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.

Evaporative Cooling Basics: from Ben Franklin's Underwear to Air Conditioning

July of 1750 was a scorcher in Philadelphia, Penn., with temperatures reaching 100 degrees Fahrenheit. 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 trousers, 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. He thought about it, and it brought to mind a century-old book that described the process of cooling water jugs in the desert by wrapping them with wet woolen cloths. Franklin theorized that he wasn't being cooled by the hot air blowing through his room, but by the perspiration evaporating off of his skin. Later, he tried some experiments -- 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 [source: History Carper].

What he describes 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.

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. 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?