period four

Fan Modulation

notes

Variable-Pitch Blade Control

variable-pitch blades

Figure 87

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.

Variable-Pitch Blade Control

static pressure

 

 

 

 

 

 

 

 

 

 

actual system

stall

 

 

 

 

 

 

 

 

resistance curve

 

 

 

 

 

 

 

 

 

 

 

 

design system

region

 

 

 

 

 

 

 

 

 

 

A

resistance curve

VAV system

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

B

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

modulation curve

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

50°

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

45°

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

40°

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

35°

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

blade pitch (angle)

airflow

Figure 88

 

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.

62

TRG-TRC014-EN

Page 67
Image 67
Trane TRG-TRC014-EN manual Variable-Pitch Blade Control

TRG-TRC014-EN specifications

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