The schematic diagram of the evaporative condenser shown in Fig. 7.1c illustrates that the evaporative condenser combines the functions of an aircooled condenser and cooling tower. Refrigerant condenses within the tubes, and these tubes are sprayed with water through which an air stream passes. The evaporation of some water into the air is the dominant process of rejecting heat to the atmosphere.
To provide a comparison of the three forms of condensing equipment, namely the air-cooled condenser, the water-cooled condenser/cooling tower combination, and the evaporative condenser—some of the characteristics of each are enumerated:
Air-cooled condenser. Usually lowest first cost of the three, and least maintenace cost as well, because no water circulates or evaporates.
Water-cooled condenser with cooling tower. Lower condensing temperature than with an air-cooled condenser, because the wet-bulb rather than the dry-bulb temperature of the air is the sink toward which the condensing temperature drives. When the distance between the compressor and the point of heat rejection is long, water can be piped to the cooling tower, rather than sending refrigerant, as must be done with the evaporative or air-cooled condenser.
Evaporative condenser. Compact and provides lower condensing temperatures than the air-cooled condenser and also lower than the water-cooled condenser/cooling tower combination. Figure 7.11 shows an evaporative condenser with a bit more detail than was presented in Fig. 7.1c.
The evaporative condenser is widely used in industrial refrigeration practice because it provides relatively low condensing temperatures. These temperatures conserve power and result in moderate compressor discharge temperatures which may be important, especially for ammonia applications. The drawback of its maintenance requirements may not be prohibitive. Most industrial refrigeration plants employ service personnel, in contrast to smaller plants where the refrigeration facility is expected to operate without regular maintenance.
Most evaporative condensers are of the blow-through type using an axialflow fan in preference to a centrifugal type. Eliminator plates are provided to avoid blowing water droplets out of the condenser. All condensers are equipped with blowdown to limit the buildup of minerals in the spray water, and this blow-down is usually installed in the pump discharge line, so that when the condenser is not in operation, the blowdown ceases.