There are basically two types of chemical fumehoods: constant air volume and variable air volume.
- Constant Air Volume (CAV) - These fumehoods exhaust the same amount of air all the time, regardless of sash position. As the sash is lowered and raised, the velocity at the face of the hood changes. This change in face velocity can result in less than optimal hood performance.
- Variable Air Volume (VAV) - This fumehood, also known as a constant velocity hood, is a hood that has been fitted with a face velocity control, which varies the amount of air exhausted from the fumehood in response to the sash opening to maintain a constant face velocity. This allows for optimal hood performance regardless of the sash position. VAV hoods also provide significant energy savings by reducing the flow rate from the hood when the sash is closed.
A true VAV system ties the fumehood exhaust with the room's air supply and exhaust. The VAV system can then ensure that the required room air changes per hour are being met, that sufficient make-up air is provided to balance the air that's being exhausted, and that appropriate room temperatures are maintained.
A VAV system is often designed for a predicted fumehood usage that is less than all hoods being in use at the same time with their sashes open. This is frequently referred to as the “diversity factor” and allows for the VAV system to be downsized, e.g., the ductwork, pumps, and chillers. When a VAV system is designed using a “diversity factor” it relies on not having all the fumehoods in use at the same time. If all hoods are in use at the same time, i.e., their sashes are fully or partway open, some of the hoods will not perform properly because they won't be able to maintain an adequate face velocity. This can result in an unsafe condition for hood users. It is very important for VAV hood users to keep the sash closed as much as possible while working at the hood and to close the sash completely when the hood is not in use. This will ensure a safe work environment for all the hood users. Another very important characteristic and a benefit of keeping fumehood sashes closed, is that they are made of fire and explosion resistant glass. The sash acts as a barrier and can protect the fumehood user from any splashes, flying glassware, and fires that can occur in the hood.
The VAV fumehoods are also supplied with an alarm that alerts the user if the face velocity has fallen outside of a pre-determined range. If your alarm is activated, discontinue using the hood, close the sash, and call EHS at 512-471-3511.
- Always work in the hood with the sash only opened as much as necessary,
- Close the fumehood sash entirely when not working at the hood, and
- Call EHS at 512-471-3511 if the hood alarm is activated.