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BackPage Primer from Environmental Building News

May 1, 2008

Evaporative Coolers

Everyone knows how cool (or cold!) it feels to have wet clothing on in a breeze. As water evaporates, it absorbs heat, leaving our skin feeling cooler. Similarly, cooling towers evaporate water to reject heat from chillers or industrial equipment.

Direct evaporative coolers (also called swamp coolers or desert coolers) use the same principle to cool buildings while offering significant energy savings over conventional compressor-based air conditioners. These coolers use a fan to blow air through an absorbent material that has water dripping through it, allowing the air to enter the building wetter and cooler. The air drops in temperature only to the extent that it can absorb moisture, though, so those coolers work only in dry climates. This principle also gives us a low-tech way to measure humidity in the air: compare the temperature measured by a regular (dry-bulb) thermometer with that measured by a thermometer that has air moving across a wet bulb. By plotting these two temperatures on a psychrometric chart we can determine the air’s humidity.

Direct evaporative coolers can chill air to a temperature nearly as low as the wet-bulb temperature, but no lower. In typical summer conditions in the western U.S., that means they can’t make air as cold as a mechanical air conditioner can. They’re designed to move a lot of air through a building, so that both the air movement and the additional volume can make up for the higher air temperature. There also has to be a way to exhaust that air from the building, and this usually takes the form of a louvered opening into an attic or through a wall.

A direct evaporative cooler for a typical home uses about four gallons (15 l) of water per hour directly, but an additional four or five gallons (15 to 19 l) per hour are often wasted when the reservoir is purged to avoid mineral buildup. (This bleed-off water can be used for irrigation if it isn’t too salty.) Annually, an evaporative cooler might represent 3% of a household’s total water use.

In some settings it is undesirable to add moisture to air entering a building, but evaporative cooling can still work—indirectly. In this approach, one airstream is cooled with evaporating water, while a separate, drier airstream enters the building. Between them is a heat exchanger that allows the wet air to cool the dry air. These indirect evaporative cooling systems tend to be less efficient than direct systems, however, because the heat exchange is never complete and because powering two airstreams requires twice the fan energy.

A new generation of more sophisticated indirect systems (see page 9), and hybrid direct-indirect systems, offers the best of both worlds, with high efficiency and low humidity. These systems function more like conventional air conditioners, providing cooler air at lower volumes than swamp coolers, but they use only one-fourth to one-third as much energy as standard air conditioners. Prices for these systems range from $3,000 to $5,000, while simple swamp coolers cost less than $1,000.

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