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Chemical Fume Hoods

Constant Volume Hoods

Conventional hoods represent the original and most simple of the hood design styles. With a conventional hood the volume of air exhausted is constant, regardless of sash height. As the sash is lowered the opening area decreases, resulting in an increase in face velocity. Since face velocity changes dramatically with sash position it is particularly important when working with conventional hoods to maintain the sash at its optimal height as indicated by the yellow sticker attached to the hood frame. Optimal sash height represents the point where face velocity equals 100 fpm.

Variable Air Volume Hoods

Variable air volume (VAV) hoods are the most sophisticated of the hood types, requiring technically proficient design, installation and maintenance. The primary characteristic of VAV hoods is their ability to maintain a constant face velocity as sash height changes. Sash height is continuously monitored and total fan exhaust volume adjusted so that the average face velocity is maintained within acceptable parameters.

Auxiliary Air Hoods

With this type of hood a dedicated duct supplies outside air to the face of a bypass hood. The main advantage of an auxiliary air hood is the energy savings realized by reducing the amount of heated or air conditioned room air exhausted by the hood. While energy savings can be substantial, the unconditioned air flow can cause discomfort for those working near the hood. It is important to keep in mind, however, that the auxiliary air supply is necessary for proper functioning of the hood.

Any alteration of the air supply system such as sealing off the auxiliary air duct will adversely affect hood operation and may result in hazardous chemical exposures. If the sash of an auxiliary air hood is kept closed most of the unconditioned air will bypass through the hood, reducing its effect on room temperature and humidity. Remember to check the optimum sash height since it will affect face velocity in a manner similar to that for bypass hoods.


2017-09-13T14:58:10.893-05:00 2017
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