Trane TRG-TRC013-EN manual Inlet Vanes

g

Inlet Vanes
		surge
	inlet vane	line
pressure	position (%)		design system
			resistance curve
			%
static	VAV system	&	$
	modulation
sensor	curve
set point
		airflow	g MC

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upward. The fan begins to “ride up” its current vane position 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 inlet vanes, instructing them to begin closing. When the inlet vanes are closed, the performance curve for the fan shifts downward until the balance point g falls along the VAV system modulation curve and the fan satisfies the system static-pressure controller.

The advantage that inlet vanes provide over discharge dampers is that the fan approaches the surge region at a much lower airflow and static pressure.

The low end of the fan’s modulating range is either the intersection of the modulation curve with the surge line, or the leakage rate through the fully closed inlet vanes, whichever is larger. Typically, a satisfactory modulating range is achieved by selecting a fan that is two or three sizes (fan wheel diameter) smaller than a fan that would be selected for a constant-volume system. Then the design operating point will fall farther to the right on the curve, permitting a larger range of modulation.

It should be noted that the addition of vanes to the inlet of a fan introduces a pressure drop that must be overcome by the fan. Inlet vane performance curves are established through fan testing, and manufacturers typically publish fan performance data with the effect of the inlet vanes included.

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