Specific systems serve isotope hoods, perchloric acid hoods, or other high-hazard sources that require isolation from the general laboratory exhaust systems.

A general ventilation system that gives 6 to 12 room air changes per hour is normally adequate. More airflow may be required to cool laboratories with high internal heat loads, such as those with analytical equipment, or to service laboratories with large specific exhaust system requirements. In all cases, air should flow from the offices, corridors, and support spaces into the laboratories. All air from chemical laboratories should be exhausted outdoors and not recirculated. Thus, the air pressure in chemical laboratories should be negative with respect to the rest of the building unless the laboratory is also a clean room (see section 8.E.2). The outside air intakes for a laboratory building should be in a location that reduces the possibility of re-entrainment of laboratory exhaust or contaminants from other sources such as waste disposal areas and loading docks. (See Chapter 7, section 7.B.6.4, for further information.)

Although the supply system itself provides dilution of toxic gases, vapors, aerosols, and dust, it gives only modest protection, especially if these impurities are released into the laboratory in any significant quantity. Operations that can release these toxins, such as running reactions, heating or evaporating solvents, and transfer of chemicals from one container to another, should normally be performed in a fume hood. Toxic substances should be stored in cabinets fitted with an exhaust device. Likewise, laboratory apparatus that may discharge toxic vapors, such as vacuum pump exhausts, gas chromatograph exit ports, liquid chromatographs, and distillation columns, should vent to an exhaust device such as a snorkel.

The steady increase in the cost of energy in recent years, coupled with a greater awareness of the risks associated with the use of chemicals in the laboratory, has caused a conflict between the desire to minimize the costs of heating, cooling, humidifying, and dehumidifying laboratory air and the need to provide laboratory workers with adequate ventilation. However, cost considerations should never take precedence over ensuring that workers are protected from hazardous concentrations of airborne toxic substances.

8.C.11.1 Supply Systems

Well-designed laboratory air supply systems approach the ideal condition of laminar airflow, directing clean incoming air over laboratory personnel and sweeping contaminated air away from their breathing zone. Ventilation systems with well-designed diffusers that optimize "complete mixing" may also be satisfactory. Usually, several carefully selected supply air diffusers are used in the laboratory. Air entry through perforated ceiling panels may also successfully provide uniform airflow, but proper air distribution above the plenum is required. The plenum, diffuser, or perforated ceiling panels must be kept free of obstructions in order for the supply system to function properly.

8.C.11.2 Constant Air Volume

Constant air volume (CAV) air supply systems are the traditional design standard for laboratories. This method assumes constant exhaust and supply airflow rates through the laboratory. Although such systems are the easiest to design, and in some cases the easiest to operate, they have significant drawbacks due to their high energy consumption and limited flexibility. Classical CAV design assumes that all fume hoods operate 24 hours/day, 7 days/week, and at constant maximum volume. Adding, changing, or removing fume hoods or other exhaust sources for CAV systems requires rebalancing the entire system to accommodate the changes. Most CAV systems in operation today are unbalanced and operate under significant negative pressure. These conditions are caused by the inherent inflexibility of this design type, coupled with the addition of fume hoods not originally planned.

8.C.11.3 Variable Air Volume

Variable air volume (VAV) laboratories are rapidly replacing traditional CAV laboratories as the design standard. These systems are based on fume hoods with face velocity controls. As the users operate the fume hoods, the exhaust volume from the laboratory changes and the supply air volume must adapt to maintain a volume balance and room pressure control. An experienced laboratory ventilation engineer must be consulted to design these systems, because the systems and controls are complex and must be designed, sized, and matched so they operate effectively together.

8.C.12 Exhaust Systems

8.C.12.1 Individual Hood Fans

In traditional exhaust systems, each fume hood has its own exhaust fan. This arrangement has the following disadvantages and advantages: There is no way to dilute the fume hood effluent before release. The possibility of cross-contamination from one fume



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