In a laboratory exhaust system, there are many reasons for keeping the total exhaust flow constant. For example, if the total flow is reduced, the air may not exit the roof line with enough momentum to be propelled above the fresh air intakes. Also, exhaust cleaning equipment like wet scrubber functions best if the air velocity through the contact bed is constant.
So fume hoods are designed with controls to keep the exhaust air flow constant, in spite of changes in the height of the sash. One method of keeping the total flow constant is a bypass grill arranged. As the sash is lowered, it exposes the bypass grill so that air can flow into the hood above the sash, keeping air flow relatively