Swamp Coolers

Written: 10/02/95
Last Updated: 28 Oct 96

How it Works

Swamp coolers are an efficient and effective machine for cooling. As a direct placement for air conditioning in dry climates like Utah and Nevada, it is an example of how man can work with nature. Being so much less expensive than air conditioning, it almost seems that we are getting something for nothing. This short essay explains part of the fascination with the evaporative cooler phenomenon.

The way a swamp cooler operates is very simple. There is a low horsepower motor which pumps the water from the floor of the cooler to the top of the cooler, where it proceeds to fall down the sides, along porous filter pads. A second motor drives a fan which pulls air from the outside, through the cooler, and then pushes it into the hot room. The significant cooling action is the water evaporating as the air passes through it. (Incidentally, the water level is kept constant with the help of a floating sphere functioning similar to the one in the toilet bowl.) The hot air enters the cooler, where two small motors power nothing more than a fan and a pump, in order to send cool air into the hot room.

The way the air is cooled in the cooler, is similar to the way evaporating sweat cools the human body. We all observe this miracle and take it for granted, but to analyze this, it is helpful to remember the bell curve: When a substance is perceived at a certain energy heat level, measured in temperature; there is really a distribution of varying levels of temperature throughout the molecules of the substance. This assortment of temperatures average out to the measured value. Most of the molecules can be around the average, and the farther from the average, the less of them there are. For example, water at room temperature has most of its molecules at approximately that temperature. But it also contains molecules that are near the boiling point of water, and also near its freezing point; however small in quantity they are, they are an important presence. Because at the boiling point of water, there are molecules that are a gas and that are a liquid. The liquid molecules will absorb energy in the form of heat t o become a gas and escape the confines of its old form. As the molecules from the higher temperature evaporate, the remaining liquid averages less heat. But there will still be water at the higher temperatures because the remaining molecules redistribute themselves along the bell curve, which enables the next molecules to evaporate. Heat is siphoned off this way from the water. More importantly, heat is extracted from the air as the boiling point liquid water grabs the needed energy for its freedom into gas.

In thermodynamic terms, the two driving forces in the universe are a simultaneous tendency for less enthalpy and more entropy. Tending to less enthalpy is to cool down and dissipate heat. Tending to more entropy is to become random and free. Personally, I strive for less tension and more freedom, although sometimes I must compromise one for the other. And that is what water is doing in an evaporative cooler. The quest for freedom on a molecular level is one of the two guiding forces in its physical existence. To gain the greater measure of entropy, it is willing to sacrifice a little enthalpy.


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