Trane TRG-TRC014-EN manual Variable-PitchBlade Control, Fan Modulation, period four

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.

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