Evaporative coolers are appliances used to cool indoor air. Evaporative coolers use much less energy than air conditioners, but they can’t cool indoor air effectively in all weather conditions.
Sometimes called swamp coolers, evaporative coolers lower the temperature of an airstream by passing the air through a moistened pad. The moving air causes water to evaporate off the pad. Evaporation requires energy (heat); in other words, the process of evaporation (a phase change process) removes heat from the air. The air exits the appliance at a lower temperature, but with more moisture, than when it entered.
To keep the pads in an evaporative cooler damp, tubing with nozzles delivers water to the top of the pads. The pads are usually made of a material called excelsior — basically, aspen wood shavings. Water trickles down the pads and drips off the bottom of the pads; the base of the metal cabinet includes a sump where the water collects. A recirculating pump pulls water from the sump and delivers it to back the top of the pads. The sump is equipped with a float valve similar to the valve in a toilet tank; the float valve opens and adds water to the sump when the water level drops due to evaporation.
These devices only work in a dry climate
Evaporative coolers work best when the outdoor relative humidity (the wet-bulb temperature) is low. If the outdoor temperature is 90ºF at 10% RH, the wet-bulb temperature is only 58ºF — good conditions for operation of an evaporative cooler. (For more information on wet-bulb temperatures, see How to Use the Psychrometric Chart.)
The bigger the difference between the dry-bulb and wet-bulb temperatures, the more effectively an evaporative cooler will operate. Most evaporative coolers can’t lower the temperature of the air below…