the nature of the materials that must be controlled compared to the filter media provided with the recirculating hood.

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.

9.C.2.10 Laboratory Chemical Hood Configurations

9.C.2.1 0.1 Benchtop Laboratory Chemical Hoods

As the name implies, a benchtop chemical hood sits on a laboratory bench with the work surface at bench height. It can be of the CAV or VAV variety and can have a bypass or nonbypass design. 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 9.7.

9.C.2.10.2 Distillation (Knee-High or Low-Boy) Chemical Fume Hoods

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 9.8.

9.C.2.10.3 Walk-In Laboratory Chemical Hoods

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. The word “walk-in” is a misnomer; one should never actually walk into a chemical hood when it is operating and contains hazardous chemicals. 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,


FIGURE 9.7 Diagram of a typical benchtop laboratory chemical hood.

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