Practically all of the remainder of this chapter will concentrate on evaporative condensers which are the predominant type used in industrial refrigeration. One of the reasons for the preference of evaporative condensers over the watercooled condenser with cooling tower is the ability to achieve lower condensing temperatures. Figure 7.12 shows a heat rejection of 548 kW (1,870,000 Btu/hr) by means of an evaporative condenser (Fig. 7.12a) and the rejection of the same magnitude with a water-cooled condenser. The capacity of both the evaporative condenser and the cooling tower are controlled by the ambient wet-bulb temperature, which in this case is 25.6°C (78°F).
The comparison shows the ability to achieve a condensing temperature of 35°C (95°F) with the evaporative condenser, while with the water-cooled condenser the condensing temperature is 40.6°C (105°F), thus 5.6°C (10°F) higher. The superior performance of the evaporative condenser is explained by the avoidance of the intermediate fluid (the cooling-tower water) in the heat-transfer processes. The temperature of water leaving the cooling tower is 28.9°C (84°F) and can only approach the ambient wet-bulb temperature, and the condensing temperature can only approach the temperature of water returning to the tower which is 35.8°C (96.5°F).
The comparison could be accused of being biased in that it would be possible to lower the condensing temperature in Fig. 7.12b by enlarging either or both the cooling tower or the water-cooled condenser. This observation is correct, but the sizes of all the components in the comparison of Fig. 7.12 are based on sizes typically chosen for this application. Nevertheless, the comparison is not complete until the comparative first costs of the condensing subsystem are evaluated. Some designers contend that the evaporative condenser in Fig. 7.12 might even cost less than the sum of the water-cooled condenser and cooling tower.
Another factor in favor of the evaporative condenser is the lower waterpumping costs. The spray-water flow rate in the evaporative condenser is typically about one-third that of the flow rate circulated between the watercooled condenser and the cooling tower. Furthermore, the length of water line between the water-cooled condenser and cooling tower will likely be much longer. This saving in water-pumping power must be balanced against the higher pressure drop in the refrigerant lines, particularly the vapor line between the compressor and condenser. Normally the compressor and water-cooled condenser will be close-coupled. The need for water treatment exists in both concepts, so this feature is not a factor.
These advantages of the evaporative condenser influence the industrial refrigeration industry to predominately favor this type of condensing system. The air-conditioning industry, on the other hand, usually chooses water-cooled condensers, so it is reasonable to ask whether the air-conditioning industry is unaware of the secret. Such is not the case, because many air-conditioning systems experience long distances between the compressor and the ultimate heat rejector. The compressor and condenser may be in the basement and the cooling tower on the roof of a multistory building. In many industrial refrigeration plants the evaporative condenser is on the roof of the machine room that houses the compressors, and the distance separating them may be only 6 to 12 m (20 to 40 ft). Also, when a centrifugal compressor serves a waterchilling system, the refrigerant chosen has a high specific volume, making condensing in the tubes less practical.