g
Fan Control Loop
static-pressure
supplysensor fan
controller
g IF
LtrpKe
°
A VAV system’s static-pressure requirement consists of a fixed component and a variable component. The system requires a minimum amount of static pressure to properly operate the VAV modulation devices and diffusers. This is considered the fixed component. The second, variable component is the amount of static pressure required to overcome the system pressure losses due to the ducts, fittings, dampers, coils, filters, etc. at various airflows. Recall from the discussion of the system resistance curve that these losses vary with changes to the system airflow.
To ensure adequate static pressure at the VAV terminal units, a simple control loop is used. First, the static pressure is sensed from a location in the system. Next, a controller compares this static-pressure reading to the system’s set point. Finally, the fan capacity is varied to deliver the required airflow at a static pressure that maintains this set point at the location of the system’s sensor.
