The Physical Principles and Designs of Evaporative Cooling

Evaporative cooling is a physical phenomenon in which evaporatin of a liquid, typically into surrounding air, cools an object or a liquid in contact with it. Latent heat, the amount of heat that is needed to evaporate the liquid, is drawn from the air. When considering water evaporating into air, the wet-bulb temperature, as compared to the air's dry-bulb temperature, is a measure of the potential for evaporative cooling. The greater the difference between the two temperatures, the greater the evaporative cooling effect. When the temperatures are the same, no net evaporation of water in air occurs, thus there is no cooling effect.
A simple example of natural evaporative cooling is perspiration, or sweat, which the body secretes in order to cool itself. The amount of heat transfer depends on the evaporation rate, however for each kilogram of water vaporized 2257 kJ of energy (about 890 BTU per pound of pure water, at 95°F) are transferred. The evaporation rate in turn depends on the humidity of the air and its temperature, which is why one's sweat accumulates more on hot, humid days: the perspiration cannot evaporate.
Evaporative cooling is not the same principle as that used by vapor-compression refrigeration units, although that process also requires evaporation (although the evaporation is contained within the system). In a vapor-compression cycle, after the refrigerant evaporates inside the evaporator coils, the refrigerant gas is compressed and cooled, causing it to return to its liquid state. In contrast an evaporative cooler's water is only evaporated once. In a space-cooling unit the evaporated water is introduced into the space along with the now-cooled air; in an evaporative tower the evaporated water is carried off in the airflow.

Evaporative cooler designs
All designs take advantage of the fact that water has the highest known enthalpy of vaporization (latent heat of vaporization) of any substance.

Evaporative cooler illustration

Direct evaporative cooling (open circuit) is used to lower the temperature of air by using latent heat of evaporation, changing liquid water to water vapor. In this process, the energy in the air does not change. Warm dry air is changed to cool moist air. The heat of the outside air is used to evaporate water.
Indirect evaporative cooling (closed circuit) is similar to direct evaporative cooling, but uses some type of heat exchanger. The cooled moist air never comes in direct contact with the conditioned environment.
Two-stage evaporative cooling, or indirect-direct. Traditional evaporative coolers use only a fraction of the energy of vapor-compression or absorption air conditioning systems. Unfortunately, except in very dry climates they increase humidity to a level that makes occupants uncomfortable. Two-stage evaporative coolers do not produce humidity levels as high as that produced by traditional single-stage evaporative coolers.
In the first stage of a two-stage cooler, warm air is pre-cooled indirectly without adding humidity (by passing inside a heat exchanger that is cooled by evaporation on the outside). In the direct stage, the pre-cooled air passes through a water-soaked pad and picks up humidity as it cools. Since the air supply is pre-cooled in the first stage, less humidity is needed in the direct stage to reach the desired cooling temperatures. The result, according to manufacturers, is cooler air with a relative humidity between 50 and 70 percent, depending on the climate, compared to a traditional system that produces about 70–80 percent relative humidity air.