CONTAMINANT SOURCES Classification
Knowledge of the process or operation and contaminant sources is essential before ventilation systems can be selected and designed. Contaminant sources affecting the working environment may be external, associated with the elements of HVAC systems, or internal.
Outdoor air is generally less polluted than the system return air. However, problems with reentry of previously exhausted air occur as a result of improperly located exhaust and intake vents or periodic changes in wind conditions. Other outdoor contamination problems include contaminants from other industrial sources, powTer plants, motor vehicle exhaust, and dust, asphalt vapors, and solvents from construction or renovation. Also, heat gams and losses through the building envelope due to heat conduction through exterior walls, floor, and roof, and due to solar radiation and infiltration, can be attributed to effects from external sources.
The HVAC system also acts as a pollutant source when it is not maintained properly. Microorganisms breed in various environments present within components (e. g., cooling coils, ducts) of the system and may be distributed throughout the building. Improper maintenance of filters leads to loss of efficiency and re-emission of contaminants.
This section discusses primarily internal sources of contaminants and other occupational hazards related to the process or the building envelope.
Among the major potential hazards affecting working environment are chemical (airborne contaminants), biological, and physical hazards. Air contaminants are commonly classified as either particulate contaminants or gas and vapor contaminants.1 Common particulate contaminants include dusts, fumes, mists, aerosols, and fibers.
Dusts are solid particles generated by such processes as handling, crushing, and grinding.
Fumes are formed when material from a volatilized solid condenses in cool air (e. g., welding fumes).
Fibers are solid particles whose length is several times their diameter, such as asbestos.
Gases are formless fluids that expand to occupy the space enclosure in which they are confined. They are atomic, diatomic, or molecular in nature, as opposed to droplets or particles, which are made up of millions of atoms or molecules.
Vapors are the volatile form of substances that are normally in a solid or liquid state at room temperature and pressure. Through evaporation, liquids change into vapors and mix with the surrounding atmosphere.
Mist is a liquid suspended in air. Mists are generated by liquids
Condensing from a vapor back to a liquid or by a liquid being dispersed by splashing or atomizing.
Aerosols are also a form of a mist characterized by highly respirable, minute liquid particles. They can be formed by atomizing, spraying, or mixing, or by violent chemical reactions, evolution of gas from a liquid, or escape of a dissolved gas when pressure is released.
Fogs are fine airborne droplets usually formed by condensation of vapor. Many droplets in fogs are microscopic and stibmicroscopic and serve as a transition stage between mists and vapors.
Smog commonly refers to air pollution; it implies an air mixture of smoke particles, mists, and fog droplets of such concentration and composition as to impair visibility, in addition to being irritating or harmful. Smog is often associated with temperature inversion« in the atmosphere that prevent normal dispersion of contaminants.
Biological hazards include bacteria, viruses, fungi, and other living organisms that can cause acute and chronic infections by entering the body either directly or through breaks in the skin. Physical hazards include thermal para-meters (temperature, relative humidity, and velocity) beyond the comfort range, excessive levels of ionizing and nonionizing electromagnetic radiation, noise, vibration, and illumination.
Airborne contaminant movement in the building depends upon the type of heat and contaminant sources, which can be classified as (1) buoyant (e. g., heat) sources, (2) nonbuoyant (diffusion) sources, and (3) dynamic sources.2 With the first type of sources, contaminants move in the space primarily due to the heat energy as buoyant plumes over the heated surfaces. The second type of sources is characterized by contaminant diffusion in the room in all directions due to the concentration gradient in all directions (e. g., in the case of emission from painted surfaces). The emission rate in this case is significantly affected by the intensity of the ambient air turbulence and air velocity. The third type of sources is characterized by contaminam movement in the space with an air jet (e. g., linear jet over the tank with a push-pull ventilation), or particle flow (e. g., from a grinding wheel). In some cases, the above factors influencing contaminant distribution in the room are combined.
The effect of buoyancy in gases released into the air can be related either to the difference in the molecular weights or to the difference in temperature. To characterize the buoyancy for gases with a molecular density significantly different from the density of air, Elterman proposed a parameter P, with units of g/(m5 K):!
1 — 29/M„
P — C——————— 2 ,7 I
1 ^ A0 ’ ‘■1|
WTiere C = gas concentration in the air, g/m3; Mg = relative molecular density of the gas; A0 = 0-0o = air temperature difference between the reference point and the air supply, °C.
According to Elterman, when P < 5 x 10-3 g/(m3 K), the distribution of gas concentrations along the room height is similar to the temperature distribution, and thus the contaminant removal efficiency and heat removal coefficients will have the same values. When 5 x 1(T < P < 0.1, gas concentration in the air of the upper zone is higher than that in the occupied zone, and the gas removal efficiency is higher than the heat removal efficiency. When P ~ 0.4, the gas concentration distribution is uniform along the room height. Only when P > 0.4 g/(m3 K) is the concentration of the gas, which is heavier than air, higher in the occupied zone than in the upper zone.
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