0.001 P»-65

Aitken nuclei Particles, generally with di­ameters less than 0.1 |xm, that are true aerosols when they form the nucleus for condensation or ice formation.

Algorithm A method of calculation that produces a control output by operating on an error signal or a time series of er­ror signals.

Alkali, alkaline A substance that neutral­izes acids.

Allergens Pollutants that may cause an al­lergy, including pollens, dusts, animal dan­der, insect debris, mold and fungi spores.

Allergic alveolitis An allergic response to inhalation of organic particles that in­volves inflammation of the small termi­nal branches of the bronchioles. Symptoms include coughing, increased production of mucus, fever, fatigue, and muscle aches.

Allergic contact dermatitis Skin condi­tion that occurs in response to exposure to sensitizing material. It is character­ized by redness, swelling and cracking and, sometimes more severe reactions involving the entire immune system.

Allergic reaction A body reaction due to exposure to an allergen.

Allergic response The release of antibod­ies by the immune system in response to foreign molecules in the body.

Allergy A hypersensitivity reaction of the human body due to a particular substance.

Allowable exposure time (AET) The rec­ommended maximum exposure time al­lowed for an operator in a workspace when subject to a physical or biological pollutant.

Alveolar fraction Particles with approxi­mate aerodynamic diameters of 0.5-3 pm.

Alveoli The small terminal air sacs in the lungs, through which gas exchange be­tween the blood and the inhaled air takes place.

Amber zone A ventilation containment zone used in the atomic energy industry.

Ambient air Air from the surroundings; used to describe the air pollution con­centrations in the open air as compared to the point of generation within the workspace.

Amorphous silica The noncrystalline forms of silica or quartz.

Amplification The reflection of a greater amount of sound than originally im­pacted the surface.

Analog A continuously variable function in a control system ranging from off to full flow.

Analyte The components of an air sample to be measured directly or indirectly,

Anechoic A room that has a low degree of acoustic reverberation, such as an anechoic test chamber.

Anemometer An instrument used to mea­sure the velocity of air or gas.

Aneroid gauge A gauge used for the mea­surement of static, velocity, or total pressure with a pitot tube.

Aneroid gauge, electronic An aneroid gauge with the advantage of being able to inte­grate the velocity pressure directly into velocity.

Anesthetic gases Narcotic gases which when inhaled give a feeling of well-being followed by unconsciousness.

Angle factor The geometrical shape fac­tor used in calculating radiation ex­change between surfaces I and /.

Angle of divergence The leaving angle of an air jet from an outlet, or the angle of change in a ductwork section.

Angle of transformation piece The angle between the opposite faces of a converg­ing or diverging ductwork section.

Anisokinetic Not isokinetic; a sample col­lected at a velocity different from that of the airflow in the ventilation system.

Antibodies Specific substances within the human body formed in response to inva­sion by an antigen.

Anti-C Anti-contamination protective cloth­ing used in the radiation industry.

Anti-sneakage baffles Baffles in an elec­trostatic precipitator that prevent un­treated gas from bypassing the active treatment zone.

Antivibration mounting A designed sup­port placed between a rotating or recip­rocating machine and the building structure to reduce the transmission of vibration.

Antidegeneration clause A term used in the U. S. Clean Air Acts implying not only that air quality standards must be achieved, but also that nowhere should air quality be allowed to worsen, even if standards are ex­ceeded.

Antoine’s equation An equation used for calculating saturation vapor pressure.

Apparatus dew point For practical pur­poses, the average temperature of a cooling coil surface.

Appliance A functional device, such as a fan or a combination of package units.

Approach velocity The velocity of air­flow into a filter bank or heat ex­changer.

Approved codes of practice (ACOPs) Legislation in the United Kingdom deal­ing with the safety aspects of dangerous materials.

Arc A high-voltage discharge to ground.

Archimedes number A dimensionless num­ber that relates the ratio of buoyancy forces to momentum forces, expressed in many forms depending on the nature of the Reynolds number.

Area, actual filter face The area of a filter perpendicular to the flow direction.

Area, body surface (ADlI) The total sur­face area of a nude person, determined by the Du Bois equation, A])u = 0.203 ^0.425]LJ0.72J

Area, duct eross-sectional The duct area perpendicular to the direction of gas flow.

Area, face The frontal face area of a filter or coil through which a gas passes.

Area, filter medium The total area of the filter medium that is available for gas flow.

Area, filter surface The area formed by the filter medium normal to the direc­tion of flow’.

Area, nominal filter face The frontal face area of a filter including the header frame.

Area samples Samples taken by placing the sampling train in a fixed location In The workplace.

Arrestance A measure of the ability of A Filter to remove a standard test dust from the air under test conditions.

Arrestance, average The weighted per­centage ratio of the total amount of standardized test dust retained bv a fil­ter to the total amount of dust fed into the filter in order to achieve a specified final pressure drop.

Arrestance, initial The arrestance value recorded during the first loading cycle in a filter test.

Arrestment The removal of a pollutant from a gas stream by an arrester or a cleaning device.

As lowr as reasonably achievable (ALARA) A standard for controlling and reducing worker exposure to pollutants.

Asbestos A natural fibrous form of sev­eral silicate minerals of the following types.

Chrysotile (white)

Amosite (brown)

Crocidolite (blue)

Each of the above forms has different OEL requirements.

Asbestosis A disease caused by the inha­lation of asbestos fibers.

Ash The nonvolatile inorganic residue left when a fuel is fully combusted.

Aspect ratio The ratio of the length of a grill or duct to the breadth.

Asphyxiant Simple asphyxiants are inert gases which deplete the oxygen supply in the breathing air to below the critical value of 18% by volume, such as gas­eous fuels or nitrogen. Chemical as­phyxiants, such as carbon monoxide and hydrogen cyanide, have a direct bio­logical effect.

Assigned protection factor (APF) The minimum level of respiratory protec­tion that a respirator can be expected to provide, assuming it is properly fitted, worn, and functioning. APFs are as­signed by NIOSH.

Assmann psych rometer An apparatus which uses a clockwork or electrical fan lo­cated above wet and dry bulb thermom­eters, positioned in a cylinder to provide a set airflow rate over the thermometer bulbs for accurate temperature readings of both bulbs.

Asthma A diseased condition of the lungs caused by pollution and other factors.

Atmosphere The gaseous envelope of air that surrounds the earth, held together by gravitational attraction. It consists of 79.1% nitrogen and 20.9% oxygen by volume, with approximately 0.03% C02, traces of the noble gases (argon, krypton, xenon, neon, and helium), water vapor, organic matter, ammonia, ozone, various salts, and suspended solid particulates.

Atmospheric dust concentration The dust burden present in atmospheric air mea­sured in mg m’3.

Atmospheric dust spot efficiency A test to measure the extent to which a filter paper is soiled after dust-laden air has passed through it.

Atmospheric pressure The pressure due to the air column above sea level at 45° latitude at 0 °C is 101.325 kPa.

Atmospheric stability The state of the at­mosphere in which vertical air move­ment is restricted.

Atom The smallest particle of an element that retains the characteristics of the ele­ment.

Atomic Absorption An analytical method in which the sample is converted into a vapor by passing it through a flame or other energy source and the absorbance at a particular wavelength is measured and compared with that of a reference substance. The absorbance measured is proportional to the concentration of that substance in the sample.

Atomic number The number of protons in the nucleus of an atom of an element. Also, the number of electrons in a neu­tral atom of the element.

Atomic weight The relative atomic mass of a substance.

Atomization The process by which a solid or liquid is reduced to very small particles or droplets, as in a fine spray.

Attenuation The reduction of sound or radiant energy.

Attenuator A unit fitted in air-handling or other noise-producing equipment to absorb sound.

Audiogram The report produced by an audiometry test, showing measured hearing threshold levels at frequencies of 500, 1000, 2000, 3000, 4000, and 6000 Hz.

Audiometry The assessment of damage to a person’s hearing.

Automatic control system A control sys­tem that reacts to a change or imbalance of a variable. It controls the imbalance by adjusting other variables to restore the system to the set control condition.

Auxiliary fuel firing Combustion of an auxiliary fuel to provide additional heat to an incinerator in order to either dry or ignite the waste material and to maintain ignition, to ensure complete combustion of solids, liquids, and gases in the incinerator.

Averaging time The time period over which the measuring procedure provides a single value.

Avogadro’s Hypothesis States that Equal volumes of different gases at the same pressure and temperature contain the same number of molecules. Hence, the volume occupied by any gas whose mass is numerically equal to its molecular weight is a constant quantity.

Axial-flow fan A fan positioned in a cy­lindrical casing in which the air enters and leaves the impeller in a direction parallel to the casing axis. The fan may have fixed-pitch blades or variable-pitch blades.


Bacharach smoke scale A scale of 10 shades of white to black for a smoke stain formed in a prescribed manner by a pumping action through a filter paper, used for the assessment of smoke from combustion appliances.

Back corona (back ionization) The dis­charge phenomenon that takes place from a particulate layer in an electro­static precipitator.

Backdraft dampers Dampers installed in a system to prevent reversed flow of gases.

Backdrafting The action of extracted air or the products of combustion returning to a space due to adverse high pressures at the point of extraction.

Background level The concentration of a particular substance present in the air without any local source of the pollut­ant, or the concentration of the pollut­ant at some distance from its source.

Bacteria The collective name for cellular microorganisms.

Bacteria Count The count of bacteria collected on a slide within a duct run or in a space.

Bad air Air that contains any form of contamination.

Baffle A device fitted in equipment or ductwork runs that changes the flow direction.

Baffle chamber A chamber consisting of baffles (slats) which present an obstruc­tion to the flow of dust-laden air. These obstructions slow down the heavy par­ticulate matter, which then settles by gravity into an adjacent hopper.

Bag house An enclosure containing a se­ries of frame-mounted bag filters and a hopper collection unit.

Balanced design method A method of duct sizing to ensure the correct airflow rate in all branches, also known as the static pressure balancing method.

Balancing The process of obtaining the design airflow in hoods and ductwork by the use of static pressure balancing or blast gates. In the case of a liquid it is achieved by means of balancing valves.

Bar The unit of pressure equal to 100 kPa, approximately one atmosphere. It is not strictly an SI unit due to its magni­tude being 102 not 105 as is required.

Barograph An instrument for recording atmospheric pressure.

Barometer An instrument used to mea­sure the barometric pressure at a given location in the earth’s atmosphere.

Basal metabolic rate The rate of oxygen consumption by a person at rest.

Basic thermal insulation of a garment

The thermal resistance provided by a given item of clothing.

Battery, heat exchange coil A device used to either heat or cool air in a duct run.

Bauxite lung See Shaver’s disease.

Beaufort scale The scale used for estimat­ing and reporting wind forces, in which

0 Is calm (velocity less than 0.5 m s-1) and 12 is a hurricane.

Bed depth The depth of adsorbent mate­rial through which the gas being treated passes.

BE1 See Biological exposure index.

B end A duct or pipe fitting which changes the direction of flow through a specified angle.

Benign Not associated with negative health effects, self-limiting.

Bernoulli effect At any point in a con­duit through which a liquid is flowing, the sum of pressure energy, potential energy, and kinetic energy is constant.

Bias A consistent deviation from the true value in the results of a measurement.

Bifurcated fan An axial flow fan that di­rects the airstream around the motor, which is enclosed in a protective casing. It is used for handling corrosive, high — temperature, and explosive dusts, va­pors and gases.

Bimetal thermometer A thermometer that uses two dissimilar bars of metals (with different rates of linear thermal expan­sion) riveted together. A variation in temperature produces a bending mo­ment on the bar, which is magnified by a lever to record temperature on a dial.

Bio-aerosol A general term relating to airborne viruses, bacteria, pollen, and fungus spores.

Bioclean classes The various standards of contaminant control to which clean rooms are held.

Biofiltration The process by which gas­eous pollutants are removed from a gas stream by aerobic digestion.

Biological agent Any of a range of micro­organisms which have an adverse effect on human health, including those genet­ically modified cell cultures and en — doparasites.

Biological exposure index (BEI) Refer­ence values developed by ACGH1 as guidelines for the evaluation of potential health hazards.

Biological half-life The time required for one-half of the material accumulated in a tissue to be removed.

Bioprocess The facility necessary in a bio­technology clean room to protect the worker, the product, and the environment.

Biosafety The safety requirements neces­sary in biotechnology clean rooms, fume cupboards, stores, etc.

Black body A hypothetical body that has an absorptance and an emissivity of unity, i. e., it absorbs all the radiation falling on it.

Blackness test A filter test for air or the products of combustion, in which a dust spot is formed on filter papei; the degree of reflection is determined by a light meter, and the result is related to a standard.

Blade shape Relating to the shape of a fan blade, either airfoil, solid, radial, forward-curved and backward-curved.

Blank-corrected Data that have had trace contamination amounts (detected in a nonexposed sample) deducted from the total amount of contaminant detected in the sampling media.

Blast gates Adjustable sliding gates (damp­ers) on extract ductwork used to balance the system.

Blender An enclosure in which two dif­ferent fluid streams are mixed.

Blower A device producing air movement.

Blow-out panels Safety panels provided to some process, such as an oven, that will protect the structure in case of an explosion by releasing the force of the explosion.

Blow-through unit An air-handling unit downstream of the supply fan.

Body core temperature increase The in­crease in body core temperature that takes place due to the inability of the body to get rid of heat.

Body heat gain or loss The positive or negative change in the heat content of the human body caused by an imbalance between heat production and heat loss.

Body heat storage The heat stored in the body due to metabolism.

Body height The standing height of a hu­man body, Hb, in m.

Body mass The mass of the unclothed hu­man body in kg, which is a measure of its inertia, or resistance to any alteration in its motion. The mass of a given body is the same anywhere on the earth or in space.

Body mass loss, gross The reduction in body mass over a given period of time, AMg, in kg.

Body mass loss, respiration The loss of body mass due to respiratory evapora­tion, in kg.

Body mass loss, sweat The body mass loss due to sweating, Amsw, in kg.

Body mass variation for solids The vari­ation in body mass due to food in­take and subsequent excretion, Amsl)1, in kg.

Body mass variation for water The body mass variation due to intake of water and excretion of urine, Amwat, in kg.

Body surface area The total surface area of the human body, determined from the Du Bois Index.

Body surface area covered by clothing The percentage of the body surface area covered by clothing.

Body temperature The temperature of a human body, either the body core tem­perature, the mean temperature of the body, or the temperature at some point on the skin. Also, the temperature of a surface which is radiating, conducting, or convecting heat.

Body thermal sensation The response of the body to changes in the thermal envi­ronment, relating to moisture, air move­ment, or temperature.

Boiling point The temperature at which the vapor pressure of a liquid is equal to the external pressure, and the liquid changes phase into the vapor state.

Boundary conditions The actual environ­mental conditions within a controlled zone.

Boundary layer A layer of fluid, extend­ing from the boundary into the bulk of the fluid, in which fluid motion is influ­enced by the frictional drag at the boundary.

Boundary layer insulation The thermal resistance at the boundary of the skin or clothing, Ia, in Clo m2 W-1.

Bourdon gauge A pressure gauge in which the sensing element is constructed from a coiled flattened tube closed at one end.

Boyle’s law See Laws of perfect gases.

Bracket A support for any item of equip­ment either on the main building struc­ture or on another item of equipment.

Branch duct entry The position At Which a branch duct enters the main duct run.

Breakthrough A condition thar exists when the backup section of A sorbent Tube is found to contain 20-25 percent Of the total amount of contaminant Cap­Tured in the front section.

Breakthrough time In the context: Of

Chemical protective clothing, The time Between initial contact of the Chemical On the barrier material surface and The Analytical detection of the chemical On The other side of the material.

Breathing zone The space around an op­erator in which breathing occurs, nor­mally taken as being a hemisphere of radius 0.3 m circumscribing the ears, the top of the head and the larynx.

Breathing zone sample The sample of a contaminant or contaminants collected in the working operator’s breathing zone.

BRI See Building-related illness.

Bronchi The larger air passages of the lungs (bronchus, singular).

Bronchioles The very small airways of the lungs that terminate in the alveoli.

Bronchitis Inflammation of the lining of the bronchi, which may be caused infec­tion or by the effect of pollution.

Bronchogenic carcinoma A lung cancer associated with asbestos exposure.

Brownian diffusion (Brownian motion) The diffusion of particles due to the er­ratic random movement of microscopic particles in a disperse phase, such As Smoke particles in air.

Bubble plate A plate or cap used in ab sorption equipment.

Buffer zone The zone of clean air in a room adjacent to a clean room; an air lock.

Building automation system Sometimes called building management system, a system that controls the mechanical

And electrical (M&E) services within a building.

Building related illness (BRI) Any health problem related to poor air quality, due To Equipment malfunction or contami­Nants In buildings. See also Sick building syndrome (SBS).

Building Services All of the mechanical, air, water, electrical, and transport ser­vices required to provide satisfactory en­vironmental conditions within a building, with due consideration of the health and safety of the occupants and energy con­servation.

Bulging or caving of a duct The maximum deflection that occurs in the sides of a duct due to negative (caving) or positive (bulging) pressure differences. The refer­Ence Plane is that existing with no pres­sure difference.

Bulk density Apparent density of bulk solids, kg nr3.

Buoyancy The upward force exerted on any object immersed in a fluid of greater density. Hot pollutant gases rising in cooler air have positive buoyancy. A vol­ume of gas denser than the surrounding air has negative buoyancy.

Buoyancy effect See Buoyancy.

Butt connection The interface between two pieces of metal that are joined to­gether by welding.

Butterfly damper or valve See Dampers and Valve.

Bypass The provision for a secondary flow path from the main flow in duct or pipe flow.

Bypass damper A damper positioned in the airstream which allows a given quantity of air to be diverted to another run or rejected to outdoors.

Bypass factor The ratio of the secondary flow’ to the sum of the main flow and secondary flow.

Byssinosis Reactive airway disease asso­ciated with inhalation of organic textile fibers, such as cotton, flax, linen, and hemp.


Ca A notation used by NIOSH to indi­cate that a substance is considered a known or potential occupational carcin­ogen.

Calibration The adjustment of a measur­ing instrument to ensure that it is giving the correct readings.

Calm A meteorological term describing a wind speed of less than 0.5 m s~J.

Calorific value The measure of the heating capacity of a fuel, usually expressed as the available heat resulting from the complete combustion of that fuel in kj kg-1 or kj nr3. Gross calorific value includes the heat of condensation of the water vapor in a hy­drogen fuel; net calorific value excludes this.

Canopy hoods A capture hood located above a process, designed to provide a suitable capture velocity to ensure the safe removal of the contaminant pro­duced by the process.

Capacity The actual duty of a fan, heater battery, filter; or other item of equipment.

Capacity, total lung (TLC) The volume of gas contained in the lungs at full in­halation.

Capacity, vital The maximum gas volume that can be expired from the lungs fol­lowing maximum inhalation.

Capture velocity The air velocity neces­sary at a point in order to capture and transport to the exhaust opening the con­taminants being emitted from a process.

Carbon dioxide (C02) The gas formed by complete combustion of carbon — containing substances. Also a product of the metabolic process.

Carbon dioxide production The quan­tity of carbon dioxide exhaled from the human body, depends on the metabolic rate.

Carbon monoxide (CO) A colorless odor­less gas formed by incomplete combus­tion. Highly toxic if allowed to accumu­late in the blood.

Carboxyhemoglobin A molecule formed by the combination of carbon monoxide and hemoglobin.

Carcinogen Any substance that has been shown to cause cancer in animals or humans.

Carnot cycle The cycle of a perfect heat engine, in which the heat is and rejected at constant temperature and the whole cycle is perfectly reversible.

Carnot principle States that no engine can be more efficient than a reversible engine when both operate between the same temperature limits.

Carrier gas An inert gas that moves the sample through the column of a gas chromatograph.

Cartridge filters Filters normally consist­ing of nonwoven V-pleated filter paper made into flat panels or cylindrical car­tridges.

Cascade impactor An instrument used to sample and separate particulates into a number of successive fractions of differ­ent sizes.

Casing An external cover of any plant item, covering the whole of the item or part of it.

Catalyst A substance used to speed up a chemical reaction, including the trans­formation of certain pollutants present in a combustion process.

Catalytic combustor A device used to re­move various solid, liquid, or gaseous pollutants from air or another gas, in which the gas is heated by an open burner to between 250 and 500 °C and passed through a catalyst bed in which the organic contaminants are oxidized into harmless by-products.

Catalytic oxidizer Used to promote oxi­dization of a combustible pollutant.

Ceiling exposure limit The maximum al­lowed concentration of a contaminant to which a worker may be exposed, set by legislation.

Celsius A temperature scale that sets the freezing point of pure water at atmo­spheric pressure at 0 °C and the boiling point of pure water at atmospheric pres­sure at 100 °C.

Cenosphere Small, hollow, spherical ash particles formed from the combustion of liquid or solid fuels.

Central chambered system A combination of components in a dedicated chamber. Central nervous system (CNS) The sys­tem of the body composed of the brain and spinal cord, which controls impor­tant body functions.

Central station A central ventilation or air-conditioning unit that provides treated air to various zones within a building.

Centrally recirculated Exhaust air from one or more treated spaces that is rein­troduced through a central unit before air treatment occurs.

Centrifugal A driving force that causes a body to move in a circular path, for exam­ple, a centrifugal fan, centrifugal separa­tor, or centrifugal pump.

Centrifugal collectors Separators for the removal of particulate matter from gas streams, classified as

Straight-through cyclones with fixed impellers,

Straight-through cyclones with moving impellers,

Scroll collectors,

Induced-draft fans combined with collectors,

Reversed-flow cyclones, or

Multiple cyclones.

See also Cyclone.

Centrifugal fan See Fan.

Chain of custody Documentation neces­sary to trace sample possession from the time of collection throughout the time of analysis.

Chamber A gas — or dust-tight enclosure. Chambering Components contained in a chamber erected on-site.

Charcoal cloths A filter manufactured from pretreated woven cellulose fiber cloth.

Charged particles Particles that have a positive or negative electrical charge. The nature of this charge effects the collection of the particles in a precipitator.

Charles’ law See Laws of perfect gases.

Chemical agent Any chemical element or compound.

Chemical asphyxiant A substance that in­terferes with the absorption or utiliza­tion of oxygen in the body, e. g., carbon monoxide.

Chemisorption An adsorption process in which the solute chemically reacts with the adsorbent to form a new compound.

Chilled water Water that is cooler than ambient temperature, obtained from a refrigeration plant, cooling tower, or a well.

Chiller A heat exchanger in which heat is removed from the warmer water or air.

Chilling temperature See Temperature, chilling.

Chimney A free-standing or built-in structure used to remove the products of combustion from a process or to pro­vide natural ventilation.

Chimney effect See Stack effect.

Chlorinated hydrocarbons Hydrocarbons in which some or all of the hydrogen atoms are replaced by chlorine.

Chlorofluorocarbons (CFCs) Aliphatic — carbon compounds containing both chlorine and fluorine atoms.

Chromatography A method of analysis using separation of mixtures based on selective adsorption.

Chronic exposure Long-term repeated exposure to low levels of pollutants, which may cause damage to the health of occupants.

Chrysotile One of the types of fibrous mineral asbestos.

Chute A device used for removing waste material or charging a furnace.

Cilia Fine hairlike structures found in the membranes that line the respiratory tract that assist in particulate removal.

Circulating fan See Fan.

Circulation time The time necessary for complete mixing of a tracer gas in A Space.

Class I—III Cabinets Containment cabi­nets used for various requirements. In a high-risk area, a Class III cabinet would be used. Class I and II cabinets are used in low-risk areas.

Classes for clean rooms Many national standards are in use for the classification of clean rooms. It is recommended that the standard of the country7 in which they are to be installed be referred to.

Clean in place (CIP) A system used in clean rooms, consisting of tanks, piping, pumps, and associated controls for the distribution of wrash and rinse solutions.

Clean-out door A door in an item of equipment or ductwork that provides access to the inside of the unit for clean­ing purposes.

Clean room A room in which the envi­ronmental conditions of air purity, tem­perature, moisture content and air movement are maintained to high and accurate standards.

Clean tunnel A tunnel providing access of operators or production components from one clean area to another.

Clean zone Any area, such as a clean room, in which set standards of air pu­rity and other environmental conditions are maintained.

Cleaning system A system or device that cleans a fluid or solid medium to a given standard.

Clearance samples Samples taken follow­ing a lead, asbestos, or other removal action, which must indicate the contam­inant concentration to be at or below a specific level before the area can be cleared for normal occupation and work activities.

Cleat A device used to connect two or more items together.

Clo The SI unit of insulation value of clothing; 1 Clo = 0.155 m2 K W"1. The term tog may be seen in the literature but tog is not an SI unit and should not

Be used.

Closed face sampling Sampling performed through a small hole in the top of a filter cassette.

Closed system The primary containment of a process in a biotechnology clean room.

Cloth-filter collectors A mechanical method of filtrat ion of particulate matter from a gas stream by the use of a number of cloth bags. Its operation is similar to a vacuum cleaner method of removal.

Clothing area factor The ratio /cl of the surface area of a clothed body to the surface area of a nude body.

Clothing insulation The resistance of a uniform layer of clothing covering the entire body that has the same effect as the actual clothing worn on the sensible heat flow under still-air conditions, /c), in Clo M2°C W"1.

Clothing insulation, effective The increased body insulation due to clothing, com­pared to the nude state, the difference between the total insulation and the boundary layer insulation, /clt, in Clo m2

°C w-1.

Clothing insulation, minimum require­ments The minimum clothing insula­tion required to maintain body thermal equilibrium at a subnormal level of mean temperature, IREQmm, in Clo m2 °C W"l„ This represents the highest ad­missible body cooling in occupational work.

Clothing insulation, neutral require­ments The thermal insulation of cloth­ing necessary to provide conditions of thermal neutrality IREQlieutra) in Clo m2 °C W_1. It represents the state of no or ab­solute minimum cooling of the body.

Clothing insulation, required The required clothing insulation to ensure a given body thermal balance.

Clothing insulation, resultant The true level of thermal insulation provided by clothing under given conditions, Icir, in Clo nr °C W-1.

Clothing surface temperature The actual mean surface temperature of clothing.

Clouds A mass of droplets of water or other liquids remaining at a more or less constant height. Clouds are usually formed by condensation after warm moist air rises by convection into cooler regions and cools by expansion to below its dew point.

Coagulation The process of particulates sticking together on coming into con­tact. As the process continues, the parti­cle size distribution becomes coarser and settles out.

Coalescence The merging of small drops of a liquid into a larger droplet.

Coanda effect When a jet becomes and remains attached to a surface due to static pressure differences, as in the case of a wall jet.

Coarse solid particles Any solid particle larger than 50 |xm, and solid particles contained in or on any liquid particle.

Coated/treated filters Filters that have been coated with a chemical specific for the con­taminant to be collected. The coatings en­hance the collection by chemically reacting with the contaminant as the air is drawn through the filter.

Cochlea A snail-shaped fluid-filled organ of the inner ear, lined on its inner sur­face with specialized hair cells that con­vert sound pressure vibrations into nerve impulses.

Coefficient of discharge A coefficient de­scribing the actual discharge of a fluid jet compared to the theoretical dis­charge.

Coefficient of hood entry The coefficient describing the pressure drop that occurs when gases flow through a collecting hood or other enclosure.

Coefficient of velocity The coefficient de­scribing the actual velocity of a jet, com­pared to the theoretical value.

Cohort A group of individuals that share a particular statistical or demographic characteristic, e. g., exposure.

Coincident error An error due to the presence of more than one particle in the measurement volume of an optical particle counter.

Cold-generated DOP A cold-generated aerosol dioctylphthalate (DOP) test, used to measure the efficiencies of High­efficiency filters. A hot DOP test may also be used.

Cold stress Physiological stress on the body created by excessive loss of body heat.

Collar A connecting piece used to con­nect two components of duct or pipe.

Collecting surface area The actual sur­face area of a filter on which particulate matter is collected, normally greater than the filter face area.

Collection efficiency The efficiency of a collection process, expressed as a per­cent of theoretical (100%) collection.

Collectors Any device used to collect par­ticulate matter, preventing it from enter­ing the environment.

Combined section of air-handling plant A Section of equipment that consists of two or more items of equipment neces­sary for its operation.

Combustion The chemical process that occurs when a given combination of fuel and oxygen is heated to a given temper­ature at which the combustible matter burns, with an increase in temperature.

Combustion air The air quantity that has to be supplied to a combustion pro­cess to ensure complete combustion. The air quantity may be either theoreti­cal or excess.

Comfort conditions The environmental conditions in a space that will ensure statistically the majority of occupants are comfortable. Relates to thermal, acoustic, and visual conditions.

Comfort indices Relating to the many comfort scales, either empirical or calcu­lated, that are in common use.

Comfort ventilation The minimum amount of air that must be provided to a worker to ensure

Relative air velocity for comfort,

Body heat removal, and

Removal of body odor and cigarette smoke, etc.

Comfytest A measuring instrument used to predict thermal comfort in a space.

Commissioning The process of setting to work an HVAC system in order to meet the operating design requirements.

Compensating control The process of au­tomatically adjusting the control point of a controller to compensate for changes in a second measured variable.

Component Any functional element of an HVAC system. See also Air diffusion and Air distribution.

Components of ventilating or air condi­tioning A single functional element forming a part of a ventilation or air conditioning system.

Compound hood An extraction hood that has two or more points of apprecia­ble entry loss.

Compressed air Air at a pressure greater than the atmospheric pressure at that lo­cation. In the case of a fan, if the outlet pressure exceeds 30 kPa it is classified as a turbo compressor.

Computational fluid dynamics (CFD) The technique of using computers to provide an assessment of the flow of air and other fluids.

Concentration The amount of a sub­stance present in a given volume of a gas or liquid, in parts per million (ppm) or p. g m-3. In the case of gases, the ppm is proportional to the molecu­lar concentration, hence the relation­ship between ppm and |xg nr3 depends on the molecular weight of the gas concerned.

Condensate The liquid formed from con­densation of a vapor generally on a cool surface.

Condensation Formation of a liquid from a gas, as when its temperature is low­ered at constant pressure.

Conductive hearing loss Hearing loss that is caused by blockage or other interference in the path by which sound energy is transferred to the inner ear.

Conductive heat exchange Heat flow that takes place by thermal conduction be­tween two surfaces in contact or along or across a solid body due to tempera­ture difference, in W nr2.

Confined space As defined by OSHA, any space that is large enough for an employee to enter and perform work that has limited or restricted means for entry or exit and is not designed for hu­man occupancy.

Connector Any device that joins two or more components together.

Consensus standards Existing standards that are voluntarily being followed by industry, typically containing the mini­mum requirements for materials, proce­dures, and applications.

Constant dryness or constant quality lines Lines on a steam, gas, or psychro — metric chart passing through all the “state points” of equal dryness fraction.

Constant volume lines Lines on a steam or psychrometric chart passing through the “state points” representing an equal volume of steam or air (dry or wet).

Containment The process of ensuring that all contaminants are contained in a room or cabinet.

Contaminant The same as pollutant, but usually used to describe indoor condi­tions.

Contamination Any unwanted material, such as radioactive material, present in a location. Also refers to loose radioactive materials that can be easily removed from surfaces.

Continuity relation The mass flow rate in unit time is the product of the density multiplied by the volume flow in unit time.

Continuous-flow mode The mode of air supply in which a regulated amount of air is supplied to the face-piece at all times.

Continuous sampling The uninterrupted sampling of air or pollutant from a space at a fixed rate.

Contra-rotating fan See Fan.

Contraction The size reduction of a duct allowing it to fit into tight spaces.

Control A manual or automatic device allowing the regulation of pressure, tem­perature, and moisture content of vol­ume flow of a system.

Control agent The medium in which the manipulated variable exists. In a steam process, the control agent is steam and the manipulated agent is the flow of steam.

Control device A manual, mechanical, pneumatic, or electrical device that con­trols any component, e. g., fan, thermo­stat, etc.

Control point The actual value of the controlled variable (set point plus or mi­nus offset).

Controlled medium The medium in which the controlled variable exists. In controlling space temperature the con­trolled variable is the space temperature and the controlled medium is the air in the space.

Controlled variable The quantity or con­dition that is measured and adjusted.

Controller Any device that senses changes in the controlled variable and provides the corrective output.

Convection The mechanism of heat transfer due to different temperatures, and hence different densities in fluids. It may be natural, dependent only on ther­mal forces, or forced, when use is made of a rotodynamic device to improve the rate of heat exchange.

Convective appliance A room-mounted device that transfers hot or cold air into the space mainly by convection.

Convective heat exchange The heat inter­change by convection between the cloth­ing surface or skin and the surrounding environment, C, in W nr2.

Convective heat exchange coefficient A com­plicated factor involving the surface ge ometry, fluid velocity, and the various fluid properties. Its magnitude governs the rate of heat exchange.

Convective heat exchange, globe The con­vective heat exchange that takes place between the surface of a globe ther­mometer and the surrounding air, CK, in W nr2.

Convective heat exchange, respiratory

The heat exchange that takes place by convection in the respiratory tract, Cres, in W m ~2.

Conveying system A system of ductwork used to convey powder, dust, or granu­lar material from the point of generation to a collection chamber.

Cooker hood A device to collect cooking fumes from above a kitchen range and discharge them to the outside. It may in­corporate a grease filter, fan, and fire damper or non-return flow damper.

Cooler See Chiller.

Cooling The removal of sensible or latent heat from a medium by means of a chiller.

Cooling coil A heat exchanger (battery) in which the chilling of a fluid occurs by means of a heat-transfer medium.

Cooling, direct A cooling system in which the medium to be cooled is not affected by another medium situated between it and the refrigerating apparatus.

Cooling effect Heat removed by a refrig­erating appliance or heat exchanger.

Cooling, indirect A cooling system in which the medium to be cooled is affected by another medium situated between it and the refrigerating apparatus.

Cooling load The quantity of heat to be removed from a process or a space in or­der to meet the plant design or environ­mental conditions.

Cooling load, latent The quantity of heat to be extracted from a medium, without tem­perature change, in order to produce a given mass of fluid or condensate from the vapor.

Cooling load, sensible The quantity of heat to be removed from a fluid stream to maintain a desired fluid temperature.

Cooling tower A packed tower in which a warm liquid is allowed to fall by gravity; cooling it to within 1 °C of the wet bulb temperature of the entering air.

Cooling water Water used as the cooling medium in refrigerating plant condens­ers. It may be from a well or river or re­cooled.

Core area of air terminal device That part of an air terminal device located within a convex closed surface of the minimum area required to include all of the air ter minal device openings inside the surface.

Core area of sand trap louver The prod­uct of minimum height H and minimum width B of the front opening in the sand trap louver assembly with the louver blades removed.

Core temperature The deep core temper­ature of a living body resulting from metabolism.

Corona The luminous discharge that ap­pears at the surfaces of a conductor in an electrostatic precipitator due to air ionization.

Corrected effective temperature An em­pirical comfort index that uses the dry bulb, wet bulb, and globe temperatures and the relative air velocity in a space.

Corrective action A control action that provides a change in the manipulated variable.

Corrosion The process of a material be­ing destroyed by chemical, electrochemi­cal, or microbiological action.

COSHH (Control of Substances Hazard­ous to Health) UK legislation regulat­ing toxic dusts, vapors, and gases.

Cotton filter cloths Woven cotton cloth stretched on a frame to produce a filter medium.

Counting rate The number of counting events per unit time.

Cowl A roof-mounted device designed to provide airflow out of a building with the minimum of flow reversal.

Crackage Cracks in the building structure through which infiltration or exfiltration can occur by means of wind forces or tem­perature and pressure differences.

Criteria Set values used to establish guide­lines for air or water quality standards.

Criterion level The 8-hour TWA limit for noise exposure, used for determining the noise dose.

Critical pressure The pressure at which the gas starts to liquefy at its critical temperature.

Critical temperature The temperature above which a given gas cannot be liquefied, regardless of the pressure.

Crocidolite See Asbestos.

Cross-drafts The unwanted or wanted movement of air within a space, which may be natural or mechanical. See Cross ventilation.

Cross-sectional area of a duct The area of a duct perpendicular to airflow, Ac. In the case of a circular duct,

Ac = 7r D2/4 = 0.7854 D2,

And for a rectangular or square duct Ac = A ■ b. Tor oval and other shapes of ductwork the appropriate equations are used.

Cross ventilation Ventilation that takes place by the circulation of air intro­duced at one side of the room and ex­tracted at the other side.

Cumulative Additive.

Cumulative effect The building up of dangerous products within the body with successive or continuing exposures.

Cumulative errors Errors in a measuring process consistently in the same direc­tion, either positive or negative.

Cumulati ve frequencies Accumulated sums of frequency values in a frequency distri­bution.

Cumulative sampling A system in which the sample is accumulated over the time, either by being taken continuously or for periods at regular intervals to a given single sample. Its composition is regarded as representative of the whole period of its accumulation.

Cunningham correction factor A factor used as a refinement to the Stokes equa­tion for falling particles of small diame­ter. These tend to slip between the air molecules and, as a result til! t ister. Cup anemometer A device used b> meteor­ologists for the measurement of wind speed.

Curie (Ci) A unit of radioactivity, related to the emission from I g of radium, it is equal to 3.7 x 1010 disintegrations per gram per second. This unit has been re­placed by the Becquerei (Bq); I Bq = 27.03 pCi.

Cuvette Small cylinder (test tube) used to hold a sample in a spectrophotometer. Cycle The sequence of events in a heat en­gine, refrigerating machine, or any process where, during the performance of mechani­cal work, heat is supplied to and rejected from the working fluid, which is returned to its original condition.

Cycling A periodic change in the con­trolled variable from one value to an­other. If uncontrolled this is known as hunting.

Cyclone A device for the removal of partic­ulate matter from gas streams by centrifu­gal force and a velocity reduction. There are three main patterns of operation

A. Descending spiral flow’.

B. Ascending spiral flow.

C. Radial inward flow.

Cyclones may be subdivided into

Dry dynamic precipitator.


High efficiency,

Inertial separation, and

Wet centrifugal.

See also Centrifugal collectors.

Daily noise dose (DND) The allowable noise exposure for an 8-hour workday.

Dalton’s law of partial pressure States that the total pressure of a gas mixture is equal to the sum of the pressures which each component gas would exert if it occupied the same space alone.

Dampers Devices fitted in ductwork to provide a flow resistance to control the air supply. Dampers may be

Butterfly: A plate which turns on a diametrical axis inside a duct, or a pair of flaps hinged to a common spindle to allow flow in one direction only.

Plate (single blade): A hinged flap that, by virtue of its position relative to airflow, creates a flow variation. This simplest form of damper, only used on small duct sizes, does not provide accurate control.

Horizontally-opposed: A multileaf damper in which adjacent blades rotate in opposite directions.

Iris: A circular damper with moving leaves that forms a variable orifice.

Parallel-blade: A damper that allows a gas to flow, in which the blades rotate in the same direction.

Damper control fan See Fan control methods.

Damper section A section of HVAC equip­ment containing a damper or valve.

Data Information collected in a given test.

Data acquisition The identification and collection of information relating to the performance of a particular piece of equipment.

DC pressurization A test of the air­tightness of a building in which a fan is used to pressurize the building to a uni­form static pressure. The pressure differ­ential between indoors and outdoors is measured to determine the air-tightness of the building structure.

DCV See Demand controlled ventilation.

Dead band A range of the controlled variable in which no corrective action is taken.

Decay The spontaneous disintegration of an unstable atomic nucleus to form an­other more stable element or isotope of a lower atomic mass.

Decay method The time from the libera­tion of a given concentration of a tracer gas into a space before its concentration decreases to a set value in the air.

Decay rate The rate at which the concen­tration of an air pollutant decreases with time, due to absorption or precipi­tation.

Decipol A unit derived in an attempt to quantify odor concentration by the per­ception of odor.

Decontamination factor A logarithmic scale used to measure the collection effi­ciency of a particulate collection device.

Decontamination index The logarithm to the base 10 of the decontamination factor.

DDC Direct digital control. See Digital control.

Deflection of a duct The largest deforma­tion of a duct subjected to an imposed load, given as the measured difference in distance between a plane through the points of support and a plane through the lowest point of the duct under a load.

Deflection of a joint The largest defor­mation of a joint subjected to a positive or negative pressure, given by the mea­sured difference in distance from a refer­ence plane outside the joint to the joint with and without pressure.

Degasser A packed tower through urhich a fluid to be degassed flows. Air is forced through the fluid stream, strip­ping the gas from the liquid.

Degree-days Temperature data recorded over a 24-hour period as deviation from a certain base temperature used to deter­mine the operating costs of a heating or air conditioning system depending on the external climatic conditions.

Degree of enclosure The actual protec­tion offered by an enclosure in the con tain men t of a generated contaminant.

Dehumidification The removal of water vapor from a gas.

Dehumidification load The mass of water vapor to be removed from a space or a process in order to meet design conditions.

Demand-controlled ventilation (DCV) A Ventilation system in which the room airflow rate is governed by an automatic control that depends on the level of a given pollutant within the space. A typi­cal example Is Allowing the C02 in a space to reach a certain level before the extract fans come into operation. How­ever, in many industrial environments other pollutants control fan operation.

Demand mode The mode of air supply in which inhalation creates a negative pres­sure inside the face-piece, causing the regu­lator to release air into the face-piece. Respirators that operate in this mode are not recommended and have been largely replaced by respirators operated in the pressure-demand mode, in which the face — piece is maintained under a slight positive pressure at all times.

Demand ventilation A system that is ca­pable of supplying varying amounts of fresh air in response to either manual or automatic control. See Demand con­trolled ventilation (DCV).

Density The measure of the amount of mass in a unit volume. The density of a gas is a function of its pressure and tem­perature. It can be determined by using the ideal gas laws.

Density Factor A factor used to correct the density of standard air or other gases for altitude, temperature, or mois­ture content.

Deposit gauge A collection device that records the deposition rate of solid or liquid particulate matter from the air.

Deposition Relating to either the dry or wet deposition of a particulate.

Dry: The interception and retention by surfaces of gases or particulate matter by diffusion, gravitational settling, or thermal forces.

Wet: The process of gas or particulate interception by sprays.

Depressurization A fan test used to deter­mine the rate of air leakage from a building by creating a negative static pressure.

Depth loading The deposition of particles mainly within the filter interstices, rather than on the filter surface. Desiccator A sealed container containing a water-absorbing substance such as silica gel or calcium chloride used to dry test materials in the laboratory.

Design standards The appropriate indus­trial, national, or international stan­dards covering


Air filters

Clean rooms


Contamination levels



Noise and vibration


Desorption The removal of adsorbed ma­terials from a solid sorbent by the use of a solvent or the application of heat, Desulfurization The removal of sulfur from flue or other sulfur-containing gases.

Detector tube A direct method for identi­fying airborne contaminants, also known as length-of-stain tube. It is a convenient tool for detecting and quan­tifying contaminants in field or emer­gency situations.

Determinant A chemical metabolic prod­uct of the change in the body’s chemistry caused by exposure to a pollutant. The level of determinant is measured in a bi­ological sample collected from the ex­posed worker, and compared to the biological exposure index (BEI). Determination The analytical measure­ment of a pollutant.

Detoxification The process of decompo­sition of toxic substances in the body to produce harmless substances, which are duly eliminated from the body.

Deviation Difference between a set point and the controlled variable in a control device at any instant.

Dew point The temperature at which a particular gas starts to condense on a cool surface.

Dew point, acid The temperature at which acid vapor in a gas stream con­denses out of the flow onto a cold sur­face or in a cold gas stream.

Dew point depression The difference in temperature between the wet and dry bulb readings.

Differential pressure The difference in pres­sure between two locations in a fluid.

Diffuser A device of variable cross-sectional area used to spread airflow into a space.

Diffusion The mixing of substances by molecular motion to equalize a concen­tration gradient. Applicable to gases, fine aerosols and vapors. (See Brownian diffusion.)

Diffusion effect The capture of particles due to Brownian motion.

Diffusion of particles The transfer of small particles and gas molecules into the surrounding air due to concentra­tion difference.

Diffusiophoresis See Stephan flow.

Digital control A control loop in which a microprocessor-based controller directly controls equipment by means of sensors. Its operation depends on a series of on-off pulses arranged to convey information.

Dilution The reduction in concentration of a solute by the addition of solvent.

Dilution equations Mathematical equa­tions that allow the determination of the decay rate of a pollutant in a space due to mechanical or natural ventilation.

Dilution ventilation or general exhaust ventilation A mixed airflow designed to dilute the contaminants within a space to required safe concentration limits. The air is extracted from the space as a whole rather than from the zone of pollution generation.

Direct-fired heater A heat generator that allows the combustion products to be mixed with the air to be heated.

Direct flame incineration A fume control device in which organic pollutants in the waste gas stream are oxidized to form nonpolluting by-products.

Direct interception Particle removal from a gas stream by a filter with geometry such that the particulate matter does not deviate from the fluid flow lines.

Direct reading A sampling approach that provides immediate or very fast feed­back such as a meter or colorimetric method.

Directivity The characteristic associated with sound energy in the form of waves moving in a straight line from the source.

Discharge or entry loss of a louver The

Reduction in airflow caused by a louver. The discharge loss coefficient is equal to the actual airflow rate divided by the theoretical airflow rate at given pressure difference across the louver. If tested with the airflow in the reverse direction, the coefficient becomes the entry loss coefficient.

Discharge coefficient A dimen sionless number describing the energy loss that occurs when a fluid is discharged from an orifice.

Discharge stack A stack that conveys combustion products or other pollutants from a space directly to outdoors. The pollutants may be removed before the remaining gas is allowed to discharge into the atmosphere.

Discharge system A system that dis­charges unwanted gaseous, solid, or liq­uid products.

Dispersion The manner in which a pol­lutant spreads from its point of genera­tion, becoming diluted with distance from the source.

Dispersoid The particles involved in the act of dispersion.

Displacement air diffusion Air diffusion where the mixing of supply air and room air external to the air terminal de­vice is at a minimum. See also Air diffu­sion and Air terminal devices.

Displacement flow, actual Actual flow pattern in an enclosure, resulting in uni­form air distribution with virtually no mixing.

Displacement flow, ideal Ideal flow pat­tern in an enclosure, in which uniform air diffusion is provided without mixing.

Displacement ventilation Room ventilation created by room air displacement, by intro­ducing air at a low level in a space at a lower temperature than the room air.

Disposal The action involved in the dis­posal of waste matter.

Distance to the V isovel (displacement air) The maximum horizontal distance L from the center of an air terminal de­vice to the rectangle circumscribing the specific isovel. It is independent of the distance from the floor.

Distribution The act of conveying a me­dium from one point to another.

Distribution ducting The supply or ex­tract air ductwork, which conveys air from the plant room to the conditioned space and vice versa.

DND See Daily noise dose.

Door and inspection panel Sealed openings in air-handling plant and ductwork pro­viding access for cleaning or maintenance.

Door air leakage The leakage due to pressure differences through the crack around a door.

DOP (Dioctylphthalate) Generated parti­cles of this chemical are used in filter ef­ficiency tests.


A. The level or amount of exposure to a hazardous chemical or physical agent.

B. The level or amount of a chemical or ionizing radiation that has been absorbed, usually expressed as

Amount per weight of the exposed organism, e. g., mg kg-1.

Dose rate The amount of a pollutant taken or received by an individual per unit of time.

Dose-response relationship The toxico — logical concept that the toxicity of a substance depends not only on its toxic properties, but also on the amount of exposure or dose.

Downdraft A natural or mechanical downward airstream, either that may, due to its temperature and/or velocity, cause thermal discomfort. In the case of a stack discharge, the term downwash may be used for the downward air current in the lee of the chimney that takes the smoke and other emissions below the emission discharge level causing ground — level pollution.

Downdraft hood A hood positioned un­der a process that receives gases, vapors or dusts from the source above.

Downstream Relating to a position after a filter has treated a gas, or some dis­tance away from a measuring device.

Downwash See Downdraft.

Draft An airstream within an occupied zone that causes thermal discomfort of the occu pants due to its temperature and/or velocity. Also, the thermal uplift caused by density differences required to provide adequate air both for the combustion process and the re­moval of the products of combustion.

Draft proofing The process of filling in air gaps around a structure to reduce the rate of air interchange.

Draft risk (DR) The percentage of people dissatisfied by a particular combination of air movement and temperature.

Draft risk rating The percentage of people predicted to be dissatisfied due to draft.

Drag anemometer An instrument used for the measurement of wind velocity by measuring the drag forces.

Drag coefficient The coefficient relating to the influence of drag over a surface in either laminar or turbulent flow.

Drain plug or cock A removable plug or valve that allows water, condensate, or other liquids to be drained from a system.

Drift velocity The velocity of the air as it drifts from a high-pressure zone to a low-pressure zone in a building.

Drop of jet The vertical distance be­tween the lowest horizontal plane tan­gent to a specified isovel and the center of the core of an air jet.

Droplet A very small particle of a liquid suspended in a gas stream.

Dry bulb temperature See under Temper­ature.

Dry heat loss The sensible heat loss from the body that takes place by raising the temperature of the air around it.

Dry scrubber An absorption system which uses a dry solvent directly in­jected into the gas stream.

Drying The process of fluid removal from a medium, either by heat or vacuum.

Drying oven An oven or stove used for the drying of a product.

Drying time The time necessary under given ambient or artificial conditions to remove a required amount of water or sol­vent from a manufactured product.

Dryness fraction of wet steam The mass of pure saturated steam contained in unit mass of wet steam.

Dual-circuit heat exchanger Combined air heater and air cooler battery, with inde­pendent pipework or ductwork circuits for the heating and cooling media.

Dual duct unit An air terminal unit as­sembly consisting of two-ducted air inlets and a means of automatically adjusting the mixing ratio of the two air streams.

Du Bois area The total body surface area of A Person,

ADu = 0.203 W0’4^ H°-ns

See Area, body surface.

Duct A conduit to distribute supply air or to extract air from a space, or a boxed run in which pipework or electrical ca­bles are carried.

Duct board A rigid board of insulation material with one or both sides faced with a finishing material. The outer face is normally a vapor barrier and air bar­rier.

Duct branch Used to subdivide the flow from one or more ducts into two or more ducts, or, conversely, to unite the flow from two or more ducts into one duct (T pieces, Y pieces, cross pieces, etc.) It may or may not include diverting elements. Note: Rigid components of ductwork allow sound and vibration transmission; the fixing of flexible sleeves on branches reduces the magni­tude of this transmission.

Duct connection component Items in­tended to facilitate the joining of two components of ductwork, including




Cleats, and

Slip joints.

Duct design The process of sizing duct­work to ensure the optimum perfor­mance in initial costs, running costs, and the distribution of air in a duct distribu­tion system. The design techniques may be

Constant pressure drop,

Constant velocity, or

Static regain.

Duct fitting Component of ductwork in­corporating a change in one or several of the following:

The length of the duct.

The orientation of the duct.

The shape of the straight length of the duct.

The area of the cross-section of the duct.

The direction of the duct, e. g., bend, elbow, or tee.

Transformation affects a change of area and/or the form of the cross-section. If the transformation is continuous, then an area reduction is termed convergent and an area increase is termed divergent. If the transformation is abrupt, then an area reduction is termed an abrupt con­traction and an area increase is termed an abrupt enlargement.

Duct sealing Measures taken to ensure that the air distribution system has an airtight seal.

Duct support The spacing of hangers or supports on a duct run to ensure that it is capable of self-support with any im­posed load.

Duct transformation See Duct fitting.

Ducted fan See Fan.

Ductwork components Individual ele­ments of ductwork, which are intended to be joined together at the time of in­stallation. These components are of var­ious types. See also Duct connection component and Duct fitting.

Duration limited exposure (DLE) The rec ommended maximum time of exposure, in h.

Dust The solid particulate matter formed by the breaking up of larger particulates by mechanical action. The particles range up to 75 |xm in diameter; larger particles are classified as grit.

Dust cake The dust layer that builds up on a fabric filter, initially improving its collection efficiency.

Dust collection mechanisms The various means by which particulate matter is collected, which may be classified as

Dynamic dry: The dust is collected under dry conditions.

Dynamic wet: The dust stream is exposed to a liquid such as water to improve the collection efficiency.

Dust disposal The methods for safely dis­posing of the collected dust.

Dust explosion An explosion caused by the ignition of certain dusts allowed to exceed a given volume for volume con­centration in air.

Dust fall The deposit rate of grits and dusts collected from the air in a measur­ing instrument.

Dust-holding capacity The weight of dust retained by a filter under specified test conditions.

Dust porosity The porosity of the dust cake, which has a direct influence on the filter pressure drop.

Dust suppression The preventive mea­sures taken to eliminate or reduce the spread of dust generated by a process into surrounding areas.

Dwelling The portion of a building in which people live.

Dynamic pressure The pressure equiva­lent of a fluid velocity at a given point.


Ecology The interactions between liv­ing organisms and their environment.

Economic velocity The velocity at which gases or fluids are conveyed to ensure that running costs are kept at an eco­nomic minimum and that damage is not caused by erosion.

ED2S The dose of a toxic product which has an effect on 25% of the exposed population.

Eddy A current in a fluid that moves in a direction contrary to that of the main stream, often having a rotary motion.

Eddy diffusion The interchange of liq­uids, gases, or vapors that takes place in an eddy current.

Effective area, air terminal The net area determined aerodynamically from an Ak Factor.

Effective drift velocity The velocity re­sulting from air flowing from one zone to another due to a pressure differential.

Effective length, duct The dimension that a straight duct contributes to the length of an air distribution installation.

Effective length, fitting The dimension that a duct fitting contributes to the length of an air distribution installation.

Effective mechanical power The energy spent in overcoming external mechani­cal forces on the body, in W, normally ignored for most activity.

Effective radiant heat flow The heat ex­change by radiation between the walls of the enclosure and the human body, Eeff, in W nr2.

Effective radiating area of a body The net effective radiating area of a body ex­posed to its surrounding.

Effective specific gravity The true spe­cific gravity of an extract gas stream, as opposed to the specific gravity of the air alone.

Effective stack or Chimney height The

Sum of the stack height and the effective plume rise, determined for the buoyancy plume and its associated efflux velocity.

Effective temperature (ET) See Tempera­ture, operative.

Efficiency The useful energy output of a device divided by the energy input into the system.

Efficiency average The efficiency of an item of equipment, such as a filter, boiler, or heating or cooling coil over its life cycle.

Efficiency, counting The proportion of particles in a volume or mass flow that are counted as they pass through the sensing element of an optical particle counter.

Efficiency, dust spot The capability of a filter to remove the staining portion of at­mospheric dust from a gas under set test conditions, expressed as a percentage.

Efficiency, filter The ratio of the number of particles retained by a filter or other air cleaning device to the number of particles entering, expressed as a percentage.

Efficiency, fractional See Efficiency, parti­cle size.

Efficiency, initial The efficiency deter­mined prior to the first loading cycle in a filter test.

Efficiency integral See Efficiency, overall.

Efficiency, local The efficiency of a given point on a filter at set operating condi­tions.

Efficiency, minimum The minimum effi­ciency obtained during the performance classification of a filter.

Efficiency, overall The average efficiency of a filter or other item of equipment under set operating conditions.

Efficiency, particle size The ability of a collection device to remove particles of a specified size or size range.

Effluent Any unwanted material, such as water or exhaust gases, discharged into the environment.

Efflux velocity The discharge velocity of waste gases from the top of a stack.

Egg crate straighteners A lattice device inserted in a ductwork run to straighten the airflow vortex after a bend or other directional change.

Ejector A device used to provide a pri­mary airstream into which the contami­nated air is entrained for subsequent removal. Used when corrosive products, high temperatures, fan blockage by par­ticulate matter, or fire or explosion risk make a fan unsuitable.

Electric control An electrical device that controls some mechanical function, such as a damper control.

Electrical properties Relating to the resis­tance, electrical capacity, and insulating characteristics of a conductor or electri­cal device.

Electromagnetic interference Interference created by rotating electrical equipment causing problems in areas such as micro­electronics clean rooms.

Electronic air cleaner A device used to clean particulate matter from a gas stream, consisting of a fan and an elec­trostatic precipitator.

Electronic control A control system oper­ating on low voltage, making use of solid-state components to amplify input signals from which the control functions are performed.

Electronic filter A filter incorporating an electrostatic precipitator. A fibrous filter that has its collection efficiency electro­statically enhanced.

Electrostatic The properties of electrically charged bodies, and the resulting associ­ated electrical phenomena that occur in the immediate vicinity of these materials.

Electrostatic charging The creation of a different electrostatic charge between par­ticulate matter and droplets. An increased efficiency in contact between the dust par­ticle and the droplets is achieved.

Electrostatic filter See under Air filter.

Electrostatic force A field in which sta­tionary electrically charged particles are subjected to a force of attraction or re­pulsion, as the result of another station­ary electric charge.

Electrostatic precipitator A filtering sys­tem for the removal of particles from an air stream by giving them an electrical charge. The charged particles are at­tracted to plates of opposite polarity onto which they adhere. The precipita­tors are classified as




Low Voltage,

Medium Voltage, or

High Voltage.

Electrostatic shocks Electric shocks expe­rienced by occupants due to a static dis­charge. Increasing the humidity and using non-static materials reduce the frequency of such events.

Element of distribution See Air distribution.

Eliminator plate A plate that mechani­cally separates droplets of moisture from a gas passing through it.

Elutriation The separation of particles in a fluid by gravity, which allows those with the greater falling speed settle as the fluid flows through an elutriator.

Emission The undesirable liberation of a dust, gas, or vapor from a process, ei­ther indoors or outdoors.

Emission factor A value representing the average amount of a pollutant that is emitted from a particular source in re la don to the amount of product.

Emission generation The volume or mass of a pollutant liberated in unit time.

Emission limit The maximum design or statutory values of the given emission of a given pollutant in a work area.

Emission rate The rate of pollutant dis­charge into the surrounding atmo­sphere.

Emission standard The allowable quan­tity of a pollutant that can be discharged from a particular process. It may be ex­pressed as

Mass discharge over a given time period, kg h_1,

Mass of pollutant per mass of processed material, g kg-1,

Parts of the pollutant in a unit volume of air, ppm, or

Mass of pollutant per unit volume of the gas in which it is discharged, mg m”3.

Emissivity (e) The ability of a surface to emit radiant heat transfer.

Emphysema, pulmonary The swelling and breaking down of the air sacs in the lungs. This reduces the area available for oxygen and carbon dioxide exchange within the lungs.

Enclosure A box, cupboard, or room in which a toxic process is carried out in safety.

Enclosing hood An extract hood, which partially or completely encloses the point of pollution generation.

End-of-service-life indicator (ESLI) A warning system that alerts a respirator user that the cartridge or canister is ap­proaching the end of its usefulness.

Energy The capacity of a body for doing work. Mechanical energy may be either potential (by virtue of the body’s posi­tion), or kinetic (by virtue of its motion).

Energy balance The arithmetic relation­ship between the energy input and out­put of a system.

Energy conservation Measures taken to reduce the use of energy.

Engineered control The removal or re­duction of a hazard through implemen­tation of an engineered solution, such as materia) substitution, process change, or installation of an exhaust ventilation system.

Enthalpy The total heat content of a gas.

Entrainment velocity The velocity in a jet stream that effectively entrains the dust or gas particles that surround it.

Entropy A function of the state of a sub­stance related to order or disorder. The entropy increases as the substance re­ceives heat.

Entry The breaking of the plane by any part of the body at the opening of a space that requires an entry permit.

Entry loss The loss of energy in the mov­ing stream that takes place as a fluid en­ters an opening.

Environment Indoor or outdoor condi­tions, including pollutants, thermal con­ditions, moisture, noise, and light.

Environment impact statement (EIS) A document used in the United States that details the influence of proposed Federal legislation on the environment.

Environmental temperature A design value used by the CIBSE in heating and cool­ing calculations, equal to 0.33 Ba + 6r.

Epidemiology The study of the develop­ment and cause of diseases.

Epidermis The outermost skin of the body, through which some dangerous chemicals can be absorbed into the body.

Epigenetic carcinogen A substance that causes cancer through a mechanism other than interaction with the genetic material.

Equilibrium dust content The amount of dust held on a clean filter cloth, which after a period of time remains approxi­mately constant.

Equivalent diameter of a duct The diam­eter Dc of a duct that will cause the same pressure drop at the same friction factor at equal flow as a given straight rectan­gular duct.

Equivalent diameter of a particle The di­ameter of a standard-densitv sphere whose motion would be similar to a given real particle, which may not be spherical.

Equivalent evaporation The evaporation that takes place in a boiler at or above

100 °c.

Equivalent exposure The exposure to a harmful product experienced by a worker. It is the sum of the exposure fraction of each component in a mixture.

Equivalent leakage area (ELA) The speci­fied design pressure difference that al­lows a given air quantity to pass through a set orifice area.

Equivalent pressure The pressure corre­sponding to standard density,

Equivalent temperature A synthetic com­fort scale that takes into account the effects of dry bulb temperature, air movement, and mean radiant temperature.

Equivalent warmth An early UK thermal comfort scale devised by Bedford.

Ergonomics The science dealing with the application of information on physical and psychological characteristics to workplace design.

Erosion control The protection of the containing material, which stops the fluid flow, from gradual removal by par­ticulates or gas bubbles.

Error An individual or cumulative mis­take made in any experiment or test.

Eupatheoscope An early device used in the UK for the assessment of thermal comfort.

Evacuated container A sealed vacuum sampling container, which is opened for the collection of a given material.

Evaporation The process of conversion of a liquid to a vapor, without necessar­ily reaching the boiling point.

Evaporative cooling The cooling of a body by virtue of latent heat removal to the surrounding environment.

Evase An increase in the size of a duct­work section.

Excess air Air supplied to a combustion process over and above the air theoreti­cally required for efficient combustion.

Exchange rate The doubling of sound en­ergy for each increase of 3 dB.

Excursion limit A time-weighted average of pollutant exposure over a length of time specified by OSHA that cannot be exceeded during the working day. See Peak limit.

Exfiltration The uncontrolled leakage of air from a building, either by natural or mechanical means.

Exhausting The process of removing gases or vapors from a space or process and discharging them safely to outdoors.

Exhaust air See Air, exhaust.

Exhaust air (EHA) classification CEN/TC 156 classifies exhaust air (EHA) into four categories related to the extract air (ETA) classifications.

EHA Classification

Pollution Level


EHA 1: Low pollution

ETA 1, or ETA 2 after cleaning.

EHA 2: Moderate pollution

EЬTA 2, or ETA 3 after cleaning.

EHA 3: High pollurion

ETA 3, or ETA 4 after cleaning.

EHA 4: Very high pollution

ETA 4.

In industrial ventilation, the bound­aries selected must be clearly stated for each application. See Extract air (ETA) classification.

Exhaust rate The controlled quantity of air, gases, vapors, and particulate matter that is removed from a space or process.

Exhaust ventilation The removal of pol­luted air from either a point source or a number of positions in a space direct to outdoors.

Expansion joint A flexible joint in a run of pipework or ductwork that allows expansion or contraction.

Experimental variance Permission granted by OSHA for the use of an alternative method of worker protection during an approved experiment to demonstrate or validate new’ safety and health techniques. The variance terminates upon study com ­pletion unless another type of variance is issued by OSHA.

Expired air temperature The air tempera­ture of the breath on leaving the nose.

Explosion, dust See Dust explosion.

Explosive limits The maximum and mini­mum concentrations of a mixture of gas, dust, or vapor in air or another gas which will explode if ignited.

Explosiveness A measure of the liklihood of a material to explode. For example, aerosol particles provide a very large sur­face area, accelerating the oxidation reac­tion, resulting in a high explosion risk.

Exposed area The particle capture area of a filter medium free from obstruc­tions, through which a gas flows.

Exposure The period of time an organism has been in contact with a certain con­centration of a pollutant.

Exposure by inhalation The toxic expo­sure of the body due to breathing con­taminated air.

Exposure limits Guidelines for worker exposure to physical agents and hazard­ous chemicals, usually expressed as an allowable time of exposure or an air concentration below which health haz­ards are unlikely to occur among most exposed workers.

Exterior hood A hood that is located close to but does not enclose the point of pollution generation.

External fan pressure difference The dif­ference between the total gauge pres­sure at the outlet of an air handling unit and the total gauge pressure at the inlet.

External fan pressurization See DC pres surization.

External work Energy used in overcom­ing external mechanical forces on the

F. TA 1: Low pollution

ETA 2; Modйra ce pollution ETA 3: High pollution

ETA 4: Very high pollution

Air Of the same quality as outdoors, with respect to humidity. From rooms with pollutant sources from humans and building materials only.

Air from occupied spaces that have impurities in addition to ETA 1.

Spaces in which moisture, chemical processes, etc., substantially lower air quantity Air containing impurities and odors detrimental to health in concen­Trations higher than regulations permit.

Offices, storage rooms, public service places with no pollution sources, including smoking.

Rooms with smoking, eating areas, etc.

Toilets, kirchens, garages, tunnels, car paries, solvent areas, laboratories, ere.

Industrial process areas, laboratories, etc.

Body, or the fraction of metabolic en­ergy related to mechanical efficiency.

Externally mounted air terminal device A unit such as a louver that prevents the ingress of rain, snow, birds, etc., into the ductwork.

Extinguishing system A system designed to extinguish fires by means of certain chemicals, gases, or water, either man­ual or automatic.

Extract air (ETA) classification Treated or untreated air that is removed from a space and discharged to outdoors. CEN/ TC 156 classifies extract air into four categories. (See top of page.)

See also Exhaust air (EHA) classifica­tion.

Extract duct Any duct through which air or another gas is removed from a space and expelled to outdoors.

Extract temperature differential The in­crease or decrease in temperature be­tween the supply air and the extract air.

Extract terminal device The grille or other device that is positioned in the main or branch duct that extracts air from a space.

Extract ventilation The mechanical ven­tilation arrangement to extract pol­luted air away from a space, either directly or by means of ductwork.

Extractor Any fan used for the extraction of air from a space.

Fabric arrester The collection of particu­late matter by means of a suitable fabric material.

Fabric collector A filter manufactured from various fabrics expanded on a frame or formed into a bag or sock.

Fabric filter See Fabric collector.

Face loading The weight of dust collected by a filter divided by the effective filter medium area.

Face velocity The average velocity across an opening or item of equipment, such as a hood, fume cupboard, heating or cooling coil, or filter.


Fan A rotodynamic bladed device that con­veys air at a given pressure and quantity in ductwork or a space by means of mechan­ical energy supplied.

Fan, abrasion-resistant A fan designed to minimize abrasion of the parts by the use of suitable materials.

Fan alignment The positioning of the fan shaft and its associated pulley in a line with the driving motor shaft and pulley.

Fan-assisted balanced ventilation Supply or extract ventilation within a space de­signed to provide the correct ratio of supply to extract air by means of one or more fans.

Speed change may be achieved by belts, pulleys, gear box: slid­ing couplings, or a variable-speed motor.

Variable speed control Damper control

Vane control

Variable blade pitch control

Adjustable pitch Fixed pitch

подпись: variable speed control damper control
vane control
variable blade pitch control
adjustable pitch fixed pitch
A damper positioned in the fan inlet or outlet will either increase or decrease the flow resistance and result in a variation in flow rate. This may be achieved either manually or automatically.

Fan inlet vanes alter fan performance by controlling the swirl.

The impeller blade angle is varied when the fan is in operation, normally only for axial flow fans.

The blade angle can be altered.

The blade angle cannot be altered from the manufacturer’s setting.

Fan-assisted exhaust ventilation Extract ventilation of a space by means of a fan, with the supply air provided by induced leakage into the building.

Fan-assisted induction terminal unit CEN/TC 156 defines these as constant flow or variable flow.

Constant flow: An assembly within which the primary airflow rate is modulated and mixed with air induced from the surroundings by means of a fan (also known as Series type).

Variable flow: An assembly within which the primary airflow rate is modulated and mixed with air induced from the surroundings by means of a noncontinuous running fan that provides a variable flow in response to thermal loads (also known as Parallel type).

Fan-assisted supply The ventilation of a space achieved by means of powered air movement components in the supply air.

Fan, conveying A fan used for the con­veying of particulate matter entrained in the air stream.

Fan, corrosion-resistant A fan constructed from materials designed to withstand the corrosive properties of the gases be­ing carried.

Fan curve A curve relating the total pres­sure and airflow rate for a fan.

Fan, dust A purpose-designed fan that is capable of extracting a dust-laden gas.

Fan energy The energy required by a fan in order for it to provide a given airflow rate against a set resistance.

Fan, flameproof A fan with a flameproof motor and bearings.

Fan, gas-tight A fan with a casing that will provide a set air leakage rate at a given operating pressure.

Fan, general-purpose A fan used to han­dle air that will not affect its working life, i. e., with no special requirements for temperature, moisture, or corrosive, abrasive, or flammable properties.

Fan, hot gas A fan capable of handling hot gases of a given temperature for a set operating time, normally designed to be installed with its motor outside the gas stream.

Fan, impeller tip diameter The maxi­

Mum diameter measured over the tips of the fan blades.

Fan inlet The opening through which the air enters the fan casing, either rectangu­lar or circular.

Fan installation

Fan laws The equations that describe the relationship between fan flow rate, pres­sure, density, power, size, rotation speed, and noise levels.

Fan motor systems The methods by which an electric motor drives a fan or pump system.

Fan, non-clogging A fan with an impeller designed to reduce the clogging of the material being handled.

Fan outlet The opening through which air lea ves the fan, either rectangular or circular.

Fan pressure

Static The fan total pressure minus rhe dynamic pressure corresponding to the mean air velocity at the fan outlet. The fan static pressure is the bursting or collapsing pressure on the enclosure.

Total The algebraic difference between the mean total pressure at the fan outlet and the mean total pressure at the fan inlet.

Velocity (or dynamic) Pressure associated with the kinetic energy in the air stream in the fan exerted in the direction of flow.

Fan section of air handling unit A unit in which one or more fans are housed.

Fan, spark-resistant A fan designed to re­duce the risk of spark generation from stationary or moving parts.

Fan, special-purpose Any fan selected to overcome the shortcomings of a General­purpose fan.




A fan in which air enters the impeller with an axial direction substantially parallel to the radial plane.

The impeller is defined as backward curved, inclined, radial, or forward curved depending on whether the outward direction of the blade at the periphery is backward, inclined, radial, or forward relative to the direction of the rotation.

Axial flow

A fan in which the air enters and leaves the impeller axial to the fan.


An axial flow fan which has two impellers arranged in series and rotating in opposite directions.

Reversible axial flow

An axial flow fan, specially designed to rotate in either direction.


A fan with an impeller with a small number of broad blades of uniform material and thickncss designed to operate in an orifice.


An axial flow fan mounted in an orifice or spigot.


A fan in which the direct drive motor is separated from the airstream, reducing corrosion rate, allowing higher operating temperatures, and reducing wear and tear on bearings.

подпись: type description
centrifugal a fan in which air enters the impeller with an axial direction substantially parallel to the radial plane.
the impeller is defined as backward curved, inclined, radial, or forward curved depending on whether the outward direction of the blade at the periphery is backward, inclined, radial, or forward relative to the direction of the rotation.
axial flow a fan in which the air enters and leaves the impeller axial to the fan.
contra-rotating an axial flow fan which has two impellers arranged in series and rotating in opposite directions.
reversible axial flow an axial flow fan, specially designed to rotate in either direction.
propeller a fan with an impeller with a small number of broad blades of uniform material and thickncss designed to operate in an orifice.
plate-mounted an axial flow fan mounted in an orifice or spigot.
bifurcated a fan in which the direct drive motor is separated from the airstream, reducing corrosion rate, allowing higher operating temperatures, and reducing wear and tear on bearings.
Fan types Classifications of fans based on specific properties.

Fan tables Data provided by the manu­facturer that describes the relationship between the volumetric output ot a fan, energy requirements, and noise level for a given fan operating at dif­ferent static pressures.

Fanger’s comfort equations The various equations devised by Professor Fanger relating to activity, clothing, vapor pres sure, mean radiant temperature, air tem­perature, and air velocity.

Federal standards Standards laid down by federal governments covering certain control aspects.

Female connection A circular sleeve used to join two duct or pipe components to­gether. The male ends of the two com­ponents are inserted into the female connection.

Fiber counting A microscopic technique which is of particular relevance to asbes­tos, where the fibers are counted on a filter paper.

Fiberglass A filler having a glass fiber me­dium.

Fibrous filter Any filter consisting of a mass of fibers as opposed to a mesh.

Pick’s law States that the molecular diffu­sion of water vapor in a gas without ap­preciable displacement of the gas is analogous to the conduction of heat, and is governed by a similar type of law.

FID See Flame ionization detector.

Field blanks Sample media that are ex­posed to the same conditions as the me­dia use for the actual sampling but are not connected to a sampling pump. See also Laboratory blanks.

Film badge A personal dosimeter con­taining photographic film that is dark­ened by ionizing radiation, used to evaluate the degree of ionizing radiation exposure in comparison to a control film.


Filter Any medium used for the separa­tion of solid, gaseous, or liquid con­taminants from a gas or fluid stream. The collection efficiency depends on the materials used. Some types of fil­ters are listed here.

Activated carbon A canister filter containing a porous form of pure carbon, which is capable of adsorbing gases and removing odors.

Automatic roll A roll filter that constantly or intermittently provides a clean portion of filter in the air stream by means of a pressure switch activating an electric motor, which winds the filter from a clean spool to a dirty spool.

Bag An extended surface filter in the form of a pocket or bag. A typical example of a bag filter is a mineral fiber bag made up of three layers, in which the first layer acts as a prefilter, the second is for fine filtration, and the third prevents fiber migration from the material used.

Brush An air filter constructed from a medium of intermeshing brushes.

Cartridge A replaceable in-line filter.

Cellular Replacement filter elements, which may be installed in a multiple, bark, or wall structure.

Cleanable A filter that, Having Collected a given amount Of Particulate matter, can Be removed From the filter frame, Cleaned, and Reused.

Coarse A filter positioned before A Fine filter to remove the larger particulates in order to extend the operating time of the fine filter.

Cylindrical A filter contained in A Cylindrical form.

Disposable The opposite of A Cleanable filter, which after collecting a certain dust burden is thrown away.

Dry cell panel A dry filter mounted in a rigid frame. In the past these were manufactured from woven fabrics and felts; however, synthetic fibers are replacing these. They have fiber diameters of 20 |im with average spacing of 300 (i. m and allow air velocity in the 2 m s-1 range.

Electret A type of filter that does not require a power supply, and depends on the use of a filter medium with a permanent charge. Best performance is achieved with dry air.

Electronic A fibrous filter that is electrostatically enhanced.

Electrostatic The filter in an electrostatic precipitator.

Fabric A filte made of either woven or felted textile.

Final The last filter in a system of A Multiple array of filters.

Fine A filter made up of fibers about 1 (j, m in diameter, with spacings of about 10 (Am, and air velocity in the 0.02 to 0.1 m s_1 range.

HEPA A high-efficiency particulate air filter designed to deal with particles below 1 (j. m with efficiencies

Of 99.95 or better. Air velocities are

0. 03 m s_i or less.

Insertable A freely removable filter fitted in a frame.

Louver A filter pleated in a louver form to increase the face surface area.

Membrane A filter that incorporates A Membrane As The collection medium.

Metal A filter constructed from metal mesh, fibers, or sintered porous metal.

Panel A shallow parallel-faced filter Element Or cell.

Passive electrostatic A mechanical filter in which the medium is electrostatically charged without the aid of A Continuous external power supply.

Pocket An extended-surface filter in which the medium is formed into pockets or bags through which the dust-laden gas flows. They may be supported by the air pressure or self­supporting.

Primary A filter that removes airborne particles 5 |xm and larger, normally supplied in panel types.

Roll A roll of filter medium on a drum that advances clean new material as the filter becomes clogged that may be manually or automatically driven.

Second-stage Filters for particulate matter from 0.5 to 5 p. m. They have An Extended face area in order to reduce the through velocity.

Self-cleaning An air-cleaning device that has the ability to be mechanically or chemically cleaned.

Sorption A filter that removes gaseous or vapor contaminants from a gas stream by an adsorptive or absorptive process.

Ultra-low-penetration air filter (ULPA) A filter that has a

Penetration of less than 0.0005%, measured under CEN test conditions.

Viscous Filters constructed of a metal or synthetic mesh wetted with oil to retain dust particles.

Filter, air See Air filter.

Filter class A certain range of filter per­formance characteristics as specified by international standards.

Filter element The supporting housing of a filter medium.

Filter insert The replacement part of A Fil­ter medium that is inserted in the filter housing.

Filter medium A material used for filtering particulate matter from gases or liquids.

Filter medium face velocity The volume flow’ rate divided by the effective area of the filter element.

Filter pack A filter medium uniformly folded and interleaved with spacers.

Filter section of an AHU The section of an air-handling unit that contains a filter.

Filtration The process of removing particu­late or gaseous matter from a fluid stream.

Final control element Any device that changes the value of a manipulated vari­able, such as a damper.

Fire damper See Fire and smoke damper.

Fire point The temperature at which a liquid gives off sufficient flammable va­por to produce sustained combustion.

Fire and smoke damper A device used to isolate one area of a building from fire or smoke in another, or one part of a duct run from fire or smoke in an­other. The device is mechanically or electrically operated in case of fire.

Fit test A method for evaluating how well a respirator seals against the wearer’s face. OSHA requires that a respirator be found to have a satisfactory fit before it can be used.

Fixed air terminal device A component that has fixed (nonadjustable) parts.

Fixed contamination A term used in the context of radiation for nonremovable contamination.

Fixed-direction grill A grill in which the discharge direction of air velocity is nonadjustable.

Fixed non-directional grill A grill in which the discharge direction may be fixed to en­sure the correct discharge pattern.

Fixing accessory of an ATD Any compo­nent that provides easy fitting and re­moval of air terminal devices.

Flame ionization detector (FID) A non­specific air-sampling instrument used to identify the amount of a substance by measuring the absorption of electrons resulting from its ionization as it passes through a hydrogen flame. It is generally used for detecting organic compounds, specifically hydrocarbons.

Flame-retardant Any item in a HVAC system that will slow the passage of flames, should a fire occur.

Flammable atmosphere Any atmosphere that represents a fire or explosive haz­ard by virtue of gases, vapors, or dusts contained in it.

Flammable limits The minimum and maximum concentrations of a gas or va­por in air which can be ignited and sus­tain a self-propagating flame.

Flange A connection device that allows two sections of duct or pipe to be bolted together.

Method of control


Mechanical, constant flow rate

Self-actuating and deriving its energy from the airstream to maintain the constant flow rate function.

Mechanical, variable flow rate

Self-actuating and deriving its energy from the airstream to maintain the constant flow rate function and having facili­ties for resetting the required value depending on an exter­nal svstem.

Pneumatic or electric

Deriving the energy for maintaining the constant flow rate function from an external source. It can be either of the constant or variable type.


Deriving its energy from the dynamic pressure in the air­stream to maintain its constant flow rate function and can be either a constant or variable type.

подпись: method of control description
mechanical, constant flow rate self-actuating and deriving its energy from the airstream to maintain the constant flow rate function.
mechanical, variable flow rate self-actuating and deriving its energy from the airstream to maintain the constant flow rate function and having facilities for resetting the required value depending on an external svstem.
pneumatic or electric deriving the energy for maintaining the constant flow rate function from an external source. it can be either of the constant or variable type.
system-powered deriving its energy from the dynamic pressure in the airstream to maintain its constant flow rate function and can be either a constant or variable type.
Flare A device used to burn rich mixtures of combustible waste gases containing pollutants.

Flash chamber A chamber provided to al­low the burning of a flammable gas in a process. In a refrigeration system, it is the separating tank between the expan­sion valve and the evaporator.

Flash point The lowest temperature at which a heated liquid fuel will ignite.

Flexible duct A non-rigid duct that can be bent, expanded, or compressed within set limits without fracturing its cloth, metallic, or plastic covering.

Floor temperature dissatisfaction risk The degree of dissatisfaction experi­enced by occupants in a space due to the floor surface temperature.

Flow The movement of a vapor, fluid, sludge, or gas in a conduit. The flow may be forced or due to gravity.

Flow coefficient The constant K used in a typical flow equation, V = K (AP)n.

Flow equalizer A component used in a conduit to reduce turbulence or eddies in the flow.

Flow exponent The exponent N of the pressure difference in the flow equation. Its value ranges from 0.5 for turbulent flow to 1.0 for laminar flow.

Flow meter A device used to measure gravimetric or volumetric flow in a con­duit.

Flow rate controller An item of equip­ment that will control the flow rate at a fixed value for a given pressure difference.

Flow rate pressure characteristic The re­lationship between the flow rate and a given pressure differential.

Flow reversal The backflow that occurs when a fluid changes its flow direction due to an imposed pressure gradient.

Flue A tube through which the gases gen­erated during the combustion process are discharged to the external environment.

Flue gas The mixture of gases produced during the combustion process.

Fluidized bed A bed of solid particles floating on air or any other gas, on which combustible matter is burnt.

Fluorescence spectroscopy Analysis in which the intensity and wavelength of the energy that is emitted from excited atoms is used to indicate the presence of certain compounds.

Fly ash Fine ash particulate matter found in flue gases.

Foam scrubber A cleaning device that uses foam as a collecting medium for particulate matter in a gas stream.

Fog A naturally occurring aerosol of wa­ter vapor containing water droplets less than 100 jxm in diameter, typically 15­35 jxm.

Forced draft The forcing of air by means of a fan into a closed chamber for com­bustion or other purposes. The pressur — ization of the chamber forces the air and combustion products up a stack.

Form view factor A factor which de­scribes the effects of the relative area of two surfaces, the geometry of the sur­faces in relation to each other, and the two emissivities on radiation heat ex­change between the surfaces.

Foul air Air that is unsuitable for respira­tion.

Fractional efficiency The efficiency of a device expressed for different fractions, e. g., the efficiency of a filter for particles of different sizes.

Free area velocity The velocity in a device where the flow’ is not influenced by changes in section.

Free delivery The actual volume flow from a fan outlet with no imposed sys­tem pressure.

Free-falling diameter Also known as sedi­mentation or Stokes diameter, the dia­meter of a sphere with the same terminal settling velocity and density as a nonspherical or irregular particle.

Freezing point The temperature at which a liquid solidifies. The same as melting point.

Freons The trade name for the series of chlorofluorocarbons (CFCs).

Fresh air Air taken directly from the exter­nal atmosphere, on the assumption that its purity is superior to that within the space to which it is being supplied.

Freshness Relating to the sensation of air entering the nose, creating a feeling of freshness rather than stuffiness.

Free area of an ATD The area available in an air terminal device for the dis­charge of air, as opposed to the actual area.

Free area ratio The ratio of an actual opening to the obstructed portion of that opening.

Friction The property possessed by two bodies in contact which prevents or re­duces the motion of one body relative to the other.

Friction factor Describes the relationship between the wall roughness, Reynolds number, and pressure drop per unit length of duct or pipe run.

Friction loss The pressure energy loss that takes place in duct or pipe flow. It is related to the Reynolds number, bound­ary layer growth, and the velocity distri­bution.

Frictional resistance The resistance to fluid flow resulting from the friction be­tween the fluid and the surrounding solid surface.

Frit A porous structure that breaks an airstream entering a solution into small bubbles, maximizing the surface area of air in contact with the solution and increasing the amount of contaminant dissolved in the airstream.

Full-face respirator A respiratory protec­tive device that covers the entire face from hairline to under the chin.

Fully adjustable air diffuser An air dif­fuser that has the provision of adjusting The Discharge flow direction through a wide angle.

Fuel A Substance suitable for the rapid and economic supply of heat by com­bustion.

Fume cupboard Cupboards of various ef­ficiency classifications in which danger­ous gases, dusts, and vapors are contained.

Fumes Small solid particulate matter nor­mally spherical in shape and ranging in size from 0.001 to 1 |xm.

Fumigation The result of a pollutant be­ing trapped under or in an inversion layer, or the process of using poisonous gases to kill insects.

Functional check A check on the perfor­mance of an item of operating equipment.

Functional measurement Relating the check on the performance of an item of equip­ment to the design specification.

Fungus A simple organism that contains No Chlorophyll, which may consist of One Cell Or Of many cellular filaments called hyphae. If allowed to grow in HVAC equipment it may cause allergic reactions.


Galvanized steel A zinc-coated steel sheet or plate with good corrosion resistance properties used for ductwork and other applications.

Gamma ray The shortest wavelength and highest energy type of all electromag­netic radiation. It originates in the nu­cleus of radioactive isotopes along with alpha particle, beta particle, or neutron emissions.

Garment insulation The degree of resis­tance to heat flow to and from the hu­man body chat a particular clothing arrangement will provide.

Gas A state of matter in which A sub­Stance completely fills the region In Which it is contained, No Matter how small the amount. Or any fuel In a gas­Eous form for use in an atmospheric or forced-draft burner.

Gas absorption The process of absorp­tion of gases that takes place in certain solids or liquids.

Gas adsorber A device for the removal of gaseous impurities from a gas or liquid phase, two methods are in use, physical adsorption and chemisorption.

Gas chromatograph (GC) An analytical instrument with an internal tube Or col­Umn that contains a solid sorbent, which allows some components of an injected sample to pass more quickly than others, separating the substances in the sample.

Gas collectors A sampling bag used to collect a sample for analysis.

Gas constant The coefficient R used in the ideal gas law, 8.3143 J mol-1 K~1.

Gas contaminants Any matter that con­taminates a pure gas or air above a given concentration level.

Gas monitoring The use of measuring or recording instruments to determine the concentration of a given gas within a space.

Gas physics The study of the various laws relating to the behavior of air or other gases.

Gas sensors Electrical or chemical devices that record the presence or level of A Cer­tain gas.

Gas scrubber A device for the removal of particulate matter from a gas stream by scrubbing the gas with a liquid.

Gaseous ion diffusion A method of charging particles in an electrostatic precipitator.

Gasket A semirigid or flexible sealing material fitted in the connection be­tween two surfaces.

Gauge (or gage) A measuring instrument used for the determination of pressure, flow, temperature, moisture, or the thickness of materials.

Gauge, altitude A pressure gauge which displays the force per unit area in terms of the height of a column of a named liquid required to exert that force.

Gauge, Bourdon See Bourdon gauge.

Gauge, compound A device that allows pressures below and above atmospheric pressure to be measured.

Gauge pressure The pressure of a system over and above atmospheric pressure.

Gauge pressure of a space The positive or negative pressure in a space with re­spect to its surroundings, due to wind or thermal forces or the relationship of supply air to extract air.

General-duty clause A clause in the OSH act that requires the employer to pro­vide a workplace that is free from recog­nized hazards likely to cause death or serious physical harm.

General exhaust ventilation (GEV) See Dilution ventilation.

Geometrical standard deviation A mea­sure of the range of particulate sizes present in a collection of particles.

Globe temperature The temperature of the surroundings (mean radiant temper­ature) as recorded by a black globe ther­mometer.

Glove box A sealed enclosure used for handling toxic products by means of long impervious gloves sealed to form part of the enclosure.

Grab sample A sample of air collected over a short time period in the workspace.

Grade efficiency See Fractional efficiency.

Gravimetric analysis The chemical analy­sis of materials by the separation of the constituents and their measurement by weight. This describes the gas mixture by giving the percentage by weight of each component gas. See also Volumet­ric analysis.

Gravitational settling The fallout of par­ticulate matter from a gas stream due to the gravity forces being predomi­nant over the flow velocity forces.

Gravimetric efficiency The efficiency of a dust collector to remove a given weight of particulate matter related to the total weight present in the air stream.

Gravity settling device A chamber in which a change in velocity and/or direc­tion of a dust-laden airstream allows the dust to settle into a collection hopper.

Grease absorption efficiency The ratio by weight of the quantity of grease retained by a grease filter to a reference quantity.

Green zone A compartment of secondary containment used in the atomic energy industry.

Greenhouse effect The retention of heat by the earth and the atmosphere due to cer­tain gases being transparent to incoming solar radiation but opaque to the longer — wave radiation back from the earth.

Grill An air terminal device with designed outlets for airflow and distribution.

Grit Particulate matter with a diameter greater than 75 ixm.

Ground level concentration The pollu­tion concentration at ground level re­sulting from the emission of a pollutant from a stack or other extraction point.

Grill type Description

Adjustable Grill intended to vary the direction or directions of the air

Delivered to the treated space consisting of one or more series of adjustable parallel ribs.

Fixed directional Grill intended to diffuse the air in one or more fixed directions,

Consisting of one or more series of fixed parallel ribs.

Fixed non Grill not intended to change the direction of air, consisting of

Directional parallel lamina ribs, perforated metal grid, wired grid, etc.

Grounding requirements All components that handle flammable gases and dusts are electrically grounded to provide a safe path for an electrostatic charge to leak away.

Guards A cover to provide occupant safety from any exposed moving parts or live electrical parts.

Guideline, air quality Any guideline that indicates the level and duration of a sub­stance above which it is assumed that the effects produced will adversely influ­ence animal or vegetable life.


Half-face respirator See Half-mask respi­rator.

Half-life The time required for the con­centration of a pollutant to decay to half its original value.

Half-mask respirator A respiratory pro­tective device that covers roughly half of the face, from under the chin to the bridge of the nose.

Halocarbon (HCFC) A class of refriger­ants that contain fluorine, chlorine, car­bon, and hydrogen.

Halogenated compound A compound that contains one or more of the ele­ments chlorine, bromine, fluorine, or io­dine as a part of its structure.

Haze The presence of particulate matter in the air, reducing visibility.

Header An element of a ductwork or pipework run which circuit branches are taken from.

Hearing threshold level (HTL) The low­est level at which a person can detect a sound or tone. Test frequencies used to establish HTLs include 500, 1000, 2000, 3000, 4000, and 6000 Hz.

Heat balance The thermal balance that occurs in a building when the heat gains equal the heat losses. Also known as balance point or break-even point.

Heat cramps Muscle cramps, usually of the legs and abdomen, caused by heavy sweating that results in an imbalance in the salts and minerals in the muscles.

Heat drop The difference between the heat contained in a working fluid at any two points in a cycle.

Heat engine A machine that enables me­chanical work to be done from heat en­ergy, usually by changes in the volume of a working fluid.

Heat exchanger A device that transfers heat from one fluid to another without allowing the fluids to come into contact with each other.

Heat exchanger, cross flow A heat ex­changer in which the fluid flow direc­tion in the shell is perpendicular to the direction of flow in the tubes.

Heat exchanger, counter flow or counter current A heat exchanger in which the flow inlet of one fluid is adjacent to the outlet of the second fluid and vice versa; the fluids flow in opposite directions.

Heat exchanger, parallel flow A heat ex­changer in which the fluids enter the same end of the heat exchanger and leave at the opposite end; the fluids flow in the same direction.

Heat exchanger, pipe A heat exchanger in which the transport medium changes between gaseous and liquid states.

Heat exchanger, plate A heat exchanger in which the fluids are separated by A Thin plate as opposed to a tube.

Heat exchanger, unidirectional A heat ex changer that is used to provide a heat exchange in one direction only.

Heat exhaustion The physiological con­dition resulting from the body suffering heat stress and loss of body fluids.

Heat island Relating to an area where the average air temperature is higher than the surroundings.

Heat load The heat input necessary to en ­sure that a treated space provides the in­ternal design temperature at a given external temperature.

Heat load, latent The heat input neces­sary to ensure that a treated space is

Maintained at given moisture content. This process is assumed to take place at a constant temperature.

Heat load, sensible The heat input neces­sary to bring about a temperature change.

Heat loss, dry The heat exchange that takes place from the human body to the surroundings by convection, radiation, and conduction but not by evaporation.

Heat output The useful heat output from a heat generator or a heat exchanger.

Heat pump A “reversed” heat engine or refrigerator that takes in heat from a body at low temperature and by the ex­penditure of mechanical work rejects heat to a body at a higher temperature.

Heat rash A rash that appears as small red spots on hot, moist skin. The spots are inflamed sweat glands.

Heat recovery The process of collecting waste heat from a gas or liquid and uti­lizing it for space heating or a process.

Heat recovery section of an AHU The part of an AHU in which a sensible or latent heat gain or loss takes place by means of a heat-transfer medium.

Heat removal lurninaire A light fitting provided with an extract duct from which the heat generated within the fit­ting, and a portion of that generated in the space, is extracted either directly to outside or for recirculation.

Heat, specific The amount of heat re­quired to raise the temperature of a unit mass of a substance one Kelvin.

Heat storage, body The amount of heat that can be stored in a body due to its tem­perature, mass, and specific heat capacity.

Heat stress index An index devised by Belding and Hatch to determine the ef­fect of extreme heat stress.

Heat stroke A serious acute condition caused by the elevation of the body tem­perature above the danger level. Symptoms can include redness of the face, reduced sweating, erratic behavior, confusion, diz­ziness, collapse, or unconsciousness.

Heat syncope Fainting that occurs in some people after standing for a Song period of time.

Heat transfer coefficient A proportional­ity factor used in an equation for deter­mining the rate of heat transfer.

Heating The process by which sensible heat is added to one medium from an­other.

Heating capacity The capacity of a heat emitter, heat generator, or heat ex­changer.

Heating coil A heat exchange coil (bat­tery) containing the primary heat trans­fer fluid positioned in a run of ductwork where it passes its heat to the secondary fluid.

Heating, direct Any heating system that does not have a heat-transfer medium, e. g., an electric fire or a gas fire.

Heating, indirect A heating system that makes use of a heat-transfer medium to convey heat from the heat generator to a heat emitter.

Heating water Water in a heat exchanger used for space and process heating. It can be low, medium or high tempera­ture, from 30 °C to 160 °C.

Heavy metal A general term relating to a specific group of metals, which as sus­pended and deposited particulates can contaminate the environment.

HEG See Homogenous exposure group.

Height allowance A percentage added to heat loss calculations to compensate for the vertical temperature gradient.

Hemoglobin The protein in red blood cells that binds with and transports oxygen.

Henry’s law States that the mass of a gas dissolved in a definite volume of liquid at constant temperature is pro­portional to the partial pressure of the gas.

HEPA (high-efficiency particulate air) fil­ters Also known as absolute filters, the large collection filter surface area pro­vides a high collection efficiency for par­ticulate matter.

Hertz (Hz) The frequency with which sound pressure changes. One oscillation per second is equal to one Hertz.

High potential hazard The health, fire, or explosion risk resulting from the pres­ence of certain materials in excess of a certain limit.

High-volume sampler A device used for extracting particulates from the air for analysis that requires a shorter sampling period than a low-volume sampler.

Histogram A diagram of the frequency of occurrences of values of a variable, grouped according to value in a number of separate ranges.

Hit and miss damper or valve A damper or valve consisting of two or more slot­ted slides operating in parallel.

Homogeneous exposure group A popula­tion or group of workers with similar exposure.

Hood A device that is located over a working area to collect any emissions generated at the source.

Hood, capturing A hood that has a suffi­cient flow rate to ensure that most con­taminants are drawn into the hood.

Hood entry loss The pressure drop (en­ergy loss) that occurs due to turbulence at the entrance to the extraction system.

Hood, receiving A hood intended to re­ceive generated contaminants at some distance from the source.

Hood static pressure The negative static pressure available at a hood.

Hopper A collection bin for the supply or disposal of powdered or granular mate­rial

Hot air column A rising column of low — density air inside or outside a building.

Hot film anemometer An instrument for the measurement of fluid velocity simi­lar to the hot wire anemometer, but more robust as it consists of a thin quartz rod covered with a film of plati­num rather than a wire.

Hot grid anemometer An instrument for the measurement of fluid velocity simi­lar to the hot wire anemometer, but with the heating and sensing elements are separated.

Hot wire anemometer An instrument for the measurement of fluid velocity by measuring the resistance of a fine plati­num or nichrome wire, which may or may not be shielded by a silica tube. The wire resistance is proportional to the temperature and the fluid flow rate.

Hot wire microphone anemometer An in­strument for the measurement of fluid flow.

Housing A device or enclosure that con­tains any item of HVAC equipment.

HTL See Hearing threshold level.

Human factors A term sometimes used synonymously with ergonomics, it may also refer to psychological and sociolog­ical aspects of ergonomic issues.

Humid air Air that is high in moisture content at a given temperature.

Humidification The process of adding moisture to air by spinning disk, ultra­sonic, steam, direct water injection, or other methods.

Humidification efficiency The ratio of the actual mass of water evaporated by a humidifier to the theoretical mass of water needed to achieve saturation at a given temperature.

Humidification load The mass of water that must be added to the airsteam in order to ensure that the design condi­tions are met.

Humidifier fever An illness caused by the growth of microorganisms in air cooling coils. These microorganisms or their generated toxins may be carried in the airstream to the conditioned space, causing an allergic response in suscepti­ble people.

Humidifier section of an AHU The part of an AHU in which water vapor is added to the air.

Humidistat A measuring and control de­vice used to control the humidity of a space.

Humidity The water vapor content present in atmospheric air.

Humidity, absolute The actual mass of water vapor present in a unit mass of air.

Humidity ratio The ratio of the mass of water vapor present in air to the mass of dry air.

Humidity ratio, saturation The humidity ratio of a gas at saturation.

Humidity ratio, expired air The mass ra­tio of water vapor to dry air in expired air.

Humidity, relative The ratio of the mole fraction of water vapor in moist air to the mole fraction of water vapor in saturated air at the same temperature and pressure.

Humidity, specific The mass of water va­por per unit mass of dry air.

HVAC The heating, ventilation and air conditioning system that circulates and delivers filtered, humidified or dehumidi­fied, and cooled or warmed air to the in­terior of a building.

Hybrid (mixed mode) ventilation A sys­tem that makes use of a mix of natural and mechanical ventilation. May be fur­ther subdivided into seasonal hybrid,

E. G., natural ventilation in summer and mechanical ventilation in winter or spa­tial hybrid, e. g., mechanical ventilation in core areas and natural ventilation at the perimeter.

Hydraulic efficiency The ratio of the ac­tual head to the ideal head.

Hydrocarbons Chemical compounds containing only hydrogen and carbon.

Hydrogen sulfide A highly toxic gas with the characteristic smell of rotten eggs.

Hydrograph An instrument that mea­sures and records relative humidity.

Hydrophobic Water-resistant or having a lack of affinity for water, usually said of a substance or material that does not absorb moisture.

Hydrostatic pressure The pressure at a point of a fluid at rest, due to the weight of the fluid above.

Hygrometer An instrument used for mea­suring the moisture content of the air.

Hygroscopic The ability of a material to absorb moisture.

Hyperbolic expansion The expansion of a fluid according to the law P V = C.

Hypothalamus The temperature control center at the base of the brain, which regulates body temperature.

Hypothermia The physiological state re­sulting when the deep core body temper­ature drops below 35 °C. It results in vasoconstriction and shivering in an at­tempt to conserve body heat.

Hypoxia A condition characterized by a deficiency of oxygen reaching the tissue.

Hz See Hertz.


IAQ See Indoor air quality.

Ideal gas A gas that obeys the ideal gas law.

Ideal gas law This relates to the proper­ties of a gas and can be represented in the form Pv = nRT.

IDLH See Immediately dangerous to life and health.

Ignition source A source that is of a high enough temperature or has enough en­ergy in a spark to cause the ignition of a gas or a material being conveyed.

Immediately dangerous to life and health (IDLH) A condition that poses a threat of exposure to airborne contami­nation likely to cause death or immedi­ate or delayed permanent adverse health effects, or that prevents escape.

Immission The rate at which a receptor of pollution encounters a pollutant.

Impact damage Damage caused to lungs, surfaces of ductwork, or fans by partic­ulate matter.

Impaction Collection mechanism where the contaminants collide with the sur­face of the filter by inertia, interception, or Brownian diffusion.

Impactor A general term for instruments that sample particles in the air by allow­ing them to impact on a retaining plate.

Impactor, cascade An instrument consist­ing of stages for producing successively increasing air velocities for collecting particles by size range.

Impeller tip diameter (fan or pump) The maximum diameter measured over the tips of the blades of the impeller.

Impingement The collision of a dust par­ticle upon a surface, leaving the particle on, e. g., a filter fiber.

Impinger A sample collector that resem­bles a graduated cylinder with a long tube fitted into a stopper. The inlet tube extends nearly ro the bottom of the outer tube, which holds the solution. The sampling pump is connected so that negative pressure is created inside the impinger, drawing air through the inlet tube into the solution, allowing the air to bubble up through the solution.

Implementation plan A plan that makes practical provision to ensure that set en­vironmental standards are met.

Impulse noise Noise of short duration,

I. E., three seconds or less. Also called impact noise.

Incineration The process of burning solid, liquid, or gaseous combustible wastes, leaving a sterile residue con­taining little or no combustible matter.

Inclined manometer A manometer in which the vertical movement of the liquid column is amplified by inclina­tion of the U-shaped reading tube.

Index number A number that is used to indicate general trends in a quantity.

Indoor air (IQA) classification Catego­ries defined by CEN 156 to classify the quality of indoor air.




Excellent air quality


Typical air quality


Low but acceptable air quality

Indoor air is also classified based on CO, concentration.





Dissatisfied (%)










Indoor air quality can be controlled using several methods.

Classification Description

No control A constantly running


Simple control A system that only runs

At the dictate of a sensor, such as an infrared sensor detecting movement

Direct control A system controlled by

A sensor that detects levels of indoor con­taminants within the space

Indoor air quality The actual quality* of air within a space compared with a given sample or standard, related to temperature, moisture, biological con­tent, and contaminant levels.

Indoor climate The actual temperature, moisture content, and air velocity within a space.

Indoor pollution Pollution inside a build­ing due to internally generated pollut­ants as well as external pollutants entering the building.

Induced air The quantity of secondary air entrained into a primary airstream.

Induced air temperature See Temperature.

Induced draft Air drawn through a fuel bed, a furnace, or a space by means of a fan situated beyond the item.

Induced leakage Leakage into an enclo­sure to equalize a pressure difference due to natural or mechanical ventilation.

Induction The process by which second­ary air is entrained into the primary jet air. The mixing process is due to the mo­mentum forces in the primary air jet.

Induction effect See Induction.

Induction ratio The ratio of entrained air to primary air.

Induction supply ATD An air terminal device in which the primary air from the duct induces secondary airflow from the treated space in such a way that a high rate of mixing between the air from these two sources takes place within the device.

Induction terminal unit An air terminal assembly which by virtue of the configu­ration of the primary air inlet(s) within the unit can induce secondary air from the surrounding atmosphere before be­ing discharged to the treated space. The flow rate of the primary air may or may not be variable. The inlet aperture(s) for the secondary air may be fixed or ad­justable by means of manual remote control.

Industrial hygiene The science and art de­voted to the anticipation, recognition, evaluation, and control of those envi­ronmental factors or stresses arising from the workplace which cause sick­ness, impaired health and well-being, or significant discomfort and inefficiency among workers or among the citizens of the community.

Industrial hygienist A person qualified in the associated sciences dealing with the industrial environment.

Inert gas A gas that does not react with other substances under ordinary circum­stances, e. g., nitrogen.

Inertia The resistance of a body to a change in its momentum or direction of motion.

Inertia bases Bases for the mounting of fans, pumps, or other rotating machines that are designed to eliminate the trans­fer of inertial forces to the structure.

Inertial deposition The deposition of par­ticulate matter that occurs under the in­fluence of an inertial force.

Inertial effects The force due to inertia equal in magnitude but opposite in di­rection to the accelerating force.

Inertial impaction This is the predomi­nant mechanism used in all particle col­lection devices.

Inertial separator A device used for sepa­rating bodies from one another, or from a fluid in which they are contained, by virtue of inertial differences.

Infiltration The leakage of air through the imperfections in a building struc­ture, due to thermal or wind forces.

Infiltration rate The rate at which outdoor air enters into a room through the imper­fections in the building structure, ex­pressed in air changes per hour or L s~*.

Influx The rate at which a gas enters a space.

Influx velocity The velocity of a gas as it enters an opening.

Ingestion The absorption of substances into the gastrointestinal tract.

Inhalable fraction Particles with aerody­namic diameters up to 10 |xm, which can enter the lungs.

Inhalation The process of breathing in, when the air enters the respiratory tract.

Inlet The opening that allows air or a contaminant into a system by either nat­ural or mechanical forces.

Inlet bells or boxes Aerodynamically shaped inlet ducts for a fan.

Inlet vane dampers Dampers inserted in the airstrearn at the inlet of a fan.

Inlet vanes Specially-designed adjustable vanes inserted in the airstrearn entering a fan inlet to control fan performance by producing a swirl of the gas in the di­rection of the rotation of the impeller.

Insertion length See Overlap length.

Insertion loss, weather louver The differ­ence in simulated rain penetration be­tween the test specimen and the calibration plate at the same test condi­tions.

Inspection panels See Door and inspec­tion panel.

In-stack filters A filter positioned in the discharge stack to remove pollutants be­fore the waste gases are discharged to outdoors.

Installation The complete plant arrange­ment.

Instantaneous Relating to the measure­ment of a variable, it may be either a peak reading or a measure at any other time during sampling.

Instrumentation A sensor that either sim­ply or automatically controls HVAC equipment or that records some particu­lar function of the plant operation, such as temperature, pressure, or flow.

Insulation of clothing The resistance to sensible heat transfer provided by a clothing ensemble, reflecting the intrin­sic insulation between the skin and the surface of the clothing, excluding the re­sistance provided by the layer of air sur­rounding the clothed body.

Integrated sampling Samples taken by drawing the air to be tested through the sampling medium, which is then ana­lyzed by a laboratory to determine the amount of contaminant transferred.

Interception A special case of impinge­ment, in which a particle is trapped on a fiber due to the effect of Van der Waals forces rather than inertia. The intercep­tion of a particle in a particle collection device occurs when the particle follows a gas streamline round a collector at a dis­tance less than the radius of the particle.

Interferent Any undesirable component in a sample to be analyzed that will ad­versely influence the instrument reading.

Interim order An official statement issued by OSHA allowing an employer to con­tinue operations under existing condi­tions while an application for a variance is being considered.

Intermittent sampling Any sampling pro­cess carried out for limited periods of time rather than continuously.

Internal energy The energy contained in a substance, which is its ability to do work.

Internal heating load Heat gains that oc­cur in a space from process loads, light­ing, solar gain, occupants, machines,


Internal leakage The leakage that takes place into an enclosure or ductwork from outside.

Internal pressure The pressure inside A Space or container, as opposed to the pressure outside.

Internal temperature The temperature in­side a space, as opposed to the external temperature.

Internally induced airflow rate of an ATD Volume of air induced into the primary airflow inside the air terminal device in unit time.

Internally mounted air transfer device See Air transfer device.

Interstitial Situated between the cells of a structure or part.

Interstitial condensation Condensation that occurs within the interstices of a mate­rial when the dew point is reached.

Intoxication The general state of the body caused by the effects of a toxic substance.

Intrinsic clothing thermal efficiency Re­duction of sensible heat exchange due to wearing clothes.

Intrinsically safe Instruments that can be safely operated in an explosive or corro­sive atmosphere.

Intermittent duty The duty of a device when operating on a part-time basis, as opposed to continuous duty.

Inversion The condition that occurs when the lapse rate is positive, i. e., tem­perature rises wTith height at a rate greater than the adiabatic lapse rate 3 °C per 300 m. In these conditions stagnant air pollution builds up and is trapped under this layer.

Involute A geometrical curve as used in a centrifugal fan casing.

Ionization The critical voltage at which gas molecules are separated into positive and negative ions in an electrostatic pre­cipitator.

Ionizing radiation Radiation that is capa­ble of causing ionization to occur, either directly or indirectly through interaction with matter.

Iris damper or valve See Dampers and


Irradiance The radiant flux striking a unit area of a surface.

Irradiation The exposure to radiation of any kind.

Isentropic operation A change in condi­tions at constant entropy.

Isokinetic A process in which the velocity at the entrance to the sample probe in a gas stream is the same as the velocity at a given point in the duct or stack at a given time.

Isokinetic sampling The sampling of a gas such that the motion of the gas en­tering the sampling device is identical to that of the gas being sampled.

Isolation The process of disconnecting a supply, electricity, fuel, or air.

Isotherm A line in a flow system or on a graph connecting points of equal tem­perature, or a mathematical or graphical relationship between two variables at constant temperature. Or a display us­ing lines on a drawing to show constant- temperature contour lines, as from ther­mal imaging with infrared techniques.

Isothermal change A process that takes place at constant temperature, such as the isothermal expansion of a gas.

Isotopes Atoms of the same element (hav­ing the same atomic number) that differ in mass number.

Isovel A line in a flow system or on a graph connecting points with constant velocity.


Jet A gaseous or liquid stream issuing from a slot, orifice, or nozzle.

Jet angle The angle at which a jet or ar­ray of jets diverges into a free space.

Jet, Coanda A jet attached to a surface.

Jet drop The downward change in the di­rection of a jet due to the difference be­tween its velocity or temperature and that of the ambient air. Also called jet fall.

Jet, enclosed A jet that is allowed to ex­pand within a channel or duct and then is constrained by the channel or duct walls.

Jet envelope The boundary between a jet and the surrounding air.

Jet fan See Fan functions.

Jet, free A jet which, on leaving an ori­fice, is allowed to expand freely without coming into contact with any surfaces.

Jet, isothermal An air jet of the same tem­perature as the space it is entering.

Jet, nonisothermal An air jet of a differ­ent temperature from that of the space it is entering.

Jet rise The upward change in direction of a jet due to the difference between its velocity and temperature and that of the surrounding air.

Jet spread The angle of divergence of a jet from its point of origin.

Jet, wall A jet that attaches itself to a sur­face. See also Coanda effect.


Kata cooling power The rate of cooling of a silvered or unsilvered kata ther­mometer due to the relationship be­tween the air temperature and the air velocity over the bulb.

Kata thermometer A thermometer that allows the air velocity to be determined by its cooling power.

Katharometer A device that compares the thermal conductivity of two gases, used to detect the presence of impurities in air.

Kelvin effect The electrical potential gra­dient caused by a temperature gradient along a conducting wire. Also known as the Thomson Effect.

Kinematic coagulation The scavenging of small particles by large particles, which increases the speed of the larger particles by differential settlement, such as in rain drops or from spray nozzles.

Kinetic energy The energy a body pos­sesses by virtue of its motion.

Kinetic theory A mathematical explana­tion of the behavior of gases on the as­sumption that gases consist of molecules in ceaseless motion in space. The molec­ular kinetic energy depends on the tem­perature of the gas.

Kirchhoff’s law The relationship that ex­ists between the absorptivity and emissiv — ity of radiating bodies. It is the capacity of a body to absorb radiation, which var­ies with the wavelength of the incident radiation and the angle of incidence.


Laboratory blanks Sample media that is not sampled 011, but is analyzed by the laboratory to detect contamination or other problems associated with prepara­tion and analysis of the samples. See also Field blanks.

Lag A delay between a change in a con­dition at one point in a system and its effect.

Lambert-Beer law The mathematical de­scription of the attenuation of a light beam by absorption and scattering by dust particles in the airstrearn.

Laminar flow Fluid flow in which the fluid particles move in straight lines par­allel to the axis of the pipe or duct.

Land breeze The air movement that takes place after sunset, when the land cools and air currents flow from the land to the cooler sea.

Langmuir equations The mathematical expressions that describe vapor adsorp­tion equilibria.

Lapse rate The rate of temperature in­crease with height.

Laser Light amplification by stimulated emission of radiation.

Laser anemometer See Laser Doppler an­emometer.

Laser Doppler anemometer An instrument for determining fluid velocity by mea­suring the difference in frequency be­tween the incident beam and that scattered from particles moving with the flow.

Latent heat The quantity of heat that is absorbed or released in an isothermal transformation of phase, in k) kg-1 °C1. Latent heat of vaporization The heat added during an isothermal change of phase from liquid to gas.

Laws of perfect gases

A. Boyle’s law The volume of a gas is inversely proportional to its pressure, at constant temperature.

B, Charles’ law

A. The volume of a gas is proportional to its absolute temperature, at constant pressure.

B. The pressure of a gas is proportional to its absolute temperature, at constant volume.

3. Joule’s law The internal energy of a given quantity of gas depends only on its temperature and is independent of its pressure and volume.

Laws of thermodynamics

A. First law Heat and work are mutually convertible.

B. Second law Heat will not pass from a colder to a hotter body spontaneously.

LCL See Lower confidence limit.

LDS0 (median lethal dose) A standard measure of toxicity indicating the dose of a substance that will kill 50% of a group of test organisms.

Lead dioxide candle A device for deter­mining the amount of sulfur dioxide in the air. The S02 reacts with a film of lead di­oxide to produce lead sulfate, which is measured to determine the concentration. Leakage The rate of fluid loss from an enclosure due to a pressure difference between the inside and outside of the enclosure.

Leakage function The relationship of the leakage occurring in a building to the pressure difference, measured in m3 h~! Pa“1.

Leakage path leeward Leakage of build­ing air that takes place due to structural openings on the downwind or sheltered side of a building.

Legionella pneumophila (LD) Infections, particularly pneumonia, caused by in­haling Legionella pneumophila and other bacteria from the family LEgionel — laceas in water droplets drifting from cooling towers, showers, etc.

LEL See Lower explosive limit.

Length-of-stain tube See Detector tube.

LEV See Local exhaust ventilation.

Lewis relationship The ratio of the con­vective heat-transfer coefficient to the evaporative heat-transfer coefficient.

Lesion An injury to the body due to the intake of certain atmospheric pollutants.

LFL See Lower flammable limit.

LIDAR An instrument that uses a Laser­radar to study the concentration and lo­cation of particulate matter by the re­flection or absorption of a laser beam.

Life cycle The design of any combination of plant items that considers the owing and operating costs of the plant.

Life cycle assessment (LCA) An analysis defined by ISO 14040 as “compilation and evaluation of inputs and outputs and the potential environmental impacts of a production system throughout its life cycle.”

Limestone scrubbing A process using a ground limestone and water mix to neu­tralize sulfur dioxide in waste gas prod­ucts.

Limit, opening The maximum sash open­ing that can be allowed on a laboratory fume cupboard to ensure safe working conditions.

Limit of detection (LOD) The smallest amount of contaminant that can be reli­ably detected by a particular analytical method.

Limit of quantification (LOQ) The small­est amount of contaminant that can be reliably quantified using a particular an­alytical technique.

Limit value A reference figure giving the allowable concentration of a chemical or biological agent in the air.

Linear air diffuser An air terminal device with single or multiple slots, each of which has an aspect ratio not less than 10:1. Each slot may consist of a number of separate elements and may or may not have an adjustable member, which allows the directions of the air delivered to the treated space to be varied.

Linear grill grill with an aspect ratio not less than 10:1.

Liquefied natural gas (LNG) Natural gas cooled to -162 °C to achieve a signifi­cant volume reduction, then stored un­der pressure.

Liquefied petroleum gas (LPG) Paraffin hydrocarbon gases comprising propane, butane, and pentanes derived from nat­ural gas wells and from the petroleum refining process that remain as liquids when stored under pressure in tanks and bottles.

Liquid entrainment separator Any device that removes and collects moisture present in an airstream.

Load In a ventilating or heating system, the magnitude of heat, airflow, or cool­ing the system must provide to meet the design conditions. The work the system must perform. The heating, ventilating, or cooling load requirement of a space or appliance.

Load, connected The sum of all the indi­vidual loads related to an HVAC system.

Load factor The ratio of the average de­mand to the maximum demand; may re­late to electrical, heating, or cooling load.

Load pattern The load change over time.

Load utilization factor Ratio of the effec­tive load in a given space to the load supplied.

Loading dust The selected synthetic dust used to determine the dust-holding ca­pacity of a filter.

Local air velocity The air velocity in the zone in which the design conditions have to be met. Or, the air velocity re­corded at a specific location in a space or in a jet stream.

Local exhaust ventilation (LEV) The re­moval of a contaminant at or near the point of its generation.

Local cooling The cooling of a given area in a space by means of chilled air jets or chilled water panels.

Local mean velocity The magnitude of the time-averaged vector of velocity at a point in an airstrearn.

Local response The response of an occu­pant or control device to changes in the Local Environment.

Local ventilation The transportation of Air into Or from a space near its point Of Use.

Long-term exposure limit (LTEL) An ex­posure limit requirement based on the as­sumption that the total body intake of a pollutant below this limit over an 8-hour working day will have no harmful effect on the worker over a working life. See also Maximum exposure limit (MEL), Occupational exposure limit (OEL), and Short-term exposure limit (STEL).

Louver An opening that allows air to enter or leave a space, which has inclined vanes to provide protection from the entry of rain, snow, and animals.

Low-leakage seal A seal on a Fluid­handling device that ensures system leakage is at or below a given level.

Low potential hazard A concentration of pollutants within a space, which will present A Very low hazard to the occu­pants or the plant.

Low-velocity ATD An air terminal device which is designed for thermally controlled ventilation, e. g., displacement flow appli­cations. See also Air terminal device.

Low-volume high-velocity (LVHV) The method of local exhaust using small hoods, which exhaust contaminants from a process at velocities of 50-100 m s-1.

Lower confidence limit (LCL) A statisti­cal procedure to estimate whether the true value is lower than the measured value.

Lower explosive limit (LEL) The lowest Concentration Of A substance In air at: Ambient temperature that will Explode if Ignited, expressed as a percentage Of The substance in the room air By Volume. See also Upper explosive limit (UEL).

Lower flammable limit (LFL) The lowest concentration of a substance in air that will sustain combustion.

Lower limit of a duct The algebraic dif­ference between the minimum limit of size and the corresponding nominal size.

Lubrication The use of oil or grease on moving parts in order to reduce the fric­tion force.

Lung function Relating to the transfer of oxygen from air into the blood and the disposal of carbon dioxide from the blood to the air.


Macropores Pores of diameter greater than 0.005 mm in the structure of an adsorbent medium.

Magnehelic dial gauges A measuring de­vice recording the static pressure of a fluid.

Make-up air Air introduced into a space to replace air that is being extracted.

Main air treatment The part of the treat­ment of air that, by virtue of the number of air handling functions involved or of the effect achieved, is considered the principal treatment.

Male connector A short circular sleeve to join two pieces of spiral duct together. The ends of the male connectors are inserted into the spiral tube ends.

Manifold A section in the exhaust air ductwork of an air treatment system into which exhaust air enters from A Number of orifices or ducts, or a header pipe in a fluid flow system that has branches.

Manometer An instrument that mea­sures pressure by fluid displacement in a U-shaped tube.

Manometer pressure The pressure re­corded on a manometer, measured in Pa or mm water gauge.

Manual damper or valve A metal or plas­tic flap used to control the rate of air­flow in a duct manually.

Manually adjusted ATD See Air termi­nal device.

Marginal irritant A material that is capa­ble of causing an irritation response af­ter repeated exposures.

Masking Relating to an additive intro­duced into the air supply in an attempt to neutralize or conceal an odor.

Masks A protective device complete with an approved filter designed to keep dan­gerous chemicals from being inhaled into the lungs.

Mass flow rate Mass of matter, which crosses a given surface, divided by time.

Mass spectrometer (MS) An instrument that identifies substances by causing them to be ionized and subjecting the resulting ions to a strong electromagnetic field.

Mass transfer The transfer of mass across a boundary, similar to heat transfer.

Maximum allowable concentration (MAC) An old American definition used before the term TLV came into use (the term is still used in Germany). See Threshold limit value (TLV).

Maximum body heat storage (Q max) The maximum value of the body heat gain achievable by the subject such that the resulting increase in body core tempera­ture does not induce pathological effect, in W h nr2.

Maximum exposure limit (MEL) The maximum concentration of an airborne substance, averaged out over a reference period to which employees may be ex­posed by inhalation.

Maximum penetrating particle size The particulate size for which a filter has minimum removal efficiency under test conditions.

Maximum-risk employees Workers who are most likely to be exposed to the highest levels of hazardous agents.

Maximum use concentration (MUC) The maximum atmospheric concentration of contaminants in which a respirator car­tridge or filter is recommended for use. Can be approximated by multiplying the PEL for the contaminant of concern by the assigned protection factor.

Mean, arithmetic The sum of a set of N Numbers divided by N, often called the average.

Mean air temperature See Temperature.

Mean diameter The geometric mean di­ameter of the size range.

Mean free length The mean free length of a particle (|xni) used in particle scrub­bing equations.

Mean free path The average distance travelled by a particle between colli­sions. In a gas it is inversely propor­tional to the pressure.

Mean, geometric The nth root of the product of N terms.

Mean particle diameter The mean value of the particle size distribution of the test aerosol.

Mean radiant temperature The average temperature of the six surfaces of a cu­bicle enclosure, used in thermal comfort work and in other heat-transfer applica­tions. It is the sum of all the surface ar­eas multiplied by the temperature of the surface divided by the total surface area.

Mean skin temperature The average tem­perature of the skin exposed to a given environment.

Means, best The best practical means for preventing the escape of noxious or of­fensive gases, smoke, grit, and dust: from a process into the atmosphere. (U. K. Al­kali Regulations.)

Measured variable A variable that is measured, and may be controlled.

Measurement station Element inserted in ductwork or pipework to facilitate the determination of temperature, humidity, flow rate, and/or pressure.

Measuring procedure Procedure for sam­pling and analyzing one or more chemi­cal agents in the air, including the storage and transportation of the sam­ple to the laboratory.

Mechanical constant flow rate controller See Flow rate controller.

Mechanical diffusion Eddy diffusion caused by mechanically-produced turbulence.

Mechanical efficiency The actual work possible by a machine, related to the work put into that machine.

Mechanical variable flow rate controller See Flow rate controller.

Mechanical rapping The mechanism that vibrates electrostatic precipitators or bag filters in order to remove the dust burden.

Mechanical shakers A table that vibrates at a given frequency in order to remove particulate matter from a casting.

Mechanical turbulence Any turbulence produced by means other than natural, such as fans. The term is also used in­correctly to define wind currents set up in and around buildings.

Mechanical ventilation Ventilation cre­ated by fans or other air-moving devices within a building, which can be divided into the following classifications:

Mechanical extract—induced inlet.

Mechanical inlet—forced outlet.

Mechanical inlet—mechanical outlet.

Mechanism An arrangement that allows rotary movement to be converted to lin­ear movement or vice versa. A linkage for dampers, etc.

Media filters Filters that collect particu­late matter on individual filter elements.

Median The central value of a series of observations ranked in order of magni­tude.

Medical surveillance program The evalu­ation of an employee’s health status, performed on a regular periodic basis by a health professional, to detect problems associated with exposure to health haz­ards, so that appropriate steps can be taken to prevent permanent or debilitat­ing injury. Medical surveillance pro­grams may also be used to ensure chat an employee’s health status will allow the continued safe use of protection equipment, or the continued safe perfor­mance of work.

Membrane A film used for collection of particulates in which the size of the mi­croscopic pores is controlled.

Mesopores Pores of diameters from 0.00005 mm to 0.005 ram that form the internal structure of an adsorbent mate­rial.

MEL See Maximum exposure limit.

Melanoma A skin tumor containing dark pigment.

Melting point The temperature at which a solid liquefies. The same as freezing point.

Mercaptans Organic compounds con­taining sulfur, which have an unpleasant odor.

Mesh A metal fiber or other material formed into a woven lattice, used to strain or filter out particulate matter from a fluid or gas.

Met unit The metabolic rate of a sedentary person at rest, 1 met = 58.2 W nr2.

Metabolic energy transformation Metabolic rate.

Metabolic heat production The produc­tion of body heat due to the intake of oxygen and carbohydrates.

Metabolic rate (M) The rate of transfor­mation of chemical energy into heat and mechanical work by aerobic and anaerobic metabolic activities within an organism, usually expressed per unit area of the total body surface, in met or W nr2.

Metabolic rate, basal (BM) Metabolic en­ergy transformation calculated from measurements of heat production or ox­ygen consumption in an organism in a rested, awake, fasting, and thermo­neutral state, in W nr2.

Metabolic rate, seated (Ms) The heat lib­erated from a body when the occupant is seated at rest, in met or W irr-.

Metal fume fever A fever suffered by workers who inhale metal fumes from a process.

Metal poisons Certain metals that cause ill­ness or death when inhaled or ingested.

Methods of air distribution Can be clas­sified into the following groups.

Crosswise Airflow that takes place from one side of a space to the other. This may be achieved by one or more jets or by allowing the air to enter the whole of one side surface and extracting the air by the whole area of the opposite side. The latter arrangement provides a piston effect, ensuring good air and contaminant transport.

Downward The supply air enters at ceiling level or high wall level and is extracted at low level. A perforated ceiling may be used to provide a piston effect. Good air mixing is achieved if cool air enters at high level at the correct temperature and velocity,

Mixed upward and

Downward Downward supply with a small proportion of high-level extraction. The largest proportion of the extraction occurs at low level.

This arrangement provides good mixing of the room air, if care is taken to ensure that short-circuiting of the high-level input and extraction does not take place.

Upward Air enters the space at low level and is extracted at high level, ideally suited for warm air supply.

Microclimate The distinctive pattern of temperature, humidity, air movement, and purity’ within a relatively small zone either inside or outside a building.

Microclimate suit A suit worn to protect an operator who is working in adverse conditions of either heat or cold.

Micrometer (p. m) The SI unit of measure­ment of particulate matter, equal to

1 X 10-6 meter. The non-SI term is the micron (jjl).

Microorganisms Organisms that can only be seen with the aid of a microscope, such as bacteria, viruses, and some fungi.

Micropores Pores of diameter less than 0.0005 mm that form the internal struc­ture of an adsorbent material.

Migration The movement of dust col­lected in a filter in the direction of flow.

Migration velocity The electrophoretic velocity of a charged particle in an elec­tric field.

Millibar (mbar) A unit of pressure equal to 100 Pa.

Minimum air change rate The lowest possible air change rate that can be used in a space in order to attain the recom­mended air purity standards.

Minimum filter efficiency The value of a filter’s efficiency relating to its perfor­mance classification under specified op­erating conditions. See also Maximum penetrating particle size.

Minimum ventilation requirements The lowest possible airflow rates that will ensure all obnoxious products are re­moved from the air by the introduction of fresh air.

Mist The suspension in air of small drop­lets of materials that are liquids at nor­mal pressure and temperature.

Mist elimination The removal of a mist from a gas stream either by condensa­tion or by the use of baffles.

Mitered elbows A bend in pipe or duct­work formed by a series of flat sections.

Mixed flow, actual An actual flow pat­tern in an enclosure resulting in the air being mixed to such an extent that con­ditions are almost the same at every point in the occupied zone.

Mixed flow, ideal The flow pattern in an enclosure in which the air is completely mixed and has the same conditions at every point.


подпись: c,


подпись: pelMixed air Air that contains two or more streams of air.

Mixing The process by which fluids of different density are combined natu­rally or by some mechanical device. One or more of the following mechanisms may produce the mixing process:


Piston flow.

Spot (local).

Laminar flow.


Mixing actuator Component designed to mix two airflows without controlling the volume.

Mixing air diffusion Air diffusion where the mixing of supply air and room air is intended.

Mixing controller Component designed to mix two airflows while controlling the volume flow.

Mixing factor A factor used in air distri­bution relating to the actual degree of mixing that takes place between the room air and a contaminant generated in that room.

Mixing height The height above an inter­nal or external pollutant source within which emitted pollutants are dispersed and mixed with the surrounding atmo­sphere. In meteorological terms, this is the area below the inversion layer.

Mixing section of an ATD A section in which two air streams of different tem­peratures or moisture content are damper controlled to provide a given flow rate before mixing occurs.

Mixing section of an AHU A section where outdoor airflow and the recirculation air­flow are mixed in a controlled manner.

Mixture rule A mathematical expression applying to w’orkers simultaneously ex­posed to chemicals that act on the same organ or organ system. The exposure level for each chemical must remain at a fraction of the permissible exposure level (PEL) so that the sum of the frac­tional exposures does not exceed unity.



Model A model of a particular flow problem, which may be either

Mathematical A mathematical simulation of the emission, dispersion, and chemical process relating to the concentration of pollutants.

Physical A model system in which tests are carried out on the emission and dispersion of a pollutant, e. g., a wind tunnel.

Modulating The control action of minute increments and decrements of adjust­ment in a system, such as in automatic control valves.

Moisture content The mass of water va­por present in a unit mass of dry air.

Moisture recovery Measures taken to prevent the moisture present in the air from leaving the treated space.

Molar diagram A plot of the thermody­namic properties of a substance that has specific enthalpy as one of its coordi­nates.

Mold diseases Diseases produced by the concentration of mold or fungi spores within a space.

Mole The SI unit of quantity; the amount of a pure element or chemical com­pound that contains the same number of atoms or molecules. It is often simpler to use moles rather than volume or mass when working with gases. Moles are given by

« — Tn MM •

For example 32 kg of 02 = 1 mole of oxygen and 16 kg of O = 0.5 mole of oxygen.

Molecular diameter of air Air at atmo­spheric pressure is 0.00037 |Jim At 20 °C. Molecular sieve Zeolites used for ion ex­change in water treatment.

Monitoring The continuous or regular observation of a fixed or variable pa­rameter.

Morbidity The incidence of disease in a community or a working group. Multiple-leaf damper or valve An air

Damper or valve that has more than one dampen arranged to provide low flow loss.


Narcotic gases Gases that produce sleep, stupor, or insensibility when inhaled in certain concentrations.

Natural atmospheric dispersoids The American Meteorological Society classi­fies natural dispersoids by size:

Haze 1.0 |jLm

Mist 1-5 |jim

Cloud or fog 5-200 |Jim

Drizzle 200-500 ^m

Rain 500-8000 |jim

Natural circulation Circulation occur­ring in a fluid due to temperature changes.

Natural ventilation Ventilation achieved by means of wind forces or density differ­ences or a combination of the two, as op­posed to mechanical ventilation, which depends on a rotodynamic device. Natural ventilation system Ventilation of a space by the influence of thermal forces and wind forces over and around a building. Under certain conditions only one of these applies, however, in the majority’ of cases it is assumed that both apply.

Necrosis The death of any cell tissue by the action of pollutants.

Negative pressure A pressure less than the ambient pressure, which may be cre­ated due to stack effect or by mechani­cal means.

Negative rated operating pressure The tested maximum negative pressure at which a duct is rated.

Nephelometer A device used to determine the suspended particulate size and con­centration by the scattering of light.

Neutral clothing insulation See Clothing insulation, neutral requirements.

Neutral solution A chemical solution that is neither acidic nor alkaline.

Neutral zone The physical state within a building where no pressure difference exists between inside and outside the building. Also used in relation to the ef­fect of a chimney in removing the prod­ucts of combustion.

Nitrogen oxides A number of different compounds of nitrogen and oxygen, normally referred to as NO,.

Noise An unwanted sound that causes annoyance or distraction.

Nominal length, flexible duct The actual length of a flexible duct after decom­pression in an unstressed state.

Nominal length, rigid duct The actual length of a rigid duct without fittings or components.

Nominal size of an ATD The nominal value of dimensions of the prepared opening (duct) into which the air termi­nal device is to be fitted. For an air dif­fuser, the nominal size is generally defined as the duct size into which the neck of the device is fitted.

Nominal size of a duct or fitting The ref­erence dimension used for the designa­tion, calculation, and application of ducts and fittings.

Nomogram A chart consisting of vari­ables, and provided two of these are known others can be determined.

Non-overloading fan A fan with back­ward-curved blades, which has power characteristics that tend to flatten with increasing flow rates, so that as the maximum volume flow rate is ap­proached the power consumed may be­come constant or even decrease. The
power characteristics of a fan with forward-curved blades steepens at high volume flow rates, and so such fans are overloading.

Nonspecific When an instrument re­sponds to more than one contaminant that is known to be present in the air.

Normal temperature and pressure (NTP) See Standard temperature and pressure (STP).

Nosocusis Hearing loss resulting from causes other than noise, such as disease, heredity, etc.

Noxious A term relating to any chemical that is harmful to the occupants of the space in which it exists.

Nozzle An air terminal device used to ob­tain the maximum conversion of static pressure to dynamic energy with mini­mum entrainment.

NTP Normal temperature and pressure. See Standard temperature and pressure (STP).

Nucleation A cleaning process using a Hu­midification and cooling cycle, causing water or another fluid to condense on sub-micrometer particles. This process in­creases particle size until impingement on packing is possible.


Distance from the inner surface of the elements, m

Typical range

Default value

External windows, doors, and radiators



External and internal walls



Floor (lower boundary)



Floor (upper boundary)



* Mainly seated occupants ** Mainly standing occupants.

Ir will be appreciated that in the industrial environment, each case will have to be considered in its own rights. Except when agreed otherwise, the default: values shall be applied.

подпись: element distance from the inner surface of the elements, m
 typical range default value
external windows, doors, and radiators 0..5-1.5 1.0
external and internal walls 0.25-0.75 0.5
floor (lower boundary) 0.00-0.2 0.1
floor (upper boundary) 1.30*-2.0*4 1.8
* mainly seated occupants ** mainly standing occupants.
ir will be appreciated that in the industrial environment, each case will have to be considered in its own rights. except when agreed otherwise, the default: values shall be applied.

Null point The distance from a generated pollution source at which the initial en­ergy or velocity of the contaminants is dissipated, and collection by a hood is possible.

Nuisance dusts Any dust that creates a nuisance, rather than being a health risk, such as dusts that cause sneezing, coughing, eye irritation, etc.


O Ring A device used to seal a shaft of a device or a pipeline that is conveying a fluid.

Obscuration The concealing from sight; lack of visibility due to dust, fumes, or smokes.

Occupied zone The volume of air confined by horizontal and vertical planes defined to include space occupied by persons.

Occupational exposure limits (OEL) The maximum time a person can work in a given polluted environment.

Occupational exposure standards (OES) U. K. standards relating to the concentra­tion of an airborne substance that can be tolerated without harmful effects on workers over a reference period. See Long term exposure limit (LTEL) and Short term exposure limit (STEL).

Octave band frequency The band in fre ­quency scale that is split into bands, each assigned a sound power level, that is twice the power level of the lower limit.

Odor Relating to the sense of smell, A Substance that stimulates the olfactory organ, allowing us to detect if a smell is pleasant or unpleasant.

Odor control The elimination of odor in a space by the use of masking chemicals or special filters.

Odor dispersion time Time taken to re­duce an odor to a defined level from a given concentration in a standard test.

Odor reduction factor The efficiency of odor reduction by a medium capable of removing odors.

Odor threshold The minimum concentra­tion of an odorous gas which 50% of a panel of trained sniffers can detect.

Off-gassing of materials The liberation of volatile organic compounds (VOC) and other gases from building products or from a manufacturing process.

Offset A sustained deviation between the control points and the set point of a pro­portional control system.

Oil slant gauge An inclined manometer tube using oil as the measuring fluid to record the pressure.

Olfaction The sense of smell or the act of smelling.

Online Relating to either

The measurement of a variable in a process that is in operation, or ® A computer arrangement to process input data without delay and to present an output result.

On/off control A simple two-position control system that is only capable of per­forming these two functions (on and off).

Once-through scrubber system A system seldom used due to the problem of using large quantities of fresh water and the resulting discharge of a large volume of polluted water.

Opacity The degree to which a plume of exhaust gases obscures the view of an ob­server, measured in terms of percentage obscuration, with 100% meaning that the plume completely obscures the line of sight through the plume.

Open face Sampling with the top portion of a filter holder or cassette removed to ensure the full filter surface is exposed.

Open rotor An open air lock with open ends between the rotor blades and the blade seal.

Operating point The static pressure and volumetric airflow that a fan is capable of producing.

Operative temperature See Temperature.

Opposed-blade damper A damper in which the adjacent blades rotate in op­posite directions, so that the leading edges seal against each other and the trailing edges seal against each other.

Optical anemometer An instrument for measuring gas flow rate using a laser, in which small frequency shifts are visual­ized as interference fringes.

Optical particle counter An optical-electronic instrument for measuring the number of airborne particles in different size ranges.

Optimum droplet size The ideal size of a water droplet in a centrifugal spray scrubber or spray tower to ensure the highest possible cleaning efficiency.

Oral temperature The temperature in the mouth recorded by a thermometer or thermocouple.

Organic Relating to or derived from living matter that has organs or an organized physical structure. Organic chemistry is the study of carbon compounds.

Organic contamination The contamina­tion of products by organic matter, par­ticularly in clean room applications.

Organic material A material from an or­ganic source.

Organic solvents An organic liquid capa­ble of dissolving an organic material or compound.

Organoleptic A material that influences a sensory organ, as in the perception of odor by the human nose.

Orifice meter See Orifice plate.

Orifice plate A metal plate with a hole of diameter smaller than the pipe or duct run in which it is fitted. The pressure drop that takes place across the plate is used to calculate the fluid velocity.

Category Description

ODA 1 Pure air, which may be temporarily dusty (e. g., pollen).

ODA 2 Air with significant concentration of dust,

ODA 3 Air with significant concentrations of gaseous pollutants.

ODA 4 Air with significant concentrations of gaseous pollutants and


ODA 5 Air with very high concentrations of gaseous pollutants or


Orifice venturi A measuring device used to determine the flow rate of a fluid by means of the pressure drop across the device.

Outdoor air Air introduced into a build­ing from a source external to the building.

Outdoor air (ODA) classification This classification covers five categories of air quality from ODA 1 to ODA 5 as shown at the top of the next page.

Outdoor pollution Natural or man-made pollution produced by sources external to a building.

Outlet An opening through which air or effluent is discharged, either by natural or mechanical means.

Outlet damper A device fitted in a duct that will allow the flow of gas to be con­trolled either manually or automatically.

Overall heat-transfer coefficient The heat flow per unit area for a given construc­tion for an overall temperature differ­ence of 1 K.

Overall uncertainty of a measuring proce­dure or of an instrument The quantity used to characterize the uncertainty of results given by an apparatus or a mea­suring procedure, expressed on a rela­tive basis by a combination of bias and precision, according to a formula.

Overbreath in using a respirator, when the wearer’s breathing rate exceeds the ability of the respirator to provide a vol­ume of air sufficient to ensure that a positive pressure is maintained inside the face piece.

Overlap length The length by which a fit­ting or duct overlaps a connecting duct.

Oxidants Substances present in air, such as nitrogen dioxide, ozone, etc., that are capable of oxidizing other chemicals or elements in oxidation-reduction type chemical reactions.

Oxygen consumption The rate at which the lungs take up oxygen.

Oxygen-deficient An atmosphere consist­ing of less than 19.5% oxygen.

Oxygen-enriched An atmosphere con­taining more than 23% oxygen.


P4SR Predicted 4-hour sweat rate. A scale used to predict the evaporation rate from a body under hot conditions.

Package units Air-handling equipment containing all the components together in a common casing.

Packed beds An absorption separator that employs a fluidized bed of plastic spheres constrained between horizontal screens.

Packed-tower wet scrubber A gas scrub­ber that removes gases and vapors, by using either water or a chemical liquid method. Efficient pollutant removal de­pends on the contact time between the entering gas stream and the wetted sur­face of the pack in the tower. This type of scrubber can be classified as

A. Concurrent flow,

B. Cross-flow, or

C. Countercurrent flow.

Paddle wheel impellers A radial blade im­peller on a fan, used for dust conveying due to its self-cleansing properties.

Panel fans A simple form of axial fan with its impeller mounted in a ring or diaphragm; it discharges air both axially and radially.

PAP See Photochemical air pollution.

Parallel flow Referring to the operation of two or more fans or pumps con­nected in parallel with each other. May also relate to a standby fan in a system.

Parameter A quantity that serves to de­termine a measurable or quantitative characteristic, or the variable feature of a measurement.

Partial air treatment Treatment that in­volves one or more of the possible meth­ods of treatment but is not complete.

Partial enclosure An enclosure used for work with toxic dusts, gases, or vapors In Which one or more of the sides may be open to the remainder of the work area.

Partial pressures See Dalton’s law of par­tial pressures.

Partial ventilation system A local exhaust system designed to provide an airflow less than that required for all the hoods that form part of the system. The appli­cation is to provide system diversity.

Particle The nature of an aerosol or other pollutant.

Particle aerodynamic diameter The dia­meter of a sphere of density 1 g cm-3 that has the same terminal velocity due to gravitational force in still air at set condi­tions of temperature, pressure, and rela­tive humidity as the particle in question.

Particle counters A manual or automatic device used to determine the particulate concentration of a given gas sample.

Particle migration velocity The velocity at which a charged particle moves in a given direction in an electric field.

Particle scrubbing A gas-cleaning device that generates large particles that can be easily collected by combining them with liquid droplets issued from fine jets.

Particle size distribution A method of re­lating the size or weight of particulate matter, e. g., 50% with diameters in the 0.1-1.0 |xm range, 25% in the 1-5 [xm range, etc.

Particulate concentration The concentra­tion of one or more particulates in a given quantity of a gas.

Particulate matter Matter consisting Of Particulate liquid and solid substances ranging in size from 0.0002 (j. m to 500 fj. m in diameter.

Parts per billion (ppb) Parts of a contami­nant in a billion parts of air or water. Care has to be taken in ensuring the term billion is the correct one. In the past in the UK, a billion was 10-12, but in the U. S. a billion is 10~9. It is now assumed that current practice relates to the latter. Other terms encountered are parts per hundred million (pphm) and parts per million (ppm).

Parts per million (ppm) The number Of Parts of a contaminant by volume in a million total parts. Volume ratio = — mole ratio = pressure ratio in the case of ideal gases.

Partition fan See Fan.

Passive sampling Sampling that depends on the diffusion of the contaminant into a solid sorbent.

Pathogen A material that is capable of producing disease in living organisms.

Pathology The study of the causes and re­sults of disease.

Peak-above-ceiling exposure limit The short-term exposure peak permitted above the OSHA standard ceiling expo­sure level.

Peak limit A pollutant or noise level that exceeds the ceiling exposure limit, but is allowed for a specific limited time dur­ing the work shift.

Penetration The distance particles of a particular size will travel into a given fil­ter before coming to rest.

Per capita air rate Volume intake of out­door air per occupant.

Percentage saturation The ratio of the moisture content of moist air at a given temperature to the moisture content of saturated air at the same temperature. Also known as degree of saturation.

Perforated plate See Flow equalizer.

Performance The operating characteris­tics of a device, as compared with those of the original design. Or the perfor­mance of an item of plant as stated by a manufacturer.

Peripheral nervous system Nerve tissues lying outside the brain and spinal cord, functions include the transmittal of sen­sory information such as touch, heat, cold, and pain, and the motor impulses for limb movement.

Permeation efficiency Reduction factor for latent heat exchange through clothing.

Permissible exposure limit (PEL) The max­imum exposure level allowed by OSHA, expressed as an 8-hour time-weighted average. These are legally enforceable in the U. S.

Permissible range The range of a physical quantity that satisfies the different pa­rameters for each of the categories of the specified environment.

Permit space As defined by OSHA, a con­fined space that contains a hazardous atmosphere, a material that could engulf an occupant, a configuration that could trap an occupant, or any other recog­nized safety or health hazard.

Personal protective equipment (PPE) De­vices and apparel worn by employees to prevent or reduce exposure to health and safety hazards in any adverse envi­ronment. Examples include respirators, gloves, chemical-resistant overalls, ear­plugs, and safety glasses.

Personal sample The result obtained from the products collected during the process of personal sampling.

Personal sampler A collection device at­tached to a person that obtains samples of air to be tested for radioactive, chem­ical, or biological agents.

Petri dish A shallow dish used to culture bacteria.

PH The unit used to relate the acidity or alkalinity of a liquid. Pure water is neu­tral and has a pH of 7.0, values below this denote acidity, and those above this denote alkalinity.

Phase equilibria The relationship be­tween contaminant solubility in the gas and liquid phases at equilibrium, which must be known for absorption separator design.

Photoallergic A reaction similar to other allergic reactions of the skin.

Photochemical Relating to a chemical re­action brought about when sunlight en­counters certain gaseous mixtures.

Photochemical air pollution (PAP) Pollut­ants such as nitrogen oxide and certain hydrocarbons that cause photochemical reactions in the air.

Photometer An analytical instrument containing a light source on one side and a light detector on the opposite side that measures the amount of light that passes through the sample.

Photophoresis Particle motion that takes place in the direction of radiation, due to the absorbed radiation warming one side of the particle more than the other.

Physical absorption The process of col­lecting a gas in water or another fluid.

Physical adsorption An exothermic phys­ical process that gives off less heat than chemisorption.

Physical testing Any process involving an actual test in order to obtain results.

Physiology Study of the function of the human body.

Piezometer tube An open-ended cali­brated glass or plastic tube that mea­sures the pressure in a pipe or vessel full of a fluid.

Piping A metallic, plastic, glass, etc. en­closure, for the conveying of a fluid, va­por, or gas.

Piston effect The ideal method of air distribution, in which uniform air­flow occurs over the whole of a room, such as when the air is injected into the room over the whole surface area of one wall and extracted from the op­posite wall.

Pitch The spacing of holes in a flange, or the angle of fan blades. That attribute of auditory sensation depending primarily on the frequency of the sound in terms of which sounds may be ordered on a scale extending from low to high.

Pitot-static traverse The set positions of a Prandtl tube in a duct run required to provide a statistically valid set of read­ings. A series of measurements of the to­tal and static pressure taken across an area of a duct to determine the air veloc­ity at that point. The sampling distance should be at least 7,5 times the diameter of the duct away from any disturbances of air flow.

Pitot-static tube A measuring device con­sisting of two concentric tubes used to measure the total and static pressures in a duct run, known as a Prandtl tube.

Plane radiant temperature See Temperature.

Plaster frame See Fixing accessory of an ATI). ‘

Plate-mounted axial flow fan See Fan types.

Plate-type design (space heaters) Type of heat exchanger characterized by a sub­stantial proportion of its heat output be­ing by way of radiant energy.

Plenum box A component forming an interface between ductwork and one or more air terminal devices. By virtue of its design or by the inclusion of acces­sories, it can also be used to equalize the pressure/velocity across air terminal devices.

Plenum chamber Any air compartment connected to one or more ducts or to a slot in an air distribution hood.

Plenum system A ventilation system that holds a space at positive pressure.

Plume Effluent discharged from a chim­ney or exhaust duct, composed of gases alone or gases and particulate matter. The plume shape depends on tempera­ture difference and turbulence. The flow of visible hot gases or vapor from an outlet.

Pneumatic Any device operated by or filled with compressed air or a liquid.

Pneumatic control A control system that operates on compressed air as the oper­ating medium for the control of valves and dampers, etc.

Pneumatic conveying The conveying of dusts, powders, or granular materials in ducts or pipes by means of a difference in air pressure.

Pneumoconiosis A lung disease experi­enced by miners caused by industrial dust.

Pollen Small particles of the male fertiliz­ing seeds of plant life, which may cause various allergic reactions especially of the respiratory tract, known as hay fever.

Pollutant Any unwanted liquid, solid, or gaseous product, resulting from the ac­tivity of man. It can be further divided in the case of air into

Primary A pollutant that is discharged into the ambient air.

Secondary A pollutant formed in the air as a result of reactions of primary pollutants.

Polluter pays principle (PPP) The term that relates to either the industry or the individual being responsible for the cost of all pollution-control measures.

Pollution Relating to any environmental constituent present in air or water to such an extent that it presents a hazard to the present or future health of hu­mans or any ecosystem.

Polonium chamber Any air compartment connected to two or more ducts, or a slot in an air distribution hood.

Polychlorinated biphenyls (PCBs) Highly toxic organic compounds used in the electrical industry, use of which is now restricted.

Polydisperse aerosol See Aerosol, Poly­disperse.

Pore diffusivity The ability of a material to diffuse gas through its pores, trapping the contaminants.

Porosity The presence of spaces within a material that can absorb gases or moisture.

Positive rated operating pressure The tested maximum positive pressure at which the duct is rated.

Potential temperature The temperature an air envelope would acquire if brought adiabatically from its initial or actual pressure to a standard pressure of 1000 mb.

Powder A substance or combination of sub­stances in the form of fine dry particles.

Ppb See Parts per billion.

Powered air-purifying respirator (PAPR) A type of air-purifying respirator that utilizes a battery-powered fan to draw contaminated air through the cartridge or filter into the facepiece.

Prandtl tube See Pitot static tube.

Precipitator, electrostatic A device for col­lecting particulate matter from a gas stream by using electric forces to impart a negative charge to the particulate matter in the gas stream. These charged particu­lates are attracted to collecting surfaces which have the opposite polarity.

Precipitation When a solid is formed from a solution, or the separation of particles from a fluid by the process of precipitation.

Precursor A substance involved in the formation of new air pollutants, i. e., a hydrocarbon is the precursor to the for­mation of ozone.

Precleaners A device to remove a contam­inant from a liquid or gas stream before the main cleaner.

Precision The degree of agreement be­tween independent test results obtained under the same conditions.

Predicted mean vote index (PMV) An in­dex used to predict the mean value of thermal sensation votes of a large group of persons, expressed on a 7-point scale.

Predicted percentage dissatisfied An in­dex that predicts the percentage of a large group of people who are likely to feel thermally dissatisfied, i. e., feel ei­ther too warm or too cold.

Prefilter A rough filter positioned before a fine filter to reduce clogging of the fine filter.

Preheat coil A heating coil in a ductwork run sized either to temper the air or to stop the filter from freezing.

Pressure The force per unit of area ex­erted in all directions by a gas or liquid on the walls of its container.

Pressure, atmospheric The pressu re of the atmosphere as indicated by a barometer, in kPa.

Pressure blowers Variations of radial — bladed fans with narrow housings and impellers, developing pressures in excess of 25 kPa at low volumes.

Pressure burst A failure in a filter, duct work, or an air handling unit due to high differential pressure.

Pressure depressions The differences in air pressure between zones necessary to stop contaminant drift.

Pressure drop The static pressure differ­ence due to friction or turbulence be­tween two locations in a ventilation system.

Pressure drop, final The value to which the filtration performance is measured in order to classify the filter

Pressure drop, final recommended The man­ufacture recommended operating pres­sure drop of a filter at rated flow conditions.

Pressure drop, initial The pressure drop obtained on a test on a clean filter.

Pressure factor The test ratio between the suction effect and air velocity passing over a cowl or roof outlet, represented by


Pressure gradient The rate of change in the pressure in a fluid at any given time.

Pressure loss See Pressure drop.

Pressure maintenance Maintenance of spec­ified (differential) pressures in spaces or pipe/duct systems.

Pressure sensor A recording or measuring device that corrects the pressure of a fluid when it varies by a given amount from its design set point.

Pressure, static The potential pressure that is exerted in all directions by a fluid at rest.

Pressure, total The algebraic sum of the velocity pressure and static pressure.

Pressure, velocity The kinetic pressure exerted in the direction of flow that is necessary to cause a fluid at rest to flow at a given velocity.

Pressure ventilation The ventilation of a space by providing air movement from a high-pressure region to a low-pressure region.

Pressure, water vapor partial The pres­sure that the water vapor in air would exert if it alone occupied the volume of the humid air at the same temperature,

In kPa.

Pressurization The process by which a space is held at a pressure greater than the surrounding areas.

Prevailing wind direction The direction from which the wind is most frequent for a given period.

Primary air The air provided to a com­bustion process to ensure efficient com­bustion, or the air leaving an outlet, forming A Jet.

Primary airflow rate The mass or vol­ume of air entering a supply air terminal device in unit time from an upstream duct or a plenum box. Or the air leaving through an opening and entering a space.

Primary air induction system Air intro­duced via an air duct to the induction unit, generally in the form of treated outdoor air.

Primary air temperature See Temperature.

Primary calibration standard A calibra­tion standard based on direct measure of a reference value.

Primary collector An air-cleaning device that removes the larger particulate mat­ter before a HEPA filter.

Primary containment The enclosing struc­ture around a red zone used in the atomic energy industry, which has A Spe­cific. leak tightness.

Priming The carryover of particles of wa­ter or other fluid in gas flow, mainly in steam boilers.

Probability A quantitative measure hav­ing values between 0 and 1 inclusive.

Process The general term that describes A Method of manufacture or control.

Process requirements The conditions, for a given process, including the boiler or cooling.

Process ventilation Ventilation provided primarily for process purposes, the require­ments of the occupants being secondary.

Product recovery The recovery of the waste products from a process reducing pollution, saving energy or materials.

Promoter A chemical substance or specific set of conditions that is favorable to, or triggers the development of, cancer.

Propeller anemometer A device used for measuring airflow in which the air ve­locity revolves a propeller, the shaft of which is connected to a gearbox and measuring dial.

Propeller fan See Fan types.

Proportional band In a proportional con­troller, the control point range through which the controlled variable must pass in order to move the final control ele­ment through its full operating range.

Proportional control A control algo­rithm in which the final control element moves to a position proportional to the deviation of the value of the controlled variable from the set point.

Proportional integral (PI) control A con­trol algorithm that combines the pro­portional response and integral response control algorithms.

Proportional integral derivative (PID) con­trol A control algorithm that enhances the PI control algorithm by adding a component that is proportional to the rate of change of the deviation Of The controlled variables.

Protective equipment, personal See Per­sonal protective equipment (PPE).

Psychrometric chart A chart constructed so that knowing any two air proper­ties the remaining thermodynamic properties of air at that condition can he determined.

Psychrometric coefficient The coefficient in the equation for the determination of the water vapor partial pressure from the wet bulb depression.

Psychometrics The study of the proper­ties of moist air.

Pulse The regular beating of the blood in the main blood vessel, a pulse of elec­tricity, the vibrating of a mechanical or electrical device.

Pulsed hot wire anemometer A device used for gas flow measurement, similar to the hot grid anemometer, in which measure­ment, are made by pulses of hot air at a downstream sensor.

Pump A device that moves a fluid by me­chanical or electrical means. The move­ment being created by a pressure differential between the inlet and outlet.

Pumping The act of forcing a fluid along a conduit by overcoming fric­tional resistance.

Push nozzle A push jet, located either on a working surface or above a process.

Push-pull exhaust See Push-pull hood.

Push-pull hood A protecting hood around a process with the air supply on one side of the contaminant source and the ex­tract on the other side.

PVC Polyvinyl chloride, a polymeric plastic material.

Pyrolysis The chemical decomposition of a material caused by the application of heat.


Quartile If a set of observations are ranked in order of magnitude, then the quartiles are those three values which divide the observations into four equal parts, i. e., the lower quartile is that value below which one quarter of the observations lie, and the upper quartile is that value below which three quarters of the observations lie.

Quenching Heat removal from A Solid By Means of a fluid, either oils or water.

Quick disconnecting ducts Ductwork provided with a quick-release mecha­nism for disconnection providing easy access for cleaning the ductwork.

Quick-release access doors Doors located in strategic areas of a plant to allow access for maintenance or cleaning.


Radial Arranged like a wheel with lines (spokes) radiating from a center point.

Radial acceleration Rate of velocity’ change with respect to time in a radial direction.

Radial-bladed fan See Paddle wheel im pellers.

Radiant cooling The cooling of A Human body or other surface by means of A Ra­diant panel.

Radiant heating The heating of a human body or other surface by means of a ra­diant panel.

Radiant temperature The temperature of a surface emitting heat to surrounding bodies by means of electromagnetic radiation.

Radiant temperature asymmetry The dif­ference between the plane radiant tem­perature and the temperatures at the sides of an element.

Radiation Energy provided to a body by electromagnetic waves.

Radiation shape factor The angle factor representing the fraction of the angular field of view from which energy ex­change is trading places.

Radiative heat exchange The heat ex­change by radiation between the cloth­ing surface, including uncovered skin, and the environment, in W m—.

Radiative heat exchange, globe The heat exchange by radiation that takes place from a black globe thermometer, in W


Radiative beat transfer coefficient The heat — transfer coefficient wholly attributed to radiative heat transfer.

Radiator A body warmer than its sur­roundings that emits its heat to the cooler surroundings.

Radioactive materials Elements that have unstable nuclei that spontaneously dis­integrate, releasing radiation in the form of subatomic particles and energy.

Radioactivity The property of spontane­ous disintegration possessed by certain unstable nucelides.

Radiometer A sensitive instrument for the measurement of heat radiation.

Radiometric forces Weak forces that cause the motion of particulate matter, including diffusiophoresis, thermo­phoresis, and photophoresis.

Radon A radioactive element, the heavi­est of the noble gases, formed by the ra­dioactive decay of radium.

Radon daughters The series of unstable isotopes that are formed as radon atoms undergo radioactive decay.

Rain louver or weather louver A fresh air inlet to a duct designed to reduce the in­gress of driving rain.

Random particle motion See Brownian diffusion.

Range The interval defined by the largest and smallest values in a set of observa­tions.

Range hood An extraction hood posi­tioned above a cooking range to provide the best possible capture velocity of the fumes.

Raoult’s law States that at equilibrium, the partial pressure of a solute vapor over a liquid mixture is equal to the vapor pres­sure of the pure solute at the given tem­perature times the mole fraction of the solute liquid component in the mixture.

Rapping A method used on bag or other filters to dislodge the dust cake from the fabric when the airflow resistance ex­ceeds a certain value.

Rated airflow The flow rate specified by the manufacturer for an item of equip­ment.

Rating conditions A set of operating con­ditions over which specified perfor­mance will result.

Rating plate The plate fitted to an item of equipment showing the operating char­acteristics.

Raynaud’s syndrome Diminished blood flow or loss of blood to the fingers, caused by vibration, also known as white or dead finger.

Reactive metal A metal that readily en­ters into a chemical reaction.

Receiving hood An extract hood in the immediate vicinity of a process that ex­tracts the generated pollutants in an ef­fective manner.

Receptor A body (e. g., a person or ani­mal) that absorbs a pollutant from the air, which may suffer some adverse ef­fects.

Receptor system A system which contam­inants enter without inducement.

Recirculated air Exhaust air returned as supply air to the air treatment system from which it originated.

Recirculation The process of returning air to the space from which it has been removed, with or without treatment.

Recovery The collection of the products of a process for reuse.

Recycling The reuse of scrap material for pollution control and conservation pur­poses.

Red zone The primary air containment zone in ventilating systems in the atomic energy industry.

Re-entrainment The induction of air that has been discharged back into a jet, at a position upstream from the discharge.

Reference method A method of analysis to which other methods of analysis are compared.

Reference period A specified time period allowed for human exposure to a spe­cific concentration of a biological agent or chemical.

Refractories A special heat-resistant heat — retaining brick used in furnaces, chim­neys, and boilers.

Reflectance A measure of the extent to which a surface is capable of reflecting radiation, defined as the ratio of the in­tensity of reflected radiant flux to the in­tensity of the incident flux.

Refrigeration The controlled removal of heat from a substance.

Regenerable adsorbers See Adsorbent, re­generate.

Regenerative blowers A fan that gener­ates pressure by centrifugal force, in which, in contrast to a centrifugal fan, the airflow is around the circumference of the impeller.

Regenerative thermal oxidizers A series of beds made from heat-resistant mate­rial which alternately store heat from the combustion chamber exhaust gases and release heat into the cooler gases en­tering the combustion chamber.

Register A combined grille and damper assembly.

Regression A statistical method used to investigate the dependence of one vari­able on one or more other variables.

Regulations Any legislation or statutes enforced by law.

Relative humidity The ratio of the partial pressure of the water vapor in moist air at a given temperature to the partial pressure of water vapor in saturated air at the same temperature.

Relative ventilation efficiency A quantity describing how the collection efficiency of a ventilation system varies between different parts of an enclosure.

Relaxation time The time necessary for a moving particle to adjust from one given steady state velocity to another, e. g., the time for a falling particle to reach its ter­minal velocity. It is independent of the na­ture of the force applied to the particle.

Rem (Rцntgen Equivalent man) The unit of measure of the radiation dose to the internal tissues.

Replacement The process of air entering a space to fill the void created by either natural or mechanical extraction.

Replacement air Air supplied to a space to replace the air removed by a combus­tion process or by natural or mechanical ventilation.

Reproducibility The extent to which any measurements taken during tests per­formed under the same conditions will provide statistically similar results.

Residence time The time a pollutant re­mains in a space after it is released.

Resistance The opposition to flow caused by friction as air passes through a duct­work or pipework system.

Resistivity A property of a material equal to the reciprocal of its conductivity.

Resistivity of dust The electrical resistiv­ity of a dust, which is one of the factors that influences the practical efficiency of an electrostatic precipitator.

Resonant frequency The sound frequency for which a particular system provides the maximum absorption. The amount of sound absorption in a system depends on the degree of damping achieved; this depends on the mass and the associated air space.

Respirable particles Particulate matter of such a size that it can pass through the body defences and into the lungs, where, depending on its nature, it will either de­posit itself or be exhaled.

Respiratory protection The use of face masks, respirators, or separate air sup­ply to reduce the intake of pollutants into the lungs.

Response A reaction of a living organism caused by a pollutant. Also, the reaction time of a measuring instrument or con­trol device to perform an action.

Respirable particulates Particulates in the size range that can pass through the de­fence mechanisms in the human body and enter the lungs during inhalation.

Resultant temperature See under Temper­ature.

Return air Air that has been removed from a space that is returned to the space either mechanically or naturally with or without treatment.

Reverberation The continuation and en­hancement of a sound caused by rapid multiple reflections between the sur­rounding surfaces.

Reverberation time The time required for a sound to fall to a given level in an en­closure.

Reverse air cleaning filters Filters that be­come self-cleaning by dislodging the im­pacted dust when the gas flow is


Reverse pulse The use of jets of high — pressure air to dislodge material from the exterior of a bag filter.

Reversible heat engine A heat engine, which will convert a certain quantity of heat into an amount of work (W), that will produce the original quantity of heat if the same amount of work is ex­pended in driving the engine backwards.

Reversible process A process that may be performed in the opposite direction to return to the initial state of the working fluid.

Reynolds number A dimensionless pa­rameter that represents the ratio of the inertia forces to the viscous forces in a flow. Its magnitude denotes the actual flow regime, such as streamline (lami­nar), transitional, or turbulent.

Ringelmann Chart A method for the vi­sual comparison of smoke from a chim­ney. The estimation is made by the comparison of the shade of smoke against shade cards.

Riser A vertical section of duct or pipe in a distribution system.

Risk assessment The sequence of events necessary to ensure that a system is de­signed to provide the safest possible working arrangement.

Rцntgen The amount of x-ray or gamma radiation that produces one unit of charge in 1 cc of dry air.

Roof ventilator A natural or mechanical unit positioned in the roof to provide air extraction from the space.

Room air conditioner A package air — conditioning unit for the air treatment of the space in which it is located.

Room air conditioner (self-contained ) A room air conditioner complete with a direct expansion (dx) system condenser and evaporator fans, filtration, and thermo­static control.

Room air distribution system The method by which air is distributed within A Space.

Rotating anemometer An instrument used to measure gas flow that depends on the rotation of vanes mounted on a spindle.

Rotation The angular displacement of a body about a specified axis in a speci­fied direction in unit time.

Rotor The rotating portion of an electric motor; the stator is the stationary part.

Route of entry Path by which toxins and other substances may enter the human body. These include inhalation, inges­tion, and absorption through the skin. Less common routes include injection and absorption through moist surfaces surrounding the eyes and ear canal.

Runaround coils The coils in a heat recov­ery system that collect heat from the hot waste steam and supply it to the cold in­coming stream via heat exchangers.


Safety cabinets A protective air enclosure (workstation) used in microbiological laboratories.

Safety factor An uncertainty factor that is used in combination with the no — adverse-effect level data to estimate the safe human dose.

Safety guards Devices to protect opera­tors from moving machine parts.

Sample badge A small clip-on device that contains solid sorbent and is used for the collection of a variety of airborne materials. It is a passive sampler that is typically clipped to the worker’s lapel and worn throughout a shift.

Sample bag A bag made from an inert polymer such as Teflon, complete with a fitting for connecting to an air-sampling pump.

Sampling The action of selecting a set of items for measurement of a given pa­rameter.

Sampling and analytical error (SAE) A nu­merical factor used in analytical meth­ods to account for uncontrollable errors. Its value is taken into consideration in the determination of whether the expo­sures are within acceptable limits.

Sampling box An enclosed chamber in which sampling is carried out or sam­ples are protected after collection for dispatch to the testing laboratory.

Sampling duration The period of time that sampling takes place at a specified sampling rate.

Sampling medium The device or mate­rial through which contaminated fluids are drawn in order to collect the con­taminants for analysis.

Sampling train The assembly of sample medium in its holder, with connecting tubing and sample pump.

Sampling volume flow rate The induced flow rate into a sampling system.

Saturated steam Steam that has the same temperature and pressure as the water from which it is formed.

Saturated vapor A vapor that can exist in equilibrium with its liquid.

Saturated vapor pressure The pressure ex­erted by a saturated vapor. This pressure is a function of the temperature.

Saturation efficiency A measure of the performance of an air washer. It is the amount of water added to the air leav­ing the washer expressed as a percent­age of the amount of water that would have been added if the air had left the washer in a fully saturated condition.

Saturation tables Tables that relate the dryness properties of a gas to its temper­ature and pressure.

Saturation vapor density The density of A Saturated vapor.

Sauter mean diameter The average ratio of the volume to the surface area used in the determination of the pressure drop in a scrubber.

Scales of sensation A simple numerical scale used to report the response of a person to temperature, humidity, air ve­locity, air purity, noise, light, taste, etc.

Scan test A test used to determine the lo­cal efficiency of an air filter.

Scatter diagram A graph in which values of one variable are plotted against the corresponding values of another prop­erty.

Scavenging The removal of an unwanted product.

Screening The separation of particulate matter by the use of filters, mesh screens, or other devices. Also, the cov­ering of a plant item to improve the vi­sual impact.

Screw conveyor A conveyor that uses an Archimedes screw to convey granular material from a hopper to point of use.

Scroll collectors A centrifugal collector in which a scroll imparts a centrifugal mo­tion to the dust stream, concentrating the dust in the peripheral layer, from where it is passed to a secondary collec­tor, and then to exhaust.

Scrubbers and absorbers Wet systems used for the removal of aerosols and other gaseous pollutants from an airstream.

Scrubbing The process of cleaning con­taminated gas by passing it through a water spray or cascade.

Seals A device on a container or conduit run that ensures that the internal prod­ucts do not escape from any joints.

Secondary air Air that is introduced above a combustion process in addition to the primary air in an attempt to ob­tain complete combustion.

Secondary containment The enclosing struc­ture around a green zone in the atomic energy industry.

Secondary filter A filter to provide final cleaning of the air after the main filter in a system.

Sedimentation The process of settling solid particulates out of suspension in a fluid or a gas.

Selectivity The degree of independence from interference.

Sensible heat The portion of heat sup­plied to a substance that produces a change in temperature without changing the state of the substance.

Sensing element A component that mea­sures of the value of a variable and pro­vides the input for control devices.

Sensitization The development of an ad­verse immune response following more than one exposure to a substance.

Sensitizer A substance that stimulates a response from the immune system.

Sensitivity The ability of a chemical analy­sis to detect low levels of the analyte. Also, any abnormal reaction of the human body to chemical substances.

Sensor A device used to measure flow, temperature, pressure, or another prop­erty of a medium.

Sensory hearing loss Irreversible hearing loss resulting from damage to the inner ear tissue that translates sound pressure into nerve impulses.

Series operation The connection of two fans or pumps in succession in order to in­crease the available pressure in a system.

Serpentine An asbestos mineral with a wavy appearance, such as chrysotile.

Set point The value on the scale of a con­troller at which the design conditions are set.

Settling The process of particulate mat­ter falling out of a gas stream or a fluid.

Settling chamber A chamber in which large particulate matter settles out of the air due to gravity. By increasing the cross sectional area of this chamber the air ve­locity decreases, allowing settling to take place.

Settling velocity The velocity that has to be attained to ensure that particles of a particular size settle at a given distance from the generation source due to the influence of gravity.

Shaver’s disease A disease of the lungs found in workers exposed to fumes or dusts containing aluminium oxide. It is a type of pneumoconiosis and results in interstitial fibrosis and decreased lung function.

Shedding The loss of fibers from a filter.

Shivering The process of metabolic regu­lation against cold by muscular action.

Shock josses The energy loss in a moving fluid stream due to one or more of the following:

Violent mixing producing eddy formation.

Separation occurring due to contractions or expansions in duct flow.

Shock ventilation The purging of a space prior to or after a process or occupancy.

Shock wave A pressure wave resulting from the rapid closure of a valve or damper in a pipeline or ductwork sys­tem, or from an explosion.

Short circuiting The process in which air from an inlet is extracted from the space before it has chance to mix, caused by the extract being positioned too near the input grille.

Shot blasting room A ventilated room de­voted to the cleaning of castings by the use of various grades of shot.

Short term exposure limit (STEL) The per­missible exposure limit for a given con­taminant averaged out over any 10- minute period during an 8-hour work­ing shift. See also Occupational exposure limit (OEL) and Long-term exposure limit (LTEL).

Sick building syndrome (SBS) The group of symptoms related to poor air quality, including headaches, irritation of mu­cous membranes of the eyes and nose, breathing difficulties, etc., experienced by occupants in poorly ventilated build­ings. See also Building-related illness (BR1).

Siderosis A benign reddish discoloration resulting from deposits of iron oxide in the lungs.

Sieving The use of sieves for the collect­ing of particulate matter or for the grad­ing by size of particulate matter for classification purposes.

Significance A statistical term relating to tests made to ascertain the probability of an effect or correlation.

Silicates Chemicals used as adsorbents.

Silicosis Pneumoconiosis resulting from inhalation of crystalline silica (quartz).

Simple asphyxiant Substance that displaces air, producing an oxygen-deficient at­mosphere.

Sink A storage device for a fluid or heat.

Sintered filters A self-supporting sintered element used in gas cleaning systems.

Size-selective sampling Industrial hygiene sampling methods that collect particles with a specific range of aerodynamic di­ameters.

Skew distribution Any set of values mea­sured during a test that is not symmetri­cally distributed.

Skin notation The word skin included as part of an exposure limit. It is used for those substances for which absorption through the skin is considered to be a significant route of entry into the body.

Slant gauge An inclined calibrated ma­nometer tube.

Sling psychrometer An instrument used to measure the dry-bulb and wet-bulb temperatures of the air, from which the humidity of the air can be determined by means of calculations, tables, or charts.

Slip (Cunningham factor) A factor used in particle physics to predict the behav­ior of small particles.

Slot venturi A device used to adjust the pressure drop in a scrubber.

Sludge The waste product formed by A Wet cleaning process.

Smell See Odor.

Smog A mixture of smoke and fog, that arises from nitrogen oxides and hydro­carbons and the photochemical action of sunlight.

Smoke Aerosols formed from minute solid or liquid particles, most less than 1 jim in diameter, generated by the in­complete combustion of a fuel or by sublimation.

Smoke bomb A firework type of device that produces smoke used for the obser­vation of airflow within a space.

Smoke damper A damper installed in a ductwork system designed to close in case of fire by means of a fusible link and electrical or magnetic device to stop the spread of smoke.

Smoke extraction A mechanical or natu­ral means by which smoke generated during a process or a fire is removed from the space to outdoors.

Smoke generator A device that electri­cally heats oil-producing smoke. The smoke is liberated from a nozzle by ei­ther thermal forces or by means of a fan and used to observe airflow patterns within a space or to observe leakage from ductwork, etc.

Smoke stain When a certain quantity of dirty air is passed through a filter paper, the degree of staining on the paper is measured and expressed as a concentra­tion of equivalent standard smoke by means of an optical reflectometer.

Smoke tube A tube or a canister con­taining a smoke-generating chemical used to observe air movements within a room.

Smuts Unburned carbon emitted from chimneys. If sulfur is present in the fuel these smuts will be acidic.

Sociocusis Hearing loss that results from exposure to the noises of everyday life.

Sodium flame test A test of HEPA filter

Efficiency using small particles generated from NaCl.

Solid phase The condition of a body be­ing a solid, such as ice.

Soiling index The degree of soiling of a filter paper fitted in a sampling device through which a contaminated gas has been passed.

Soot The aggregates leaving a combus­tion chamber due to incomplete com­bustion of a carbonaceous fuel.

Sorbent Any agent that is used in a sorp­tion process.

Sorbent tubes Small glass tubes that con­tain sampling media such as silica gel or activated charcoal.

Sorption Adsorption (a surface process) or absorption (a volume process).

Sound A physiological sensation re­ceived by the ear, which may or may not cause annoyance.

Sound intensity The sound power distrib­uted over unit area, in units of W m~2.

Sound power The rate at which sound energy is produced at the source, given in watts.

Sound pressure The average variation in atmospheric pressure caused by a sound, given in pascals.

Sound rating The manufacturer’s rating of the noise level produced by an item of rotating or reciprocating plant.

Source Where any form of pollution is generated.

Source sampling The testing and mea­surement of an emission at its point of generation.

Space requirements The footprint (m2) or volume for a given item of equipment.

Sparking The production of sparks; oc­curs under certain conditions on electro­static precipitators.

Speciation The process of determining the different species present in a chemi­cal agent.

Species The different forms in which a bi­ological or chemical agent may be present.

Specific flow The volumetric air change rate within a space, denoted by A. the flow volume rate into and out of the space divided by the volume of the space.

Specific gravity The weight of a material in kg that would occupy one cubic meter under a definite state of condi­tions.

Specific heat The amount of heat (or me­chanical work) required to raise the temperature of a unit mass of a sub­stance one degree Celsius. In the case of gases there are two specific heats, ac­cording as to whether the heating takes place at constant pressure or at constant volume.

Specific humidity The mass of water va­por that is present in a unit mass of moist air.

Specific leakage Relating to the leakage that takes place from an enclosure due to a pressure difference created either mechanically or naturally. It is the ratio of leakage area in m2 divided by the floor area in m2.

Specific measuring range The range of concentration values for which the over­all uncertainty of a measurement proce­dure is intended to lie within specific limits.

Spectrophotometry A photometer for comparing two light radiations, wave­length by wavelength.

Spirometry A test method used to evalu­ate lung function that measures volume of exhaled air passing through a tube during a given time.

Splitters Turning vanes inserted in a fluid flow system to reduce frictional losses.

Spontaneous combustion Any combus­tion that takes place without a source of ignition due to the nature of the mate­rial and its packing arrangements.

Spread The angle of divergence of a jet or plume from its origin.

Squirrel cage fan A fan with a squirrel cage rotor, which has the advantages of lowr cost, low maintenance, and sturdi­ness.

Stability The state of being stable.

Stable Relating to a chemical that is not readily decomposed, or a stable condi­tion in a working environment.

Stack Effect The pressure forces set up in space due to the density differences be­tween the hot and cold columns.

Stack solids The solid content of a gas stream leaving a chimney or exhaust duct.

Stagnation A situation in which flow is assumed not to occur.

Standby fan A fan or fans included in a system to provide a backup in case of failure, and/or to provide a rapid air change in the case of a chemical spill.

Standard The maximum level of an air contaminant allowed in workplace or external air as defined by a legal author­ity. Or any national or international standard relating to a product or code of practice.

Standard air Air at standard temperature and pressure (STP) which is the same as normal temperature and pressure (NTP)

0 °C and 101.325 kPa, with the force due to gravity 9.80665 N kg~J.

Standard conditions Relating to normal conditions such as standard temperature and pressure (STP), which is the same as normal temperature and pressure (NTP).

Standard deviation A statistical measure of the scatter of a series of numbers or measurements about their mean value.

Standard temperature and pressure (STP) This is the same as normal tem­perature and pressure (NTP), 0 °C and 101.325 Pa.

Standard threshold shift (STS) An increase of 10 dB or more in a person’s HTL in the 2000 to 4000 Hz range, significant because it represents loss of a significant proportion of hearing.

Static efficiency The ratio of fan static pressure to fan total pressure.

Static electricity Phenomena associated with electric charges at rest, due purely to the electrostatic field produced By The charge.

Static head The difference between the total fluid pressure and the dynamic pressure.

Static pressure curve A graphical repre­sentation of the static pressure and Vol­Ume flow of a fan at a set speed.

Static pressure regain The increase in static pressure that takes place due to A Decrease in the air velocity causing the velocity pressure to be converted into static pressure.

Static sample The result of the process of static sampling.

Static sampler A device not attached to a person that samples air in a particular location.

Static sampling The use of a static sam­pler to determine a particular property.

Stator The stationary portion of an elec­tric motor.

Steam Water in gaseous state above its boiling point.

Steam tables Tables containing the ther modynamic properties of steam over A Range of pressures and superheat.

STEL See Short-term exposure limit.

Step control A control method in which a multiple switch assembly sequentially switches on or off various stages of a device, such as a heater battery.

Stephan flow The flow of molecules to­wards or away from the surface of a vol­atile liquid due to either evaporation or condensation.

Stokes diameter The equivalent spherical diameter of the particle being considered.

Stokes law This relates to the factors that control the passage of a spherical parti­cle through a fluid. The Stokes diameter of a particle is the diameter of A Sphere of unit density, which would move in A Fluid in a similar manner to the particle in question, which may not be spherical.

Stoichiometric The exact quantity of reac­tants required to completely react accord­ing to a particular chemical equation. If the reaction were complete only products and no reactants would remain.

Stopping distance The maximum distance a moving particle will travel in still air af­ter all the external forces are removed. In the Stokes region it is the velocity of the particle times the relaxation time.

STP See Standard temperature and pressure.

Straighteners Vanes fitted in ductwork or air handling units before or after a change in section to produce a reduction in pressure drop.

Stuffing box A box that provides a seal for a fan shaft.

Subcutaneous The deepest layer of skin, containing fatty and connective tissue that provides a cushion and insulative base for the skin and also binds the skin to the underlying tissues.

Superheated steam Steam that has a tem­perature above that corresponding to boiling temperature, corresponding to the pressure at which it exists.

Superheated vapor See Superheated Steam.

Supersaturation An unstable condition in which the concentration of a solution or a vapor is greater than that correspond­ing to saturation.

Supply air Treated or untreated air entering the space. For the purpose of drawings it is color-coded to show the various ther­modynamic treatments.

Number of thermodynamic treatments

Color code for drawings





2 or >




Supply air (SUP) classification

Category Description

Supply system An arrangement that pro­vides the distribution of a fluid, vapor, gas, electricity, or another medium.

Surface contaminant Any contaminant that adheres to a surface in A clean Room.

Surface tension A characteristic of a liq­uid surface, with effects at liquid-gas or liquid-liquid interfaces.

Suspended matter (particulates) Particles that remain in suspension in A Gas or A Fluid for a sufficient time in to be De­Tected by physical means.

Sutherland’s equation An equation that allows the effect of temperature on the viscosity of a gas to be determined.

Swirl nozzles Nozzles used to distribute primary air into a space by creating a swirl. This arrangement provides good entrainment and mixing of the air.

Synergism The phenomenon in which the effect produced by two causes together is greater than the sum of the effects that would be produced by the causes sepa­rately.

System curves The graphical representa­tion of the resistance (static pressure) that occurs in a ventilation or pump sys­tem at different flow rates.

System effect The effect that system com­ponents and/or the room have on the air quantity and pressure delivered to the space.

System life The duration of the life cycle of an item of equipment or a complete system.

System specification The engineering specification produced by the manufac­turer of the equipment, or by the system designer for the plant as a whole, stating what the system is capable of achieving.

System toxin A substance that affects tar­get organs or entire organ systems.

SUP 1 Supply air containing only out­

Door air

SUP 2 Supply air containing a mixture of

Outdoor and recirculated air

Table exhaust Mechanical extractions of pollutants generated on a worktable during a process. The extraction takes place through a perforated workbench or from the sides or back of the table.

Tachometer An instrument used to deter­mine the speed of rotation of a shaft, normally in revolutions per second (rps),

Tackifier A substance applied to a partic­ulate collection device to increase its ef­ficiency in dust retention.

Take-off Any pipework or ductwork branch taken from a main run.

Target A desirable air quality or tempera­ture to aim for. Also items affected by pollutants.

Tangential acceleration Acceleration of a fluid tangentially to a vane or impeller due to rotary motion.

Target level The predetermined concen­tration of a dominant contaminant to be achieved by air technology using chemi­cal or other control methods. It may re­late to an entire room volume or a building zone.

Target organ A specific organ where the toxic effect of a substance is manifested.

Teflon filter A chemical-resistant hydro­phobic filter composed of polytetraf — louroethylene (PTFE) used for industrial hygiene sampling.

Telemetering Signal transmission from a measuring instrument by telephone or radio to a distant point for recording or display.


Temperature The degree of molecular ac­tivity in a body; high activity gives a high temperature, low activity a low tempera­ture. The degree of activity is based on the assumption that absolute zero has no mo­lecular movement at all. The following are some specific temperatures:

Absolute The temperature relative to absolute zero, expressed in Kelvin. Also called thermodynamic temperature.

Asymmetry (radiant) The difference between the plane radiant

Temperatures of the two opposite sides of a small plane element.

Dew point The temperature of a mixture of air and water vapor at which further cooling or the addition of more water vapor will cause moisture to condense from the air.

Difference (vertical air) The difference in air temperature measured at 1.1 m and 0.1 m above the floor.

Differential in occupied zone The largest value of the difference between the measured air temperatures in the occupied zone.

Dry bulb The air temperature recorded by a dry bulb thermometer, a sensory device excluding any effects of moisture or radiation.

Effective See Operative

Environmental The sum of two — thirds of the mean radiant temperature and one-third of the air temperature.

Equivalent The temperature as recorded by an Eupatheoscope, taking into consideration air temperature, air velocity, and thermal radiation.

Globe The temperature recorded by a 100 mm black bulb (globe) thermometer.

Gradient risk The percentage of people predicted to be dissatisfied due to a difference in air temperature between the ankle and the head.

Induced The temperature of the internally induced air flow.

Jet The leaving temperature of a jet from an opening or the average jet temperature at a given cross-sectional area of a jet.

Mean in occupied zone The arithmetic average of the measured values of air temperature in the occupied zone.

Mean radiant The theoretical uniform surface temperature of an enclosure iri which an occupant would exchange the same amount of radiant heat as in the actual nonuniform enclosure.

Operative The theoretical uniform temperature of an enclosure in which an occupant would exchange the same amount of heat by radiation and convection as in the actual nonuniform space.

Optimum operative The temperature that satisfies the greatest possible number of people at a given clothing and activity level.

Plane radiant The uniform temperature of an enclosure where the radiance on one side of a small plane element is the same as in the non-uniform actual environment.

Primary The temperature of the primary airflow.

Primary difference The difference between the primary air temperature and the reference air temperature in the reference zone.

Resultant The temperature recorded by a thermometer in the center of a 100 mm diameter blackened globe, corrections for the air velocity.

Resultant temperature For air velocities less than 0.1 m s_1, it is given by

0res = 0.50,.+ 0.5

And if the air velocity is above 0.1 m s-1,


Res 1….. + JtQ~v

Where 0re, = dry resultant temperature, = mean radiant temperature, 03 = air

Temperature and v = velocity in m — s~1

Room reference The average of at least five measurements of the air temperature at a height of 1.1 m from the floor and outside the area directly influenced by the device.

Thermodynamic A Kelvin is 1/273.16 of the triple point of water.

Total The air temperature of a total flow from an ATD.

Wet Bulb The temperature as recorded by a thermometer bulb covered by a wet wick.

Tempering The process of heating or cooling make-up air being supplied to a space in order to provide the required temperature limits.

Temporary threshold shift ( IT S) A tem­porary shift in hearing threshold level that goes away after the person has been in a quiet environment for a few hours. It is confirmed by an audiogram retest follow­ing a suspected standard threshold shift (STS) after at least 14 hours away from high levels of noise.

Temporary variance An OSHA variance issued to an employer who is unable to comply with a standard by its effective date for reasons beyond their control. The employer must demonstrate a plan for coming into compliance within a pe­riod not to exceed one year and provide all available measures to protect the em­ployees in the interim.

Teratogens Toxins that cause abnormal development or birth defects.

Terminology Relating to current word usage of technical terms.

Terminal Falling Velocity See Terminal settling velocity.

Terminal Settling Velocity The maximu m velocity attained by falling particulate matter, at which the gravitational force is balanced by the viscous drag created by the fluid in which it is fall­ing. Terminal free-fall velocity of a sphere moving through a fluid, derived by using Stokes’ law, Newton’s drag coefficient, and Reynolds number, is proportional to the radius of the sphere squared and to the density, and is inversely proportional to the fluid viscosity.

Test aerosols Aerosols for testing the effi­ciency of filters generated from various grades of dusts.

Test airflow rate The rated airflow in equipment testing.

Test dust Various grades of dust used to test the collection efficiency of filters.

Test procedures The arrangements of the various sequences of events necessary for carrying out a test on a system dur­ing commissioning or for any other testing.

Test volume flow rate See Test airflow


Theoretical air quantity The stoichiomet­ric quantity of air required for complete combustion of a given quantity of a spe­cific fuel.

Thermal anemometer An anemometer that employs the principle that the quantity of heat removed by a gas stream passing a heated element has a direct relationship to the velocity of the gas stream.

Thermal coagulation The process by which Brownian movement causes par­ticulate matter to collide and adhere.

Thermal comfort That state in which the human body is in a state of thermal equi­librium. also called thermal neutrality.

Thermal currents Natural convection currents set up in a fluid due to density differences.

Thermal discomfort Discomfort experi­enced due to excessive heat loss or gain from or to the human body due to radi­ation, convection, conduction, evapora­tion, or air movement.

Thermal ignition sources A source that will cause the ignition of a flammable gas, vapor, or dust, such as an electric spark, flame, or hot surface.

Thermal load The heating or cooling re­quirements of a space due to structural gains or losses and the air infiltration and mechanical ventilation. The load produced by a process in a working environment.

Thermal oxidizers Devices used for the destruction of hazardous or toxic gases by oxidation at elevated gas tempera­ture, producing primarily carbon diox ide and water.

Thermal pollution The effect of the emis­sion of high emission temperature waste products into the environment, creating significant temperature change.

Thermal updraft The air movement that is created by a thermal plume.

Thermocouple An instrument for the mea­surement of temperature consisting of two wires of different metals joined at each end. An electrical electromotive force is generated, the magnitude of which allows the temperature to be measured.

Thermodynamics The branch of engi­neering that deals with the relationship between heat, power, and gases

Thermograph A device for measuring and recording air temperature.

Thermography The use of a tube or 1R film to determine surface temperatures

Thermohydrograph A mechanical or elec­trical device that records simultaneously the relative humidity and temperature of the air throughout the day.

Thermometer A device used to determine the temperature of a medium.

Thermometer anemometer An instrument that measures the heat removed by an air stream passing over a heated element or bulb to determine the air velocity. See Kata thermometer.

Thermophoresis The motion of particulate matter in the direction of a cooler gas or surface due to the hot side of the particles exerting more force than the cool side.

Thermostat An instrument used to de­tect temperature changes and provide corrective output.

Thoracic fraction Particles with aerody­namic diameters of 5-10 pm, which en­ter the lungs but not the alveoli.

Threshold A level or dose of a pollutant below which it is assumed to have no ef­fect on life.

Threshold dose A dosage or exposure level below which the adverse effects of a substance are not realized or expressed by the exposed population.

Threshold limit concentration (TLC) The concentration of an air pollutant al­lowed in the work space.

Threshold limit value (TLV) The limits of airborne concentration of chemical sub­stances that are allowed in workplaces published by the American Conference of Governmental and Industrial Hygien — ists (ACGIH). Also known as MAC.

Threshold limit value-ceiling (TLV-C) The concentration that should not be exceeded during any part of the working exposure.

Throttling The expansion of a fluid through a constricted passage (across which there is a pressure difference), during which no external work is done. The initial and final velocities of the fluid are equal, and there is no heat ex­change with external sources. A change in entropy will, however, take place.

Throttling range In a proportional con­troller, the control point range through which the controlled variable must pass to move the final control element through its full operating range.

Through ventilation Ventilation that takes place through a space due to wind forces, normally resulting in poor mixing of the room air due to short circuiting.

Throw The distance a fluid stream trav­els on leaving an outlet before its veloc­ity is reduced to a specific value.

Tidal volume The volume of gas inhaled or exhaled during each cycle of breathing.

Tight building syndrome See Sick build­ing syndrome (SBS).

Time constant The time required for a dynamic component, such as a sensor, to reach 63.2% of the total response to a change in its input.

Time-weighted average (TWA) exposure The average exposure to contaminant that an operator is exposed to over a work period.

Tinnitus The perception of high-pitched noises in the ears, such as ringing, roaring, or hissing caused by a bodily condition rather than an eternal source.

Tip speed The velocity of the tip of a fan or pump impeller.

Titration An analytical method involv­ing the quantitative addition of reagents to a solution until an endpoint is reached as indicated by a color change or a precipitate.

TLC See Capacity, total lung or threshold limit concentration.

Tolerance The ability of a person to with­stand adverse conditions of air quality, infectious agents, noise, vibration, or light without showing signs of infection or disease.

Total airborne particles All the particles surrounded by air in a given volume of air.

Total efficiency The ratio of the power added to the airstream to the power put into the fan or other device at the shaft.

Total energy The sum of the internal en­ergy, pressure energy, and kinetic energy of a fluid or substance.

Total lung capacity (TLC) See Capacity, total lung.

Total pressure The algebraic sum of the velocity and static pressure, recorded at a set position within the system.

Toxicity The ability of a substance to be poisonous or injurious to living organisms.

Toxicology The study of the body’s re­sponses to toxic substances.

Tracer gases Gases used with an instru­ment to determine the air change rate within a space.

Tracer technique The use of a tracer gas in air for the study of air movement within a space.

Tracheobronchial region The middle re­gion of the respiratory system, com­prised of the trachea and the bronchi.

Trajectory The actual path taken by air or particulate matter due to its velocity and density when allowed to enter a space either naturally or mechanically.

Transition A change in a duct from square or rectangular to round or vice versa.

Transitional flow The nature of flow in the zone between laminar and turbulent flow.

Transport velocity The air velocity re­quired in an extract duct conveying dust to ensure that the particulate matter re­mains in suspension and does not settle.

Traversing The process of moving across a grid line in a duct or on a hood with a Pitot tube in order to determine the ve­locity or pressure distribution.

Trickle valves Manual or automatic valves or openings that allow a given quantity of air or other gas to pass from one space to another.

Trigger finger An industrial injury caused by a constriction of the tendon charac­terized by the inability to bend or straighten a finger.

Troubleshooting The sequence of events carried out by a service engineer to de­termine the reason for faulty operation of a system.

True value A theoretical value that ex­actly relates a quantity for specific con­ditions.

Tube-axial fans An axial flow impeller mounted in a tubular housing, which contains the rotational velocity.

Turbulence Fluid motion made up of ran­dom eddies as opposed to streamline flow.

Turbulence loss The energy loss that takes place in A ventilation system through air turbulence.

Turbulent ventilated rooms The method of air distribution within a space that ensures the maximum amount of air mixing.

Turn-down ratio The lowest percentage of capacity at which a fluid flow device can be set in order to obtain suitable de­sign flow conditions.

Turning vanes Curved vanes added to ductwork elbows in an attempt to en­sure streamline flow and, by so doing, reduce turbulent losses.

Tyndall lamp A parallel light beam pro jected onto a cloud of dust particles gen erated from a process to produce scattering of the light, allowing an as­sessment of the magnitude and path of the cloud.


U-Tube manometer An instrument used for measuring pressure differences in a fluid or a gas by means of a U-shaped tube containing a fluid such as mercury or oil.

Ultraviolet (UV) analyzer An instrument using the wavelength of UV light to deter­mine the properties of a gas or vapor.

UV Radiation Electromagnetic radiation in the wavelength range of approxi­mately 4 x 10~7 to 5 x 10~9 m, i. e., be­tween visible light waves and X-rays.

UV sterilization Sterilization of air or water by means of UV rays.

U value The overall heat-transfer coeffi­cient, in W nr2 °C.

Ultraclean room A high-efficiency clean room used in surgical operations.

Uniform mixing The mixing of two or more fluids that ensures complete uni­formity of the mix.

Unit A quantity or dimension adopted as a standard of measurement.

Unit collector A particulate-collection de­vice that is self-contained with fans, fil­ters, etc.

Unidirectional flow See Laminar flow.

Unidirectional jet Airflow in one direc­tion only. It is essential that the tempera­ture differential between the jet air and the ambient air is small and the leaving air velocity is high.

Unstable Relating to a fluid in a state that is not stable. See Stable.

Upstream The location of a fluid before entering a process.

Upper confidence limit (UCL) A statisti­cal procedure used to estimate whether the true value is higher than the mea­sured value.

Volume of air entering or leaving the lungs in one respiratory cycle.

Ventilation, dilution See Dilution ventila­tion.

Ventilation effectiveness The ability of a ventilation system to remove the pollu­tion generated within a space.

Ventilation efficiency Indices that provide A method of assessing the mixing char­acteristics of incoming air with the room air. It presents a means of determining the pollutant distribution within the space.

Ventilation heat loss or gain The quantity of sensible and latent heat lost or gained from an enclosure due to natural or me­chanical ventilation.

Ventilation, local exhaust See Local ex­haust ventilation.

Ventilation, mechanical Air movement created in a space by a fan or other air — moving device.

Ventilation, natural Air movement cre­ated by wind forces, thermal forces, or a combination of both.

Ventilation rate The actual mechanical or natural air change rate within a space, expressed in L/s or air changes per hour. The supply air may be all fresh air, or a mixture of fresh and recirculated air.

Ventilation strategy The method of plan­ning to ensure that the best method of ventilation is provided to a space.

Ventilation system Any natural or me­chanical system that provides some form of ventilation.

Ventilation thermal load The heating or cooling load required to compensate for thermal losses resulting from natural or mechanical ventilation.

Venturi A tube that is constricted and then opens out again, used to either measure the flow of a fluid or to assist the scrubbing of a gas by a liquid.

Venturi meter A measuring instrument used to determine the fluid velocity, achieved by the comparison of pressure differentials across its throat.

Venturi Scrubber A scrubber with water velocities of between 60 and 100 m s~1 or higher, which create shear stresses, break­ing up a gaseous air stream to provide ef­fective particulate removal.

Vibration The rapid oscillating move­ment of a solid body due to an alternat­ing force, for example, a rotating piece of machinery that is out of balance.

Vibration isolator A flexible cloth or plastic connection placed between the source of a vibration, such as a fan housing, and a potential conductor of the vibration, such as ductwork.

Viruses The causative agent of many in­fectious diseases.

Viscosity The resistance to fluid flow caused by the shear forces between lay­ers in the fluid.

Viscosity, dynamic Sometimes called ab­solute viscosity, the shear stress in a fluid divided by the velocity gradient.

Viscous flow See Laminar flow.

Viscosity, kinematic The ratio of dynamic viscosity to density.

Visibility The measure of the clearness of air.

Vital capacity (VC) The volume of air that can be taken in and pushed out of the lungs.

Vitiated air Bad or polluted air.

Volatile organic compounds (VOC) A var­ied group of pollutants that are liberated from certain synthetic building materials and fabrics. They are assumed to be re­sponsible for some of the aspects of sick building syndrome.

Volumetric analysis The determination of the amount of a particular gas in a mix­ture of gases, as the percentage of the to­tal volume. See Gravimetric analysis.

Volumetric efficiency The ratio of the net volume flow rate handled by the ma­chine to the volume flow rate handled by the impeller.

Volumetric heat The heat required to raise the temperature of a unit volume of a gas one degree Celsius.

Volumetric flow rate The flow of a fluid

Expressed in m3 s_1 or L s’1.

Vorticity Rotary motion, such as in a tor­nado.

Vortex Fluid flow that takes place with rotary motion, such as that observed in the wakes of buildings.

Vortex breakers 1. A device used to straighten out rotary flow in a duct a short distance after a fan. 2. A device found in a cyclone discharge fitted to re­duce shell erosion by particulate abra­sion.

Vortex gas cleaners A gas pollutant sepa­rator that makes full use of a vortex for the separation of particulate matter.

Vortex shedding anemometer A device for measuring air velocity by placing an obstruction in a gas flow and measuring the frequency of vortex is formation downstream of the obstruction.


Walk-in fume cupboard A fume cup­board that has the sash opening from the floor of the laboratory.

Walk-through survey An examination or inspection of a workplace involving a review of hazardous materials present and/or used, observation of work prac­tices, and consversations with individu­als to identify all of the actual or potential chemical, physical, biological, and ergonomic hazards.

Wall function See Boundary conditions.

Wall ventilator A wall air outlet or inlet with a weatherproof cover to provide air exchange between inside and outside by natural forces.

Warning properties The physical and chemical characteristics of a substance that allow it to be tasted or smelled at unsafe concentration levels.

Warmth The state or quality of the body being warm; it is not necessarily a state of thermal equilibrium.

Washing See Scrubbing.

Washer, air A device for adding moisture to an air stream by means of spray or capillary action. This term should not be used to relate to any wet cleaning pro­cess of a gas or other air stream.

Washout Equipment attached to dust col­lectors, that allows the collected particu­late matter to be washed away.

Waste energy Any waste liquid, solid, or gas that contains useful energy after a process.

Waste heat Heat from a process that is surplus to requirement; this heat may be supplied to a heat recovery device for use in another part of the plant or process.

Water chiller A water-cooling device op­erating either by the direct expansion of a refrigerant by an absorption system or by evaporative cooling.

Water hardness The hardness of a water sample due to the salt content.

Water make-up The extra treated or un­treated water required to replace water lost by evaporation or absorption into dust particulates.

Water purification The treatmen t neces­sary to ensure that water used for a pro­cess or discharged from a process is of an approved standard.

Water raw Water as supplied from a source before any treatment.

Water, softening A chemical process used to change the chemical composition of water by the removal of hardness salts.

Water, vapor Water in the gaseous state that can be liquefied by increasing the pressure without altering the temperature.

Weather strip A purpose-made strip fit­ted around doors and windows to re­duce infiltration.

Weight, arrestance Tests carried out with synthetic dusts to determine the weight of dust collected on a filter during testing.

Weighting A The frequency-selective de­vice on a sound level meter used to mea­sure the A frequency netw’ork.

Weighting scale The A, B, or C weighting scales, used to approximate the response of the human ear at different ranges of sound pressure levels.

Weld fumes The fine fumes that are pro­duced and liberated into the room air during the welding process.

Wet air filter Any filter that depends on water or another fluid to improve the ef­ficiency of collection of contaminants in a gas stream.

Wet bulb globe temperature (WBGT) A heat stress index, which provides infor­mation on comfort conditions.

Wet bulb temperature See under Temper­ature.

Wet centrifugal A dust collector that uses water or other fluid in a centrifugal ac­tion in order to improve the particulate collection efficiency.

Wet collector A gas-cleaning device that uses a fluid for the removal of a pollut­ant from an air or gas stream.

Wet chemical processes A gas-cleaning process that uses certain chemicals to ensure the maximum retention of a given pollutant in the cleaning fluid.

Wetted wall column An experimental ap­paratus used to determine the mass transfer that takes place through lami­nar boundary layers.

White zone A ventilation containment zone used in the atomic energy industry.

Wide-open volume The maximum flow volume a fan is capable of delivering writh no resistance on the outlet side.

Wind Air motion relative to the earth’s surface caused by thermal forces and the earth’s rotation.

Windbreak A natural or artificial barrier that protects a building from the pre­vailing winds.

Wind chill index An empirical scale that correlates well with the sensation of bare dry skin due to the chilling effect of the outdoor air temperature and wind speed.

Wind infiltration Infiltration of outdoor air into a building caused by the pres­sure difference across the faces of the building.

Wind pressures The resulting positive or negative pressures due to the wind ve­locity set up on the walls and roof of A Structure.

Wind rose A graphical method of. show­ing the direction, velocity, and frequency of wind for a given location over set time periods.

Wind shear The change in the wind ve­locity and direction with height above some reference plane.

Wind speed The wind velocity measured at a point in open undisturbed country 10 m from the ground. Corrections have to be made for other locations, and for heights above 250 m.

Wind stop A flat plate or a cone fitted over an outlet or inlet duct in order to reduce the possibility of flow reversal due to wind pressure.

Wind tunnel A fan-assisted test rig used to determine the air forces and flow pat­terns acting on model buildings or com­ponents.

Wind vane A revolving instrument used to indicate the direction in which the wind is blowing.

Windward The side of a building or duct opening on which the wind is blowing, also known as upwind.

Work benches A bench on which a pro­cess is carried out.

Work field analysis The actual analysis of a given task or the determination of the concentration of pollutants within a work region.

Work region Any area in which an allot­ted work task is carried out.

Work room A room in which an operator carries out some allotted process.

Work pattern The sequence of activities carried out by the worker over a given work period.

Workplace The defined working area or areas in which work is carried out.

Work platform A platform provided around a plant item for safe mainte­nance of the unit.

Work space The area or volume of a Space in which work on a given process Is Carried out. The term is also used for the clearance area provided around equipment for maintenance.

Work station Any area in which some as­pect of work activity is carried out.

Working level (WL) The allowable level of exposure of a person to an atmo­sphere that contains any combination of Radon daughters.

Working level month (WLM) An expo­sure of 1WLM can be taken to be re­ceived by a person working in a Radon daughter concentration of 1WL for 170 hours.

Working procedures Set requirements on how an industrial work process is car­ried our.


X-rays Short-wavelength electromagnetic energy that originate outside the nucleus as electrons suddenly move from higher to lower energy levels, giving up energy.


Yield point The point at which material extension is no longer proportional to tension applied. Up to this limit, elastic deformation occurs; after this limit, plastic deformation occurs.


Zeolite Pellets or granules of aluminum silicate, used in water treatment or air — cleaning applications.

Zero count rate The number of counts re­corded in unit time by an optical particle counter when a particle-free gas is passed through the measuring chamber.

Zero exposure standards Relating to car­cinogens, which have essentially zero al­lowable exposure levels.

Zone A set area or volume, either in­doors or outdoors, in which work may or may not be carried out, depending on pollutant levels.

Zone, capture The area or volume in which a capturing device contains the generated emissions around a process. The capture velocity in this zone must be high enough to ensure the efficient collection of pollutants.

Zone, comfort The area or volume of a space that has its thermal and acoustic environment held at a set standard for the comfort of occupants.

Zone, control The area or volume of a space that has its acoustic, visual, ther­mal, and air purity conditions con­trolled to specified levels.

Zone, dead The part of an environment in which the influence of air interchange can be considered negligible.

Zone, exterior Any area or volume out­side a building or process.

Zone, interior The most central portion inside a building or process.

Zone, local The area or volume in which the air is controlled locally. The control requirements may be for

Worker protection and comfort,

Process control, or

Product protection.

Zone, main Normally the largest area, often the same as the occupied zone, in which the specified pollutant levels must be maintained.

Zone, multi Any area or volume within a building that may have more than one requirement for the set environmental conditions.

Zone, occupied The zone of a building in which humans or animals are housed.

Zone, perimeter The inside portion of a building or process that is nearest to the outside environment.

Zone, pressure A zone within a building or a process that is held at a given posi­tive, negative, or neutral pressure in or­der to contain process contaminants.

Zone, protection The area or volume in a working space in which protection of the process or occupants is maintained to set conditions.

Zone, trapped air Any area in the working environment in which the air movement is inadequate to remove the pollutants gen­erated within the space.

Zone, uncontrolled Any area in which the space conditions are not specified or controlled.

Zone, working The volume or area of a building in which a given activity is be­ing carried out.

Zoning The practice of dividing a build­ing into sections for heating and cooling control, in order to ensure that one con­troller is capable of dealing with ОHe re­quirements of one zone alone.

[1] Constant friction method. The principle is that the friction losses per meter run of the duct are taken as being constant. Values in the 1 to 5 Pa/m range are typical. The friction losses of connecting elements are expressed as an equivalent length of the straight ductwork runs. This is a simple method. It is ideally suited as a preliminary design method where it is combined with another approach.

• Constant velocity method. This is a simple but not very cost effective approach for systems with a wide range of duct diameters.

• Gradual velocity reduction method. This method is a variation of the constant friction approach, where a maximum velocity is used for the main and branch ducts. This procedure provides a reasonable solution and choice between the velocity, diameter, and resistance. The method is not useful to provide the same static pressure at each outlet.

• Static pressure recovery method. The diameters are selected in such a way that the same static pressure is available before every connection. The duct reduction is selected in such a way that the gain of static pressure is in balance with the friction losses up to the next connection point. This method may result in fewer control devices at connection points or outlets. Low velocities and large diameters at the end of the system may be the result of this design approach.

[2] Total capture of emissions is possible (equivalent to total building evacuation). The enclosure offers total containment, unaffected by in­plant drafts.

[3] Geometric Modeling

This is the first step, and a very important one, in every CFD study, as it is the basis for the computational mesh. The engineer has to choose the resolution and the details needed for capturing the flow effects desired from the available data. In many cases, the data available are too few; in other cases, the amount of data has to be reduced to a level that can be handled by the resources available.

[4] FIGURE 12.21 A simple calibration facility for small thermal probes.

Tem. The instrument to be calibrated is placed downstream from the nozzle, where an almost flat velocity profile is formed. The reference velocity may be based on the measured flow rate and a known nozzle velocity profile, or it may be measured close to the calibrated sensor using a Pitot-static tube. High — quality calibration wind tunnels are expensive facilities requiring skilled per­sonnel to carry out the calibrations.

Very low (below 0.5 m s_I) velocities are difficult to achieve in normal wind tunnels. In this range a slide tunnel is a better alternative. In a slide tun­nel the air stands still and the sensor moves with a constant velocity on a slide. The reference velocity is based on the calibration distance and the traveling time. As the length and time can be measured very accurately, the calculated reference velocity value is of good quality’. Convective currents inside the slide tunnel restrict the lowest calibration velocities to around 0.05 m s~*. On the other hand, high velocities are restricted by the length of a straight tunnel, be­cause a certain length of the tunnel is required for acceleration and decelera­tion of the slide.