Trane TRG-TRC013-EN manual “Riding the Fan Curve”, VAV System, S” p eKe

g
VAV System			actual system
			resistance curve
			design system
			resistance curve
pressure	surge
	region
static
			modulation
			range
		airflow	g IC
cwx
S” p eKe
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"Riding the Fan Curve"

static pressure

static pressure		surge	actual system
increase		line	resistance curve
	%				design system
					resistance curve
				$
						airflow
						reduction

airflow g ID

“Riding the Fan Curve”

The simplest form of fan capacity control is called “riding the fan curve.” This method involves no direct form of control but simply allows the fan to react to the change in system static pressure.

During operation, a VAV system experiences changes in resistance as the VAV terminal units modulate closed. This increases the system resistance, creating a new actual system resistance curve. In response, the operating point of the constant-speed fan simply “rides up” on its performance curve from the design operating point e to balance the new system condition. This new operating point f is at a lower airflow and a higher static pressure.

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