How Evaporative Coolers Work

1. How does air move through an evaporative cooler?

Evaporative cooling systems are a very popular and energy-efficient way of cooling a home or business in dry climates (typically regions with below 50% humidity). Hot outside air enters the evaporative cooler and passes over water in the form of saturated pads. The water evaporates into the air and the energy used removes heat from the air. From there, the 15-to-40-degree-cooler air 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 relative humidity in summertime. An evaporative cooler uses less than one-third the amount of energy that an air conditioner would, and it costs about half as much to install. Unlike central air conditioning systems that circulate the same air, evaporative coolers provide a steady stream of fresh air.

2. How many types of evaporative coolers are available?

There are two types of evaporative coolers available. Direct evaporative coolers add moisture to the air to cool, 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 since no air is being re-circulated (most central units work this way as well). Indirect evaporative coolers cool the area without adding moisture to the air, but are the more expensive type. However, indirect evaporative cooling units provide an energy-saving alternative in situations where the direct evaporative cooling may not be practical or useful.

3. How do evaporative coolers work?

Evaporative cooling adds moisture to the air to reduce air temperature and increase relative humidity. It occurs when moisture is added to air that has a relative humidity of less than 100%. The lower the relative humidity in the air, the greater the cooling effect possible when moisture is added. The technology is versatile and saves energy. In favorable climates (most of the western United States and several other worldwide areas), evaporative cooling can meet most or all building cooling needs using one-fourth of the energy of usual conventional equipment. Evaporative coolers also can be integrated with conventional chiller systems to save on cost. Finally, using evaporative technology can also improve a facility's load profile.

4. How should I choose the right evaporative cooling system?

Evaporative cooling systems are rated by the cubic feet per minute (CFM) of air that is 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.

When choosing an evaporative cooler, again, consider your surrounding climate. For instance, if you live in a 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 damper, humid area like Boston, Massachusetts or New York City, an evaporative cooling system would be fine for the dry weather, but would then need to be enhanced with a compressor-based cooling system during warmer seasons.

***The following chart shows some examples of temperatures relating to wet bulb (which accounts for the moisture in the air) and dry bulb (how a standard thermometer measures 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 ideal candidates for evaporative cooling systems:

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