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TRG-TRC014-EN 63
period four
Fan Modulation
notes
In response to reduced airflow and rising system static pressure, the variable-
pitch vaneaxial fan responds similarly to a centrifugal fan with either inlet-vane
or variable-speed capacity control.
By changing the angle of the fan blades, the performance curve for the fan
shifts downward, and the airflow and resulting pressure decrease until the
operating point (B) balances along the fan modulation curve.
These curves describe the performance characteristics of each fan modulation
method, in terms of the fan power required versus the percent of design
airflow. Realize that these are generalized curves based on an arbitrary set of
test conditions.
On a generalized basis, the forward-curved (FC) centrifugal fan with inlet vanes,
the variable-pitch vaneaxial fan, and fan speed control are similar in
performance.
To obtain a more accurate comparison for specific operating conditions, a life-
cycle cost analysis should be performed.
Fan Modulation Comparisons
100
100
90
90
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
0
10
10
20
20
30
30
40
40
50
50
60
60
70
70
80 90 100
80 90 100
BI fan with
BI fan with
discharge
discharge
dampers
dampers
AF fan with
AF fan with
inlet vanes
inlet vanes
FC fan with
FC fan with
discharge
discharge
dampers
dampers
FC fan with
FC fan with
inlet vanes
inlet vanes
fan
fan
-
-
speed
speed
control
control
% design airflow
% design airflow
% design power
% design power
variable
variable
-
-
pitch
pitch
vaneaxial
vaneaxial
Figure 89