Where the air is dry, evaporative coolers can provide cooling less expensively than operating an air conditioner. They are most common in desert areas with arid climates like in the Southwest where many homes use them. And they are sometimes seen in other parts of the country where the situation calls for some lower cost cooling, like commercial kitchens, laundries and garages. Evaporative coolers work because when dry air absorbs moisture, its temperature is lowered. They cost about half as much to purchase as conventional air conditioners and one quarter as much to operate.
The two most common types of evaporative coolers are single-stage and two-stage and units. Single-stage or direct-type coolers are the most common. They consist of a large fan and absorbent pads to hold the water. Outside air is cooled by about 20 degrees as it is pulled through the wet pads by a fan and blown into the home. Some have rigid pads, and others have rotating pads that turn on wheels, picking up moisture, and giving it up to the air. Two-stage coolers are more complicated, more expensive, and only used where daytime temperatures regularly surpass 100°F.
Besides lower operating costs, another advantage of evaporative coolers is they don't require as much power as air conditioners. You can plug an evaporative cooler into a 120-volt wall outlet on most units. Only on larger units is special wiring needed.
Unlike air conditioners, you must keep at least one window open when an evaporative unit is running to let some air escape and make way for the cooler air to blow in. Depending which window you open, you can direct the cooler air. Some people open an attic hatch to allow air to escape into the attic, but do this only if sufficient attic venting is installed.