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40 TRG-TRC013-EN
notes
period three
Fan Capacity Control
This modulation causes the actual system resistance curve to shift.
In a VAV system, therefore, the fan no longer operates at a single point on its
performance curve but must operate over a range of such points.
“Riding the Fan Curve”
The simplest form of fan capacity control is called “riding the fan curve.” This
method involves no direct form of control but simply allows the fan to react to
the change in system static pressure.
During operation, a VAV system experiences changes in resistance as the VAV
terminal units modulate closed. This increases the system resistance, creating a
new actual system resistance curve. In response, the operating point of the
constant-speed fan simply “rides up” on its performance curve from the design
operating point A to balance the new system condition. This new operating
point B is at a lower airflow and a higher static pressure.
airflow
airflow
static pressure
static pressure
surge
region
surge
surge
region
region
design system
resistance curve
design system
design system
resistance curve
resistance curve
modulation
modulation
range
range
actual system
resistance curve
actual system
actual system
resistance curve
resistance curve
VAV System
Figure 60
"Riding the Fan Curve"
static pressure
static pressure
increase
increase
airflow
airflow
reduction
reduction
%
airflow
airflow
static pressure
static pressure
surge
line
surge
surge
line
line
$
design system
resistance curve
design system
design system
resistance curve
resistance curve
actual system
resistance curve
actual system
actual system
resistance curve
resistance curve
Figure 61