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62 TRG-TRC014-EN
notes
period four
Fan Modulation
Finally, the capacity of vaneaxial fans can be modulated by adjusting the pitch
angle of the actual fan blades. This diagram shows a set of fan blades that can
swivel to modulate the capacity of the fan.
This method of fan modulation is typically only found on very large systems.
Most vaneaxial fans modulate their capacity by varying the speed of the fan.
Note that the fan performance characteristics of this type of fan are slightly
different from those of a centrifugal fan. Performance is plotted based on
airflow at various blade angles versus total pressure (static pressure plus
velocity pressure).
The broken line defines the airflow and total pressure range that produces a
stall condition for each blade angle. Operation in the stall region is
characterized by separation of the air at the fan blade surface, resulting in
instability.
Variable-Pitch Blade Control
variable
variable
-
-
pitch
pitch
blades
blades
Figure 87
35°
40°
45°
blade pitch (angle)
blade pitch (angle)
stall
stall
region
region
50°
Variable-Pitch Blade Control
airflow
airflow
static pressure
static pressure
A
B
design system
design system
resistance curve
resistance curve
actual system
actual system
resistance curve
resistance curve
VAV system
VAV system
modulation curve
modulation curve
Figure 88