Evaporative cooling systems have been used to condition facilities since before refrigerated cooling was invented. There are two types of evaporative cooling techniques: indirect and direct. The indirect approach removes sensible energy only from the air stream with an exchanger that has the air stream flow on one side of a plate and water on the other. The direct approach removes energy from the air stream adiabatically by adding moisture to the air stream. These approaches can be combined to provide cooling and humidification that displaces energy from a conventional compressor-type HVAC system. Annual energy savings of more than 15 percent for a one-stage, direct system and over 38 percent for a two-stage indirect/direct system can be realized when compared to a conventional HVAC system. Brown (1993) says, "The use of direct evaporative cooling to humidify increases the opportunity to recover heat by 36 percent and reduce energy consumption during the heating season." One- to two-year paybacks are possible. The justification of evaporative cooling should not be based solely on the ability to replace mechanical cooling but should be considered as an opportunity to displace mechanical cooling/humidification energy. [Brown, 1992; Brown, 1993]
Evaporative cooling should always be considered for all make-up air systems, regardless of geographic region, as noted in the descriptions of the approaches below:
Indirect evaporative cooling is applicable when there is a large process equipment-cooling load, the HVAC system operates on a continuous basis, and a direct evaporative cooling opportunity exists. [Brown, 1993]
Direct evaporative cooling can be justified on the basis of eliminating the need for a steam humidification system, enhancing economizer operation, reducing humidification energy used in conjunction with heat recovery, and controlling the temperature of a space independent of controlling its humidity. Direct evaporative cooling for both make-up air and recirculation-type systems should always be considered when the outdoor wet-bulb temperature is 53°F (11.7°C) or less for 3,500 or more hours per year. Direct evaporative cooling of air conditioning condensers, which are normally simply air-cooled, can reduce electrical energy and demand consumption by 20 to 40 percent. [Knebel, 1997] [Brown, 1990; Brown, 1993]
Typically, a two-stage evaporative cooling system comprises a direct evaporative cooler downstream of an indirect evaporative cooler. The two coolers can be combined in a myriad of arrangements to increase energy efficiency. [Brown, 1992; Brown, 1993; Brown, 1990]
Evaporative cooler configuration
