Cooling tower is one type of heat exchanger. It received hot water from source & decrease the temperature of the same & supply it to source. it decrease the temperature by decreasing the pressure of incoming water.
 
A cooling tower’s function is to remove heat from a building or process application that generates heat and must be dissipated. The fundamental principal employed in accomplishing this removal of heat is the evaporation of water. How this evaporation is accomplished varies among differing types of cooling towers but nearly all towers rely upon the atomization of the water over sprinkler heads, allowing the water to “rain” over fills which are a honeycomb-like structure that increases the surface area of the water , and then pulling air across the surface of the water through the use of fans.
 
The entire purpose of this is to create a medium into which a refrigerating device such as a chiller can eject heat.
 
Depending upon the outside air temperature and the outdoor relative humidity, the temperature at which water can be theoretically cooled can be determined. This is known as the wet-bulb temperature. Unless it is raining, this temperature is always cooler than the outdoor temperature (dry-bulb). In practice, cooling towers do not achieve temperatures as cool as the wet-bulb temp and but produce water 5 to 7 degrees warmer. This difference is called the approach.
 
Chillers can use this cooled water to remove heat from its condenser, pump the water over the cooling tower and evaporate a portion the water to then eject said heat to the surrounding outdoors to the tune of about 970 btus per pound of water evaporated.