FIGURE 8.1 Effect of baffles on face velocity profile.

FIGURE 8.2 Non-Bypass fume hood.

has the undesirable characteristic of producing very large face velocities at small sash openings. As the sash is lowered, face velocities may exceed 1,000 fpm near the bottom. Face velocities are limited by the leakage of the hood through cracks and under the airfoil and by the increasing pressure drop as the sash is closed. A common misconception is that the volume of air exhausted by this type of hood decreases when the sash is closed. Although the pressure drop through the hood increases slightly as the sash is closed, no appreciable change in volume occurs. This does not mean that these fume hoods should not be closed when