It was the conclusion in Section 7.6 that the leaving water temperature from a cooling tower is controlled by the wet-bulb temperature of ambient air. Because the same process of heat and mass transfer occurs in both devices, the wet-bulb temperature also has a dominant influence on the capacity of evaporative condensers. Figure 7.13 shows relative capacities of an ammonia evaporative condensers to changes in wet-bulb temperature and condensing temperatures. The capacities are relative to a condenser operating with a condensing temperature of 35°C (95° F) and a wet-bulb temperature of 25°C (77°F). The trends are as expected, namely the capacity increases with a given wet-bulb temperature as the condensing temperature increases. Furthermore, at a given condensing temperature the capacity increases with a reduction in wet-bulb temperature.
Even though Fig. 7.13 indicates that the temperature difference between the condensing refrigerant and the entering wet-bulb influences the capacity, it is not to be assumed that the heat-rejection capacity is proportional to this difference in temperature. For an air-cooled condenser the heat-transfer rate is proportional to the temperature difference between the condensing refrigerant and the dry-bulb temperature of the entering air. For a water-cooled condenser the capacity is also proportional to the temperature difference between the refrigerant and entering water. For an evaporative condenser, as Fig. 7.14 shows, the level of temperatures as well as the temperature difference controls the capacity. This trend indicates that if an evaporative condenser develops a certain heat-rejection capacity with a temperature difference between condensing refrigerant and the ambient wet-bulb temperature, for example, 40°C to 25°C (104°F to 77°F), the capacity of the condenser will be less if the same temperature difference exists at a lower level, for example, 30°C to 15°C (86°F to 59°F).
The reason for this behavior lies in the evaporation process on which the evaporative condenser operates. The major heat-transfer mechanism in the evaporative condenser is due to the vaporization of water from the condenser tubes, and the rate of this vaporization is proportional to the difference of watervapor pressure of the liquid water on the tube and the water vapor pressure in saturated air that surrounds the tube. Examination of a psychrometric chart (Fig. 6.18 or 6.20) shows that at the low level of temperature, the saturation curve flattens out so that a given difference in temperature translates to a lower difference in water-vapor pressure.