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ASHRAE TC Laboratory Classification Subcommittee ISBN (PDF) . and ASHRAE Laboratory Design Guide (ASHRAE ). This second edition of ASHRAE Laboratory Design Guide is a comprehensive reference manual for the planning, design, and operation of laboratories. It gives . ASHRAE LABORATORY DESIGN soundofheaven.info - Ebook download as PDF File .pdf ) or read book online.
Auxiliary air hoods and ductless chemical hoods are not considered conventional and are used less often. Effect of baffles on face velocity profile in a laboratory chemical hood. Clean rooms, on the other hand, are normally operated at a positive static pressure to prevent infiltration of particulates. These laboratories often support large or multiple teams and are configured with relocatable furnishings. Blocked or plugged air intakes and exhausts, as well as control system calibration and operation, alter the performance of the total ventilation system.
When multiple similar chemical hoods are installed at the same time, at least half should be tested, provided the design is standardized relative to location of doors and traffic, and to location and type of air supply diffusers. Thirty to fifty percent of a face velocity of fpm, for example, is 30 to 50 fpm, which represents a very low velocity that can be produced in many ways.
The rate of 20 fpm is considered to be still air because that is the velocity at which most people first begin to sense air movement. Most people walk at approximately fpm approximately 3 mph [4. If a person walks in front of an open chemical hood, the vortices can overcome the face velocity and pull contaminants into the vortex, and into the laboratory. Therefore, laboratory chemical hoods should not be located on heavily traveled aisles, and those that are should be kept closed when not in use.
Foot traffic near these chemical hoods should be avoided when work is being performed. This air usually enters the laboratory through devices called supply air diffusers located in the ceiling. Velocities that exceed fpm are frequently encountered at the face of these diffusers.
Normally, the effect is not as pronounced as the traffic effect, but it occurs constantly, whereas the traffic effect is transient. Relocating the diffuser, replacing it with another type, or rebalancing the diffuser air volumes in the laboratory can alleviate this problem. Exterior windows with movable sashes are not recommended in laboratories. Wind blowing through the windows and high-velocity vortices caused when doors open can strip contaminants out of the chemical hoods and interfere with laboratory static pressure controls.
Place hoods away from doors and heavy traffic aisles to reduce the chance of turbulence reducing the effectiveness of the hood. Laboratory chemical hoods should be regarded as safety devices that can contain and exhaust toxic, offensive, or flammable materials that form as a result of laboratory procedures.
Just as you should never flush laboratory waste down a drain, never intentionally send waste up the chemical hood. Do not use the chemical hood as a means of treating or disposing of chemical waste, including intentionally emptying hazardous gases from compressed gas cylinders or allowing waste solvent to evaporate. When checking if laboratory chemical hoods are performing properly, observe the following guidelines:.
Box 9. Many factors can compromise the efficiency of chemical hood operation, and most are avoidable. Be aware of all behavior that can, in some way, modify the chemical hood and its capabilities.
Keep laboratory chemical hoods and adjacent work areas clean and free of debris at all times. Keep solid objects and materials such as paper from entering the exhaust ducts, because they can lodge in the ducts or fans and adversely affect their operation. The chemical hood will have better airflow across its work surface if it contains a minimal number of bottles, beakers, and laboratory apparatus; therefore, prudent practice keeps unnecessary equipment and glassware outside the chemical hood at all times and stores all chemicals in approved storage cans, containers, or cabinets.
Furthermore, keep the workspace neat and clean in all laboratory operations, particularly those involving the use of chemical hoods, so that any procedure or experiment can be undertaken without the possibility of disturbing, or even destroying, what is being done.
Except when adjustments to the apparatus are being made, keep the chemical hood closed, with vertical sashes down and horizontal sashes closed, to help prevent the spread of a fire, spill, or other hazard into the laboratory. Horizontal sliding sashes should not be removed. The face opening should be kept small to improve the overall performance of the hood.
If the face velocity becomes excessive, the facility engineers should make adjustments or corrections. For chemical hoods without face velocity controls see section 9.
This range should be determined and marked during laboratory chemical hood testing. Do not raise the sash above the working height for which it has been tested to maintain adequate face velocity.
Doing so may allow the release of contaminants from the chemical hood into the laboratory environment. Chemical hood sashes may move vertically sash moves up and down , horizontally sash is divided in panes that move side to side to provide the opening to the hood interior , or a combination of both.
Although both types of sash offer protection from the materials within the hood and help control or maintain airflow, consider the following:. The laboratory chemical hood must provide adequate containment at that sash height.
Thus, the chemical hood must be tested in that position.
With the sash completely raised, it no longer provides a barrier between the chemical hood user and the materials within the hood.
If the only way to keep the sash in a fully raised position requires the use of a sash stop, the laboratory personnel may get into the habit of leaving the sash in this position, potentially reducing the safety and energy efficiency of the chemical hood. For chemical hoods with horizontal sashes, the intended operating configuration is to open the panes in such a way that at least one pane is between both arms, providing a barrier between the user and the contents of the chemical hood.
In addition,. Sash panes should be equal width with a maximum of 15 in. Sash panes and viewing panes constructed of composite material safety glass backed by polycarbonate, with the safety glass toward the explosion hazard are recommended for chemical hoods used when there is the possibility of explosion or violent overpressurization e. For all laboratory chemical hoods, the sash should be kept closed when the hood is not actively attended.
Lowering or closing the sash not only provides additional personal protection but also results in significant energy conservation.
Some chemical hoods may be equipped with automatic sash-positioning systems with counterweighting or electronic controls see section 9. Although turning laboratory chemical hoods off when not in use saves energy, keeping them on at all times is safer, especially if they are connected directly to a single fan. Because most laboratory facilities are under negative pressure, air may be drawn backward through the nonoperating fan, down the duct, and into the laboratory unless an ultralow-leakage backdraft damper is used in the duct.
If the air is cold, it may freeze liquids in the hood. The ducts are rarely insulated; therefore, condensation and ice may form in cold weather. When the chemical hood is turned on again and the duct temperature rises, the ice will melt, and water will run down the ductwork, drip into the hood, and possibly react with chemicals in the hood.
Chemical hoods connected to a common exhaust manifold offer the advantage that the main exhaust system is rarely shut down. Hence, positive ventilation is available on the system at all times. In a constant air volume CAV system see section 9.
Some laboratory chemical hoods on variable air volume VAV systems see section 9. The setback may be triggered by occupancy sensors, a light switch, or a timer or a completely lowered sash.
Understand what triggers the setback and ensure that the chemical hood is not used for hazardous operations when in setback mode. They should only be turned off when they are empty of hazardous materials. An example of an acceptable operation would be a teaching laboratory where the empty chemical hoods are turned off when the laboratory is not in use.
The OSHA lab standard includes a provision regarding laboratory chemical hoods, including a requirement for some type of continuous monitoring device on each chemical hood to allow the user to verify performance and routine testing of the hood. It does not specify a test protocol. They should pass the low- and high-volume smoke challenges with no leakage or flow reversals and have a control level of 0. The test includes several components, which may be used together or separately, including face velocity testing, flow visualization, face velocity controller response testing, and tracer gas containment testing.
These tests are much more accurate than face velocity and smoke testing alone. Performance should be evaluated against the design specifications for uniform airflow across the chemical hood face as well as for the total exhaust air volume. Equally important is the evaluation of operator exposure. The first step in the evaluation of hood performance is the use of a smoke tube or similar device to determine that the laboratory chemical hood is on and exhausting air.
The second step is to measure the velocity of the airflow at the face of the hood. The third step is to determine the uniformity of air delivery to the hood face by making a series of face velocity measurements taken in a grid pattern. Leak testing is normally conducted using a mannequin equipped with sensors for the test gas.
As an alternative, a person wearing the sensors or collectors may follow a sequence of movements to simulate common activities, such as transferring chemicals. It is most accurate to perform the in-place tests with the chemical hood at least partially loaded with common materials e. For the ASHRAE leak testing, the method calls for a release rate for the test gas of 4 liters per minute Lpm , but suggests that higher rates may be used. One-liter per minute release rate approximates pouring a volatile solvent from one beaker to another.
Eight liters per minute approximates boiling water on a W hot plate. The 4-Lpm rate is an intermediate of these two conditions. If there is a possibility that the chemical hood will be used for volatile materials under heating conditions, consider a higher release rate of up to 8 Lpm for worst-case conditions. If the laboratory chemical hood and the general ventilating system are properly designed, face velocities in the range of the design criteria will provide a laminar flow of air over the work surface and sides of the hood.
Higher face velocities fpm or more , which exhaust the general laboratory air at a greater rate, waste energy and are likely to degrade hood performance by creating air turbulence at the face and within the chemical hood, causing vapors to spill out into the laboratory Figure 9.
Laminar versus turbulent velocity profile. Velocity data are from a single traverse point on two separate hoods. An additional method for containment testing is the EN , which is the standard adopted by the European Union and replaces several other procedures that were in place for individual countries. Document the results in order to maintain a log showing the history of chemical hood performance. Laboratory personnel should request a chemical hood performance evaluation any time there is a change in any aspect of the ventilation system.
Thus, changes in the total volume of supply air, changes in the locations of supply air diffusers, or the addition of other auxiliary local ventilation devices e. Visually divide the face opening of a laboratory chemical hood into an imaginary grid, with each grid space being approximately 1 ft 2 in area. Using an anemometer, velometer, or similar device, take a measurement at the center of each grid space.
Face velocity readings should be integrated for at least 10 seconds 20 is preferable because of the fluctuations in flow. The measured velocity will likely fluctuate for several seconds; record the reading once it has stabilized.
Calculate the average of the velocity for every grid space. The resulting number is the average face velocity.
Such readings indicate the possibility of turbulent or nonlaminar airflow. Smoke tests will help confirm whether this is problematic. Traditional handheld instruments are subject to probe movement and positioning errors as well as reading errors owing to the optimistic bias of the investigator. Also, the traditional method yields only a snapshot of the velocity data, and no measure of variation over time is possible.
To overcome this limitation, take velocity data while using a velocity transducer connected to a data acquisition system and read continuously by a computer for approximately 30 seconds at each traverse point. If the transducer is fixed in place, using a ring stand or similar apparatus, and is properly positioned and oriented, this method overcomes the errors and drawbacks associated with the traditional method. The variation in data for a traverse point can be used as an indicator of turbulence, an important additional performance indicator that has been almost completely overlooked in the past.
Most laboratory chemical hoods are equipped with a baffle that has movable slot openings at both the top and the bottom, which should be moved until the airflow is essentially uniform.
Larger chemical hoods may require additional slots in the baffle to achieve uniform airflow across the face.
These adjustments should be made by an experienced laboratory ventilation engineer or technician using proper instrumentation. Effect of baffles on face velocity profile in a laboratory chemical hood.
One important factor to consider is acceptable sash position. However, one must understand how the chemical hood will be used to determine the range of sash positions needed. Anemometers and other instruments used to measure face velocity must be accurate in order to supply meaningful data.
Instruments should be calibrated at least once a year and the calibration should be National Institute of Standards and Technology traceable. If there is any concern that a laboratory chemical hood or other ventilation device may not provide enough protection to the trained laboratory personnel, it is prudent to measure worker exposure while the hood is being used for its intended purpose. By conducting personal air-sampling using traditional industrial hygiene techniques, worker exposure both excursion peak and time-weighted average can be measured.
The criterion for evaluating the hood should be the desired performance i. A sufficient number of measurements should be made to define a statistically significant maximum exposure based on worst-case operating conditions. Direct-reading instruments may be available for determining the short-term concentration excursions that may occur in chemical hood use.
When specifying a laboratory chemical hood for use in a particular activity, laboratory personnel should be aware of the design features. Assistance from an industrial hygienist, ventilation engineer, or laboratory consultant is recommended when deciding to purchase a chemical hood.
Construct laboratory chemical hoods and the associated exhaust ducts of nonflammable materials. Locate the utility control valves, electrical receptacles, and other fixtures outside the chemical hood to minimize the need to reach within the chemical hood proper.
Other specifications regarding the construction materials, plumbing requirements, and interior design vary, depending on the intended use. See Chapter 7 , sections 7. Although chemical hoods are most commonly used to control concentrations of toxic vapors, they can also serve to dilute and exhaust flammable vapors. Although theoretically possible, it is extremely unlikely even under worst-case scenarios that the concentration of flammable vapors will reach the lower explosive limit LEL in the exhaust duct.
However, somewhere between the source and the exhaust outlet of the chemical hood, the concentration will pass through the upper explosive limit and the LEL before being fully diluted at the outlet. Both the designer and the user should recognize this hazard and eliminate possible sources of ignition within the chemical hood and its ductwork if there is a potential for explosion. The use of duct sprinklers or other suppression methods in laboratory hood ductwork is not necessary or desirable.
Since the invention of the chemical hood, two major improvements have been made in the design—airfoils and baffles. Include both features on any new purchases. Airfoils built into the bottom and sides of the sash opening significantly reduce boundary turbulence and improve capture performance. Fit new chemical hoods with airfoils and retrofit any hoods without airfoils.
When air is drawn through a laboratory chemical hood without a baffle see Figure 9. All chemical hoods should have baffles. When baffles are installed, the velocity distribution is greatly improved.
Adjustable baffles can improve hood performance and are desirable if the adjustments are made by an experienced industrial hygienist, consultant, or technician. The first chemical hoods were simply boxes that were open on one side and connected to an exhaust duct.
Since they were first introduced, many variations on this basic design have been made. Six of the major variants in airflow design are listed below with their characteristics. Conventional laboratory chemical hoods are the most common and include benchtop, distillation, and walk-in hoods of the CAV, CAV bypass, nonbypass, and VAV, with or without airfoils.
Auxiliary air hoods and ductless chemical hoods are not considered conventional and are used less often. Trained laboratory personnel should know what kind they are using and what its advantages and limitations are. A CAV chemical hood draws a constant exhaust volume regardless of sash position. Because the volume is constant, the face velocity varies inversely with the sash position.
The laboratory chemical hood volume should be adjusted to achieve the proper face velocity at the desired working height of the sash, and the chemical hood should be operated at this height. A nonbypass chemical hood has only one major opening through which the air may pass, that is, the sash opening.
The airflow pattern is shown in Figure 9. A CAV nonbypass chemical hood has the undesirable characteristic of producing very large face velocities at small sash openings.
As the sash is lowered, face velocities may exceed 1, fpm near the bottom. Face velocities are limited by the leakage through cracks and under the airfoil and by the increasing pressure drop as the sash is closed.
Effect of sash placement on airflow in a nonbypass laboratory chemical hood. A common misconception is that the volume of air exhausted by this type of chemical hood decreases when the sash is closed.
Although the pressure drop increases slightly as the sash is closed, no appreciable change in volume occurs. All chemical hoods should be closed when not in use, because they provide a primary barrier to the spread of a fire or chemical release.
Many trained laboratory personnel are reluctant to close their CAV nonbypass chemical hoods because of the increase in air velocity and noise that occurs when the sash is lowered. This high-velocity air jet sweeping over the work surface often disturbs gravimetric measurements, causes undesired cooling of heated vessels and glassware, and can blow sample trays, gloves, and paper towels to the back of the laboratory chemical hood, where they may be drawn into the exhaust system.
Exercise care to prevent materials from entering the exhaust system where they can lodge in the ductwork, reducing airflow, or can be conveyed through the system and drawn into the exhaust fan and damage the fan or cause sparks.
Because of numerous operational problems with the design of nonbypass hoods, their installation in new facilities is discouraged. If present in existing facilities, their replacement should be considered. In many instances, the cost of replacement can be recouped from the resulting reduction in energy costs. A bypass chemical hood is shown in Figure 9. It is similar to the nonbypass design but has an opening above the sash through which air may pass at low sash positions.
But the face velocity stops increasing as the sash is lowered to the position where the bypass opening is exposed by the falling sash. The terminal face velocity of these types of hoods depends on the bypass area but is usually in the range of to fpm—significantly higher than the recommended operating face velocity. Therefore, the air volume for bypass laboratory chemical hoods should also be adjusted to achieve the desired face velocity at the desired sash height, and the hood should be operated at this position.
This arrangement is usually found in combination with a vertical sash, because this is the simplest arrangement for opening the bypass. Varieties are available for horizontal sashes, but the bypass mechanisms are complicated and may cause maintenance problems.
For a well-designed bypass hood, the face velocity will stay relatively constant until open about 12 in. Effect of sash placement on airflow in a bypass laboratory chemical hood. A VAV chemical hood, also known as a constant velocity hood, is one that has been fitted with a face velocity control, which varies the amount of air exhausted from the chemical hood in response to the sash opening to maintain a constant face velocity.
In addition to providing an acceptable face velocity over a relatively large sash opening compared to a CAV hood , VAV hoods also provide significant energy savings by reducing the flow rate when it is closed. These types of hoods are usually of the nonbypass design to reduce air volume see below. Even though the face velocity responds to the position of the sash, the face velocity may drop off as the sash height increases, depending on the design. As a result, there is a maximum sash height above which the chemical hood becomes less effective.
Quantitative tracer gas testing of many auxiliary air laboratory chemical hoods has revealed that, even when adjusted properly and with the supply air properly conditioned, significantly higher personnel exposure to the materials used may occur than with conventional non-auxiliary air chemical hoods.
They should not be purchased for new installations, and existing ones should be replaced or modified to eliminate the supply air feature. This feature causes a disturbance of the velocity profile and leakage of fumes into the personnel breathing zone. The auxiliary air chemical hood was developed in the s primarily to reduce laboratory energy consumption and is a combination of a bypass hood and a supply air diffuser located at the top of the sash.
Ideally, this unconditioned air bypasses the laboratory and significantly reduces air-conditioning and heating costs. In practice, however, many problems are caused by introducing unconditioned or slightly conditioned air above the sash, all of which may produce a loss of containment.
There are other design differences from a traditional chemical hood; thus, it is usually not possible to simply reduce the flow of a traditional hood to a lower face velocity and expect it to meet the same performance criteria as these specially designed hoods.
Like any other chemical hood, the design criteria and limitations need to be fully understood before one is selected for the laboratory. For example, if the chemical hood is designed to meet performance criteria at a sash height of 18 in.
Reviews by users have been mixed. For best results, be sure that the engineers, trained laboratory personnel, and the vendors understand how the chemical hoods are intended to be used. Their design and function continue to improve. Ductless laboratory chemical hoods are ventilated enclosures that have their own fan, which draws air out and through filters and ultimately recirculates it into the laboratory.
The filters are designed to trap vapors generated in the chemical hood and exhaust clean air back into the laboratory. They frequently use activated carbon filters, HEPA filters, or a combination of the two. Newer filter materials on the market claim that they capture a larger variety of chemicals.
These ventilated enclosures do not necessarily achieve the same level of capture and containment as a chemical hood. Unlike a conventional laboratory chemical hood, it is not possible to conduct tracer gas studies to measure containment even with the newer technology ductless hoods. Because the collection efficiency of the filters decreases over time, the filters must be monitored and replaced routinely. Depending on the materials and the laboratory environment, chemicals can desorb from the filter and reenter the laboratory over time.
Ductless chemical hoods have extremely limited applications and should be used only where the hazard is very low, where the access to the hood and the chemicals used in it are carefully controlled, and under the supervision of a laboratory supervisor who is familiar with its serious limitations. If these limitations cannot be accommodated, do not use this type of device. The benefits of recirculating chemical hoods are that they are much more energy efficient than a ducted chemical hood and they do not require a ventilation system that relies on a fan on the roof or upper levels.
Some urban buildings retrofitted with laboratories on lower floors, buildings with limitations on the ventilation system or laboratories with minor chemical use have successfully used these ductless hoods, under the limited conditions cited above and with rigorous filter maintenance programs.
They can also be used for control of particulate material where a chemical hood or even Class 1 or 2 biosafety cabinets provide too much turbulent air see section 9. To determine whether recirculating hoods are appropriate, an industrial hygienist or safety professional should conduct a risk assessment that includes. Individuals using recirculating hoods need training on the use and limitations of the recirculating hood. Each ductless chemical hood should have signage explaining the limitations, how to detect whether the filter media are working, and the filter maintenance schedule.
As the name implies, a benchtop chemical hood sits on a laboratory bench with the work surface at bench height. The sash can be a vertical-rising or a horizontal-sliding type or a combination of the two. Normally, the work surface is dished or has a raised lip around the periphery to contain spills. Sinks in chemical hoods are not recommended because they encourage laboratory personnel to dispose of chemicals in them.
If they must be used, to drain cooling water from a condenser, for instance, they should be fitted with a standpipe to prevent chemical spills from entering the drain. The condenser water drain can be run into the standpipe. Spills will be caught in the cupsink by the standpipe for later cleanup and disposal.
A lip on the cupsink could be used as an alternative to a standpipe to prevent spills from getting into the sink. A typical benchtop chemical hood is shown in Figure 9.
The distillation hood is similar to the benchtop hood except that the work surface is closer to the floor to allow more vertical space inside for tall apparatuses such as distillation columns.
A typical distillation hood is shown in Figure 9. A walk-in hood stands on the floor of the laboratory and is used for very tall or large apparatus. The sash can be either horizontal or double- or triple-hung vertical. These hoods are usually of the nonbypass type. Once past the plane of the sash, the personnel are inside with the chemicals.
If the personnel are required to enter the hood during operations where hazardous chemicals are present, they should wear PPE appropriate for the hazard.
It may include respirators, chemical splash goggles, rubber gloves, boots, suits, and self-contained breathing apparatus. A typical walk-in chemical hood is shown in Figure 9. The California chemical fume hood is a ventilated enclosure with a movable sash on more than one side.
They are usually accessed through a horizontal sliding sash from the front and rear. They may also have a sash on the ends. Because their configuration precludes the use of baffles and airfoils, they may not provide a suitable face velocity distribution across their many openings. A ventilated enclosure is any site-fabricated chemical hood designed primarily for containing processes such as scale-up or pilot plant equipment.
Most do not have baffles or airfoils, and most designs have not had the rigorous testing and design refinement that conventional mass-produced chemical hoods enjoy. Working at the opening of the devices, even when the plane of the opening has not been broken, may expose personnel to higher concentrations of hazardous materials than if a conventional hood were used.
The perchloric acid laboratory chemical hood, with its associated ductwork, exhaust fan, and support systems, is designed especially for use with perchloric acid and other materials that can deposit shock-sensitive crystalline materials in the hood and exhaust system. These materials become pyrophoric when they dry or dehydrate see also Chapter 6 , section 6. Special water spray systems are used to wash down all interior surfaces of the hood, duct, fan, and stack, and special drains are necessary to handle the effluent from the washdown.
The liner and work surface are usually stainless steel with welded seams. Perchloric acid hoods have drains in their work surface. Water spray heads are usually installed in the top, behind the baffles, and in the interior. The water spray should be turned on whenever perchloric acid is being heated in the chemical fume hood. The ductwork should be fabricated of plastic, glass, or stainless steel and fitted with spray heads approximately every 10 ft on vertical runs and at each change in direction.
The fan and stack should be fabricated of plastic, fiberglass, or stainless steel. Welded or flanged and gasketed fittings to provide airtight and watertight connections are recommended. Avoid horizontal runs because they inhibit drainage, and the spray action is not as effective on the top and sides of the duct. Any washdown piping, which is located outside must be protected from freezing.
A drain and waste valve on the water supply piping that allows it to drain when not in use is helpful. Route the drain lines carefully to prevent the creation of traps that retain water. Write special operating procedures to cover the washdown procedure for these types of hoods. The exhaust from a perchloric acid hood should not be manifolded with that from other types of chemical hoods.
Design chemical hoods used for work with radioactive sources or materials so that they can be decontaminated completely on a regular basis. A usual feature is a one-piece stainless steel welded liner with smooth curved corners that can be cleaned easily and completely. The superstructure of radioisotope hoods is usually made stronger than that of a conventional hood to support lead bricks and other shielding that may be required. Special treatment of the exhaust from radioisotope hoods may be required by government regulations to prevent the release of radioactive material into the environment.
This treatment usually involves the use of HEPA filters see section 9. Another practical way to handle radioactive materials that require special exhaust treatment is to use a containment chamber within a traditional chemical hood. Several safety supply companies offer portable disposable glovebag containment chambers with sufficient space to conduct the work and then dispose of them in accordance with applicable nuclear regulatory standards. Chemical hoods in clean rooms are generally no different than traditional chemical hoods, except that they are usually made of polypropylene or thermoplastics.
Some have hinged sashes rather than sliding sashes. Most require separate chemical hoods for acid work and solvent work. Polypropylene hoods burn easily, melt quickly, and may become fully involved in a fire. There are fire-retardant polypropylene and other thermoplastics available, but they cost more. Alternatively, an automatic fire extinguisher may be installed inside.
Until recently, treatment of laboratory chemical hood exhausts has been limited. Because effluent quantities and concentrations are relatively low compared to those of other industrial air emission sources, their removal is technologically challenging.
And the chemistry for a given chemical hood effluent can be difficult to predict and may change over time. Nevertheless, legislation and regulations increasingly recognize that certain materials in laboratory chemical hoods may be sufficiently hazardous that they can no longer be expelled directly into the air.
Therefore, the practice of removing these materials from exhaust streams will become increasingly more prevalent. A number of technologies are evolving for treating chemical hood exhaust by means of scrubbers and containment removal systems.
Whenever possible, experiments involving toxic materials should be designed so that they are collected in traps or scrubbers rather than released. If for some reason collection is impossible, HEPA filters are recommended for highly toxic particulates. Liquid scrubbers may also be used to remove particulates, vapors, and gases from the exhaust system.
None of these methods, however, is completely effective, and all trade an air pollution problem for a solid or liquid waste disposal problem. Incineration may be the ultimate method for destroying combustible compounds in exhaust air, but adequate temperature and dwell time are required to ensure complete combustion.
Incinerators require considerable capital to build and energy to operate; hence, other methods should be studied before resorting to their use. Determine the optimal system for collecting or destroying toxic materials in exhaust air on a case-by-case basis. Treatment of exhaust air should be considered only if it is not practical to pass the gases or vapors through a scrubber or adsorption train before they enter the exhaust airstream.
Also, if an exhaust system treatment device is added to an existing chemical hood, carefully evaluate the impact on the fan and other exhaust system components. These devices require significant additional energy to overcome the pressure drop they add to the system. All of the titles in the well-received and highly poplular Advanced Energy Design Guide series are available for free download.
To learn more:. The table describes criteria, design attributes, and operating specifications associated with each LVDL. The purpose of the LVDL classification system and Table A are to foster communication between stakeholders when evaluating the protective capability of an existing LACS or during discussions about design, construction, and operation of a new LACS intended to manage and control airborne chemical hazards.
Using this Guide will help achieve cost-effective and cost-efficient refrigeration systems for new projects, expansions, remodels, and existing systems that simply need a tune-up. Department of Energy. The reader service card provides the opportunity to request information from new products chosen by editors as well as advertised products in each issue.
Using common information in the form of a building model for all analysis related to building performance is critical to ensure a holistic, manageable, and verifiable result. Building Information Modeling is focused on eliminating significant amounts of redundant and wasted effort currently embedded in the design, construction, and operations of facilities due to the lack of software interoperability.
BIM Guide. Summary "The book covers topics such as exhaust hoods, primary air systems, process cooling, air treatment, exhaust stack design, airflow patterns and system balancing, energy recovery, the laboratory commissioning process, and the economics of both initial and life-cycle costs. A dedicated chapter gives guidance on laboratories that specialize in biological containment and animal research, addressing envelope design, system reliability, redundancy, proper space pressurization, biohazard containment and control, product protection, and sanitation.
Updated to reflect current standards and industry practices, this second edition also adds two new chapters: Bibliographic information. Publication date Copyright date Note Revised edition of: McIntosh, Chad B.
Dorgan, Charles E.
ISBN electronic bk. Librarian view Catkey: