Evaporative Cooler - Evaporative Cooler Designs

Evaporative Cooler Designs

Most designs take advantage of the fact that water has one of the highest known enthalpy of vaporization (latent heat of vaporization) values of any common substance. Because of this evaporative coolers use only a fraction of the energy of vapor-compression or absorption air conditioning systems. Unfortunately, except in very dry climates, the single-stage (direct) cooler can increase relative humidity to a level that makes occupants uncomfortable. Indirect and Two-stage evaporative coolers keep the RH lower.

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. The Relative Humidity increases to 70 to 90% which reduces the cooling effect of human perspiration. The moist air has to be continually released to outside or else the air becomes saturated and evaporation stops.

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 air. The moist air stream is released outside or used to cool other external devices such as solar cells which are more efficient if kept cool. One indirect cooler manufacturer uses the so-called Maisotsenko cycle which employs an iterative (multi-step) heat exchanger that can reduce the temperature to below the wet-bulb temperature. While no moisture is added to the incoming air the RH does rise a little according to the Temperature-RH formula. Conditioned air without added moisture increases the evaporation of perspiration improving the cooling effect of Indirect compared to Direct.

Two-stage evaporative cooling, or indirect-direct. 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 transferred in the direct stage, to reach the desired cooling temperatures. The result, according to manufacturers, is cooler air with a relative humidity between 50-70%, depending on the climate, compared to a traditional system that produces about 70–80% relative humidity in the conditioned air.

Hybrid. Direct or Indirect cooling has been combined with vapor-compression or absorption air conditioning to increase the overall efficiency and /or to reduce the temperature below the wet-bulb limit.

Materials. Traditionally, evaporative cooler pads consist of excelsior (wood wool) (aspen wood fiber) inside a containment net, but more modern materials, such as some plastics and melamine paper, are entering use as cooler-pad media. Wood absorbs some of the water and has a larger surface area which allows the wood fibers to cool passing air to a lower temperature than some synthetic materials, but natural fibers also can pose a problem with harboring or supporting mildew growth.