How Swamp Coolers Work | HowStuffWorks - Home And Garden

July 1758 was a scorcher in Philadelphia, Pennsylvania, with temperatures reaching 100 degrees Fahrenheit (37 Celsius). As he later wrote in a letter, Benjamin Franklin was in his room, reading and writing with "no other cloaths on than a shirt, and a pair of long linen drawers, the windows all open, and a brisk wind blowing through the house." Even founding fathers sweat, and as he changed to a dry shirt, he noticed something — it felt warm, like it had been near a fire, compared to the damp shirt he had just removed. Franklin theorized that he wasn't being cooled by the hot air blowing through his room, but by the perspiration evaporating off his skin. Later, he tried some experiments — wetting the bulb of a thermometer and watched as the temperature dropped. He discovered that the evaporation of liquids caused heat loss. What he described in his letter is evaporative cooling.

Liquid evaporates by shedding molecules into the air, changing from a liquid state to a gas. As they become suspended in the air, the molecules draw some of the heat from the hotter air, cooling it down as the water and air find equilibrium. The process also cools the remaining liquid, as hotter, faster-moving molecules are the most likely to escape into the air.

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Swamp coolers work by harnessing that cooling reaction — you just need a way to circulate the now-cooler, more humid air through the house.

Picture an air conditioner — just a sheet metal box on the outside of a window, really. In a standard air conditioner, there are some fairly complicated refrigerants inside, but with a swamp cooler, it's much simpler. The main thing inside is a blower — a fan at one end of the box that brings air in from the outside and pushes it into the house at the other end. Before the air goes into the house, it passes through a set of damp pads, where the evaporation takes place. The cooler air goes into the house and the warmer air is pushed out. A small pump keeps the pads moist, so the water doesn't just evaporate away completely. It works just like the Egyptians' woolen blankets or Ben Franklin's sweaty shirt.

So how exactly does a swamp cooler compare to an air conditioner?

Other Uses

It may be an old technology, but evaporative cooling still has some life in it. For example, inventors have improved on the centuries-old water cooler that inspired Ben Franklin, using evaporative cooling to keep fruits and vegetables cold from spoiling in the heat. The coolers can be made from a variety of local materials. In India, for instance, an earthenware bowl is placed inside a larger, water-filled bowl, and then covered with a cloth to drain the water into the open. In Sudan, a similar cooler was tested and found to keep tomatoes fresh for 20 days — 18 days longer than they would have lasted without the cooler. Elsewhere, coolers are made from cloth-covered bamboo, wooden frames or bricks.

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