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Prudent Practices in the Laboratory: Handling and Disposal of Chemicals
aware of all the design features of the hood. Assistance from an industrial hygienist, ventilation engineer, or laboratory consultant is recommended when deciding to purchase a fume hood.
8.C.6.1 General Design Recommendations
Laboratory fume hoods and the associated exhaust ducts should be constructed of nonflammable materials. They should be equipped with either vertical or horizontal sashes that can be closed. The glass within the sash should be either laminated safety glass that is at least 7/32 inch thick or other equally safe material that will not shatter if there is an explosion within the hood. The utility control valves, electrical receptacles, and other fixtures should be located outside the hood to minimize the need to reach within the hood proper. Other specifications regarding the construction materials, plumbing requirements, and interior design will vary, depending on the intended use of the hood. (See Chapter 6, sections 6.C.1.1 and 6.C.1.2.)
Although 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 most 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 hood, the concentration will pass through the upper explosive limit (UEL) and the LEL before being fully diluted at the outlet. Both the hood designer and the user should recognize this hazard and eliminate possible sources of ignition within the hood and its ductwork if there is a potential for explosion. The use of duct sprinklers or other suppression methods in laboratory fume ductwork is not necessary, or desirable, in the vast majority of situations.
8.C.6.2 Special Design Features
There have been two major improvements in fume hood design-airfoils and baffles-since the fume hood was invented. Both features should be included on any new fume hoods that are purchased.
Airfoils built into the fume hood at the bottom and sides of the sash opening significantly reduce boundary turbulence and improve capture performance. All fume hoods purchased should be fitted with airfoils.
When air is drawn through a hood without a baffle (see Figure 8.1), most of the air is drawn through the upper part of the opening, producing an uneven velocity distribution across the face opening. When baffles are installed, the velocity distribution is greatly improved. All fume hoods should have baffles. Adjustable baffles can improve hood performance and are desirable if the adjustments are made by an experienced industrial hygienist, consultant, or hood technician.
8.C.6.3 Fume Hood Airflow Types
The first fume 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 fume hood airflow design are listed below with their characteristics. Conventional hoods are the most common and include benchtop, distillation, and walk-in hoods of the constant air volume (CAV), variable air volume (VAV), bypass and non-bypass variety, with or without airfoils. Auxiliary air hoods and ductless fume hoods are not considered "conventional" and are used less often. Laboratory workers should know what kind of hood they are using and what its advantages and limitations are.
Constant Air Volume (CAV) Hoods
A constant air volume (CAV) fume hood draws a constant exhaust volume through the hood regardless of sash position. Because the volume is constant, the face velocity varies inversely with the sash position. The fume hood volume should be adjusted to achieve the proper face velocity at the desired working height of the sash, and then the hood should be operated at this height. (See section 8.C.4.)
Variable Air Volume (VAV) Hoods
A variable air volume (VAV) fume hood, also known as a constant velocity hood, is any hood that has been fitted with a face velocity control, which varies the amount of air exhausted from the fume 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 from the hood when it is closed. These types of hoods are usually of the non-bypass design to reduce air volume (see below).
A non-bypass hood has only one major opening through which the air may pass into the hood, that is, the sash opening. The airflow pattern of this type of hood is shown in Figure 8.2. A CAV non-bypass hood