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TRG-TRC013-EN 43
period three
Fan Capacity Control
notes
An exaggerated example is used to illustrate this system operation. Assume
that the load on the system decreases, causing all or some 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” its constant-speed
performance curve toward B, from the design operating point A, trying to
balance with this new system resistance curve. As a result, the fan delivers a
lower airflow at a higher static pressure. The system static-pressure controller
senses this higher static pressure and sends a signal to the supply fan to reduce
its capacity. Modulating the fan capacity results in a new fan – system balance
point C, bringing the system static pressure down to the sensor’s set point.
This action results in the fan unloading along a curve, the VAV system
modulation curve. This curve represents the fan modulation needed to
balance the static pressure required to offset these variable system losses
(demand) with that produced by the fan (supply). The equation used to
calculate the VAV system modulation curve is:
where,
■ Static Pressure and Airflow are points along the VAV system modulation
curve
■ Static Pressure
d
and Airflow
d
describe the design operating point
■ Static Pressure
c
is the system static-pressure control set point
VAV System Modulation Curve
$
&
design system
design system
resistance curve
resistance curve
airflow
airflow
static pressure
static pressure
sensor
sensor
set point
set point
VAV system
VAV system
modulation curve
modulation curve
%
Figure 64
Static Pressure (SP)
Airflow
Airflow
d
-----------------------
2
SP
d
SP
c
–()× SP
c
+=