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Condenser Water Temperature & Flow

Chillers larger than about 100 tons usually have water circulating through the condenser. This water removes the heat from the chiller (the contribution due to the cooling as well as removing the heat from the motor, engine or absorber). Therefore a high efficiency electric chiller rated at 0.6 kW per ton rejects approximately 14,000 Btuh/ton of cooling. A high efficiency absorption chiller with a COP of 1.0 rejects about 24,000 Btuh/ton of cooling.

The circulating cooling tower water flow is determined primarily by the range in temperature. For example, with a high efficiency electric chiller, a 10°F range (e.g., supplying 85°F water to the chiller condenser and heating that water to 95°F), requires a condenser water flow of about 2.8 gpm per ton. With the same 10°F range, it would have to be about 4.8 gpm per ton for the high efficiency absorption chiller. This flow can be reduced by widening the range, but that would decrease the efficiency of the chiller itself.

The temperature of the water sent to the chiller condenser from the cooling tower is determined, largely by the ambient wet bulb temperature and the efficiency of the cooling tower (the amount of air drawn through the tower and the efficiency of air-water contact). Dry bulb temperature has only a minimal impact on cooling tower performance. The cooling tower is normally specified to meet the design wet bulb temperature in any geographic area -- commonly 75° to 78°F. The cooling tower manufacturer then designs the tower to produce 85°F water under this condition for the design heat rejection level and water flow. The temperature difference between the water sent to the condenser (i.e. coming off the cooling tower) and this wet bulb figure (say 78°F) is defined as the approach temperature 7°F (= 85 - 78).

This design day wet bulb condition occurs relatively few hours a year in most parts of the United States. During dryer times, the cooling tower can produce colder water than 85°F. And, most chillers will operate with reduced power input if this water temperature is reduced (down to a given limit as specified by the equipment manufacturer). This concept is called "floating the condenser" and holds the potential of conserving energy and reducing operating costs.

Selecting the condenser water parameters and cooling tower design is very complicated. Design of these systems requires experience, careful analysis, and consideration of initial investment and operating costs. The material presented here is simply an explanation of several design parameter opportunities. Please refer to the specific cooling design modules elsewhere in this information system for additional details.

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