Filter
Top Products
52 TRG-TRC014-EN
notes
period four
Fan Modulation
To accommodate the variable-volume requirements imposed by a VAV system,
the system supply fan must be selected and controlled in such a manner that it
is capable of modulating over the required airflow range without entering an
unstable area of operation.
First, certain VAV system basics should be reviewed, including:
n The fan performance curve
n The system resistance curve
n The fan modulation curve
Fan Performance Curve
The fan performance curve of a centrifugal fan is a plot of the airflow
capacity at a given speed (rpm) versus the static pressure it generates. While a
centrifugal fan produces both static and velocity pressures, it is traditional to
VAV Systems
period four
Fan Modulation
Figure 71
Fan Performance Curve
airflow
airflow
static pressure
static pressure
1
,
0
0
0
r
p
m
1
,
0
0
0
r
p
m
24,000 cfm
24,000 cfm
[11.3 m
[11.3 m
3
3
/s]
/s]
2.7 in. H
2.7 in. H
2
2
O
O
[672.5 Pa]
[672.5 Pa]
Figure 72