Low-Momentum Supply with Exterior Hoods
Exterior hoods intended to capture contaminants should be placed as close to contaminant sources as possible. In actual practice, however, the hoods can not always be placed close to the source due to circumstances such as working conditions. In such cases, to enhance the exhaust efficiency of exterior hoods, it is useful to use a low-momentum air supply directed toward the exhaust outlet. The supply airflow, which functions to transport contaminants emitted from sources located at a distance from the exhaust outlet, should be relatively low with a uniform velocity but high enough so that it is not disturbed by the surrounding air motions. The advantages of using low-momentum supply with exterior hoods are that (1) a lower supply airflow rate to the workspace is possible, (2) a lower exterior hood exhaust flow rate is possible, and (3) it is possible to supply clean air to the breathing zone of the worker.
10.4.5.2 Principle
Gases, vapors, and fumes usually do not exhibit significant inertial effects. In addition, some fine dusts, 5 to 10 micrometers or less in diameter, will not exhibit significant inertial effects. These contaminants will be transported with the surrounding air motion such as thermal air current, motion of machinery, movement of operators, and/or other room air currents. In such cases, the exterior hood needs to generate an airflow pattern and capture velocity sufficient to control the motion of the contaminants. However, as the airflow’ pattern created around a suction opening is not effective over a large distance, it is very difficult to control contaminants emitted from a source located at a distance from the exhaust outlet. In such a case, a low-momentum airflow is supplied across the contaminant source and toward the exhaust hood. The
Contaminants emitted within the supplied airflow are transported to the exhaust opening and the exterior hood can easily exhaust the contaminated air.
The low-momentum air, which is supplied from a relatively wide supply inlet, functions to transport contaminants to near the exterior hood. In addition, it functions to change the direction of contaminants toward the exterior hood when the direction of contaminated air is initially different from the exhaust direction. The momentum or velocity of the supply air to reach to the exterior hood will be sufficient when the motion of contaminants can be neglected. However, when the contaminated air exhibits significant motion or flows in a direction different from the exterior hood, the supply air velocity should have a sufficient momentum to control the contaminant flow.
The exterior hood could be placed beside, below, or above the contaminant source, and the direction of the supply air could be horizontal, vertical, or diagonal toward the exhaust inlet. The sources should be within the supply airflow, and in some cases, a worker could also be located within the flow. When the worker is within the supply airflow, how’ever, a region of a recirculating airflow, a wake region, can be created downstream. If the breathing zone of the worker and the contaminant source are within this wake region, high exposures may occur. Therefore, the relation between the direction of supply airflow and the position of the worker should always be considered, as the source should never be within the wake region (see Section 10.2.3).
10.4.5.3 Applicability of Sources
Since the low-momentum supply system should enhance the efficiency of an exterior hood by supplying low-momentum airflow to a source, the system can be applied to practically any sources where an exterior hood can be used. In particular, it is effective to apply the system when an exterior hood cannot be placed close to a source or the exhaust direction is different from the initial contaminant release direction.
Many processes generate contaminants in casting plants. Some practical applications of the low-momentum supply system in these plants will be introduced here. Figure 10.84 shows an example of the system applied to the process where molten metal is being poured from a low-frequency furnace to a ladle. The temperature of molten metal is about 1800 K and a highly buoyant plume containing metal fumes is formed above the tapping nozzle and the ladle. To control the plume, a supply inlet is placed horizontally from the center of the ladle by 2.5 m and supply airflow is blown at a uniform velocity of 3.0 m s_1 toward the ladle. The direction of plume is turned toward the exhaust outlet by the supply airflow, and the flanged exterior hood exhausts the fume-containing plume. The dimensions and operating conditions of the system are indicated in the figure. The velocities of supply and exhaust airflow were determined using CFD simulations.40 41
Figure 10.85 shows an example of the system applied to a shaking-out process in a casting plant. In this process, when molding sand around castings is shaken off, high concentrations of dust rise above the shakeout machine in a buoyant plume. To remove the dust, an exterior hood was placed beside the source and a supply inlet was placed on opposite side. Air is blown toward the exhaust outlet to change the direction of the dust toward the exterior hood. The temperature of castings is about 700 K, the
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800—1*— — 1700 |
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To dust collector |
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FIGURE 10.85 Practical application to remove dusts from a shaking-out machine. Supply airflow rate is i.88m3s_l, exhaust airflow rate is 9.18 m3 s-1. |
Supply with exterior hoods system is similar to the push-pull ventilation system. In the push-pull system described in other sections (10.4.3 and 10.4.6), the main function of the jet is as an injector or as a curtain, whereas in the low-momentum supply system the main function of the supply flow is as a carrier or a transporter of contaminants. The main difference in supply airflow between the push-pull system and the low-momentum supply system is that the supply air is blown at relatively low velocity from relatively wide openings, whereas in the push-pull systems the jet is blown at high velocity from narrow slots.
The low-momentum supply system could also be applied to operations inside booths. If a worker must be inside a booth, to protect the breathing zone, a supply inlet with a relatively wide area is placed above the worker and the low-momentum clean air is blown toward the worker. At the same time, the airflow could transport contaminants to the exhaust outlet.
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FIGURE 10.86 Practical application to remove dusts generated during handwork. Depth of the system is 6 m, supply airflow rate is 4.95 m3 sr1, and exhaust flow rate is 9.08 m3 s_l. |
In the low-momentum supply system, the contaminants are emitted within the low-momentum airflow blown from the supply inlet and they are transported to near the exhaust opening. If the contaminants diffuse into the whole of the supply airflow, the exterior hood must exhaust the whole of the airflow. To diminish the exhaust flow rate, some methods to prevent the contaminants from diffusing into the whole of the airflow are required. One possible method is to supply the air as slowly as possible but with enough velocity to reach the exhaust outlet and to control the surrounding air motion. Another method is to blow supply air with uniform
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FIGURE 10.87 The fundamental operation and dimensions of the low-momentum supply with exterior hood system. |
Velocity, but with low turbulence intensity.42 The exhaust outlet, which is an exterior hood, must exhaust only the contaminated airflow transported from the source; it is not necessary to exhaust the whole of the supply airflow. A flanged exhaust outlet should always be used to enhance the efficiency of the exterior hood.
Workers could be either inside the supply airflow or outside. If a worker is inside the airflow, it is possible that the breathing zone of the worker is within the wake region and he or she may be exposed to the contaminated airflow. To reduce the exposure the worker should always be upstream of the source and should try not to have his or her back to the supply airflow.
10.4.5.6 Design Equations and/or Parameters
An established design method for this type of system is not available. The practical design of the low-momentum supply with exterior hood system described in the previous part of this section used the flow ratio method.6 However, the actual exhaust flow rate was adjusted visually to the appropriate value in order to exhaust only the contaminants transported by the supply airflow.
The flow ratio method was first suggested for use in designing receptor hoods and then it was suggested for design of push-pull systems. The concept of the method is described as follows.
Figure 10.87 shows the fundamental operation of the push-pull flow’. The suction hood should simultaneously exhaust the pushed air (contaminated supply
Airflow, Qv) and the air from the surroundings (qv). The total Exhaust Flow rate (qv,) Is Calculated using Eq. (10,122), provided Qvis less than the Exhaust Flow rate:
Qv‘ = 4r‘ + 4vi П 0.124)
= +<7 V/Йу) = <?„,(!+A’),
Where K = Qv. y qV{ is the flow ratio.
Here, one considers the limit value for Qv /qt, v It is expressed as
= (10.125)
In designing the push-pull hood, one always applies a safety factor, N, resulting in the exhaust flow rate for design, Q„fD, which is expressed as the following:
<ay, D = ^,(1 + nKL), (10.126)
Where KL is a function of many factors such as the flow and surrounding air motion. It is normally determined from the results of experimental investigations.
The limit value of the flow ratio, KL, is expressed as the following experimental equation for two-dimensional push-pull flows:
KL = 0.55(LBS/LB)XA{0A6((LFi/LBy1A + 0.13)}
{5.8(v0/vi) (LBS/LB) +1},
(10.127)
Where V1 is the velocity of push airflow, V0 is the velocity of surrounding air motion or contaminant motion, LBS is the distance between push and pull openings, LB is the width of push opening, and Lra is the length of the flange on the exterior hood.
For practical design, the recommended aspects of the push-pull flow (hood) and the safety factor are as follows:
0 < LBS/LB < 15.0,2.0 < LFi/LB < 10.0,0 < V0/v^ < 1.0.
For two-dimensional flow N = 1.1 and for three-dimensional flow N — 2.2.
Applying the flow ratio method to the low-momentum supply system, the required exhaust flow rate is often in excess of practical values. This is because the value of Qv3 is given as the value at which all the supplied airflow should be exhausted by the exterior hood. In the low-momentum supply system, contaminant sources should usually be between the supply inlet and the exterior hood. The supply airflow is contaminated at the position of the sources and it flows to the exterior hood. Therefore, all of the airflow is not always contaminated. Unfortunately, a design method considering such cases (the diffusion of contaminants within the airflow) has not been established yet, and the appropriate exhaust flow rate has to be adjusted after the system is installed.
A partial enclosure can be combined with a plane air jet in order to increase the containment of the hood. The jet can be thin or wide, depending on function and placement, and can be single or multiple. The exhaust hood must be very carefully designed since the exhaust velocity in the opening is usually low and the jet’s velocity is usually high.
When using a thin air jet, one common problem is disturbance of the jet when walking or reaching through the jet or passing material through the jet. These activities could deflect the jet and result in a large escape of contaminated air from inside the hood to the worker and the surroundings.
When total isolation of the surroundings is necessary the contaminant source (or rhe worker) has to be placed in a volume that is physically closed and has both supply and exhaust air. When these systems are situated in small rooms they function as both local and general ventilation. Some smaller systems that normally are of bench size are included in this section.
The systems described illustrate some applications for the use of air jets combined with exhausts. There are many potential combinations of jets and exhausts and most of them have been used at some time. It is not possible to include all potential descriptions here and the ones included were chosen to illustrate the advantages and disadvantages of these systems. More examples are described in the literature (e. g, Hayashi6 and Goodfellow2).
The description of biological safety cabinets is also included in this section. Biological safety cabinets include those with combined supply and exhaust airflow, but also include other designs which are also described.
10.4.6.2 Workbenches
General
A workbench makes use of a local air supply in conjunction with exhaust air ro ensure good control of the contaminants generated on a bench process. The local exhaust removes the contaminants, while the local supply air protects the operator and/or the products against airborne contaminants. The local supply air improves the thermal environmental conditions by introducing cool dehumidified air in a hot environment. This ensures that the operator’s thermal comfort is maintained in areas of high temperature, where full air conditioning of the entire workspace is not economically feasible.
Principle
Low-velocity supply air enters the space above the operator, providing a clean air zone around an operator working in a contained area. It is arranged so that the contaminated air flows toward the exhaust openings. The low-ve- locity supply air is usually discharged vertically above the worker, although horizontal flow can be used. A typical example of a workbench is shown in Fig. 10.88.
Applicability of Sources
Workbenches are suitable for area or point sources with low momentum; typical applications are sanding, painting, laminating, soldering, and powder mixing and handling. They are especially useful for industrial operations that, due to operating conditions, cannot be enclosed or isolated. These systems are unsuitable for high-momentum sources such as high convective heat releases or large particles with high release velocities. In such cases, the capture efficiencies
Nant source, and the other opposite the source (Fig. 10.89). The figure shows a typical industrial system in which the measured dust concentrations and standard deviations in the worker’s breathing zone are presented with various local ventilation configurations. Without local ventilation, the operator’s average dust exposure was 42 mg m-3. With local exhaust only, the exposure was reduced below 1 mg nr3. There was no statistically significant difference between the side draft and downdraft exhausts. The addition of local supply air further reduced the exposure to below 0.5 mg nr3. The best results were achieved when both exhaust inlets were used in combination with supply air. With this combination, the exposure was below 0.2% of the exposure from the dilution ventilation alone. Interestingly, this reduction was obtained without increasing the total airflow rate; hence, a better performance can be achieved for the same energy usage.
Location of Workers Relative to Exhaust Openings and Sources
The standing or sitting worker is normally close to the contaminant source, which is on the working table. The relative location of the exhaust openings should be designed to ensure that contaminants are forced away from the operator’s breathing zone.
Disturbances
Any obstructions in the flow field may cause recirculating airflows, which reduce the efficiency of the local ventilation system. The operator’s body also causes wakes when bending over the table. This recirculating flow may transport contaminants from the source region to the worker’s breathing zone, resulting in potentially high exposures. Ambient disturbances such as airflows due to supply air jets, moving machinery, and drafts from windows and doors may interact with the supply and exhaust openings velocity field and reduce the performance. The worker’s physical movements also produce air currents, which may influence the flow field and exposure.
The influence of air disturbances on performance can be minimized by locating the supply air device as close to the operator’s breathing zone as practical to improve protection and by using walls or side baffles near the contaminant source.
Specific Issues
High supply air velocities or cool supply air can cause uncomfortable drafts on the worker. Nonuniform supply air velocities with high turbulence intensity may result in decreased capture efficiency, increased contaminant spread, and increased thermal discomfort.
Changing Flow Rates
For a constant exhaust flow rate, an increase in supply airflow provides better operator protection or production protection, but it also increases contaminant spread and risk of draft. Any decrease in supply airflow rate will result in a reduction of the design conditions of the operator or product protection.
With a constant air supply flow rate, an increase in exhaust airflow will result in increased operating costs. Any decrease in exhaust flow rates will result in
Reduced capture efficiency and increased contaminant spread. Reduced exhaust flow rate will cause dust buildup in the ducts due to the reduction in velocity.
Design Equations and/or Parameters
Supply Air When designing workbenches, it is essential that the supply air face area be large enough to cover the contained area. Therefore it is important to have some indication of the operator’s range of movements for all intended operations. Moreover, for efficient protection the supply airflow must be adequate to get a stable flow field that will not be affected by ambient disturbances. In industrial applications the suitable mean supply air velocities are typically between 0.2 and 0.45 m s~l. Low velocities should be used when the distance between the supply air unit and the operator is small or for cool supply air. High velocities are applicable at greater distances and in hot environments, with thermal comfort being considered.
The supply air velocity should be uniform to protect against ambient air contaminants. Nonuniform flow results in high velocity gradients and turbulence intensities and rapid supply air mixing with the ambient air. As well as reducing protection efficiency, this will adversely influence thermal comfort. An even velocity distribution can be achieved using distribution manifolds and a filter at the face of the supply air unit.
The flow field created within the protection zone depends mainly on the density difference between supply air and room air (Fig. 10.90). With vertical flow the supply air should be isothermal or cooler than ambient air. If it were warmer, the extension of the controlled flow would be reduced due to buoyancy effects, resulting in the supply air not reaching the operator’s breathing zone. As the supply air cannot be used for heating, the operator’s thermal comfort should be maintained, preferably with radiant heaters in cold environments. If the supply air temperature is lower than the room air, the denser supply air accelerates down to the operator, and for continuity reasons the supply flow contracts. Excessive temperature differences result in a reduced controlled flow area with thermal discomfort, and should only be used in special cases.
Exhaust Openings The locations of exhaust openings are chosen to suit the particular application, with the hood as close to the generating source as possible. The exhaust opening location depends on the direction of contaminant release and its momentum. For low-momentum sources, exhaust openings should rest on the worktable or appropriate surfaces, thus reducing the amount of air drawn from surrounding clean air regions.
Exhaust airflow rates and exhaust opening locations are designed to ensure air velocities near the contaminant source are in the 0.3-0.5 m s-i range for low-momentum sources. If the exhaust is through a table grille and the source is near the table surface, the exhaust airflow rate is calculated by
Qv = A-v, (10.128)
Where A is the area of the working table and V is the mean air velocity at the opening.
To ensure that contaminants are not spread into the ambient air, the exhaust airflow rate must be higher than the supply flow rate.
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Supply air. |
Evaluation Procedures
The ability of the ventilation system to protect the worker efficiently can readily be determined by personal samples. The PIMEX method (see Chapter 12) can be used to determine the worker’s exposure during various work phases. The capture efficiency as well as the supply air fraction can be measured using tracer gas techniques. Simple evaluation is carried out visually with smoke tube or pellet tests. Daily system evaluation is recommended using airflow or static pressure measurements at appropriate parts of the system. The air velocities, turbulence intensities, air temperature, mean radiant temperature, and air humidity should also be measured to provide an assessment of thermal comfort.
10.4.6.3 Wall Jet-Enhanced Exhausts
General
To enhance the efficiency of a partial enclosure it is possible to let a plane supply air jet blow inside and/or into the hood along one or more wails or along the table. Other advantages of this system are a reduction in needed supply flow to the room or a reduction in necessary exhaust hood flow for the same level of control. The supply flow (jet) inside the hood usually makes the flow into the hood (through the hood opening) more stable. As for all exhaust hoods with supply air inside, the supply flow rate must be less than the exhaust flow rate and the difference must be large enough to ensure sufficient velocity into the hood.
Principle
A rectangular or a canopy hood is placed close to a wall with the opening tacing down. This could be called an open exhaust hood. The hood may have
Partial shielding by placing walls on the sides, resulting in a partial enclosure. A wall jet is added to one of these configurations (open or partially enclosed). One possibility is to have a wall jet blow vertically upward along the inner back of the hood (along the inner wall) and then into the exhaust opening (Fig. 10.9’1). Another possibility is to have the wall jet directed horizontally along the working table (bottom of the open exhaust hood) from the opening toward the inner wall. The jet turns upwrard at the end of the table, at the inner wall, and then travels into the exhaust outlet (Fig. 10.92). The wall jet can be generated by a fan, from the general supply air system, or from a pressurized air system. The air in the jet could come from either outside or inside the room.
Exhaust plenum
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FIGURE 10.91 Vertical jet along inner wall. Dimensional example for kitchen hood. Width is 1.2 m, depth is 0.8 m, height between table and exhaust is 1. 15 m, jet inlet width is 3.5 mm, jet flow rate is 28 L s ‘1, and exhaust flow rate is 278 L s_l. |
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FIGURE 10.92 Horizontal jet along table. Welding table with grid. Exhaust flow rate is 278 L s 1 and jet flow rate is 50 L s_l. Dimensions are approximately the same as in Fig. 10.91. |
The contaminants are transported into the exhaust opening, from the table, by suction into the supply air (jet) and the movement of the exhausted flow over the table.
Figure 10.91 is shows the principle for a jet along the inner wall. Figure 10.92 shows the principle for a jet along the table, in this case a welding table with grid, where the jet is blown below the grid. Figure 10.93 shows the design of a kitchen hood with the jet along the inner wall, where the grease filter is a part of the connection to the exhaust system. These examples were first described by Lind and Aourell44 and Petersson and Vikstrom.45
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FIGURE 10.93 Design of exhaust with wall jet for a kitchen hood wfth partly shielded sides and circular grease filter. |
Applicability of Sources
These systems can be used for relatively small sources that generate a large amount of contaminants at not too high temperatures and velocities, and where the process is not influenced by cross-drafts.
These systems have been applied to welding tables with open sides, even if the welding processes normally generate too much heat to be appropriate for this kind of local ventilation. In these cases, a plane jet blows along the welding table (from front to back) or an initially plane jet blows from front to back, along the lower part of the welding table (below the grid). These types of systems have also been used for kitchen hoods with open sides. In these cases, the plane jet is blown upward along the back wall. An air jet along the inner back wall of a laboratory fume hood has often been used to increase the flow stability inside the hood.
Different Forms and Boundaries Relative to Other Types
The opening to the exhaust duct is either a horizontal rectangular exhaust opening facing down (with air direction upward) or one or more slots connected to the exhaust duct. These slots should preferably also be placed horizontally, facing down, but could be at some angle to the horizontal plane.
The opening that generates the plane jet could either be a narrow slot or a line of holes placed so close to each other that the jet air forms a continuous plane jet.
The system with a horizontal jet (Fig. 10.92) is similar to the push-pull system used on open surface tanks (Section 10.4.3). One difference is that in this system the jet functions as an injector, whereas in push-pull systems the main function is as a curtain.
If the supply jet is inclined upward, while situated at the front of the table, its function could be similar to an air curtain in an opening. However, the inclination from the horizontal for the jet is usually very slight, since angles above 5 to 10 degrees sharply deteriorate the function of the ventilation system. If the jet is directed vertically, it should be placed along the back wall, blowing upward.
The connection to the exhaust duct (the exhaust opening) is usually a horizontal surface with suction vertically up, and this connection should be situated above the jet inlet and the source generation point. Sources with heat generation should be on or slightly above the surface and between the jet inlet and the exhaust opening. For a vertical wall jet, the source should be as close as possible to the jet.
The worker must always be outside the hood or in front of the table, that is, the jet should not pass over the worker.
The tools and the contaminant generation should be on or slightly above the working surface. Preferably they should not be directly in the jet, but close to it, when using a horizontal jet. For a vertical jet they should be as far away from the table’s front as possible—in other words as close as possible to the vertical jet along the inner back wall, without being in the jet itself. At the same time, they should be as far away as possible from the worker, who naturally is placed behind the jet opening.
The relation between the two flow rates is specific for each hood design. The flow rate into the exhaust duct must be larger than the sum of the jet flow rate and the induced flow rate. The induced flow rate should ideally be of such a size that the flow rate into the hood, passing the operator, keeps the contaminants in the flow into the exhaust.
The jet velocity should fulfill two demands. First, it must be large enough to generate an induced flow rate, which transports the contaminants into the exhaust (for a horizontal jet along the table, it should be large enough to turn upward at the inner wall and continue into the outlet); second, the velocity should not be so large that the jet bounces at the inner wall and returns toward the operator or for a vertical jet that it turns outward along the exhaust opening plane and into the surroundings.
If the hood has side walls the opening to the outside must be large enough to permit work inside the hood and it must also be designed to facilitate flow into the opening and diminish the possibilities for the moving air to transport contaminants to the surroundings.
Pressurized air could be used for the supply jet. Since this normally gives higher initial velocities, the hood becomes more sensitive to changes in the jet flow rate and velocity. Pressurized air systems could also result in more noise problems than a jet resulting from a fan.
The largest disturbances can occur when the supply jet is disturbed or deflected by the process. In this case the jet can blow in unwanted directions, such as to the side or backward into the surroundings. The operator can disturb the horizontal jet by standing too close to the jet inlet and also by preventing the induced flow from passing the contaminant source.
The difference between the supply and exhaust flow rates of the room’s general ventilation must be large enough to handle the difference between the hood’s supply and exhaust flow rate. If the difference between the room flow rates is too small (the available flow rate into the hood is too small) the hood’s containment diminishes and the air inside the hood could be pulled into the room. If the difference is too large, the room supply flow can influence the flow into the hood and possibly inside the hood, which can result in increased contamination outside the hood. The same condition could occur when the direction of the room supply flow disturbs the flow into and inside the hood.
If the hood’s exhaust flow rate is too small (smaller than the sum of the initial jet flow rate and the induced flow rate) the jet will carry some contaminants into the room. The same condition will occur with a jet flow rate or jet velocity that is too large.
An increase in exhaust hood flow rate is not normally a problem, as long as the room supply airflow rate is high enough. If the exhaust hood flow rate diminishes, the same types of problems arise as described above when the initial exhaust flow rate is too low.
An increase in the jet flow rate gives the same result as a supply rate that is initially too high. A decrease in the jet flow rate could result in diminished functioning of the hood, but a small decrease has no deleterious effect.
Design Equations andlor Parameters
The designs that have been published are all empirical. However, it should be possible to use the wall jet equations from Chapter 7 and the equations for velocities outside exhaust openings (Section 10.2.2) to design these types of hoods.
The exhaust flow rate from the hood must always be much larger than the initial flow rate in the jet, since the jet always induces a larger flow rate than the initial jet flow rate. One recommendation is to have the initial jet flow rate approximately equal to 10% of the exhaust flow rate.
The design should start with the necessary air velocity through the hood opening or near the source, from which the induced flow rate can be calculated. This is used, together with operating restrictions on velocities, to calculate the necessary initial jet flow rate and jet opening area, which, together with the distances (height, depth, width), give the jet’s velocity and flow rate. These give the necessary minimum exhaust flow rate and the equations in Section 10.2.2 may be used to calculate the size of the exhaust opening. It is possible to start with exhaust opening size and height above the table to calculate the maximum velocity and flow rate of the jet. The same equations presented earlier are used to calculate the velocity gradients outside the exhaust openings. These gradients are used to check that the velocities from the jet are not higher than the velocities from the exhaust, except where the exhaust velocities are too small to capture or transport the contaminants. This usually happens near the contaminant source. Both the velocity and the momentum (or flow rate) into and near the exhaust opening (maintained by the exhaust flow) must be higher than the velocity and momentum generated by the jet at the same place. See Figs. 10.91-10.93 for design examples.
The pressure drop for the exhaust opening, the jet supply opening, and for the hood opening if there are side walls, can be calculated using the normal equations for flow inside ducts and into ducts and openings.
An important design parameter is the jet angle. Normally the jet should be parallel to the table or the inner back wall and can thus be treated as a normal wall jet. If the jet has a small angle upward from the table, the wall jet equations may be unsatisfactory and experiments may be necessary.
The contaminant and heat generation rates must also be assessed in order to determine if the generation rates are too high for the calculated airflow rates or if they influence the passing flow into the exhaust. Generation rates That Are too high could make it necessary to increase the hood’s Exhaust Flow rate or to change to another hood type. It may also he possible to increase The Jet flow rate, but since this means increasing velocities at the Exhaust, the result Must be evaluated before use.
Evaluation
A visual evaluation of ventilation system performance can Be Performed By Injecting smoke into the jet. No quantitative evaluation methods for These Systems have been reported, but it should be possible to measure The Containment of a hood with side walls (partial enclosure) using one of the containment indices (see Sections 10.2.1 and 10.5). Additional information May be Obtained by measuring capture efficiency.
10.4.6.4 Biological Safety Cabinets
General
Biological safety cabinets (BSCs) used in the United States are divided into three classes: I, II, and III. Class II cabinets are further divided into four types: A, Bl, B2, and B3.46 In other countries, other categorization schemes are sometimes used, but usually follow the same general operating conditions. This division is quite unlike the rest of the local ventilation chapter, but since these descriptions are used whenever BSCs are used, it is practical to describe them here.
Class I BSCs are, from the functional view, similar to a fume cupboard (Section 10.2.3). Class II cabinets are used for product and worker protection. Class III cabinets are used for work with very dangerous microbiological or radioactive agents and provide maximum protection to the environment and the worker. The class and type of BSC used is dependent on the demands for worker and product protection.
Principle
A document from the U. S. Centers for Disease Control and Prevention and National Institutes of Health46 describes, in detail, the different BSC designs and operating parameters. This document is easily available on the Internet and only a short summary will be provided here. For more detail regarding operation, design, use, testing, protection, etc. the reader is referred to the latest edition of the CDC-NIF1 document.
Class I The Class I BSC provides personnel and environmental protection, but no product protection. It is similar in air movement to a chemical fume cupboard, but has a HEPA filter (see Chapter 9) in the exhaust system to protect the environment (Fig. 10.94). In the Class I BSC, unfiltered room air is drawn across the work surface. Personnel protection is provided by this inward air velocity as long as a minimum velocity of 0.37 m s~J is maintained through the front opening (see the discussion on fume cupboards in Section 10.2.3.3). In many cases Class I BSCs are used specifically to enclose equipment.
The Class I BSC is hard-ducted to the building exhaust system, and the building exhaust fan provides the negative pressure necessary to draw room
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U Room air
Contaminated
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HEPA-filtered
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FIGURE 10.94 |
Side view
The Class I BSC (A: Front opening. 8: Sash. C: Exhaust HEPA filter. D: Exhaust pie —
Air into the cabinet. Cabinet air is drawn through a HEPA filter as it enters the
Exhaust plenum. A second HEPA filter may be installed at the terminal end of
The exhaust.
Some Class I BSCs are equipped with an integral exhaust blower; the cab
Inet blower must be interlocked with the building exhaust fan. In the event
That the building exhaust fan fails, the cabinet exhaust blower must also turn
Off so that the exhaust ducts are not pressurized. If the ducts are pressurized
And the HEPA filter develops a leak, contaminated air could be discharged
Into other parrs of the building or the environment.
Class II The Class II (Types A, Bl, B2, and B3)46 biological safety cabinets provide personnel, environmental, and product protection. Airflow is drawn around the operator, through the hood opening and into the front grill of the cabinet, which provides personnel protection. In addition, the downward flow of HEPA-filtered air provides product protection by minimizing the chance of cross-contamination along the work surface of the cabinet. Because cabinet air has passed through the exhaust HEPA filter, it
Downflowing air stream just above the work surface. In the Type B cabinet, approximately 70% of the downflow air exits through the rear grill, passes through the exhaust HEPA filter, and is discharged from the building. The remaining 30% of the downflow air is drawn through the front grill. Since die air that flows to the rear grill is discharged into the exhaust system, activities that may generate hazardous chemical vapors or particulate should be conducted toward the rear of the cabinet.
Type B1 cabinets must be hard-ducted, preferably to their own dedicated exhaust system, or to a properly designed laboratory building exhaust. Blowers on laboratory exhaust systems should be located at the terminal end of the duct work. A failure in the building exhaust system may not be apparent to the user, as the supply blowers in the cabinet will continue to operate. A pressure-dependent monitor should be installed to sound an alarm and shut off the BSC supply fan, should failure in exhaust airflow occur. Since this feature is not supplied by all cabinet manufacturers, it is prudent to install a sensor in the exhaust system as necessary. To maintain critical operations, laboratories using Type B1 BSCs should connect the exhaust blower to the emergency power supply.
Class I1B2 The Class 11B2 BSC is a total-exhaust cabinet; no air is recirculated within it (Fig. 10.97). This cabinet provides simultaneous primary biological and chemical containment. The supply blower draws in room air or outside air at the top of the cabinet, passes it through a HEPA filter, and down into the work area of the cabinet. The building or cabinet exhaust system draws air through both the rear and front grills, capturing the supply air plus the additional amount of room air needed to produce a minimum calculated or measured inward face velocity of 0.5 in s~l. All air entering this cabinet passes through a HEPA filter, and perhaps some other air-cleaning device such as a carbon filter, and is exhausted to the outside. Exhausting as much as 0.5 m3 s_1 of conditioned room air makes this cabinet expensive to operate.
Class 1IB3 A Class IIB3 BSC (Fig. 10.98) is a ducted Type A cabinet having a minimum inward air velocity of 0.5 m s_1. All positive-pressure contaminated plenums within the cabinet are surrounded by a negative air pressure plenum. Thus, leakage in a contaminated plenum will be into the cabinet and not into the environment.
Class ITT The Class III biological safety cabinet (Fig. 10.99) was designed for work with microbiological agents demanding the highest biosafety level, and provides maximum protection to the environment and the worker. It is a gas-tight enclosure with a nonopening view window. Access for passage of materials into the cabinet is through a double-door passthrough box that can be decontaminated between uses. Reversing that process allows for safe removal of materials from the cabinet. Both supply and exhaust air are HEPA filtered. Exhaust air must pass through two HEPA filters, or a HEPA filter and an air incinerator, before discharge to the outdoors. Airflow is maintained by a dedicated independent exhaust system
Scribed in Section 10.2.3.6. A totally enclosed glove box with added cleaning of inlet and outlet air is a Class III BSC.
Horizontal laminar flow clean air benches are not BSCs (Section 10.3.4). They discharge HEPA-filtered air across the work surface and toward the user. These devices only provide product protection. They can be used for certain clean activities, such as the dust-free assembly of sterile equipment or electronic devices. These benches should never be used when handling potentially infectious materials. The worker can be exposed to materials on the clean bench. Horizontal clean air benches should never be used as a substitute for A Biological safety cabinet.
Vertical laminar flow clean benches also are not BSCs. They may be useful, for example, in hospital pharmacies when a clean area is needed. Although these units generally have a sash, the air is usually discharged into the room under the sash, resulting in the same potential problems as the horizontal laminar flow clean benches.
Specific Issues
The CDC-NIH document describes, in detail, the different uses of the different classes and types of BSCs and the type of protection (personnel, product, and environmental) each type provides. The document also provides a detailed comparison of filtration (air cleaning), airflow pattern (into the cabinet from the room or from the supply duct), and necessary performance tests (leak, velocity profile, differential pressure, etc.) for each type of BSC (see also Simons4 /).
BSCs should be designed to operate 24 hours per day, although energy’ conservation efforts may suggest BSC operation only when needed. The room air balance is of importance in operation of BSCs since room air is removed by ducted BSCs.
Design Equations and/or Parameters
No design parameters for flow rate and pressure loss have been found. The CDC-NIH document specifies the velocity through openings to the surroundings. Class I and IIA BSCs should have a velocity directed into the cabinet larger than 0.37 m s_1. All other Class II BSCs should have a minimum inward face velocity of 0.5 m s-1. Class III BSCs should have no connection to the surrounding work area and should maintain a negative pressure of at least 125 Pa, relative to the surrounding areas.
Most of the BSCs are connected to ducted exhaust systems and have HEPA filters for the supply and the exhaust air. The result is high pressure losses for the BSCs, even if low air flow rates are used. The filters must be handled with care, since much of the protection depends on the proper function of the filter(s).
10.4.6.5 Fume Cupboards with Auxiliary Air
General
Laboratory fume cupboards extract a significant volume of air. The replacement air has to be heated during the cold season and cooled (and
Sometimes dehumidified) during the hot season before being supplied to the room. One way to reduce the energy cost associated with fume cupboards is to supply air directly into the fume cupboard. This air, which is treated to a lesser extent than the room supply air, is callcd auxiliary air. This method of supplying air has some similarities to the distribution of air at workbenches (Section 10.4.6.2) and to the use of air curtains in an opening. In actual practice, these systems are somewhere between the other two systems. Use of one of the other two systems is recommended before using an auxiliary air fume cupboard. The main difference between an auxiliary air fume cupboard and a normal laboratory fume cupboard is the use of both exhaust and supply duct systems. This could result in higher initial costs at the time of construction, but should result in lower energy costs.
Principle
The principle of this system is to connect a supply duct to the fume cupboard and introduce the air along the upper edge of the front opening. The air should be supplied outside of the hood, although there are models with supply on the inside. The air is directed down across the front opening and all the air should flow into the opening. Sometimes air is introduced along the interior side walls of the cupboard and along and over the work surface. Both the air velocity and the flow rate must be low enough to prevent disturbances to the flow into the opening, but high enough to reduce the need for conditioned supply air. The air is commonly distributed through a plenum above the opening when the sash is open and is, by use of a valve, introduced directly into the fume cupboard when the sash is closed. This means that the normal bypass used when the sash is closed is replaced with a duct-connected air supply, which works similarly to the bypass when the sash is closed.
The principle for the auxiliary cupboard is shown in Fig. 10.100. Two other types are shown in Figs. 10.101 and 10.102.48
Applicability of Sources
This special type of laboratory fume cupboard is used for the same work as a normal laboratory fume cupboard (Section 10.2.3.3). There is some evidence to suggest that auxiliary air systems are better for walk-in fume cupboards49 and generally result in better protection than normal fume cupboards.50 However, several articles have been published which point out the inherent problems with an auxiliary cupboard.4,i’51
Different Forms and Boundaries Relative to Other Types
The main forms of this type of hood are shown in Figs. 10.100-10.102. The difference between this type of hood and workbenches is that for fume cupboards the auxiliary air is only a part of the total exhaust flow rate and the flow is not intended to cover the person standing in front of the exhaust opening. The differences between this type of hood and an air curtain are that the air curtain is thinner, has a higher velocity covering the whole opening, and
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FIGURE 10.100 Principle for auxiliary air supply to a laboratory fume hood.51 |
Normally has an exhaust slit in the lower part of the opening to keep the curtain plane over the opening. Auxiliary air fume cupboards look like ordinary laboratory fume cupboards with an added supply air outlet above the opening.
Specific Problems
Auxiliary air cupboards have many problems, most of which have been reported in the literature. A main problem that does not receive much attention is coordination of the flow into the opening controlled by the exhaust with the supply flow directed down immediately above the opening. This includes the complex and simultaneous relationships between velocities, flow rates, flow widths, flow directions, flow stability, turbulence, and temperatures. To this should be added the same problems that exist for normal fume cupboards, such as necessary exhaust flow rate and velocity, flow pattern inside the cupboard, working procedures, and the influence of people on the flow pattern outside and into the opening. The auxiliary air outlet may also be a source of noise.
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Aux. air supply Exhaust
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It is quite common to find comments on the problem with different temperatures, different flow rates, and supply flow stability. Most of the other problems seem not to have been investigated.
Comments on the temperature of the auxiliary air are common. Auxiliary cupboards were first introduced to save energy and unconditioned outside air has often been used for the auxiliary supply air. This is usually not acceptable because unconditioned supply air will be too cold during the heating season for a person to work at the hood. It may also be too hot during the cooling season. Today, most auxiliary air systems operate at temperatures close to room temperature.
Even with the use of conditioned auxiliary supply air, these systems have advantages over traditional fume cupboards. It may be possible to lower the
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FIGURE 10.102 Auxiliary air supplied along the interior side walls of the hood.51 |
Supply airflow rate to the room, to have a constant flow rate to the room, and to achieve better protection of the worker from the contaminants inside the cupboard. To achieve this better protection, it is necessary to have a high air velocity, which could disturb the airflow into the cupboard. This also put demands on design and maintenance.
Design Equations andfor Parameters
There are no specific design equations for this type of hood. Usually the exhaust flow rate is similar to the flow rate for ordinary fume cupboards. The different recommendations for auxiliary cupboards do not generally agree, most likely because all parameters influencing the performance have not been taken into account.
Albern51 recommends the auxiliary airflow rate be one-third to two-thirds of the exhaust flow rate through the cupboard.
Saunders49 recommends the auxiliary air have a uniform velocity distribution across the discharge area (±20%). The discharge area must be of a size that the air velocity does not exceed the face velocity of the fume cupboard by more than a factor of 1.2.
Fuller50 claims that an auxiliary cupboard has better performance than an ordinary cupboard, if properly designed and used. The auxiliary flow rate should be 50% to 75% of the total exhaust flow from the cupboard. Below 50% there is no beneficial effect and above 75% the auxiliary air will aspirate contaminants out of the cupboard. The auxiliary air should enter the hood through the upper one-half to two-thirds of the opening. This should fill the volume between the cupboard operator and the opening and assist in the containment. The auxiliary air should be distributed uniformly across the length of the cupboard for a vertically sliding sash or above only the open sash of a cupboard with a horizontally sliding sash. It should also have a temperature within ±1.5 °C of the room temperature and have a constant flow rate without pulsations.
According to Mitchell48 the auxiliary flow rate should be between 20% and 40% of the total exhaust flow rate from the cupboard: below 20% results in only small improvements and above 40%, leakage rises quite fast with increasing flow rate.
Based on literature reports and practical experience, the following recommendations are made. The auxiliary air supply should
Be stable and uniformly distributed air with temperature ±1.5 °C of the room temperature,
Have an airflow rate between 30% and 50% of the total exhaust air from the cupboard, and
Be directed vertically or slightly into the opening with a velocity at the upper end of the opening less than the velocity into the opening.
Meeting the final recommendation will dictate the area of the auxiliary air inlet. A common velocity through a fume cupboard opening is 0.5 m s-1. The upper end of the air supply opening is at about the same level as the head of an operator. People are quite sensitive to drafts (too high an air velocity or too low an air temperature or both) on the neck and on the back of the hands. For office work it is recommended that air velocities near a person be less than 0.3 m s_1 and preferably less than 0.15 m s_1 (see Chapter 5). Laboratory work is often similar to office work, and the design of the auxiliary air supply must be done with great care to prevent conditions that are detrimental to the flow into the cupboard or to the persons working in front of the cupboard.
The advantages and disadvantages of fume cupboards with auxiliary air are probably best summarized by Burgess:52
A well-designed auxiliary-air-supplied hood reduces air-conditioning costs and improves hood capture efficiency. The disadvantages of this hood include higher initial cost and maintenance due to the additional replacement air system, the possible introduction of airborne dust into the laboratory, discomfort
To operators during winter conditions unless the air is tempered, disturbance of the exhaust velocity profile at the hood face with poor canopy designs, and the possible hazard that exists in a poorly designed system if the exhaust tan tails and the auxiliary fan continues to operate. Although these objections can be resolved through appropriate system design, they do demonstrate the complexity of the auxiliary-air-supply system. One facility requirement that frequently cannot be met is the overhead space required for the air supply duct and the auxiliary air plenum.
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