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Oregon State University

Fume Hood Types

Standard Fume Hood

A standard fume hood is a constant air volume (CAV) hood, an older, traditionally less elaborate hood design used for general protection of the worker. Because the amount of exhausted air is constant, the face velocity of a CAV hood is inversely proportional to the sash height. That is, the lower the sash, the higher the face velocity. CAV hoods can be installed with or without a bypass provision which is an additional opening for air supply into the hood.

Standard Hood
standard hood

Bypass Fume Hood

The bypass fume hood is an improved variation on the conventional fume hood. The bypass is located above the sash face opening and protected by a grille which helps to direct air flow. The bypass is intended to address the varying face velocities that create air turbulence leading to air spillage. The bypass limits the increase in face velocity as the sash nears the fully closed position, maintaining a relatively constant volume of exhaust air regardless of sash position.

bypass hood

Auxiliary Air Hood

This fume hood, sometimes referred to as a makeup air fume hood, was developed as a variation on the bypass fume hood and reduces the amount of conditioned room air that is consumed. The auxiliary fume hood is a bypass hood with the addition of directly ducted auxiliary air to provide unconditioned or partially conditioned outside makeup air. Auxiliary air hoods were designed to save heating and cooling energy costs, but increase the mechanical and operational costs due to the additional ductwork, fans, and air tempering facilities. Unless the volume (and therefore velocity) of auxiliary air is carefully adjusted, the air curtain created will affect the hood operation and may pull vapors out of the hood interior. Installation of this type of hood is not permitted at OSU since the disadvantages outweigh the benefits.

Aux Air hood

Last Update 01/04/2006