Two-stage Evaporative Cooler A two-stage evaporative cooler has been developed that does not produce levels of indoor humidity as high as traditional single-stage evaporative coolers.  Getting out of a pool and standing in a breeze will help you feel cool, even on a hot day. This is the principle behind evaporative cooling. Evaporative coolers are cooling systems that use only water and a blower to circulate air. As the water evaporates a cooling effect on the surrounding air occurs. That is why evaporative coolers may increase indoor humidity and make occupants uncomfortable. A two-stage evaporative cooler has been developed that does not produce levels of indoor humidity as high as traditional single-stage evaporative coolers. Evaporative coolers use a fraction of the energy of traditional air conditioning systems.
Evaporative cooling systems use the cooling effect of evaporation to condition air, water, or both. A single-stage evaporative cooler, often called a "swamp cooler," cools air by pulling warm, dry air through a water-soaked pad. The water thereby cooling the air and evaporating the water in the air system absorbs heat in the air. Sometimes, the indoor air can pick up too much humidity from the evaporative cooler and cause discomfort inside the conditioned space. One solution is a two-stage cooler in which the 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 second stage, the precooled air passes through a water-soaked pad and picks up humidity as it cools. Because the air supply to the second stage evaporator is pre-cooled, less humidity is added to the air, whose affinity for moisture is directly related to temperature. The result, according to the manufacturer, is cool air with a relative humidity between 50 and 70 percent, dependent on the regional climate, compared to a traditional system that would produces about 80 percent relative humidity air. An advanced two-stage evaporative cooler uses 100 percent outdoor air and a variable speed blower to circulate cool air. Direct and indirect evaporative coolers can reduce energy consumption 60 - 75 percent improvement over conventional air conditioning systems according to American Society of Heating and Engineers, Inc. (ASHRAE). Yet this relative improvement depends on location and application. For example homes using a 2.5 - 3.0 ton conventional air conditioners in the Sacramento area of California were refitted with two-stage evaporative coolers. One manufacturer reports that the system uses about 3,000 to 6,000 gallons of water per year. Using one the most expensive California water rates from another locale, Santa Barbara, water costs are less than $16 to $32 per year respectively, using $3.92 per hundred cubic feet rates. 
Currently two models are available from one manufacture. Another supplier is redesigning a model to be re-introduced in 2003. Because water evaporates more readily in drier air, two-stage evaporative cooling is more efficient in climates where typical summertime relative humidity is low. The trade off for using less electricity is the need for water. The places where evaporative coolers are most effective are typically where water is scarce.
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Check with your local building official to determine if code constraints limit application of evaporative coolers in your location.
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Two-stage evaporative coolers can be mounted on the exterior wall, in an attic, or on an exterior concrete pad. Supply air can be either ducted or discharged to a central location in the home. A heating source can also be integrated.
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When ducted, evaporative coolers are quieter than traditional air conditioners since no compressor operates. They use less energy than traditional air conditioners to achieve the same level of cooling capacity. Currently, an evaporative cooler with a cooling capacity equivalent to a three-ton conventional system retails for about $1,800, and total installed cost is estimated at about $3,900. According to the manufacturer, installed cost is lower than conventional high efficiency air conditioners. According to one manufacturer, a commercially-available two-stage cooler can condition a 1,700 square foot house in Sacramento, California during peak cooling season for less than $30 per month at 8 cents per kWh. | 
