Evaluating Evaporative Coolers

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How exactly do evaporative coolers work?
An evaporative cooler adds moisture to the air by increasing relative air humidity and decreasing air temperature. Evaporative cooling occurs when moisture is added to air that has a relative humidity of less than 100%. The lower the relative humidity of the air, the greater cooling effect possible when moisture is added. The technology used in evaporative coolers is a versatile and energy efficient alternative, adjunct to compressor-based cooling. In favorable climates (most of the western United States and other dry climates worldwide), evaporative coolers can meet most, or all, of a building's cooling needs, and they only use one-fourth the energy of conventional equipment. Evaporative coolers can also be cost-effectively integrated with conventional chiller systems.

What factors should I be watching out for when looking for an evaporative cooler?
Consider the surrounding climate when choosing an evaporative cooler. For instance, if you live in a very dry, arid climate like Albuquerque, New Mexico or Tucson, Arizona, a direct evaporative cooling system would be the perfect solution. However, if you live in a humid area like Boston, Massachusetts or New York, New York, an evaporative cooler would be fine for the dry weather, but it would need to be enhanced by a compressor-based cooling system during the humid season.

Evaporative coolers are rated by the cubic feet per minute (CFM) of air delivered to a building. Most models range from 3,000 to 25,000 CFM. Manufacturers recommend providing enough air-moving capacity for 20 to 40 air changes per hour, depending on climate.

The following chart shows some examples of temperatures relating to wet bulb (accounts for moisture in the air) and dry bulb (standard thermometer's temperature) conditions in various cities in the United States during the month of July. The cities where the wet bulb range is well below the dry bulb range are top candidates for evaporative coolers. 

What is the internal structure of an evaporative cooler like?
Hot, outside air enters the evaporative cooler and passes over water-saturated pads. The water evaporates into the air and this energy is used to remove heat from the air. From there, the cooler air (15-40 degrees lower) is directed into the home, pushing the warmer air out through windows. Since this process also humidifies the air, evaporative coolers are best used in areas with low, summertime humidity.

What types of evaporative coolers are there to choose from?
Direct evaporative coolers add moisture to the air while increasing the relative humidity. Direct systems require a building's exhaust system to match the rate at which the conditioned air is introduced to the space without circulating the same air. (Central air conditioning systems sometimes work similarly to direct evaporative coolers.)

Indirect evaporative coolers lower the temperature without adding moisture to the air. However, this type is more expensive than the direct type. Keep in mind that indirect systems provide the same energy efficient alternative as a direct cooler, in situations where the direct type may not be practical or useful.

How do direct and indirect systems differ?

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