In a wet
cooling tower (or open circuit cooling tower), the warm water can be cooled to
a temperature lower than the ambient air dry-bulb temperature, if the air is
relatively dry (see dew point and psychrometrics). As ambient air is drawn past
a flow of water, a small portion of the water evaporates, and the energy
required to evaporate that portion of the water is taken from the remaining
mass of water, thus reducing its temperature. Approximately 2,300 kilojoules
per kilogram (970 BTU/lb) of heat energy is absorbed for the evaporated water.
Evaporation results in saturated air conditions, lowering the temperature of
the water processed by the tower to a value close to wet-bulb temperature,
which is lower than the ambient dry-bulb temperature, the difference determined
by the initial humidity of the ambient air.
To achieve
better performance (more cooling), a medium called fill is used to increase the
surface area and the time of contact between the air and water flows. Splash
fill consists of material placed to interrupt the water flow causing splashing.
Film fill is composed of thin sheets of material (usually PVC) upon which the
water flows. Both methods create increased surface area and time of contact
between the fluid (water) and the gas (air), to improve heat transfer.