If the fan varies in speeds to keep the flow rate constant, then what is the function of the Pneumatic Damper Actuator?
This question is about the main building exhaust fans and not an individual fume hood exhaust fan. The main building exhaust fans are typically variable speed fans that are controlled to maintain a certain duct static pressure downstream of the fume hood. A laboratory will have multiple fume hoods tied into one main building exhaust fan system. Each fume hood has a damper in order to control the airflow leaving the hood as the sash is moved in order to maintain a constant face velocity. Each hood works independently from the main building exhaust system. The main building exhaust system is variable speed to compensate for what the fume hoods are doing (e.g., if all of the sashes are closed, then the fume hood dampers would be closed and the main building exhaust system would slow down in order to maintain its static pressure setpoint (less air is needed to be pulled from the fume hoods). As the sashes are opened, the fume hood dampers would open, the main exhaust static pressure would
