Evaporative Coolers

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.