Trane TRG-TRC014-EN manual Fan Control Loop, VAV Fan Modulation, period four

period four

Fan Modulation

notes

Fan Control Loop

static-pressure

supplysensor fan

controller

Figure 78

To achieve this balance, a simple control loop is used. First, the static pressure is sensed from a particular location in the system. Second, a controller compares this static pressure reading and the set point of the system. Finally, the fan capacity is controlled to deliver the required airflow, at a static pressure that maintains this set point at the location of the system sensor.

VAV Fan Modulation

static pressure

sensor set point

actual system resistance curve

design system resistance curve

A

VAV system
modulation curve	B

airflow Figure 79

We will use an exaggerated example to illustrate this system operation. Assume that the load on the system decreases, causing all or part of the VAV terminal units to modulate closed. This causes the system resistance curve to shift upwards.

In response, the fan begins to “ride up” the constant-speed performance curve from the design operating point (A), trying to reach the balance point with this new system resistance curve. As a result, the fan delivers a lower airflow at a higher static pressure.

TRG-TRC014-EN 57