Cooling towers (Figure 3.19b) are like evaporative condensers, working on the principle of cooling by evaporating water into a moving air stream. The effectiveness of this evaporative cooling process depends upon the wet bulb temperature of the air entering the unit, the volume of air flow and the efficiency of the air/water interface.
As mentioned above, cooling towers are essentially large evaporative coolers where the cooled water is circulated to a remote shell and tube refrigerant condenser. Note that the cooling water is circulating through the tubes while refrigerant vapor condenses and gathers in the lower region of the heat exchanger. Notice also that this area subcools the refrigerant below the temperature of condensation by bringing the coldest cooling tower water into this area of the condenser. The warmed cooling water is sprayed over a fill material in the tower. Some of it evaporates in the moving air stream. The evaporative process cools the remaining water.
The volume of water used by cooling towers is significant. Not only does water evaporate just to reject the heat, but water must be added to avoid the buildup of dissolved solids in the basins of the cooling towers. If these solids build up to the point that they foul the condenser surfaces, the performance of the unit can be greatly reduced.