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TRG-TRC014-EN 59
period four
Fan Modulation
notes
As the VAV terminal units modulate shut, the system resistance curve shifts
upwards. 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.
The system static pressure controller senses this higher pressure and sends a
signal to the discharge dampers, instructing them to begin closing. This has no
effect on the performance curve for the fan, but it does add a new static-
pressure loss to the system. This moves the system resistance curve back down
to where the system balances at an operating point along the desired fan
modulation curve (B). This brings the system static pressure back down to its
set point.
Other methods are more energy efficient, so discharge dampers are rarely used
for supply fan modulation.
Discharge Dampers
airflow
airflow
static pressure
static pressure
A
B
design system
design system
resistance curve
resistance curve
actual system
actual system
resistance curve
resistance curve
discharge
discharge
damper
damper
SP loss
SP loss
Figure 82