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TRG-TRC013-EN 45
period three
Fan Capacity Control
notes
As the VAV terminal units modulate shut, the system resistance curve shifts
upward. 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 discharge dampers, instructing them to begin closing. This
results in a build-up of static pressure at the fan outlet and causes the fan to
“ride up” further on its performance curve until it reaches its new operating
point C at a higher static pressure and lower airflow. The system balances at D
along the desired VAV system modulation curve, bringing the system static
pressure (downstream of the discharge damper) down to its set point.
This method of control is essentially the same as “riding the fan curve,” except
that the static-pressure drop takes place across the discharge damper instead of
across the VAV terminal units. While discharge dampers can be used with all
types of centrifugal fans, they are most effectively used with the FC fan for the
same reason mentioned with “riding the fan curve.” Other methods of supply
fan capacity control are more energy efficient, so discharge dampers are rarely
used.
Discharge Dampers
discharge
discharge
damper
damper
SP loss
SP loss
airflow
airflow
static pressure
static pressure
$
'
design system
design system
resistance curve
resistance curve
&
surge
line
surge
surge
line
line
%
VAV system
VAV system
modulation
modulation
curve
curve
sensor
sensor
set point
set point
Figure 67