Decoupled condenser-water system

System Controls

The June 2006 ASHRAE Journal article, “Prescription for Chiller Plants,” (Baker, Roe, and Schwedler) details a chilled- water plant with variable-speed condenser water pumps.

These three energy consumers must be balanced to minimize overall energy use. This makes varying condenser water flow complex, but the strategy below has been implemented on projects.

Control of the condenser water pumps and cooling tower fans is based on chiller load, which is calculated using the chilled-water flow rate and temperature differential. When the chiller load is less than 80 percent, the control system varies the speeds of both the cooling tower fan and condenser-water pump in proportion to chiller load, until the minimum allowable condenser-water flow rate is reached. As chiller load decreases further, the condenser-water pump remains at the minimum speed, while the cooling tower fan speed continues to decrease.

Figure 55. Effect of chiller load on water pumps and cooling tower fans

	100										100
	100
	80										80
	80
	60	pump speed									60	%
	60											load,
%												load,
speed,	40										40	chiller
speed,												chiller
		fan speed
	20										20
	20
		chiller load
	0	10	20	30	40	50	60	70	80	90	100
		10	20	30	40	50	60	70	80	90	100

chiller load, %

The minimum allowable condenser water flow rate is the highest of the following:

•Minimum water flow rate required through the chiller condenser (consult chiller manufacturer)

•Minimum water flow rate required through the cooling tower fill (consult cooling tower manufacturer)

•Minimum water flow rate required to properly cool the pump motor (consult pump manufacturer)

•Minimum water flow rate that produces enough pressure to overcome the static lift from the base of the cooling tower to the top of the tower

To overcome possible problems with variable-water flow across a cooling tower, some designers decouple the condenser-water system as shown in Figure 56. This arrangement allows energy optimization through a reduction of pumping power without excessively complicating the system. Because cooling towers work best with full water flow over the transfer media, a

SYS-APM001-EN

Chiller System Design and Control

SYS-APM001-EN specifications

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Trane SYS-APM001-EN manual Decoupled condenser-water system

Models: SYS-APM001-EN

System Controls

Figure 55. Effect of chiller load on water pumps and cooling tower fans