Air Handling > Demand-Based Control of Laboratory Fume Hood Exhaust and Makeup

Existing Conditions

The laboratory fume hood exhaust system incorporates a xxx hp exhaust fan to draw xxx cfm from xxx fume hoods <incorporates a separate exhaust fan for each of xxx fume hoods, extracting approximately xxx cfm from each hood while in operation>. The fume hood users control air flow in a limited way by raising or lowering the glass sashes at the front of each fume hood. The fan runs <fans run> at full speed regardless of fume extraction requirements. A make-up air unit with a xxx hp fan replaces the exhausted air with heated or cooled outside air.

The exhaust and makeup air systems are designed for peak requirements, so they move more air than necessary most of the time.

Retrofit Conditions

Modern laboratory exhaust systems make use of digital automation systems to vary system capacity in response to requirements at the fume hoods. We recommend that the existing system be equipped with modern controls to minimize the exhaust and makeup air flows without compromising safe laboratory conditions.

Each fume hood is equipped with a special-purpose controller incorporating sash position sensing, airspeed sensing, and detection of operator presence directly in front of the hood. The controller modulates an air valve above the hood to throttle air flow in response to requirements. The exhaust fan<s> and makeup air fan would be equipped with either variable frequency drives or bypass arrangements to control air flow to the exact requirements indicated by the fume hoods.

Makeup air is modulated to track total exhaust flow and ensure a slight positive pressure in the laboratory at all times.

Further Benefits

With an overall reduction in air flow through the laboratory, temperature control in the space becomes easier. If the retrofit is well designed and commissioned, the conditioned space is likely to be more comfortable for occupants.