Trane TRG-TRC014-EN manual Operating Point, Riding the Fan Curve

period four

Fan Modulation

notes

Operating Point

		performance
		curve
				system
				resistance
pressure			A	curve
	[672.5 Pa]2
static	2.7 in. H O		0
			1
			,
			0
			0
				r
				p
				m

airflow	24,000 cfm	Figure 74
	[11.3 m3/s]

When the system resistance curve is superimposed on the fan curve, the intersection predicts the airflow and static pressure at which the system will balance (operating point “A”).

Running the fan in this example at 1,000 rpm will satisfy the design requirements of the system by overcoming 2.7 in. H2O [672.5 Pa] of static pressure resistance while moving 24,000 cfm [11.3 m3/s] of air.

"Riding the Fan Curve"

					actual system
3.15 in. H2O				B	resistance curve
								design system
	static pressure
[784.6 Pa]								resistance curve
	increase					A
2.7 in. H2O
[672.5 Pa]							1
							,
							0
							0
							0
									r
									p
									m
								airflow reduction
		18,000 cfm
						24,000 cfm				Figure 75
			[8.5 m3/s]			[11.3 m3/s]

During operation, a VAV system experiences changes in resistance as the VAV terminal units modulate. This creates a new system resistance curve.

In response, a constant-speed fan “rides up” the performance curve from the design operating point (A) to a new balance point (B). This new operating point is at lower airflow (18,000 cfm [8.5 m3/s]) and a higher static pressure (3.15 in. H2O [784.6 Pa]). This method of modulating the airflow of the fan is called “riding the fan curve.”

54 TRG-TRC014-EN