Evaporative cooling is an energy-efficient method of cooling the air in a building or space without using a traditional refrigerant-based air conditioning system. Evaporative coolers (also called swamp coolers) work by passing air over or through water-soaked pads of evaporative cooling media. As water evaporates from the pads, the air is cooled. The cool air is then blown into the space, while warmer air escapes through open windows or vents.

Advantages and Disadvantages of Evaporative Cooling

Evaporative coolers and swamp coolers are typically less expensive to buy and run than traditional air conditioners that use refrigeration. According to Energy Saver, a publication of the U.S. Department of Energy's Office of Energy Efficiency & Renewable Energy, installation costs can be about one-half as much as central air conditioners, while energy use can be about one-quarter as much.

Alongside these clear cost advantages, evaporative coolers have a significant limitation. They need to be able to add moisture to the air in order to cool it, so they work better in areas where the outdoor air is drier during the warm season. For this reason, some manufacturers recommend evaporative cooling as an effective strategy only in low-humidity areas of the western U.S. Another disadvantage of evaporative coolers is that they require more frequent maintenance than traditional air conditioners. That said, evaporative cooler maintenance is comparatively simple.      

Evaporative Cooling and Ventilation

Evaporative coolers work by continually bringing in air, which is cooled using evaporation and circulated with a fan, while warmer indoor air is allowed to escape through open windows or other ventilation. This is a contrast to the way air conditioning systems work, treating and circulating air that's already inside a building and operating most efficiently when the building is sealed.

For this reason, when a building has few or no windows, doors or vents that can remain at least partially open to the outdoors, an evaporative cooler won't be able to cool it effectively. This also means that an evaporative cooler or swamp cooler can't provide the same level of air-cleaning filtration that can be obtained with high MERV filters in the air handler of a central air conditioning system. However, many evaporative coolers have swamp cooler filters to help remove particles from the outdoor air that's coming in, even if these filters are less effective than high MERV filters.

Connecting to a Water Source

Evaporative coolers have a water tank or reservoir that directs water onto the pads, keeping them saturated while the cooler is operating. On portable swamp coolers, this tank can be filled manually with a bucket or hose, but many units can also be connected to a continuous water supply for automatic filling, and non-portable units typically require a connection for continuous supply.

Sizing an Evaporative Cooler

Evaporative coolers and swamp coolers are rated in cubic feet per minute (cfm), which measures the amount of air they deliver. According to Energy Saver, manufacturers typically recommend 20 to 40 air changes per hour, depending on climate.

Here are two rules of thumb that can help you size an evaporative cooler:

• If you multiply the square footage of a space by the ceiling height and then divide by two, that will give you the cfm rating necessary for 30 air changes per hour.
• If you provide 500 cfm of cooling capacity per square foot of area to be cooled, that will result in 30 air changes per hour in a space with 10-foot ceilings.

Many manufacturers also provide model-specific guidance on how much space units can be expected to cool.

Evaporative Cooler Types: Portable Coolers, Window Coolers, Whole-Building Systems

There are a few basic designs for evaporative coolers.

Portable evaporative coolers or portable swamp coolers are commonly used in warehouses and manufacturing facilities, as well as in outdoor areas where spot cooling is desired.

Window evaporative coolers are commonly used in homes and workshops and can be easily stored during the winter months.

Central evaporative coolers are permanently installed, connecting to ductwork to distribute air throughout a building:

• Down-discharge central evaporative coolers are installed on building roofs.
• Side-discharge central evaporative coolers can be installed on building sides for easier maintenance access, and they can also be installed on building roofs.

Central evaporative coolers use one of two basic designs:

• Multiple-inlet evaporative coolers have louvered inlets on three or four sides. Air enters through these inlets and passes through comparatively thin, fibrous pads for cooling.
• Single-inlet evaporative coolers draw air through a single opening. The then passes through a thicker, honeycombed pad. Single-inlet units typically use a larger blower motor, resulting in greater airflow and, in combination with the thicker pads, more cooling power.