A cooling tower cools water by spraying it through a stream of ambient air. A schematic diagram of a cooling tower and the manner in which it serves the refrigeration condenser are shown in Fig. 7.6. The air- and water-flow patterns suggested in Fig. 7.6 are counterflow of air and water, a frequently used configuration. Another popular geometry is crossflow, in which the air is blown horizontally through the falling stream of water. Because some water evaporates into the air, a supply of makeup water must be provided. Also, because the makeup water contains some dissolved minerals, the concentration of these minerals in the sump water would progressively increase were a blowdown not provided.
The explanation of the heat- and mass-transfer process in a cooling tower starts with the recollection of the straight-line law first introduced in Sec. 6.13. The straight-line law states that when air is in contact with water, the change in air conditions is a straight line on the psychrometric chart directed toward the saturation line at the water temperature. This information is used to examine what happens to the enthalpy (heat content) of the air. If the enthalpy of air increases in the process, the enthalpy and temperature of the water must decrease. Consider first the special case shown in Fig. 7.7, where the wet-bulb temperature of the air equals the water temperature.
The path of the air moves toward the saturation line at the water temperature, which is along the wet-bulb temperature line. The wet-bulb temperature lines and the enthalpy lines are essentially parallel, so there is no change in the enthalpy of air, and the temperature of water does not change either. This is the process that takes place in evaporative coolers that reduce the air temperature in homes in arid regions.
If the temperature of the water is higher than the wet-bulb temperature of the air, as in Fig. 7.8, the enthalpy of the air increases from point 1 to point 2, so an energy balance requires that this heat must come from the water by cooling it from point 1′ to point 2′.
When these elementary processes are expanded to a complete counterflow cooling tower, they show the pattern of air and water conditions as in Fig. 7.9. The air progressively increases in enthalpy, and while its dry-bulb temperature is shown decreasing in Fig. 7.9 as it rises through the tower, there could be situations where the temperature increases in passing through the tower.
The key concept implicit in Figs. 7.7 through 7.9 is that the leaving water temperature can approach the wet-bulb temperature of entering air. For this reason, catalog data for cooling towers show the ambient condition that affects cooling tower performance as the wet-bulb temperature, and dry-bulb temperatures may not even be indicated. When a constant heat load is imposed on the condenser and its cooling water, the leaving water temperature rides up as the ambient wet-bulb temperature increases in a trend as shown in Fig. 7.10. Because the heat load and the water-flow rate are constant, a fixed drop in water temperature (5°C or 9°F in this case) prevails over the entire range shown in the graph.