Trane TRG-TRC013-EN manual Surge

g

Discharge Dampers

static pressure

sensor set point

discharge	surge
damper	line
SP loss	&
		%
VAV system	'	$
modulation
curve

design system resistance curve

airflow g IM

KKe

upward. 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 discharge dampers, instructing them to begin closing. This results in a build-up of static pressure at the fan outlet and causes the fan to “ride up” further on its performance curve until it reaches its new operating point g at a higher static pressure and lower airflow. The system balances at h along the desired VAV system modulation curve, bringing the system static pressure (downstream of the discharge damper) down to its set point.

This method of control is essentially the same as “riding the fan curve,” except that the static-pressure drop takes place across the discharge damper instead of across the VAV terminal units. While discharge dampers can be used with all types of centrifugal fans, they are most effectively used with the FC fan for the same reason mentioned with “riding the fan curve.” Other methods of supply fan capacity control are more energy efficient, so discharge dampers are rarely used.

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