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42 TRG-TRC013-EN
notes
period three
Fan Capacity Control
Because of this issue, and since many VAV systems are large with high static
pressures, some form of system static-pressure control is generally used.
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.
Fan Control Loop
static-pressure
static-pressure
sensor
sensor
controller
controller
supply
supply
fan
fan
Figure 63