How Evaporative Coolers Work
Evaporative coolers (also called swamp, desert, or air coolers) are devices that cool air through the simple evaporation of water. They differ from refrigeration or absorption air conditioning, which use the vapor-compression or absorption refrigeration cycles. In the United States, small-scale evaporative coolers are called swamp coolers by some users due to the humid air conditions produced. The name sump cooler is also used. Air washers and wet cooling towers utilize the same principles as evaporative coolers, but are optimized for purposes other than air cooling.
Evaporative cooling is especially well suited for climates where the air is hot and humidity is low. For example, in the United States, the western/mountain states are good locations, with swamp coolers very prevalent in cities like Denver, Salt Lake City, Albuquerque, El Paso and Phoenix, where sufficient water is available. Evaporative air conditioning is also popular and well suited to the southern (temperate) part of Australia. In dry climates, the installation and operating cost of an evaporative cooler can be much lower than refrigerative air conditioning, often by 80% or so. But evaporative cooling and vapor-compression air conditioning are sometimes used in combination to yield optimal performance. Some evaporative coolers may also serve as humidifiers in the heating season.
In moderate humidity locations there are many cost-effective uses for evaporative cooling, in addition to their widespread use in dry climates. For example, industrial plants, commercial kitchens, laundries, dry cleaners, greenhouses, spot cooling (loading docks, warehouses, factories, construction sites, athletic events, workshops, garages, and kennels) and confinement farming (poultry ranches, hog, and dairy) all often employ evaporative cooling. In highly humid climates, evaporative cooling may have little thermal comfort benefit beyond the increased ventilation and air movement it provides.
Evaporative Cooling History
Civilizations throughout the ages have found ingenious ways to combat the heat in their region. An earlier form of air cooling, the windcatcher (Bâd gir), was invented in Persia (Iran) thousands of years ago in the form of wind shafts on the roof, which caught the wind and passed it through water and blew the cooled air into the building. Nowadays Iranians have changed the windcatcher into an evaporative cooler (Coolere Âbi) and use it widely. There are around 9,000,000 evaporative coolers in central Iran.
Evaporative cooling was in vogue for aircraft designs for some time in the late 1930s. In this case the system was used in order to reduce, or eliminate completely, the radiator which would otherwise create considerable drag. In these systems the water in the engine was kept under pressure with pumps, allowing it to heat to temperatures above 100 Celsius, as the actual boiling point is a function of the pressure. The super-heated water was then sprayed though a nozzle into an open tube, where it rapidly boiled and released its heat. The tubes could be placed under the skin of the aircraft, resulting in a zero-drag cooling system.
However these systems also had serious disadvantages. Since the amount of tubing needed to cool the water was large, the cooling system covered a significant portion of the plane even though it was hidden. This led to all sorts of added complexity and the systems were always terribly unreliable. In addition this large size meant it was very easy for it to be hit by enemy fire, and practically impossible to armor. British and US attempts to use the system turned to ethylene glycol instead. The Germans instead used streamlining and positioning of traditional radiators. Even its most ardent supporters, Heinkel’s Günter brothers, eventually gave up on it in 1940.
Evaporative cooling was used in some automobiles, often as aftermarket accessories, until modern vapor-compression air-conditioning became widely available.
How Evaporative Cooling Works
Evaporative cooling is a physical phenomenon in which evaporation of a liquid, typically into surrounding air, cools an object or a liquid in contact with it. Latent heat describes the amount of heat that is needed to evaporate the liquid; this heat comes from the liquid itself and the surrounding gas and surfaces. When considering water evaporating into air, the wet-bulb temperature, as compared to the air’s dry-bulb temperature, is a measure of the potential for evaporative cooling. The greater the difference between the two temperatures, the greater the evaporative cooling effect. When the temperatures are the same, no net evaporation of water in air occurs, thus there is no cooling effect.
The simplest example of how evaporative cooling works would be perspiration, or sweat, which the body secretes in order to cool itself. The amount of heat transfer depends on the evaporation rate, which in turn depends on the humidity of the air and its temperature, which is why one sweats more on hot, humid days.
Another, recent application of evaporative cooling is the “self-refrigerating” beverage can. A separate compartment inside the can contains a desiccant and cooling liquid. Just before consumption, the desiccant comes into contact with the cooling liquid, inducing evaporation.
Evaporative cooling is a very common form of cooling buildings for thermal comfort since it is relatively cheap and requires less energy than many other forms of cooling. However evaporative cooling requires an abundant water source as an evaporate, and is only efficient when the relative humidity is low, restricting its effective use to dry climates. Evaporative coolers are colloquially referred to as swamp coolers in the U.S. In other places they are known as desert coolers.
Evaporative cooling is commonly used in cryogenic applications. The vapor above a reservoir of cryogenic liquid is pumped away, and the liquid continuously evaporates as long as the liquid’s vapor pressure is significant. Evaporative cooling of ordinary helium forms a 1-K pot, which can cool to at least 1.2 K. Evaporative cooling of helium-3 can provide temperatures below 300 mK. Each of these techniques can be used to make cryocoolers, or as components of lower-temperature cryostats such as dilution refrigerators. As the temperature decreases, the vapor pressure of the liquid also falls, and cooling becomes less effective. This sets a lower limit to the temperature attainable with a given liquid.
This process has recently been observed to operate on a planetary scale on Pluto and acts as an Anti-Greenhouse Effect. Evaporative cooling is also the last cooling step in order to reach the ultra-low temperatures required for Bose-Einstein Condensation (BEC). Here, so-called forced evaporative cooling is used to selectively remove high-energetic (“hot”) atoms from an atom cloud until the remaining cloud is cooled below the BEC transition temperature. For a cloud of 1 million alkali atoms, this temperature is about 1μK.
Evaporative Cooler Designs
There are multiple evaporative cooler designs on the market today. Direct Evaporative Cooling (open circuit) is used to lower the temperature of air by using latent heat of evaporation, changing water to vapor. In this process, the energy in the air does not change. Warm dry air is changed to cool moist air. Heat in the air is used to evaporate water.
Indirect Evaporative Cooling (closed circuit) is similar to direct evaporative cooling, but uses some type of heat exchanger. The cooled moist air never comes in direct contact with the conditioned environment.
Two-stage Evaporative Cooling, or Indirect-Direct, is another form of evaporative cooling. Traditional evaporative coolers use only a fraction of the energy of vapor-compression or absorption air conditioning systems. Unfortunately, except for in very dry climates, they may increase humidity to a level that makes occupants uncomfortable. Two-stage evaporative coolers do not produce humidity levels as high as that produced by traditional single-stage evaporative coolers.
In the first stage of a two-stage cooler, warm air is pre-cooled indirectly without adding humidity (by passing inside a heat exchanger that is cooled by evaporation on the outside). In the direct stage, the precooled air passes through a water-soaked pad and picks up humidity as it cools. Because the air supply to the second stage evaporator is pre-cooled, less humidity is added to the air (because cooler air can’t hold as much moisture as warmer air). The result, according to manufacturers, is cool air with a relative humidity between 50 and 70 percent, depending on the climate, compared to a traditional system that produces about 80 percent relative humidity air.
Evaporative Cooler pads
Traditionally, evaporative cooler pads consist of excelsior (wood wool, aspen wood fiber) inside a containment net, but more modern materials, such as some plastics and melamin paper, are entering use as cooler-pad media. Wood absorbs some of the water, which allows the wood fibers to cool passing air to a lower temperature than some synthetic materials. The thickness of the padding media plays a large part in cooling efficiency, allowing longer air contact. For example, an eight-inch-thick pad with its increased surface area will be more efficient than a one-inch pad.
Comparison to air conditioning
Comparison of Evaporative cooling to phase-change air conditioning:
Less expensive to install
- Estimated cost for installation is 1/8 to 1/2 that of refrigerated air conditioning
- Estimated cost of operation is 1/4 that of refrigerated air.
- Power consumption is limited to the fan and water pump vs. compressors, pumps, and blowers
- Estimated cost of operation is 1/4 that of refrigerated air.
- Power consumption is limited to the fan and water pump vs. compressors, pumps, and blowers.
- The constant and high volumetric flow rate of air through the building reduces the age-of-air in the building dramatically.
- Evaporative cooling increases humidity, which, in dry climates, may improve thermal comfort.
- High temperature, high humidity outside conditions decrease the cooling capability of the evaporative cooler.
- No dehumidification. Traditional air conditioners remove moisture from the air, which is usually a design requirement except in very dry locations. Evaporative cooling adds moisture, which, in dry climates, may improve thermal comfort.
- The air supplied by the evaporative cooler is typically 80–90% relative humidity.
- Very humid air reduces the evaporation rate of moisture from the skin, nose, lungs, and eyes.
- High humidity in air accelerates corrosion. This can considerably shorten the life of electronic and other equipment.
- High humidity in air may cause condensation. This can be a problem for some situations (e.g., electrical equipment, computers, paper/books, old wood).
- Evaporative coolers require a constant supply of water to wet the pads.
- Water high in mineral content will leave mineral deposits on the pads and interior of the cooler. Water softeners, bleed-off, and refill systems may reduce this problem.
- The water supply line needs protection against freeze bursting during off-season, winter temperatures. The cooler itself needs to be drained too, as well as cleaned periodically and the pads replaced.
- Pollen, odors, and other outdoor contaminants may be blown into the building unless sufficient filtering is in place.
- The vents that allow air to exit the building may pose a physical security risk.
- Asthma patients may need to avoid evaporatively cooled environments.