10.2.3.! General

The classification of hoods into exterior hoods and enclosures could sometimes make it difficult to specify a hood. This classification is only an at­tempt to describe the hoods. Enclosures can be separated into partial and total enclosures: partial enclosures have an opening to the surroundings big enough to use for work, and total enclosures do not. Both have the contaminant source inside a physical volume and for some of these hoods this volume is large enough for some workers to work inside. See Fig. 10.39.

Exhaust opening inside enclosure Contaminant source


To fan and cleaning




Air inlet through leakage or filter or separate fan


Exhaust opening inside enclosure Contaminant source


To Fan and cleaning


Enclosures Enclosures

Partial enclosures are a compromise between containment and access. Most people misunderstand the function of partial enclosures. It is not possible to completely separate the interior from the surroundings with partial enclosures. Complete separation is only possible with total enclosures. The function of a partial enclosure is as dependent on the flow rate, the flow field, the working procedures, the contaminant generation process, etc. as is the function of exte­rior hoods. The advantage with a partial enclosure is that the physical walls di­minish the possibilities for the contaminants to escape from the hood to the surroundings. Thus these hoods could be used when relatively high demands are put on the contaminant concentration outside the hood.64 Some of the most commonly used enclosures, such as fume cupboards and booths, are described. Many variations of these exist, e. g., enclosure of the complete process, and some of these are described here.

The total enclosure is the natural choice when very high demands on the containment of generated contaminants exist (e. g., glove boxes). Usually these are not totally closed, since some air must enter the volume before it is ex­hausted. The connections to the surrounding are either small openings in the walls (cracks) or a designed opening, which could include a filter to ensure clean air inside the enclosure. If the hoods are combined with internal supply airflow inlets they are described in Section 10.4. The main differences between partial enclosures and total enclosures are that the pressure drop for the total enclosure, to get an acceptable flow rate, is much higher than for a partial en­closure, and that the necessary flow rate for adequate contaminant control is usually smaller for the total enclosure than for the partial enclosure.

Access to the interior of the enclosure is much more restricted for a total enclosure than for a partial enclosure. So-called totally closed hoods, where all contact between inside and outside is through air locks or by robot or remote control (see Section, these are not only expensive to construct and operate, they also need specialized ventilation systems to function properly.

If the contaminated airflow rate that is to be exhausted, or the internal pressure, varies too much it could be advantageous to use an exhaust connec­tion with a small distance between tube and duct, acting as an opening for ad­ditional air when contaminant flow rare is low. This could be in the form of a large exterior hood covering the outlet from the process and leaving only a very small opening gap for external air (thimble). See Fig. 10,40.

There are many different types of these enclosures, such as hoods around transfer points for conveyor belts, hoods connected to electric furnaces, hoods for large storage bins, etc. These hoods are usually designed to use the actual equipment’s walls and existing openings as exhaust connections. The contam­inant generation rate often varies in these volumes. This makes it necessary to have some kind of volume or flow regulator to prevent leakage. In principle an exhaust flow rate that varies with the generation rate or is large enough for the largest generation rate could be used. There are two other solutions to this problem. One is to make some part of the hood, the wall, or the exhaust con­nection of flexible material. In this way the volume could vary with the inter­nal pressure, the exhaust flow rate could be kept constant, and leakage could be prevented. The second is to have a large enough volume in the hood to function as a reservoir. This volume stores some of the contaminated air inside
the hood, while the airflow continues to exhaust at a constant rate. When the contaminant generation rate decreases, the stored contaminant is exhausted. However, this construction could lead to high concentrations, or “dead spots,” in some sections of the enclosure. This could result in some spreading of the stored contaminant to the surrounding.

The location of the exhaust opening inside the enclosure should be in the main direction of the expected emission direction. The exhaust opening is usu­ally located in the back wall, but many other locations are possible, including the ceiling, side wall, floor, or combinations of these. These other locations are used in practice.

Under normal operating conditions the worker will be situated outside the enclosure. The operator will enter the hood only with his or her hands/arms. During the setting up of equipment it may be necessary to enter the enclosure, but entry should be kept to a minimum and, whenever possible, before the emission of contaminant has commenced.

It has long been recognized that the presence of a worker close to an en­closure, especially a fume cupboard, can have a significant effect on the ex­haust hood performance (see Section However, one aspect of
operator interaction that can be modified is operator movements. These create unstable eddies that can have a significant adverse effect on the containment performance of fume cupboards. The effects found for fume cupboards are similar for other partial enclosures. With total enclosures, workers do not nor­mally influence the performance of the hood.

Equipment and supplies should be placed in the enclosure before a con­taminant-generating procedure commences. Unnecessary equipment should be removed. High-input heat sources within an enclosure will cause convection currents that can disturb the flow and should be avoided, if possible, or ac­counted for with correct placement of baffles and/or exhaust connections. Disturbances to the airflow into and in an enclosure can be caused by

Equipment and the process being carried out in the enclosure,

The presence and movements of the operator,

Movement of other personnel,

Drafts or cross-flows external to the enclosure, and Fluctuating conditions downstream of the enclosure.

Measures should be adopted to minimize these effects.

Disturbances can be caused by all processes that take place in or that pass through the remaining openings of the enclosure. If the location of the worker is in the opening, between the contaminant source and the surrounding room, recirculation areas in front of the worker can be created that could carry con­taminants into the breathing area.

Working locations between the contaminant source and the capture open­ings dramatically reduce the efficiency of the capture system and should there­fore be avoided. If the hood is enclosed on three vertical sides the sensitivity to cross-draft is low.

Enclosures and especially fume cupboards normally remove large quantities of air, need to be replaced via a well-designed makeup air system. The enclosure and the make-up air supply should be regarded as an integral system and the supply must not compromise the performance of the enclosure. As room air currents can have a significant effect on the containment of an enclosure, the problem is to re­place the extracted air with a minimum of disturbance to the room air. This may require the use of special diffusers or a perforated ceiling to achieve sufficiently low air velocities. It is usually recommended that the air speed from such devices not exceed 50 to 60% of the face velocity of the enclosure. This would lead to a value much lower than that usually found with conventional ceiling diffusers or grills.

Drafts from windows and doors can have speeds that exceed that through the face of the enclosure. This can be especially so where the makeup air does not balance the amount extracted by the enclosure(s). Doors should be kept closed unless the room has been specifically designed to operate otherwise.

An increase of the capture airflow rate normally results in increased cap­ture efficiency, but the relationship between these quantities is not linear. A case-by-case evaluation is necessary to establish this relationship. In every case an increase of the airflow rate causes an increase of the operating costs. Anal­ogously, a decrease in airflow rate leads to a decrease in capture efficiency and in some cases, a total breakdown of the capture effect (e. g., capture devices working with the vortex principle require minimum airflow rates).

Evaluation can be performed by measuring capture efficiency using real contaminants and applying the real process or by substituting with tracer materials. A simpler, but qualitative, method of evaluation is the visualiza­tion of the airflow. If the relationship between capture efficiency and airflow rate is known, a measurement of the airflow rate can be used for frequent evaluation. See Section 10.5. Booths


At many workplaces, emissions occur randomly across a certain emission area (e. g., across the area of a workbench or grinding workpiece). In many cases these emissions are difficult to control using exterior exhaust systems be­cause of the undefined emission location and because of work procedure flex­ibility. Additionally, severe influences (cross-flows) from the surrounding room often reduce the efficiency of single exhaust elements to a minimum. In such cases, booths are the appropriate choice for a local exhaust system.

Booths are partially enclosed workplaces with one or more open face(s) for access by workers. These openings at one or more sides of the enclosure function not only to capture air contaminants directly through their Short-dis­tance capture capability but also to cause an airflow in a certain direction (normally away from the worker/work process and into the enclosure). The capture efficiency could be increased by using an existing main flow direction (e. g., thermal flows caused by heat sources) to support the capture process.


Medium-sized workpieces (workbench size up to 4-5 m square base) often need to be treated during work processes. Depending on the treatment process, different types of emission areas are possible. The actual emission area due to the treatment may be small (e. g., during welding) but time dependent, moving across the whole workpiece, or the emission area could be the whole surface of the workpiece (e. g., during spray painting). In both cases it is nearly impossible to realize an effective capture of the air contaminant while using external cap­ture elements. Either the elements have to be readjusted every moment to ensure proper function or the exhaust airflow has to be increased to unrealistic values in order to increase the grasp of the capture elements.

When using a booth the capture efficiency of one or more basic capture openings (slot, bellmouth inlet, etc.) is enhanced by shielding it against influ­ence from the surrounding airflow (cross-drafts) from at least one side and therefore restricting the flow toward the opening. Since the direct grasp of ex­haust openings is very short, the main effect is obtained from the ambient air entering the booth while following the air volume flow deficit, thus generating a general draft into the booth. The aim is to establish a flow across the entire remaining open faces of the booth, uniformly directed into the booth and to­ward the capture openings.

Generally the capture openings are located in the back wall of the booth but may also be in the ceiling, side wall(s), floor, or a combination of these lo­cations. The location of the exhaust opening depends on the type and direc­tion of the emissions.

If a significant thermal stratification is expected inside the booth, the pres­sure difference between the inside and the outside of the booth, which in­creases with height has to be taken into account during the design process. Appropriate design features include efficient capture devices in the ceiling ot the booth and an overall dense structure of the booth.

Applicability of Sources

Booths are generally suitable for all sources where the location of the emission can not be restricted to a fixed point (e. g., area sources) or moving point sources (e. g., polishing, grinding, welding, spray painting) and for sources with high momentum-driven emissions (e. g., grinding, spray painting).

Booths are often used for work procedures with momentum-driven emis­sions. fn such cases the capture devices must be placed to take advantage of this momentum. For example, a spray paint booth would have the exhaust lo­cation downstream of the painting location, most likely at the back of the booth. The capture devices in the back wall should be suitable to reduce the momentum of the emitted particles in such a way that they are not reflected back into the work area. Floor exhaust should be able to keep the heavier par­ticles down so that they cannot be a source for secondary emissions.

Different Forms and Boundaries Relative to Other Types

Based on the manufacturing process, the existing emission processes, and on workers’ demands (e. g., ergonomical aspects), different types of booths are possible. Many of these are commonly used. There can be a division according to the booth type (the shape of the booth), the position of the worker, the type of emission process, and the applied types of capture devices with the corre­sponding airflow pattern inside the booth.

The essential booth types are

Floor type or cabin: The floors of these booths are at or very near to the facility floor level. They are often very large to accommodate very large workpieces. In some cases, these booths may have flexible or hydraulically movable walls or roof to ensure access to the booth by a crane (see Fig. 10.41).



Workbench type: Booths of this type have at least one open face and thus may be laboratory fume cupboards, safety cabinets, or similar equipment.

Special-purpose type: Booths of this type are specially designed, in shape and function, for a certain production facility and/or process.

The position of the worker could be inside or outside the booth. In some cases, the worker is no longer necessary because of automation. The essential types of emission processes are described in the Design Equations and/or Pa­rameters section.

In addition to the location of the capture systems, booths could be as­signed to different design categories:

Systems with extraction only: The most common of this type use slot exhaust devices that are more or less uniformly distributed across the exhaust wall of the booth (e. g., the back wall). Some attention has to be paid to maintaining a uniform distribution of the exhaust volume flow across the slit openings. Rows of bellmouth inlets or even vortex hoods are more advantageous because they provide a more uniform flow field.

Combined systems (extraction with additional supply air): It is advantageous to increase the range of the capture system by an additional supply system. The supply air blows the contaminated air toward the capture openings (see Section 10.4.6).

Booths are also available that recirculate the exhaust air, after internal fil­tering, to the surrounding room or other areas outside the room.

Specific Issues

Problems will arise if the capture principle is not matched to the process or through the process itself (e. g., clogging of floor capture openings, clog­ging of filters). The structure of the booth could also be used for purposes of noise reduction.

Design Equations and/or Parameters

The size of the booth, accessibility, and other necessary openings have to be designed according to the demands of the manufacturing process.

The design of the exhaust system is first influenced by the type of contami­nant emission caused by the process. Contaminant emissions are possible due to

Momentum (e. g., grinding, spraying),

Density differences due to heat (e. g., shaker roast in foundries) or due to high-density vapors,

Pressure differences (e. g., processes with compressed air), or

Diffusion and evaporation (e. g., solvents from painting).

Capture opening(s) locations should be chosen to take advantage of the initial release direction of the contaminant. This leads to locating exhaust openings in the back, floor, ceiling (e. g., for heat-emitting processes), or side walls of the booth. In many cases it is useful to combine exhaust openings in different sides of the booth.

Generally the airflow rate per cross-sectional area of openings or the in­flux velocity in the opening (equal distribution across the whole area assumed)
is used as a design base. As a first assumption an influx velocity of V-m = 0.1 to

0. 2 m s~! should be used. Based on the total area of all openings /ltot, the nec­essary airflow rate, Qex, can be calculated


подпись: 10.71)As

Qex — A

At this step in the design procedure, it is necessary to consider the type of contaminant emission that the booth is designed to control. With the above assumption of influx velocity (0.1 to 0.2 m s-1), all emitted material should re­main in the booth. With these velocities, it will not be possible to draw in any contaminants that escape the booth. In case of doubt, the influx velocity, Vm, Should be increased and the necessary hood flow rate should be recalculated.

Additional calculations are necessary if significant heat loads inside the booth cause thermal stratification. A capture system in the ceiling would be advantageous in this case. A check of the pressure in the booth is necessary to avoid spilling of contaminated air near the top of face opening due to the ther­mal pressure. The height-dependent inflow or spilling velocity due to pressure differences can be calculated as


V{z) =

2 •





Where Pj(z) and P^z) are ^le linearized functions of the pressure versus height Z.

Pol -z + b-i

подпись: pol -z + b-i


подпись: (10.73)P(z) = Ptot, oi -g

Where Ptot01 and p01 are the values for pressure and density at floor level and BJ is the linearity factor for the increase or decrease of the density with the height.

It is necessary to check that no outward-directed velocity occurs across the open face of the booth. In such a case, another capture principle must be chosen or the exhaust airflow has to be increased until the needs are fulfilled. The possibility of spilling contaminated air out of the booth could also be re­duced by the use of a flexible curtain covering the open face of the booth. Fume Cupboards


A fume cupboard is a boxlike, partially enclosed workstation that is used to protect operators and other personnel by limiting the spread of airborne contami­nants generated within it. It is ventilated by a mechanically induced inward flow of air through an adjustable working opening or openings (working aperture). This inward airflow is designed to contain the contaminant within the cupboard and to dilute it. The contaminated air is usually ducted away by means of an ex­tract system which should provide for its safe release. Because fume cupboards are partial enclosures, there will be some leakage in die form of transient emissions of contaminant from the opening(s). A fume cupboard should therefore be capable, not only of generally containing the contaminant generated within it, but also of reducing these transient emissions to acceptable levels.

Fume cupboards are frequently referred to as laboratory fume hoods and are a primary method of contaminant control within laboratories.

Fume cupboards are mostly used for small-scale laboratory work by a single operator either seated or standing outside the cupboard. The opening! s) is fur­nished with a transparent sash or sashes that shields the operator from the process being carried out within the cupboard. The movement of air around and away from the worker creates a clean-air work zone while at the same time containing the contaminated air within the enclosure. The sash(es) on the working aper- ture(s), together with the side, back, and top walls, reduce the amount of air needed to produce a sufficiently high inward airflow capable of overcoming the tendency of the contaminant to escape from the cupboard. The sash offers protec­tion against chemical splashes and also shields the source from the effects of cross­drafts. The sides of the cupboard are sometimes transparent, especially if the fume cupboard is to be used for demonstration work. See Fig. 10.42.

A number of design features that are usually associated with good con­tainment by a fume cupboard:

Good entry of the air into the cupboard reduces the size of eddies at the boundaries and can be achieved by using rounded jambs at the side walls, an aerofoil on the sill, and a well designed sash handle.65

A back baffle at the rear of the enclosure forms a low-level extract slot in addition to a slot at a high level. The low-level slot helps to make the velocity over the open aperture more uniform as well as producing a flow across the floor of the enclosure which will scavenge any heavy vapors that may not have mixed with the incoming air. The high-level slot removes air and pollutants rising into the upper part of the cupboard.

Applicability of Sources

Fume cupboards are widely used in chemistry laboratories, both in schools and in industry, to control moderately dangerous contaminants gener­ated by small-scale processes. The releases usually have low momentum and ideally the cupboard should be used with the sash in the closed position.

Different Forms and Boundaries Relative to Other Types

Walk-in Fume Cupboards A walk-in fume cupboard is a cupboard in which the working chamber extends to the floor level. Large equipment can be set up in these cupboards before active working commences; it is not intended that they be entered during working. The design is basically the same as that of the bench — mounted cabinet but with either double-hung vertical sashes or horizontal sashes. Horizontal sashes have the advantages of being lighter and easier to move. Walk-in fume cupboards are often fitted with a removable table or shelf as a work surface at normal bench height. This type of cupboard cannot be expected to function well with the front sash fully opened; it would require a large extract volume flow rate and would be susceptible to room air currents and external movements. In practice they should be used with the sash as near to its closed position as possible.

Perchloric Acid Fume Cupboards Use of concentrated perchloric acid gives rise to special hazards and these special fume cupboards should be for per­chloric acid use only. Contact of hot perchloric acid with organic materials and certain metals (especially copper) can lead to the formation of perchlorates which,


In the dry state, can be highly explosive and shock sensitive. In addition, when Ex­Posed to perchloric acid certain organic materials, for example wood and cotton, can be very unstable and possibly ignite spontaneously when dry. Perchlorates are water soluble and both this type of fume cupboard and its ductwork system are usually fitted with water wash-down systems. It is particularly important that the area behind the back baffle is thoroughly washed. In consideration of the above comments, the following design features should be incorporated:

1. The materials of construction, from the cupboard to the fan, should be inorganic and resistant to attack by perchloric acid. For the cupboard itself suitable materials include stainless steel of types 316 or 317, solid epoxy resin, and rigid PVC. Stainless steel has been popular for this application as it is easy to form, weld, and polish. It is, however, attacked by the acid, which causes discoloration of the metal surface and the formation of iron(III) perchlorate, which can be explosive. Ductwork, separate from other extract systems, is usually made from stainless steel or plastic materials. Fire regulations may preclude the use of plastic ductwork or require it to be sheathed in an outer casing of metal or GRP. The fan casing and impeller can both be made of plastic.

2. Surfaces should be impervious, smooth, and crevice-free, joints between walls and the base should be rounded to facilitate cleaning.

3. The whole system should be designed for ease of decontamination. The cupboard will incorporate a wash-down system. Adequate drainage in the cupboard and ductwork is required. Water drained from the system, including that from the fan casing, should be collected and properly disposed of. It should not be possible for water to get onto the work surface.

4. Ignition sources in the cupboard and duct system should be avoided. Plastic fans, electrical sockets external to the cupboard, and explosion — proof light fittings reduce the likelihood of ignitions.

Recirculating Fume Cupboards The recirculating fume cupboard, some­times called a ductless or filtration fume cupboard, is a type of fume cupboard that recirculates air back to the workspace after it has been filtered. It therefore eliminates the need for an extract and dispersal system. The design and perfor­mance of these cupboards has improved greatly since the 1980s, when some early models were found to be inadequate at containing vapors and greatly re­duced worker protection.66 Recirculating fume cupboards have built-in fans that pass the contaminated air through one or more types of filters, e. g., acti­vated carbon, molecular sieves, electret filters, HEPA filters, etc. A pre-filter re­moves particulate contamination while the main filter, which may be made up of several different types of filter media, removes fine particles, vapors, and gases. The most widely used filters are activated carbon filters, which are used for specific groups of chemicals. The heart of the recirculating fume cupboard is the filtration system and this needs to be selected carefully. It can also be the weakness of this type of cupboard. They are not appropriate for every applica­tion. Success and safe use depend on a thorough appraisal of their limitations.

Auxiliary Air Fume Cupboards In this type of fume cupboard, partially treated air is supplied externally to the cupboard directly to or near the face of the cupboard so as to reduce the demand for fully conditioned air from the workplace, thereby reducing operating costs. The supply air should be distrib­uted uniformly over the height or width of the cupboard and the airflow should be directed into the cupboard at all points over its face. Auxiliary air fume cupboards are dealt with more in Section 10.4.6.

Variable Air Volume Fume Cupboards This type of cupboard incorporates a variable air volume (VAV) controller that regulates the amount of air exhausted from the cupboard such that the face velocity remains essentially constant irrespec­tive of the sash position. A sensor detects either the sash position, the pressure dif­ferential between the fume cupboard interior and the room, or the velocity at. some point in the cupboard. This information is used to control either the exhaust fan speed or the position of a control damper. The supply air volume flow rate into the laboratory or workspace should also be regulated. It should be remembered that with the sash in the closed position the amount of air to dilute contaminants in both the fume cupboard and the laboratory is reduced and that there could, for ex­ample, be difficulty in reducing contaminant levels below the lower explosive level.

Location of Workers and Tools Relative to Exhaust and Source

Under normal operating conditions the worker will be standing or seated outside the fume cupboard and will enter the cupboard only with his or her hands/arms. During the setting up of equipment it may be necessary to enter the fume cupboard, but entry should be kept to a minimum and, whenever possible, should be carried out before any emission of contaminant has commenced.


Disturbances to the airflow into and in a fume cupboard can be caused by

1. Equipment and the process being carried out in the cupboard,

2. The presence and movements of the operator,

3. Movement of other personnel,

4. Fluctuating conditions downstream of the cupboard, and

5. Drafts or crossflows external to the cupboard.

Measures should be adopted to minimize these effects.

Equipment and Processes Equipment and supplies should be placed in the cupboard before a procedure commences. Unnecessary equipment should be removed. High-input heat sources within a cupboard will cause convection currents that can disturb the flow and should be avoided if possible. Work should be carried out well within the cupboard, at least 150 mm from the plane of the sash whenever possible. It should not however be placed closer than 50 mm to the lower extract slot of a back baffle. Large pieces of equip­ment should if possible be raised 25 to 50 mm above the working surface of the cupboard to improve the flow in the cupboard.

Presence and Movements of the Operator It has long been recognized that the presence of a worker at a fume cupboard can have a significant effect on its performance. Chang and Gonzalez67 showed that with a person standing in an

Airflow, a complex flow pattern is set up in the wake of the body. In the case of A Fume cupboard the flow is usually from behind the body so that a region of low pressure develops between the person and the cupboard face. This flow combines with that in the person’s thermal boundary layer. Johnson, Fletcher, and Saunders68 showed that the near-body airflow field could be dominated by these thermal effects, which caused velocities of about 0.2 m s-1. The possibility exists therefore that pollutants could be drawn outward through the aperture of the cupboard and pass into the breathing zone of a person working there. These dis­charges have been visualized using smoke and are described by several authors.69’70 It is difficult to do much about the fact that the presence of a person working at a fume cupboard will interfere with the airflow into it. However, one aspect of operator interaction that can be modified is that of operator movement. These movements create unstable eddies that can have a significant adverse effect on the containment performance of fume cupboards.71 Ljungqvist showed how leakage increased considerably when an operator performed movements as op­posed to standing still. Movement of the sash from the closed to the open position causes contaminated air to be released from the cupboard in a pulse lasting for several seconds.72 Closing the sash has a negligible effect. With systems of work­ing where the closed position is the norm, the sash may need to be opened and closed at frequent intervals, resulting in a relatively high operator exposure.

Movement of Other Personnel A major factor that can influence the con­tainment of a fume cupboard is the movement of personnel in its vicinity. A per­son walking past a fume cupboard creates eddies in his or her wake with local velocities of the same order as that of the airflow through the face of the cup­board. Measurements of the effects of these eddies on containment of cupboards72 show that leakage depends on the distance of the moving person from the cupboard. BS7258 recommends that “the distance from the sash to any circulation space should be at least 1000 mm, so as to preserve a zone undisturbed by anyone other than the operator.” The size of the leakage will also depend on the speed of the person as well as the air velocity through the face opening. There­fore, although a fume cupboard may perform well under ideal test-room condi­tions at a low face velocity, movement of personnel close to its face needs to be considered carefully when selecting an operational value for the face velocity.

Conditions Downstream of the Cupboard Variations in conditions downstream of the cupboard may affect the performance of the cupboard. The variations can arise from three causes: atmospheric conditions, multi-ex­tract systems, and VAV controls.

On-site containment measurements73 made on a fume cupboard on a particu­larly exposed site showed that very large day-to-day variations in the degree of con­tainment could be attributed to the wind blowing across the stack. A suitable design of stack termination and a high discharge velocity should reduce this problem.

Problems can arise if several cupboards, each with its own fan, are con­nected to a single common duct and discharge stack. If a fume cupboard was switched off or a fan failed, flow through the cupboard could be reversed causing contaminants to be discharged into the room. The collecting duct should be kept at a lower pressure by its own fan to reduce this risk.

The operation of flow dampers can cause pressure fluctuations in the ductwork system. Measurements by Melin74 indicate that pressure oscillations in an exhaust system can cause instabilities in the airflow through a fume cup­board sufficient to give rise to outward leakage of contamination, especially when a person stands in front of the cupboard.

Drafts and Cross-Flows Fume cupboards remove large quantities of air, which need to be replaced via a well-designed makeup air system. The fume cupboard and the makeup air supply should be regarded as an integral system and the supply must not compromise the performance of the cupboard. As room air currents can have a significant effect on the containment of a fume cupboard, the problem is to replace the extracted air with a minimum of dis­turbance to the room air. This may require the use of special diffusers or a per­forated ceiling to achieve sufficiently low room-air velocities. It is usually recommended that the air speed from such devices does not exceed 50 to 60% of the fume cupboard face velocity. This would lead to a value much lower than that usually found with conventional ceiling diffusers or grills.

Drafts from windows and doors can have speeds that exceed that through the face of the cupboard, especially where the makeup air does not balance with the amount extracted by the cupboard(s). Doors should be kept closed unless the laboratory has been specifically designed to operate otherwise.

Location of Fume Cupboards

A fume cupboard should be sited to avoid disturbances to the fume cup­board, its operator, and other personnel. It should not interfere with the es­cape route from the area in the event of an emergency. It should be sited to maximize the working efficiency of the cupboard and the area in which it is installed. Although specified dimensions and layout cannot guarantee perfor­mance, the values in Table 10.875 are based on experience and are widely used.

A fume cupboard should not be positioned where the only escape route from the area would necessitate passing directly in front of the fume cup­board. Siting with respect to makeup air grills should be considered carefully (see above on drafts and cross-flows) and a minimum separation distance of 1500 mm is recommended.75

Changing Flow Rates

Control of contaminant at the face of a fume cupboard depends on the move­ment of air through the working aperture. The velocity must be sufficiently high to resist the effects of disturbances created by the operator, other personnel, equip­ment, cross-drafts, etc. It should not be so high, however, as to cause disturbances to equipment in the cupboard or to create eddies in the wake of the operator or from the cupboard itself that will adversely effect containment.

The recommended value of the face velocity has been the subject of much de­bate and variation over the years. The recommended values have also varied widely with use. For example, CIBSE76 recommends values as low as 0.2 and 0.25 m s’1 for fume cupboards for teaching and research purposes, respectively, and as high as 2.0 m s_1 for radioactive work depending on grade. Middleton 77 quotes the optimum range for the face velocity of laboratory fume cupboards to lie between


Recommended minimum distance (mm)

Separation from traffic route


Lateral distance from large


Equipment, pillars, or adjacent wall

Distance from opposite wall


Distance from opposite fume cupboard


Distance from opposite bench top used


By fume cupboard operator

Distance from door in same wall


Distance from door in wall at right angles


0. 5 and 1.2 m s_1. However, ACGIH25 suggests that face velocities in the range of

0. 3 to 0.5 m s_1 are suitable. BS725875 notes that velocities less than 0.3 m s’1 are unlikely to gjve good containment. A face velocity of 0.3 m s-1 is seen by ANSI78 as being adequate only under near-perfect conditions and not appropriate as a general standard. ANSI sees the value of 0.5 m s-1 as adequate and perhaps more than so. The range suggested by ACGIH would therefore seem to be generally acceptable with caution being exercised at the lower end of the range.

Design Equations and/or Parameters

The size of a fume cupboard should be chosen to maximize performance. The height of the work surface above floor level and the overall internal height are de­termined by the intended usage. Where the required internal height is 2 or more it is worth considering the need for a walk-in cupboard. Widths of cupboards are usually in increments of approximately 300 mm. The choice between vertical and horizontal sashes is to some extent a national preference, e. g., in the UK most sashes are of the vertical type. However, there can be practical reasons for the choice. Some laboratory procedures may require a relatively large vertical reach. To do this with a vertical sash could require several times the airflow rate required using a horizontal sash. The hybrid vertical/horizontal sash could meet most needs. Specification of a face velocity is now regarded as less important than containment but it is worth noting the advice of BS725875 that a value less than 0.3 m s-1 is un­likely to give good containment. The back baffles should be adjusted to give an even distribution of face velocities; velocities measured at the face should not differ from the mean by more than 15%.

Where a cupboard is fitted with a bypass, it should be designed with an air velocity that is sufficiently high to prevent the escape of contaminants. This is particularly important where the cupboard is likely to be subjected to high heat loads, which could cause strong thermal currents in the upper part of the cupboard. However, the velocity should not be high enough to disturb the flow inside the cupboard and cause loss of containment.

Fume cupboards in themselves generate virtually no noise due to their low air velocities, but they can, and do, amplify noise generated by the exhaust system. The noise can originate from the ductwork itself, bends, flow control­lers, fans, or belt drives. More information on noise generation and amplifica ­tion in fume cupboard installations is given by Haugen79-

Evaluation Procedures

Qualitative Testing The use of smoke released around the boundaries and inside a fume cupboard can be a simple and effective way of visualizing the local airflow patterns and gives an indication of the likely containment. It is however a very subjective test. Air movement around and under sash han­dles can be shown using either titanium tetrachloride applied with a cotton wool swab or a smoke tube. The latter is far more convenient to use and is in the form of a sealed pencil-like glass ampoule. To use, the ends of the tube are broken off. Air is passed through the tube and emerges as a thin plume of dense white smoke. Flow patterns inside the cupboard are better shown by the use of smoke pellets or a smoke generator. The latter has several disadvantages over smoke pellets: it is bulky and heavy, usually requires a power supply, and tends to be fairly uncontrollable in the volume flow rate of the smoke.

The flow should move smoothly into the cupboard and should not re emerge into the laboratory. It should be remembered that the smoke test is a negative test, i. e., if smoke is seen to emerge from the cupboard, the cupboard does leak; if no smoke is seen to emerge, the cupboard may leak.

More information on flow visualization can be found in Chapter 12.

Quantitative Testing

Face Velocity Measurements Although it is generally accepted that face velocity is not sufficient to specify or describe fume cupboard performance, it is a relatively easily made measurement that is readily understood and widely quoted. Low face velocities make a fume cupboard sensitive to outside distur­bances (for example drafts) whereas excessively high velocities can cause ed­dies in the wakes of operators and under sash handles which can lead to contaminant being drawn out of the cupboard.

There are two main types of instrument in general use for face velocity mea­surements: vane anemometers and thermo — or heated-element anemometers. The latter type is becoming increasingly popular although the traditional vane ane­mometer is still widely used. The vane anemometer has a number of disadvan­tages. It is relatively large and this may physically restrict its use in certain circumstances. The value measured is an average over a large area. The instru­ment has a slow response to velocity fluctuations and cannot be used to investi­gate turbulence levels. In addition, the calibration of vane anemometers needs to be checked frequently. With a thermal anemometer the loss of heat from the sens­ing element is a function of the airstream velocity past it, resulting in a fairly fast response. Commercial instruments usually have two sensing elements to give tem­perature compensation. This type of instrument will tolerate moderate contami­nation by dust, can be cleaned (with care), and is in common use in the field.

To make measurements, the working aperture of the cupboard is divided into equal areas by imaginary lines parallel to the sides of the aperture. The air velocity is measured in each of these areas and sometimes its variation with time. If individual velocities are within about ±15% of the mean, there should be little cause for con —

Cem.80 (The accuracy1 of individual readings is likely to be ±10%.) These measure­ments can identify problems caused by crossdrafts and badly adjusted back-baffles.

Containment Tests Fume cupboards offer only partial containment, It: is therefore logical to seek a test that measures how good the partial containment is. Such information can be useful not only in assessing the safety of a fume cupboard but also in comparing different design features and different manufacturers’ cup­boards. Although there are several different methods of determining containment, the basic method is the same and involves the release of a tracer inside the cup­board and detection of leakage at a point or points outside of it. The tracer is usu­ally a gas mixture, 10% sulfur hexafluoride (SF6) in 90% nitrogen. SF6 is a very dense gas and mixture with nitrogen makes the density of the tracer closer to that of air. The tracer is supplied from a pressurized cylinder via a pressure gauge and flowmeter, at a flow rate between 1 and 8 liters per minute (L min1), to a gas ejec­tor. The ASHRAE design has been quite widely used in the past. It is used for ex­ample in the American ASHRAE 110-1995 and German DIN 12 924 standards. The gas passes through a critical orifice, entrains air through openings in the body of the device, and is ejected through wire-mesh gauze. The tracer is ejected at rela­tively high speed (over 1 m s~’) into the fume cupboard. In a second type of ejector, the tracer gas passes into the cupboard through a sintered material and emerges into the cupboard at relatively low speeds. Devices of this type are used as silencers when venting air from compressed air systems and have the added advantages of being small, readily available, and cheap. Use of this type of ejector is proposed in a draft CEN Standard on fume cupboards currently under preparation.

Some standards (for example ASHRAE 110-1995,81 DIN 12 924,82 and Nordtest83) specify the use of a manikin at the face of the fume cupboard during containment while others (for example, BS725875) have an unobstructed entry. Under the ASHRAE protocol the tracer gas leakage is sampled through a collec­tion probe at the breathing zone (nose) of the manikin, whereas the DIN stan­dard specifies sampling over a grid of points outside the cupboard 100 mm from the plane of the sash. The British standard specifies a sampling grid in the plane of the sash. The draft CEN standard will not specify the use of a manikin for this type of containment test and will have sampling grids both in the plane of the sash and at some distance in front of it to allow for a dynamic sash test.

The gas sample taken is analyzed usually by either an infrared or an electron capture gas analyzer. The infrared gas analyzer is the more widely used device and is capable of analyzing a broad spectrum of gases. It is heavy’ and bulky but porta­ble. Some models have built-in calibrations but on-site calibration will give more accurate results. The gas sample usually passes through a large cell (2 to 5 liters) at a flow rate on the order of 15 L min-1 and the response time of the instrument is therefore several seconds. As contaminant is ejected from fume cupboards in bursts lasting a matter of seconds,63 the output signal of the analyzer is attenuated and does not accurately reflect the true course of events. Electron capture devices will detect any electron capture gas. The instruments are small, portable, can of­ten be battery operated, and have a very small measuring volume. The response time is very short and the output of the instrument gives a close representation of the real-time events. The output signal of early instruments tended to drift with time but this problem seems to have been overcome. Both types of instrument will measure SF6 concentrations in the likely range of interest (about 0,01 to 20 ppm). The output of the analyzer can be fed to a chart recorder, datalogger (for down­loading into a PC), or directly into a PC for processing.

The effectiveness of the fume cupboard can be expressed in a number of ways. Several of these ways are discussed by Melin74 and Olander.1 j Nordtest83 defines an escape safety parameter as

E = -r C-f.———- r x 100%, f 10.74)

Where Cx is the calculated tracer gas in the exhaust air and Cniax is the maxi­mum recorded tracer gas concentration.

In the ASHRAE standard the fume cupboard performance is summarized by expressions of the following type:

XxAUyyy ;m75s

Xx AM vyy, ‘

Where xx is the tracer gas release rate in L mirr1, yyy is the average gas con­centration measured in ppm; AM is “as manufactured,” usually tested in test room with no equipment in the cupboard; and AU is “as used,” installed in a laboratory with equipment as in actual use.

In Section 10.5 other efficiency measurements are described. Gas Storage Cabinets


Gas storage cabinets were originally developed for the semiconductor in­dustry in the 1970s. These early storage cabinets consisted of a box that en­closed the tank and connections; they were operated under negative pressure and exhausted to the outside. Gas storage cabinets have become more sophis­ticated, adding gas detection, fire sprinklers, alarms, and pneumatic con­trols.84 Some cabinets have point-of-operation air cleaners such as scrubbers.

Many building and health and safety codes require the use of gas stor­age cabinets, exhausted enclosures, and/or separately ventilated gas stor­age rooms for toxic gases. These controls are also recommended for flammable and corrosive gases.

Compressed gases are defined in several ways. In the UK compressed gases are defined as “gases with a gauge pressure exceeding 1.5 kg cm-2” and “liquids with a vapor pressure exceeding 3.0 kg cm-2.” The U. S. De­partment of Transportation defines a compressed gas as “any material or mixture having in the container either an absolute pressure greater than 276 kPa at 21 °C or an absolute pressure greater than 717 kPa at 54 °C, or both, or any liquid flammable material having a Reid vapor pressure greater than 276 kPa at 38 °C.”85


Gas storage cabinets consist of a box that encloses the tank(s) and all connections. Many include change out capabilities and an access door. The cabinets are exhausted to remove any contaminant that may leak into the cabinet and to maintain the cabinet under negative pressure relative to the surrounding area. The gas cabinet is designed to control fugitive Emissions from the Gas cylinder and gas distribution system. In most cases, a Violent re ­Lease Of The cylinder contents will not be controlled.

Applicability of Sources

Gas storage cabinets are used to contain compressed gas cylinders Con­Taining toxic, flammable, or corrosive gases.

Different Forms and Boundaries Relative to Other Types

Gas storage cabinets are designed to contain one to four gas cylinders. The cylinders are connected to a gas distribution system which is also con­tained in the cabinet. Very sophisticated systems are available from cabinet manufacturers. These may include automatic or semiautomatic change-over capabilities, fire sprinklers, purging systems, and gas detection systems which may include alarms and automatic shutoff. Point-of-use scrubbers may also be incorporated into the design, depending on the gas being used (see Fig. 10.43).

Specific Issues

In addition to the toxic and/or flammable properties of the gas, the gas cylinder may also pose a hazard due to the potential high-energy release of the contents. For this reason, additional safety considerations are necessary. The cylinders are specifically designed to contain the contents under high pressure. Careful inspection and maintenance is necessary. The fittings used on the cyl­inder are often specific for the gas of concern and mixing of components among different cylinder types is not recommended and is, in some cases, pro­hibited. Cylinders need to be restrained to prevent accidental damage to the valve or regulator. During transport the cylinder valve should be covered.

Gas storage cabinet use may be required by local, state, or national codes. These codes vary by location and the designer or user of the cabinet is referred to these codes for further information. One source of building code informa­tion in the U. S. is the Uniform Building Code86 and the Uniform Fire Code.*17

The Uniform Fire Code requires that pyrophoric, flammable, or highly toxic gases be within ventilated gas cabinets, laboratory fume hoods, or ex­hausted enclosures.87

Gases with a health hazard, flammability, or reactivity ranking of 3 or 4 (toxic or highly toxic) should also be used and dispensed from a ventilated gas cabinet. The cylinder and any fittings subject to leakage should be enclosed by the cabinet.88

Toxic gas monitoring including visual and audible alarms and communi­cation with a constantly attended Emergency Control Station may also be nec­essary in areas that use toxic or highly toxic gases. The monitoring system must also automatically close the shutoff valve on toxic or highly toxic gas supply lines. Smoke detectors are required in rooms or areas where highly toxic compressed gases are stored indoors. ss

The Uniform Mechanical Code89 requires that ducts carrying explosive or flammable gases be ducted directly to the outside of the building.

Design Equations And/or Parameters

The average face velocity at all openings into a gas cabinet should be at least 1 m s_1 with a minimum velocity at any point of 0.76 m s-1.8’’ These

InnovantR Cone shaped top t>m

Vides Maximum air flow efficiency

Substantiated by many hours of

Actu. il ori-ute testing.

Interior i5 Finished with white

2-part polyurethane paint to add

Brightness and better risibility

Rugged extenor construction of

JI gauge cold ridled steel with

Continuously welded seams.

Low-profile, one-inch reinforced

Threshold makes cylinder Installa

Non and removal easy.

Make installation of gas control

Panels, cylinder supports, shelv

Ini’, And other equipment easy.

Modular Unis tat‘w Supports

Full length, sturdy plane binge

Located on the front edge of the

Remain within the side dimensnms

Door surface allows dottr to

Of the cabinet

Around door to ensure a posistive

Neoprene gaskets fit snugly

• Hat-top design with exhaust


L:irr control sprinkler head, for

Extra protection with a fuse rating

Of LIS0 F.

• Rugged ex tenor constmction <*/

JI — gauge cold rolled steel with

Continuously welded seams.

• Interior and exterior are finished

With gray 2-part polyurethane

• Automatic door closure to

Ensure containment t>f leaks.

Non-protruding paddle type

Latch prevents accidental opening

And snagging. It slams and latches

At three points and is fixed with a

Lock for security.

With an inlet filter {pin VEN-010

Standard inlet air louver

Optional diffuser plate fitted

XX} lets air into the cabinet.

Access panel and wire reinforced

Have steel frames and are fully

Safety glass riewing window


Low-profile, one inch reinforced

Threshold makes cylinder Installa

Tion and removal easy.

Neoprene gaskets fit snugly

Around door to ensure a positive

• Cylinder restraints to ensurt

That all cylinders are held securely

In place during storage and opera

Motlular Unistatiu supports

Make installation of gas control

Panels, cylinder supports, shelv­

Ing, and other equipment easy.

Special diffuser plate fitted with

An inlet filter lets air into the Cabi­

Net And acts in tandem with the

Cone shaped top to produce a con-

Cross-sectional area of the cabinet

Without channelling or dead spots.

Tinuous flow of air over th

on-pmtrudmg paddle type

Latch prevents accidental opening

And snagging. It slams and latches

At three points and is fixed with a

Lock for security.

Access panel and wire reinforced

Have sled /Rames And arc fully

Safety gl. iss viewing wmdou


Automatic door closure to

Ensure containment of leaks.

Hxtenor ts finished with blue

2-part polyurethane paint.

Cylinder restraints to ensure

That all cylinders are heui securely

In place during storage and opera

• hire control sprinkler head, for

Extra protection with a fuse ratine

Of 1JS° I


Enclosures Enclosures

Velocities must be maintained through the access port at the valve, but also through the door of multiple cylinder cabinets when the door is opened to replace a cylinder. The cabinet may be equipped with a two-position flow operation interlocked with the door which will increase the airflow when the door is opened.88

Burgess et al.40 describe a study of gas storage cabinets. In the study, coefficient of entry (CJ for various inlet/outlet configurations was mea­sured. A tracer gas study is also described. The tracer gas study involved releasing sulfur hexafluoride (SF6) at 0.032 L s-1 at a critical leak position in the cabinet and measuring SF6 concentration in the exhaust stream. The tracer gas was turned off when a steady exhaust stream concentration was observed and the time for the concentration to decay to 5% of steady state was measured.

The study found that the slot-type inlet at the bottom of the cabinet door resulted in higher pressure losses (lower Ce) than the diffuser or perforated plate inlet. The exhaust configuration had little effect on Ce or tracer gas clear­ance time. The study also concluded that an exhaust rate 0.118 m3 s”[2] for a two-cylinder cabinet was sufficient as little improvement was seen with an in­crease to 0.165 m3 s-1.40 The slotted inlet took longer to clear a leak than ei­ther the perforated plate or diffuser inlet. Measured coefficients of entry for a two-cylinder gas storage cabinet are shown in Table 10.9.

Pyrophoric gases deserve special consideration. Considerable research on storage and use of silane gas has been performed. A summary of some of this research is provided by the Semiconductor Safety Association.90

Building codes also require that systems conveying explosive or radioactive materials be prebalanced through duct sizing. Other systems may be designed with balancing devices such as dampers; however, dampers provided to balance airflow must be provided with securely fixed minimum position blocking devices to prevent restricting flow below the required volume or velocity.88

10.2.3.S Enclosures of Complete Process (Buoyant and Nonbuoyant Sources)

Enclosures for Buoyant Sources

Enclosures are used throughout industry to capture emissions from buoyant sources. The following discussion pertains mainly to large enclosures such as those used on metallurgical process vessels. Many of the design equations and procedures developed for buoyant source hoods described by Goodfellow91»92 apply to large enclosure design. Process vessels successfully using enclosures in­clude electric arc furnaces, top and bottom blown-oxygen steel conversion fur­naces, and nonferrous converters.

The use of enclosures for fugitive emission capture on process vessels such as furnaces offers the following advantages:

Inlet configuration

Outlet configuration

Coefficient of entry, C„




Shallow cone


Deep cone





Shallow cone


Deep cone


Perforated plate



Shallow cone


Deep cone


3. On small and low-production furnaces, the enclosure can be used as both primary and secondary control, thereby reducing the need for other equipment.

4. As a side benefit, on electric arc furnaces the enclosure offers a great potential for noise control.

The main disadvantages of using an enclosure are as follows:

1. The potential for interference with the normal operation and maintenance of a process vessel is high. A major design effort is required to overcome this disadvantage. All aspects of the process vessel operation must be considered. Lines of sight for process vessel and crane operators, access for crane-held ladles and buckets, process vessel movements, and maintenance access must be accommodated by the enclosure design. This is more easily achieved in a new installation.

2. Enclosure design on retrofit cases becomes very difficult and may require a compromise between process vessel operation and fume capture performance.

The design methodology is based on dealing with the following three main aspects:

1. Process and layout,

2. Fume capture, and

3. Mechanical design.

Discussion of design aspects will concentrate on electric arc furnace enclosures, although the design methodology generally applies to most large enclosures.

Process and Layout Requirements The key to a successful enclosure in­stallation is to consider all process and layout requirements during the initial design stage. When planning an enclosure for a metallurgical furnace, the fol­lowing questions should be asked:

1. Are primary and/or secondary emissions to be controlled by the enclosure?

2. What is the extent of furnace and related equipment movements?

3. How will the enclosure affect the furnace operation process control?

4. Where must enclosure openings be located?

On oxygen steel conversion furnaces, primary fume control is usually achieved by a separate close-capture hood positioned over the vessel mouth. The enclo­sure is then used for secondary fume control during charging, turndown, tap­ping, and slagging.

On electric arc furnaces, an enclosure can be used for both primary and secondary fume control. However, for large, high-production furnaces, It Is more economical to provide separate direct evacuation control for primary melting emissions. Use of separate gas cooling equipment to handle the high heat content from primary emission off-gas on high-production furnaces is of­ten less expensive than directly quenching with large amounts of dilution Air From an enclosure. The amount of air dilution is dictated by fabric filter tem­perature limitations. Generally, if the enclosure exhaust rate for secondary fume capture is similar to or more than that required for primary Control, The enclosure system is designed to handle both emissions.

If primary control by the enclosure is under consideration, the extra wear and tear on electrode holding equipment by the escaping fume must be taken into account. This potential problem is particularly evident on Ultra-high­power (UHP) furnaces where the holding equipment would be constantly ex­posed to high-temperature flame. As a possible solution, the furnace could be equipped with a roof-mounted water-coiled stub stack, which naturally draws fume from the furnace and into the enclosure. This approach would divert the fume and prevent damage to the electrode equipment.

When primary fume capture is performed by the enclosure, furnace off-gas combustion efficiency is lower than experienced by furnace direct evacuation control. The off-gas, rich in carbon monoxide (CO), rises from furnace roof openings and partially burns and cools with enclosure air. Significant levels of CO have resulted in the enclosures and exhaust ducting from this type of combi­nation. These levels are not explosive but present a potential hazard to person­nel working in the enclosure or in downstream fume cleaning equipment.

Various furnace movements must be accommodated by the enclosure. Op­erations such as furnace tilting for tapping and slagging, electrode vertical lift, and direction of furnace roof swing must be coordinated with respect to enclo­sure shape and the location of the exhaust off-take.

Movement of related equipment must also be considered. Tapping ladle and slag pot positioning may be done by overhead cranes or transfer cars and will dictate the extent of enclosure doors and roof slots. Charge bucket positioning and approach by overhead crane will also determine door and slot require­ments. Routine removal of the furnace roof and water-cooled panels by over­head crane may finally dictate the size of openings. Emergency procedures mus’t also be accommodated in the design. For example, a full ladle trapped in the en­closure because of a doorjamb must be removed before the metal solidifies.

The following items affecting furnace operation and process control should be addressed as the enclosure shape is established:

1. Line of sight for crane operators and furnace attendants,

2. Furnace control points and attendant location,

3. Method of charging additives,

4. Furnace ancillary equipment location, and

5. Equipment maintenance access requirements.

Furnace control points and ancillary equipment location must be con­sidered as to whether they are positioned in or out of the enclosure. If the bulk of furnace ancillary equipment is located in the enclosure, layouts must allow for proper servicing. If attendants must work in the enclosure during furnace operation, emission capture design must provide a relatively fume-free work environment. In general, enclosure-opening requirements should be minimized during the layout stage. Bucket charging of an electric arc furnace requires a roof slot for crane access. Sliding doors can be used to cover these openings. After the bucket has entered the enclosure, the side doors are closed but the roof slot doors remain open. An air curtain blow­ing across the roof slot, as shown in Fig. 10.44, can be used to prevent charging emissions from escaping through the roof slot. Ample clearance is required to fit doors and air curtain equipment on the enclosure roof. A roof slot is also required during tapping if the ladle is held by the crane.

Fume Capture Fume capture is accomplished by a combination of the following enclosure features:

1. Containment and storage of the emission,

2. Air extraction from the enclosure, and

3. Air curtain and exhaust off-take.

If acceptable working conditions must be maintained in the enclosure dur­ing furnace operation, attention must be given to internal airflow patterns,

I. E., minimization of fume recirculation in the enclosure.

Containment and Storage of the Emission The main function of the physical enclosure is to contain secondary furnace emissions from tapping, slagging, and charging and perhaps primary emissions from melting. These emissions are thermally driven against the enclosure roof and can overcome the in-draft effect of the extraction system if the enclosure is not built tightly. Gaps around roof slot doors can also present a severe leakage problem. When the roof doors are open for crane rope access, an air curtain can be effectively used to contain emissions.

The enclosure is also capable of storing fume surges during bucket charg­ing. With proper design, the top of the enclosure will fill with fume while the lower working level remains clear. The key to producing this effect is to reduce fume recirculation in the enclosure by proper placement of the air curtain with respect to the exhaust off-take.

Tapping, slagging, and melting are prolonged, continuous operations. During these periods, the enclosure should not be used for fume storage. The enclosure exhaust capacity must be greater than the emission plume flow rate to avoid fume buildup in the enclosure during these operations.

Air Extraction from the Enclosure A textbook approach to enclosure de­sign for a hot process would follow a procedure of determining the in-draft veloc-

Air curtain across roof slot


Ity required to overcome the stack or “chimney” effect. This calculation could serve as a check, but does not account for plume updrafts in the enclosure.

To determine the air exhaust rate from the enclosure the following steps are recommended.

Step 1: Determine primary emission heat content. This step should be taken early in the design stage to determine if the enclosure will capture both primary and secondary emissions. The heat content of furnace emissions and the temper­ature limitation on the fume collector are considered for this task. The off-gas heat content is calculated for furnace reactions during melting and refining peri­ods. The maximum heat content should be used for design. Assuming a fabric

Filter collector with polyester cloth is used, a 130 °C temperature limit is im­posed for continuous operation. The fume volume flow rate after dilution Is Then determined from the elementary thermodynamic equation:

— “ * (10.76)

Pcp(Ts-Tamh) ’ where

Q = actual volume flow rate after dilution (m3 s-1)

(l> = heat release rate from furnace off-gas (kj s_l)

P — air density at Ts (kg nr3)

Cp = specific heat of air at Ts (kj kg-1 °C_1)

Ts = specified air temperature after dilution (°C)

Tamb = ambient air temperature (°C)

For a high-production furnace the fume volume flow rate after air dilution to 130 °C will be considerably higher than for secondary fume control by en­closure. A separate primary fume capture system would be used for this case.

For the remaining steps, a small, low-production furnace is under consider­ation with both primary and secondary emissions being captured by the enclosure.

Step 2: Determine secondary emission plume flow rate. The plume flow rate for charging and tapping is predicted by design equations for plume flow rates (compare Section 7.5). The enclosure height is taken as the limit of plume rise. The plume rise from the open furnace before charging should also be calculated. This event is also considered as a prolonged emission.

Step 3: Determine enclosure exhaust rate. The volume flow rate for pro­longed emissions during melting, tapping, and periods with the roof swung open sets the minimum exhaust rate required to ensure a relatively fume-free en­closure environment. The fume volume flow rate after dilution (from Step 1) is compared to the highest of the calculated plume flow rates for the prolonged emissions. The greater of these two amounts determines the enclosure exhaust rate. Although the charging plume flow rate can be higher than the tapping plume flow rate, it does not set the enclosure exhaust rate. The enclosure is used to store this approximately 30-second surge for the charging operation.

Air Curtain and Exhaust Off-Take Air curtain design and exhaust off­take location are very important considerations. The air curtain is applied on roof openings that are typically 2 to 3 m wide and used for crane rope access. The opening may extend over the length of the enclosure and should, therefore, be served by two sets of independently operated doors— one for tapping and one for charging. This feature minimizes the pen area when one of the two events occurs.

The optimum position for the exhaust off-take is directly opposite the air curtain discharge. Rising fume with the highest concentration is directed straight into the off-take without excessive recirculation in the enclosure.

The main purpose of the air curtain is to contain the vertical updrafts from charging and tapping emissions. The air curtain slot discharge should therefore be pointed downward (e. g., 15 to 25 degrees from the horizontal) in order to achieve an approximately horizontal resultant flow.

The air curtain design equations are outlined in Chapter 7. The plume data for furnace charging are used in this calculation step. Note that the plume volume flow impinging on the width of the slot should be used rather than the whole plume flow.

During melting, the air curtain should efficiently direct fume towards the exhaust off-take without allowing recirculation within the enclosure. The air curtain design should therefore also consider the fume trajectory when a lower updraft velocity from melting is experienced.

The air curtain supply air can be taken from either inside or outside of the en­closure. However, there is a net flow advantage to taking this air from the outside.

Elevated work area temperatures at operating floor level in the enclosure may be a problem. Limited louver openings or wall-type fans can be used for cooling if operators must normally spend prolonged periods in the enclosure.

Mechanical Design The success of an enclosure installation depends heavily on acceptance by operation and maintenance personnel. Mechanical and structural integrity and reliability must therefore be designed into the enclosure.

The following are a few design details to be considered:

1. After opening locations and proper clearances have been established, the enclosure frame support system should be considered. Major support beams placed at the edge of openings will provide extra strength against the rubbing of crane cables. The overall construction should be light, which allows fast easy repair in the event of collision by crane-held objects. Collision with a robust enclosure would still result in damage and probably be more difficult to repair.

2. Enclosure doors should be designed with generous clearances and be easily operated by simple mechanisms. Wheels, guide rollers, and pneumatic cylinders can be used as part of door mechanisms.

3. Enclosure roof doors that are directly susceptible to fume drafts must be positioned beneath the roof overlapping the opening. Rising fume will then strike, deflect, and disperse within the enclosure, instead of leaking through door clearances. The rest of the roof construction must be tightly sealed.

4. Easy maintenance access must be provided. Removable roof panels for access to furnace subassemblies are desirable. Water-cooled equipment, electrode, and roof movement mechanisms, etc. all require overhead access for proper maintenance.

5. Material selection for the enclosure shell should consider the corrosiveness of the environment. Aluminized sheeting is preferred over zinc-coated material in a steel-production environment.

6. The high sound levels produced by an electric arc furnace can be contained within a furnace enclosure if a proper acoustical design is carried out. Any design should be checked by a competent acoustician. The following points should be considered:

A. The material of construction of the enclosure should be of sufficient thickness to ensure a heavy-duty and robust design. In most cases structural requirements already ensure this.

B. The cladding should be sufficiently stiffened or damped to preclude resonance at the furnace frequency and its first few harmonics.

C. The inside of the enclosure should be lined with sound-absorbing material (e. g., fiberglass) selected for the frequencies involved and suitably protected from damage.

D. Holes, openings, and air leaks should be minimized, treated, or At Least located away from people, where possible.

E. Operating practices should minimize the amount of time operators need to spend inside the enclosure or near an opening While the Furnace is operating.

Enclosures for Nonbuoyant Sources

General Design Considerations Design of enclosures for nonbuoyant or inertial sources is completely different from design for buoyant sources. The dust produced by inertial sources arises from the motion of the particulate mat­ter itself, rather than from the thermal head of the air as in the case of buoyant sources. Enclosures are practically the only control technique suitable for large — scale inertial sources such as bulk materials-handling operations. Unlike buoy­ant sources, dust generated by these operations does not travel in predictable paths, and the range of travel is usually limited. These considerations preclude the use of remote hooding. Local hooding (i. e., a receptor hood) is sometimes used for inertial sources that have a single direction of travel, such as particles projected from a grinding wheel. For large-scale inertial sources, dust generation takes place in all directions and exterior hoods could be used to alter the motion of coarse particulate matter that is projected away from the source.

As with any hood system, design methods are used to obtain required ex­haust rates and hood dimensions.93 The main mechanisms of dust generation are air induction, material splash, air displacement, and air entrain ment.

Of these dust-producing mechanisms, air induction and air displacement are important for determining the exhaust rate for enclosures. Air entrainment and material splash are important for determining the size and shape of the enclosure.

A common and important application of exhausted enclosures is to bulk materials transfer points such as at chutes, bins, and dumping sites. Design equations for estimating the required exhaust rate are summarized below. Goodfellow16 presents further details on design equations. Consideration Is Also given to sizing the enclosure and positioning the off-take.

The exhaust rate for an enclosure controlling emissions from a falling ma­terials operation should equal the sum of the following quantities:

1. Flow rate of air induced by the falling material,

2. Flow rate equal to the volumetric flow rate of material (i. e., flow rate of displaced air), and

3. Flow rate sufficient to provide a working in-draft velocity through all openings (i. e., control velocity).

Working design equations for each of these flow rates follow. Hemeon61 developed equations for estimating the volumetric flow rate of induced air based on the power generated by the stream of falling particles (i. e., the work done per unit time by the drag force over the distance

Fallen). The recommended equation for estimating the induced airflow Rate Is The Following:94"95



Qv = flow rate of induced air (m3 s~[) Qm = material flow rate (kg s_l) H = drop height (m)

= cross-sectional area of falling stream (m2) Ps = solids density (kg nr3) D = particle mass median diameter (m)

0.631 = empirical constant (m1/3 s~2/3)

The flow rate of displaced air is given by the material flow rate divided by the bulk density




подпись: (10.79)Lastly, for a recommended control velocity of 0.5-1.0 m s-1 through the total area of the openings, A, the flow rate (qv ) is given by

Qv = Av,

Where V — control velocity (0.5 m s_1 for well-protected sources, 1.0 m s-1 for

Vigorous motion operations).

Sizing the enclosure is more important than might first appear. If the enclo­sure walls are close to the compacting pile of material, material spiash effects will cause losses through openings in these walls. Therefore, the use of a larger enclo­sure allows the velocity of these air streams to decrease before reaching the walls. Since quantitative estimates cannot be made as to the magnitude of the material splash effects, field observations of an existing system and experience are the only guides. Air entrainment becomes a factor when the enclosure has large areas or complete sides that must remain open. Winds or local air currents can then enter and exit the enclosure, thereby removing dust. The flow rate can be calculated in a straightforward manner from the wind velocity, open area, and loss coefficient of the opening. However, the ingress airflow rate is usually found to be quite large so that it may not be practical to attempt to counteract it by enclosure exhaust atone. Positioning the exhaust off-take close to the active zone of dust generation may capture the most concentrated portion of airborne dust before recirculation and mixing with entrained air can occur. This approach reduces the exhaust volume needed for air induction and control velocity.

Selection of the off-take position is important from the standpoint of the amount of material removed. Locating the off-take in the proximity of the ma­terial stream or at points of splash will result in greater removal of materials. This positioning may be desirable as a means to control splash effects pro­vided that the off-take velocity is kept low.

Nonexhausted enclosures may be used to contain dust arising from inertial sources and to protect against entrainment by winds. All the difficulties attendant

In the use of exhausted enclosures apply equally to nonexhausted enclosures. Since nonexhausted enclosures do not maintain an inward airflow through openings, tight sealing is the only means for restricting the escape of dust. No design proce­dures for nonexhausted enclosures can be given, but provisions should be made for removal of settled dust and for access to any equipment inside the enclosure.

The capture efficiency of an existing enclosure installation can be estimated by measuring the portions of captured and spilled dust. The measurement pro­gram can be quite involved depending on enclosure size, intermittence of opera­tion, dust settlement in the enclosure, and the extent of air entrainment. The measurement program must be custom designed to best suit the operation.

Gilbert, Hunter, and Ross96 describe a modeling technique used to im­prove capture of lime dust from a clamshell unloading operation. The source is typical for bulk materials handling at receiving terminals throughout indus­try. Large amounts of loose material are handled in the open, thus making control of dust generation and dispersion a constant challenge. In order to de­sign an accurate physical model, it was necessary to identify important vari­ables that were affecting the fugitive emission problem. A detailed account of the variables affecting performance is presented. This makes their paper an ex­cellent reference for demonstrating the design aspects for this type of non — buoyant source. The paper also has a qualitative description of performance before and after modifications to the hood capture system.

The lime unloading operation simply consists of using a clamshell to un­load a barge. The lime is carried by the clamshell into an enclosed hopper and dropped. From this transfer point, conveyors carry the lime to storage silos.

Figure 10.45 illustrates the lime dumping hood. A three-sided enclosure contains the discharge area over the hopper. The top is fitted with a slot for the clamshell trolley. In the original design, the exhaust duct to the dust collection baghouse is located at the enclosure midpoint. A description of the fugitive emissions from this source, as reported by Gilbert, Hunter, and Ross96 follows:

During the lime unloading operation when the clamshell is dumped into the hopper in side the enclosure, fugitive emissions of lime dust can sometimes be seen escaping over the front lip of the hopper, escaping at the middle and upper elevation out the front of the enclosure, escaping through the open trolley slot at the top of the enclosure and/or pulled out in the wake of the clamshell. There are many variables that affect the flow patterns inside the hopper and the enclosure to cause these fugitive emissions.

There are several important characteristics of the flow patterns and dust generation that are obvious from watching the field unit in operation. Almost all of the entrained lime dust comes up out of the hopper from below the grizzly starting about 1-2 seconds after the lime starts to fall through the grizzly. The amount of dust, the plume velocity, and the re­gion where it comes up out of the grizzly depend on where the load was dropped, how large a load was dropped, and the elevation of the clamshell above the grizzly. The plume travels upward in the enclosure and sometimes directly out of the front of the enclosure. As the plume rises in the enclosure, it is caught by the wind swirl patterns and carried higher in the enclosure where it can escape through the front or out of the trolley slot at the top of the enclosure. As the plume rises it may move in front of the clamshell, into the clamshell, in back of the clamshell or to the sides of the clamshell, depending on where the drop was made. Because the clamshell is brought out of the enclosure as soon as it is empty, it will generally push or carry out lime dust as it exits from the enclosure. From field observa­tions, it was also obvious that a full clamshell load drop produced more dust in the enclo­sure than a partially full clamshell. For a severe dust generation drop, it would take 30 to 40 seconds for the enclosure exhaust flow’ to clear the enclosure of airborne dust.


10. Location of enclosure ventilation openings

11. Degree of material dampness

12. Enclosure open-area control velocity

The original design was based on a control velocity recommended by dust control design manuals. The original design of 28 m3 s-1 exhaust flow induced an inward velocity of about 0.5 m s-1 through the enclosure entrance and trol­ley shots. This was not sufficient to overcome plume trajectories aimed out­ward, or to overcome the effect of moderate wind levels.

A design based on the enclosure open-area control velocity does not con­sider all the other variables listed as affecting dust control. Calculation proce­dures to predict many of the other variables would be very complicated if not impossible to perform. Physical modeling of the problem and solution was therefore used as the basic design tool.

A large one-sixth-scale model of the unloader hopper was selected so that flow patterns in the enclosure could be evaluated.96 Smoke was used to simu­late the behavior of the lime dust in the enclosure. The lime drop from the clamshell was simulated by releasing coarse sand, thus modeling the flow pat­terns caused by the volume displacement and the air entrainment. The effects of local wind speed and direction on the enclosure were also simulated.

Conclusions concerning the causes of the fugitive emissions were devel­oped from extensive model testing. The emissions escaped from the enclosure by direct plume trajectory and wind flow patterns. Lime dropped into the back of the grizzly creates a plume towards the front of the enclosure, whereas a drop near the front produces a plume to the rear. The plume is caused by the rapid displacement of air and dust from the hopper.

Conclusions concerning the elimination of fugitive dust escape were also developed from model testing. The baghouse capacity of 28 m3 s_1 is sufficient to capture most of the emission by implementing the following considerations:

Capture of dust is improved by repositioning the exhaust duct at a lower elevation closer to the grizzly, thus reducing the influence of wind.

By dropping lime in front of the hopper the dust plume is directed to the back, where a baffled off-take effectively captures the lime dust.

A downward flowing exhaust through the grizzly and into the hopper directly counteracts the plume velocity.

Slow opening of the loaded clamshell at as low an elevation as possible minimizes the emission.

The final recommended configuration for improving dust capture is shown in Fig. 10.46. The design change was rather simple and the model test showed a significant reduction in visible fugitive emissions.

This design review example has illustrated the following points:

Dust emissions result from the creation of local airflow caused by wind and from air displacement and entrainment resulting from material movement.

Capture system performance on a non buoyant source is influenced by enclosure (hood) design and location of the exhaust point.

Analytical design techniques, which predict the magnitude of local airflow and provide a basis for sizing exhaust systems, are not immediately available for


Where baghouse flow is drawn from the back of the hopper under a single baffle, which is raised off of the grizzly.

Every situation. The designer is, therefore, often forced to use a rule-of — thumb approach or the modeling approach to size a capture system.

The modifications shown in Fig. 10.46 were installed in the field unit. Re­ports from field unit operators and observers indicated that the significant im­provement shown by the model tests was also exhibited in the field.96

Partially Enclosed Systems It is common that in different kinds of continu­ous production lines, only some part of the process is emitting air contaminants. It may not be necessary to enclose the whole production line but rather the part of it where the emission takes place. Examples are different types of surface coating lines and printing lines where organic solvents are emitted from the paint or the printing ink, or in the woodworking industry where slabs of wood are trans­ported and processed in different machines in a production line. These types of production lines are often designed as a long conveyor belt passing the different processing machines. The normal solution is then to enclose the machine as effec­tively as possible and to connect it to the general exhaust system of the factory. It is not possible to totally encapsulate the machine that is emitting the contaminant since there must be openings for the transportation of material into and out of the enclosure. The inlet opening is normally easier to control than the outlet. One rea­son is that, many times, it is possible to cover the main area of the opening with some soft material such as brushes or rubber strips that keep the opening almost

Closed but allow the material to pass. Another reason is that the conveyor belt causes a slight airflow in the direction it is moving. It is more complicated to con­trol the outlet opening. If the enclosed machine, for example, is applying paint to the material, it is not possible to use brushes or anything else that come in contact with the wet paint. The area of the opening on the outlet side must be larger than on the inlet side. The conveyor belt movement also causes air to be pulled out of the enclosure, bringing the contaminant with it. In this example, as always when paint or ink with organic solvents is applied, the treated material may be a source of contaminant emissions. The enclosure must therefore be extended, covering the conveyor belt until the emission is acceptable or to the next step of the process, e. g., a drying oven. The dimensions of the machine and the need to have access to different parts of it influence the dimensions of the enclosure. The airflow rate needed to achieve a high capture efficiency is normally low. Glove Boxes


One way to minimize or eliminate exposure to contaminants is to have a completely closed box with a glass or plastic panel to look through and gloves mounted in one or more walls. This type of local exhaust hood makes it possi­ble to have a completely shielded workplace available nearly anywhere.


Glove boxes are connected to an exhaust. The form of the box could lie simi­lar to any other partially closed hood. Instead of the opening through which the work is performed in a partially closed hood, the box has a wall with two holes attached to a pair of arm-length gloves. This enclosed volume normally does not have any specific opening for entrance of air from the surroundings. The air nor­mally enters through existing cracks. If there is a specific opening to the surround­ings a nonreturn valve is placed there to prevent flow from inside the box to the general work area. This valve could be very simple, e. g., a plastic sheet hanging on the inside covering the openings. The opening could also be covered with a filter to prevent contaminants from leaking to the outside, if air somehow should flow’ backwards. Glove boxes are ready-made from sheet metal or plastic. Work is per­formed by putting the hands into the fixed gloves and manipulating the tools or materials inside the box by hand. See Fig. 10.47.

Applicability of Sources

Glove boxes are commonly used for small abrasive blasting activities and to control exposure to highly toxic and radioactive materials. Glove boxes used for abrasive blasting are quite similar to the abrasive blasting room (Section But are much smaller and usually do not have mechanical air supply into the box. The same type of system, but with much greater tightness, is used for radioactive and highly toxic biological or chemical materials. For these ap­plications an air supply that flows directly into the box is often used or needed.

Different Forms and Boundaries Relative to Other Types

The transparent surface and the gloves could all be situated in the same wall. Quite often the transparent surface is in one wall and the gloves are on


Each of the adjoining sidewalls. This could be advantageous when working with both hands in the box. Strippable plastic liners on the interior of the box and filters or other air cleaners at the exhaust connection are often used to fa­cilitate decontamination. For microbiological work, glove boxes are called Class 3 microbiological safety cabinets97 or Class III Biosafety Cabinets.98 These have HEPA-filtered inlet air and HEPA-filtered exhaust and are nor­mally a recirculation unit (Section They could be connected to an exhaust duct instead of recirculation. To protect the process or the product,

E. G., in the electronics industry, the same construction could be used but with a higher pressure inside than outside the box. These positive-pressure boxes are for product or process protection and do not offer worker protection.

Specific Problems

In order to remove the generated contaminants associated with abrasive blasting and to ensure visibility into the box, a high exhaust airflow rate is necessary. High flow rates may also be necessary to maintain a large pressure difference to prevent leaks of highly toxic and radioactive materials.

The size of the box is usually small, since the whole interior of the box must be reached with the hands in the gloves. Larger boxes could be supplied with more pairs of gloves, and different gloves could be used for different areas inside the box.

Glove box failures are most likely to occur at the glove connection. Nor­mal flexing of the glove material, as well as contact with abrasive, corrosive, or radioactive materials, reduces the integrity of the glove material. Caps on the glove ports are recommended when the gloves are not in use. Periodic in­spection and glove replacement are recommended.

For abrasive blasting it is common to have a horizontal grill inside the box for collecting the abrasive material which functions as the stand for the mate­rial to be treated. Abrasive blasting generates large amounts of dust with large particles, which should be exhausted as fast as possible. In these boxes, the ex­haust connection is above the working area (grill).

Sometimes gases or vapors are used or generated inside the box. Gloves could be permeable for some gases, which may be dangerous to the skin. Selec­tion of glove material must consider the type of contaminant. In cases where suitable glove material is not available, it may be necessary to use a completely closed volume with remote controlled manipulators instead of gloves.

When openings for air supply are used, they should be small, but not too small, since this could generate high, disturbing velocities of incoming air.

To get equipment and the material to be treated into and out of the box it is necessary to have some kind of door. This door should be made in such way that it tightens when negative pressure is applied inside the box. Instead of one door, an airlock volume with one door to the box interior and one door to the exterior can be used.

When these boxes are used to control highly toxic and radioactive materials, provision for emergency power is necessary to ensure continuous exhaust ventila­tion. In some locations, seismic safety considerations may also be necessary.99

Depending on use, the tightness and containment of the box can be tested with a tracer gas or with generated contaminants. Since the pressure inside the box is lower than the pressure outside it could be easier to check the tightness without the exhaust airflow. Measurements similar to those for fume cupboards can be used.

Design Equations and/or Parameters

When glove boxes are used for abrasive blasting, their design must take into account that the ejector of the abrasive material is driven by compressed air. This influences the chosen flow rate, which should be large enough to gen­erate an air velocity of at least 3 m s-1 in the openings.100 The necessary ex­haust airflow rate recommended by INRS is then

Q = Sv + Qp , (10.80)

Where Q is the total exhaust flow rate, m3 s-1, S the total area of openings to the surroundings, m2, V the necessary air velocity in the openings, 3 ms’1, and Qp the airflow rate blown into the box as pressurized air and expanded to at­mospheric pressure, m3 s_1.

The ACGIH25 recommends 20 air changes per minute for a glove box for abrasive blasting. With a volume of 1 m3 this is equal to 0.33 m3 s-1. They also recommend an inward air velocity at all openings of at least 2.5 m s’1.

Several recommendations exist for glove boxes used for highly toxic and ra­dioactive materials. The ACGIH25 recommends a minimum flow rate equal to 0.25 m3 s’1 per m2 of open door area. The box should also operate with a pres­sure of at least 62 Pa below atmospheric pressure. A minimum duct velocity of 10 to 20 m s_1 is also recommended. Lawrence Livermore National Laboratory99 recommends a minimum velocity through openings of at least 0.5 m s_1 and nega­tive pressure relative to the surroundings. In this case, the openings include both operational openings, such as doors and filters, and noncatastrophic accidental

Openings, such as glove failure or tear-off of Bagout Covers. No specific Pressure Requirements are given. LLNL requires a detailed safety analysis to determine Nec­Essary ventilation system requirements and airflow if a large catastrophic failure, such as loss of a window, is credible." Filtration is usually necessary when toxic Or Radioactive particles are used or may be generated. The filter resistance Needs To be considered when sizing the ventilation system.