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TRG-TRC014-EN 55
period four
Fan Modulation
notes
This “riding the fan curve” method of modulation can be used with any
centrifugal fan. However, it is most energy efficient when applied to forward-
curved (FC) fans. The configuration of the input power curves of the
FC fan are such that input power requirements drop as the fan operating point
moves upward along the constant-speed performance curve.
“Riding the fan curve” can be used successfully when the airflow modulation
range for the system is small. On the other hand, if the system is required to
modulate over a wide range of airflows, the increased static pressure
experienced at reduced airflows may overpressurize the VAV terminals,
resulting in greater-than-desired space airflow and noise problems.
Additionally, if the fan enters an unstable area of operation at this high static
pressure, fan vibration, called “surge”, and severe air pulsation within the duct
system, may result. This may lead to objectionable noise levels and potentially
a mechanical fan failure.
input power
input power
Forward-Curved Centrifugal Fan
A
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]
1
,
0
0
0
r
p
m
1
,
0
0
0
r
p
m
B
3.15 in. H
3.15 in. H
2
2
O
O
[784.6 Pa]
[784.6 Pa]
18,000 cfm
18,000 cfm
[8.5 m
[8.5 m
3
3
/s]
/s]
Figure 76