sash positions needed. For example, if the users will need to sometimes use the hood with vertical sash fully open, then the test criteria should be for 100% sash opening.
It may be prudent to set the acceptance criteria with the sash 100% open and 80% open, ensuring adequate containment at both of these positions.
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.
9.C.2.8.7 Additional Exposure Monitoring
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.e., does the hood contain vapors and gases at the desired worker-exposure level?). 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.
9.C.2.9 Laboratory Chemical Hood Design and Construction
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.
9.C.2.9.1 General Design Recommendations
Construct laboratory chemical hoods and the associated exhaust ducts of nonflammable materials. Equip them with vertical, horizontal, or combination vertical/horizontal sashes that can be closed. For the glass within the sash, use either laminated safety glass that is at least 7/32-in. thick or other equally safe material that will not shatter if there is an explosion inside. 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.C.1.1 and 7.C.1.2) Information regarding the minimum flow rate through hoods can be found in ANSI Z9.5.
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.
9.C.2.9.2 Special Design Features
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 9.4), most of the air is drawn through the upper part of the opening, producing an uneven velocity distribution across the face opening. 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.
9.C.2.9.3 Laboratory Chemical Hood Airflow Types
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.