Cooling towers may be classified on the basis of the fluid used for heat transfer and
on the basis of the power supplied to the unit. In wet cooling towers, the condenser
cooling water and ambient air are intimately mixed. Cooling results from the evaporation of a portion of the water and to a lesser extent from the loss of sensible heat to the
air. In dry cooling towers, the temperature reduction of the condenser water depends
upon conduction and convection for the transfer of heat from the water to the air.
Mechanical draft cooling towers either force or induce the air which serves as the
heat—transfer medium through the tower. For their driving force, natural-draft cooling
towers depend upon the density difference between the air leaving the tower and the
air entering the tower. Cooling ponds are generally considered for heat removal only
when suitable land is available at a reasonable price, since such systems are simple,
cheap, and frequently less water-intensive. It is normally assumed that all heat
discharged to a cooling pond is lost through the air-water interface. With low heattransfer rates, large surface areas are required.
When land costs are too high, spray ponds often provide a viable alternative to
cooling ponds. It is estimated that a spray pond requires only about 5 to 10 percent of
the area of a cooling pond due to the more intimate air-water contact. In addition, drift
losses and corrosion problems are less severe than in cooling towers.