Trane TRG-TRC013-EN manual VAV System Modulation Curve

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VAV System Modulation Curve

static pressure

sensor set point

VAV system modulation curve

	design system
%	resistance curve

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airflow g IG

K” t

Ke

upwards. In response, the fan begins to “ride up” its constant-speed performance curve toward f, from the design operating point e, trying to balance with this new system resistance curve. As a result, the fan delivers a lower airflow at a higher static pressure. The system static-pressure controller senses this higher static pressure and sends a signal to the supply fan to reduce its capacity. Modulating the fan capacity results in a new fan – system balance point g, bringing the system static pressure down to the sensor’s set point.

This action results in the fan unloading along a curve, the “e“

. This curve represents the fan modulation needed to balance the static pressure required to offset these variable system losses (demand) with that produced by the fan (supply). The equation used to calculate the VAV system modulation curve is:

Static Pressure (SP) =

where,

 Airflow 		8	⋅ (SPs – SPr)
 Airflow---------------------	--
	s

+b_r

■Static Pressure and Airflow are points along the VAV system modulation curve

■Static Pressures and Airflows describe the design operating point

■Static Pressurer is the system static-pressure control set point

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