FIGURE 8.3 Bypass fume hood.

not in use, however. A closed hood provides a primary barrier to the spread of a fire or chemical release.

Many workers are reluctant to close their CAV non-bypass hoods because of the increase in air velocity and noise that occurs when the sash is lowered. This high-velocity air jet sweeping over the work surface often disturbs gravimetric measurements, causes undesired cooling of heated vessels and glassware, and can blow sample trays, gloves, and paper towels to the back of the hood, where they may be drawn into the exhaust system. Care should be exercised to prevent materials from entering the exhaust system. They can lodge in the ductwork, reducing airflow to the hood, or can be conveyed through the system and drawn into the exhaust fan and damage the fan or cause sparks. Variable-volume non-bypass hoods avoid these undesirable features by maintaining the velocity at a constant rate except at very low sash positions, where it may increase to a few hundred feet per minute. Hoods with horizontal sashes are usually of the non-bypass variety, because this arrangement is more difficult to connect to the bypass mechanism.

A bypass fume hood is shown in Figure 8.3. It is similar to the non-bypass design but has an opening above the sash through which air may pass at low sash positions. Because the opening is usually 20 to 30% of the maximum open area of the sash, this hood will still exhibit the increasing velocity characteristic of the non-bypass hood as the sash is lowered. But the face velocity stops increasing as the sash is lowered to the position where the bypass opening is exposed by the falling sash. The terminal face velocity of these types of hoods depends on the bypass area but is usually in the range of 300 to 500 fpm—significantly higher than the recommended operating face velocity. Therefore, the air volume for bypass hoods should also be adjusted to achieve the desired face velocity at the desired sash height, and the hood should be operated at this position. This arrangement is usually found in combination with a vertical sash, because this is the simplest arrangement for opening the bypass. Varieties are available for horizontal sashes, but the bypass mechanisms are complicated and may cause maintenance problems.

Quantitative tracer gas testing of many auxiliary air fume hoods has revealed that, even when adjusted properly and with the supply air properly conditioned, significantly higher worker exposure to the materials used in the hood may occur than with conventional (non-auxiliary air) hoods. Auxiliary air hoods should not be purchased for new installations, and existing auxiliary air hoods should be replaced or modified to eliminate the supply air feature of the hood. This feature causes a