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If you have ever enjoyed a breeze on a too hot day as it cools your skin, you felt as Ben Franklin did back in 1750, when he changed out of damp clothes and into dry ones on a 100+ day. He noted that he was cooler in the damp clothes and realized that the warm breeze in the room was not cooling him, but rather, his sweaty clothes. Not an attractive thought, but one that led him to experiment by wetting the bulb of a thermometer with spirits that evaporated quicker than water, and then blowing air across it. He managed to bring the temperature down so far that ice froze on the bulb.

An evaporative cooler, also known as a swamp cooler works on this principle. It is essentially a large fan with water-moistened pads in front of it. The fan draws warm outside air through the pads and blows the now-cooled air throughout the house. The pads can be made of wood shavings – wood from aspen trees is a traditional choice – or other materials that absorb and hold moisture while resisting mildew.

Small distribution lines supply water to the top of the pads. Water soaks the pads and, thanks to gravity, trickles through them to collect in a sump at the bottom of the cooler. A small recirculating water pump sends the collected water back to the top of the pads.

Since water is continually lost through evaporation, a float valve – much like the one that controls the water in a toilet tank – adds water to the sump when the level gets low. Under normal conditions, a swamp cooler can use between 3 to 15 gallons of water a day.

A large fan draws air through the pads, where evaporation drops the temperature approximately 20 degrees. The fan then blows this cooled air into the house.

Here’s a little lesson to help determine if a swamp cooler will cool your house enough:

Wet and Dry Bulb Temperature

To predict how much a swamp cooler will cool the air, you need to know the wet and dry bulb temperature. The dry bulb is easy — it’s just the regular temperature of the air. The wet bulb temperature tells you what the air temperature would be at 100 percent humidity, and it’s measured with a thermometer covered with a wet cloth sock and exposed to airflow.

The wet bulb temperature is always lower than the dry bulb temperature, and the difference between the two is the wet bulb depression. Depending how efficient your swamp cooler is, it can bring the temperature down as much as 95 percent of the wet bulb depression. Imagine you and your evaporative cooler are in Las Vegas, and it’s 108 degrees outside with a wet bulb temperature of 66 degrees. A swamp cooler operating at 85 percent efficiency can bring the temperature down to a nice, cool 72.3 degrees, right in the human comfort zone.

Unfortunately, evaporative air coolers don’t work everywhere. Swamps, for instance, are lousy places for swamp coolers. It’s not entirely clear where they got the nickname, but it probably refers to the humidity they add to the air or the swampy smell that can develop when they aren’t cleaned often enough. In order to work, they need a hot, dry climate. In the U.S., swamp coolers work well in the arid southwest.