In addition to the major task of condensing refrigerant, evaporative condensers are sometimes called on to cool some other fluid as well. The prime example of this supplementary cooling requirement, and a frequent one, is to cool the oil injected in screw compressors. To accomplish this assignment, pump water from the sump to the oil-cooling heat exchanger and return the warmed water to the sump. This additional cooling load reflects itself in the performance of the condenser, as shown in Figure 7.34, where the solid lines show the performance without, and the dashed lines with external cooling by the sump water.
When no supplementary cooling is demanded, the water temperature in the sump at A is the same as that sprayed over the tubes at the top of the condenser, point B. This equality of temperatures is inherent because, as Figure 7.1c shows, there is a direct connection from the sump to the sprays. When the sump water provides supplementary cooling, the spray water temperature at B’ will be higher than temperature A. The consequence of the higher spray-water temperature is a greater rise in air enthalpy through the condenser, and a higher condensing
temperature of the refrigerant.
If an additional load is imposed on the condenser by adding heat to the sump water, the condenser will accommodate the addition—afact which is shown in Fig. 7.34 by the increased rise in air enthalpy through the condenser. The increased capacity does not come without a cost, however, and that cost is the increased condensing temperature of the refrigerant.
Manufacturers of evaporative condensers usually recommend a separate closed circuit evaporative cooler for the supplementary cooling duty to permit separate control of the refrigerant temperature and the fluid temperature, and to allow for dry operation during cold weather.