Trane TRG-TRC016-EN manual Condenser-Water Temperature, Tower

period four

Chiller-Plant Control

notes

Condenser-Water Temperature

kWh	300,000
consumption,
							cooling
energy	200,000						tower
							chiller
annual	100,000
		85°F	70°F		55°F	optimal
	[29.4°C]		[21.1°C]	[12.8°C]		control
	condenser-water temperatureset point							Figure 106

Lowering the temperature of the condenser water can also reduce the energy consumption of the chiller. Depending on the system load and outdoor conditions, cooling towers typically have the ability to supply colder condenser water than at design conditions. This, however, increases the energy consumption of the cooling tower fans. The key to maximizing energy savings is knowing the relationship of cooling-tower energy consumption to chiller energy consumption.

At design conditions, a chiller typically uses five to ten times more energy than a cooling tower. This would suggest that it might be beneficial to use more cooling-tower energy to save chiller energy. However, there is a point of diminishing return where the chiller energy savings is less than the additional energy used by the cooling tower. Figure 106 shows the combined annual energy consumption of a chiller and cooling tower in a system that is controlled to various condenser-water-temperature set points. The third column shows a system that attempts to supply 55°F [12.8°C] water from the cooling tower at all times. Of course, at design conditions, the cooling tower may not be able to supply this temperature, but it will supply the water at the coldest temperature possible.

The fourth column shows a system that uses a control system to dynamically determine the optimal condenser-water temperature that minimizes the combined energy use of the chiller plus cooling tower. It is obvious that this method of optimal control minimizes overall system energy consumption.

98 TRG-TRC016-EN