ing and periodically as part of a recommissioning or maintenance program.

Once a chemical hood is tested and determined to be acceptable via the ASHRAE/ANSI 110 method or an equivalent means, the face velocity should be noted and used as the reference point for routine testing. Each chemical hood, laboratory, facility, or site must define the acceptable average face velocity, minimum acceptable point velocity, and maximum standard deviation of velocities, as well as when ASHRAE/ANSI 110 or visualization testing is required. These requirements should be incorporated into the laboratory’s Chemical Hygiene Plan and ventilation system management plans (see section 9.H).

When first installed and balanced, a laboratory chemical hood must be subjected to the ASHRAE/ANSI 110 or equivalent test before it is commissioned. 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.

9.C.2.2 Factors That Affect Laboratory Chemical Hood Performance

Tracer gas containment testing of chemical hoods reveals that air currents impinging on the face at a velocity exceeding 30 to 50% of the face velocity reduce the containment efficiency by causing turbulence and interfering with the laminar flow of the air entering the chemical hood. Thirty to fifty percent of a face velocity of 100 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.

9.C.2.2.1 Proximity to Traffic

Most people walk at approximately 250 fpm (approximately 3 mph [4.8 kph]) and as they walk, vortices exceeding 250 fpm form behind them. 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.

9.C.2.2.2 Proximity to Supply Air Diffusers

Air is supplied continuously to laboratories to replace the air exhausted through laboratory chemical hoods and other exhaust sources and to provide ventilation and temperature/humidity control. This air usually enters the laboratory through devices called supply air diffusers located in the ceiling. Velocities that exceed 800 fpm are frequently encountered at the face of these diffusers. If air currents from these diffusers reach the face of a chemical hood before they decay to 30 to 50% of the face velocity, they cause the same effect as air currents produced by a person walking in front of the chemical hood. 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.

9.C.2.2.3 Proximity to Windows and Doors

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.

9.C.2.3 Prevention of Intentional Release of Hazardous Substances into Chemical Hoods

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.

For some operations, condensers, traps, and/or scrubbers are recommended or necessary to contain and collect vapors or dusts to prevent the release of harmful concentrations of hazardous materials from the chemical hood exhaust.

9.C.2.4 Laboratory Chemical Hood Performance Checks

When checking if laboratory chemical hoods are performing properly, observe the following guidelines:

   Evaluate each hood before initial use and on a regular basis (at least once a year) to visualize airflow and to verify that the face velocity meets the criteria specified for it in the laboratory’s Chemical Hygiene Plan or laboratory ventilation plan.

   Verify the absence of excessive turbulence (see section 9.C.2.6, below).



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