Evaporative Coolers: What's the Big Idea?

Many people are confused by evaporative coolers. They don't know how exactly they work. They aren't sure why or if they should use evaporative coolers instead of air conditioners. They don't know how simple it all really is.

Evaporative cooling is when the evaporation of liquid, usually into surrounding air, cools an object or a liquid in contact with it. Some examples are:

-Perspiration or sweat, which the body secretes in order to cool itself.

-The mist from a waterfall making people feel cool.

-"Self-refrigerating" beverage can. There is a separate compartment inside the can that contains a desiccant (a substance that induces or sustains a state of dryness) and a cooling liquid. Just before consumption, the drying agent comes into contact with the cooling liquid which induces evaporation.

Most evaporative coolers have a vertical pad of cellulose fiber, a system for delivering water to the top of the pad, and a fan to draw air through the porous pad as the water runs down the medium and is absorbed. The fan draws ambient air through vents on the unit's sides and through damp pads. Heat in the air evaporates water from the pads which are constantly being re-dampened to continue the cooling process. Moist air is delivered to the room through a vent.

The pads inside of evaporative coolers are made of excelsior (wood wool or aspen wood fiber) inside of a containment net, but modern materials, such as some plastics and melanin paper, are entering use as cooler pad media. The wood absorbs some of the water, which allows the wood fibers to cool passing air to a lower temperature than unnatural materials. The thickness of the pads affects the cooling efficiency of the pads. The thicker the pad, the longer the air contact, so an eight inch thick pad with its increased surface area will be more efficient than a one inch pad.

A drop in temperature as a result of using an evaporative cooler depends on how much water the air can absorb. This means that in highly humid climates, evaporative cooling may have little thermal comfort benefit beyond the increased ventilation and air movement it provides. The effectiveness of the evaporative cooler is also affected by how evenly the pad media is wetted, how long the air is exposed to the pad, the evaporability of the water (which depends on its temperature and hardness), and the ability of the building to vent the warmer exhaust air back into the atmosphere.

Evaporative coolers are relatively cheap and require less energy than other forms of cooling for the most part. It might not hurt to give them a try.