An evaporative cooler is a device that chills air by evaporating liquid water into water vapor. Other names for an evaporative cooler are a swamp cooler, desert cooler, evaporative air conditioner, or swamp box. While highly effective coolers, evaporative cooling is not appropriate for every situation. For example, if you start with humid air, the process just won’t work. Take a look at examples of evaporative cooling and learn how evaporative coolers work.
- An evaporative cooler chills air by turning liquid water into water vapor.
- This works because the phase transition from a liquid to a gas absorbs energy.
- Evaporative coolers are effective in hot, dry environments, but do not work in humid areas because water doesn’t evaporate very well.
What Is an Evaporative Cooler?
An evaporative cooler takes many forms, depending on its use. Usually, it cools air or another hot gas. But, industries also use a form of evaporative cooler to chill hot water.
One way of making a basic evaporative cooler is by placing a damp towel, cardboard soaked in water, or large container of water in front of a fan. Increasing the surface area of the wet object improves the effect. If you don’t have a fan, hanging a damp towel in an open window helps.
Commercial evaporative coolers are a bit more sophisticated, but the principle is the same. A fan blows air across a porous damp frame. A pump refreshes the water supply.
Evaporative Cooling Examples
Evaporative cooling occurs in nature, as well as applications for personal and business use:
- Human perspiration cools the body as sweat evaporates. So, a “dry heat” really is cooler, in a way, because perspiring is more effective.
- The air around a river, lake, or ocean often is cooler than surrounding warm air because evaporating water reduces the temperature.
- One reason a forest is cool is because transpiration occurs in trees. The plants take in water from the ground and release water through openings on their leaves. This effect also has consequences for the global climate.
- Another example is the chill you feel when you step out of the shower.
- One way ancient humans cooled buildings was filling porous earthenware vessels with water and either placing them in breezy locations or fanning them.
How Evaporative Cooling Works
The key principle for evaporative cooling is that the phase change from a liquid to a gas absorbs energy from the surroundings. It works for all liquids changing into vapor, but water is especially effective because of its high heat of vaporization. Molecules in liquid water connect to each other by hydrogen bonds, which are fairly strong. It takes a lot of energy to separate them and turn the liquid into a gas. One kilogram of water absorbs 2257 kilojoules (~890 BTU) of energy when it vaporizes. This is a lot of cooling power, but the rate of cooling depends on the humidity of the surrounding air. In dry air, cooling is highly effective. In humid air, the process still works, but not as well.
Evaporative Cooler Advantage and Disadvantages
Evaporative cooling is great for some circumstances, but it isn’t a good option for others. In a nutshell, it’s good for hot and dry environments, and not an ideal solution for hot and humid ones.
Advantages
- For home cooling, it is less expensive than an air conditioner in terms of installation costs, cost of operation, and power usage.
- Because there is no compressor, turning it on does not cause a power spike.
- It does not require any special refrigerants.
- It increases humidity, which decreases static electricity in dry environments and may improve comfort for people and pets.
- The water-soaked pad acts as a filter, removing dust, pollen, and other irritants from air.
Disadvantages
- Evaporative coolers don’t lower air temperature as low as air conditioners.
- It increases humidity. In an already humid environment, this prevents perspiration from evaporating, increases the rate of corrosion, may promote condensation of water and surfaces, swells wooden doors and furniture, and may promote mold and mildew growth.
- It requires a constant supply of water. Hard water leaves mineral deposits that necessitate frequent pad replacement and plumbing maintenance.
- Dry pads in commercial units pose a fire hazard.
- When improperly maintained, a swamp cooler offers a breeding ground for mosquitoes.
- Evaporative cooling works when there is good air circulation that moves humid air out of the area. Adding a swamp cooler to an energy efficient home decreases its efficiency, often significantly, because of the need for chimneys or vents.
References
- Bonan, Gordon B. (13 June 2008). “Forests and Climate Change: Forcings, Feedbacks, and the Climate Benefits of Forests”. Science. 320 (5882): 1444–9. doi:10.1126/science.1155121
- Lu, Zhengmao; Leroy, Arny; Zhang, Lenan; Patel, Jatin J.; Wang, Evelyn N.; Grossman, Jeffrey C. (2022). “Significantly enhanced sub-ambient passive cooling enabled by evaporation, radiation, and insulation”. Cell Reports Physical Science. 3 (10): 101068. doi:10.1016/j.xcrp.2022.101068
- Maheshwari, G.P.; Al-Ragom, F.; Suri, R.K. (2001). “Energy-saving potential of an indirect evaporative cooler”. Applied Energy. 69 (1): 69–76. doi:10.1016/S0306-2619(00)00066-0