An evaporative cooler is a devices that cools air through the simple process of water evaporation. These units – also known as swamp, desert or air coolers – differ from refrigeration or absorption air conditioning, which use vapour-compression or absorption refrigeration cycles.
In the US, small-scale coolers of this type are known as swamp coolers by some because they produce humid air conditions. Another name for these is “sump cooler”. Air washers and wet cooling towers use the same principles as our evaporative coolers – but these are optimised for purposes other than cooling air.
Evaporative cooling is the perfect fit for climates where air is hot – but humidity is low. Mountain or western US states are a good example of this. Swamp coolers are very popular in cities such as Denver, Salt Lake City, El Paso and Phoenix. This is because sufficient water is available.
This type of air cooling also suits the southern (or temperate) parts of Australia. In dry climates, the installation and running cost of this cooler type is often much lower than refrigerative cooling. It can be as high as 80% in some cases! But you can also use evaporative cooling and vapour-compression cooling together for the best performance. And there are some evaporative coolers that find use as humidifiers during the heating season
If you are in a location with moderate humidity, there are many cost-effective uses for this type of cooling. Industrial plants, commercial kitchens, laundries, dry cleaners, spot cooling and confinement farming are just some examples where you may see this cooling in use. In very humid climates, however, evaporative cooling may have little thermal comfort aside from the greater ventilation and air movement in provides.
A History of Evaporative Cooling
People throughout the ages have found ways to overcome the heat in their part of the world. An earlier form of air cooling, the windcatcher (Bâd gir), was invented in Persia (Iran) thousands of years ago. These came in the form of wind shafts on the roof that caught the wind, before passing it through water and blowing cool air into a building. Now in Iran, however, the windcatcher is an evaporative cooler (Coolere Âbi) and these are widely in use. There are around 9 million of these coolers in central Iran, in fact.
This cooling was popular in the design of aircraft for some time in the late 1930s. The system was used to reduce – or cut out – the radiator, which otherwise led to significant drag. The water in the engine was kept under pressure with pumps, which let it heat up to more than 100° Celsius. The actual boiling point was a function of that pressure. A nozzle was then able to spray this water into an open tube, boiling quickly and releasing its heat. Tubes could sit under the skin of the aircraft to provide a zero-drag cooling system.
These systems also had serious downsides, however; covering a significant part of the plane even though it was hidden. The amount of tubing needed to cool the water was quite large. It led to all sorts of extra difficulties, so the systems were unreliable. Not only that, but the large size made it easy to be hit by enemy fire. It was also tough to armour. After trying to use the system, the UK and the US chose ethylene glycol instead. Germans, meanwhile, were streamlining and positioning traditional radiators instead. Even its most ardent fans – Heinkel’s Günter brothers – gave up on it in 1940.
Evaporative cooling did find use in some cars, often as after-market accessories, but only until modern vapour-compression air conditioning became available on a wider basis.
How Evaporative Coolers Work
Evaporative cooling is a phenomenon in which the evaporation of a liquid – normally into the surrounding air – cools an object or liquid. Latent heat describes the amount of heat you need to evaporate a liquid – and this heat comes from the liquid itself, as well as the surrounding gas and surfaces.
If you think about the water vapour, the wet-bulb temperature (in comparison to the air’s dry-bulb temperature) measures the potential for this type of cooling. The greater the difference, the greater the effect. If the temperatures are the same, there is no net evaporation of water – and, therefore, no cooling.
A simple example of this in action is with sweat. This is where the body secretes in order to cool itself. And the amount of heat transfer will depend on the rate of evaporation, which in turn will rely on the air humidity and temperature. That’s why you sweat more on hot, humid days.
Another, more recent use of evaporative cooling is in the “self-regulating” drinks can. A compartment inside the can contains a desiccant and a cooling liquid. Just before you drink from the can, the desiccant comes into contact with the cooling liquid to induce evaporation.
What Do We Use Evaporative Coolers For?
This style of cooling is quite common in keeping buildings cool for thermal comfort. This is because it is relatively cheap – and you need less energy in comparison to other forms of cooling. It does, however, require plenty of water and it is only efficient if the relative humidity is low. It’s why this type of cooling is most effective in dry climates.
Evaporative cooling will also be used in cryogenic applications. The vapour above a reservoir of cryogenic liquid pumps away, with the liquid evaporating as long as the vapour pressure is there. Cooling ordinary helium forms a 1-K pot, which can cool to at least 1.2-K; with helium-3, it provides temperatures below 300mK. You can use each technique to make cryocoolers – or as parts of lower-temperature cryostats, such as dilution refrigerators.
As the temperature goes down, the vapour pressure of the liquid also falls – making cooling less effective. This sets a lower limit to the temperature you can attain with a given liquid.
You can see this process operate on a planetary scale on Pluto. It also acts as an Anti-Greenhouse Effect and evaporative cooling is the last step in order to reach the ultra-low temperatures you need for Bose-Einstein Condensation (BEC). This is where so-called forced evaporative cooling is used to remove high-energetic (“hot”) atoms from an atom cloud until the remaining cloud is below the BEC transition temperature; this is about 1μK in a cloud of 1 million alkili atoms.
Evaporative Cooler Designs
You can find many designs on the market today. With Direct Evaporative Cooling (open circuit), you can reduce the temperature of air with the latent heat of evaporation – changing water to vapour. In this process, the energy in the air does not change. Warm, dry air changes to cool, moist air, while heat in the air evaporates the 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 vapour-compression or absorption air conditioning systems. Except for in very dry climates, however, they may increase humidity to a level that makes people uncomfortable. Two-stage evaporative coolers do not produce humidity levels as high as traditional single-stage evaporative coolers.
In the first stage of a two-stage cooler, the unit pre-cools warm air indirectly without adding humidity by passing inside a heat exchanger. In the direct stage, that air passes through a water-soaked pad and will pick up humidity as it cools. As the air supply to the second-stage evaporator is pre-cooled, less humidity is added to the air. The result, say manufacturers, is cool air with a relative humidity between 50% and 70%. This will depend on the climate and compares with a traditional system that will produce relative humidity of around 80%.
Cooler Pads
It is normal for cooler pads to consist of excelsior – wood wool, aspen wood fibre – inside a containment net. But more modern materials, such as some plastics and melamin paper, are now entering use. Wood can absorb some of the water, which lets the wood fibres to cool passing air. The thickness of this padding will play a large part in the efficiency of the process as it allows longer air contact. An eight-inch-thick pad, for example, will be more efficient than a one-inch pad.
Comparison to Air Conditioning
Advantages
Less expensive to install
- Estimated cost for installation is 1/8 to 1/2 that of refrigerated air conditioning
Less expensive to operate
- Estimated cost of operation is 1/4 that of refrigerated air
- Limits power consumption to the fan and water pump vs. compressors, pumps and blowers
Ventilation air
- 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
Disadvantages
Performance
- High temperature, high humidity decreases the cooling capability of the evaporative cooler
- No dehumidification – Traditional air conditioners remove moisture from the air, which can be a design requirement except in very dry locations; evaporative cooling adds moisture, which, in dry climates, may improve thermal comfort
Comfort
- Air from 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; affecting the life of electronic and other equipment
- High humidity in air may cause condensation; a problem in some situations (e.g. electrical equipment, computers, paper/books, old wood)
Water
- Evaporative coolers need a constant supply of water to wet the pads
- Water high in mineral content will leave 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. You will need to drain the cooler, as well as clean it periodically and replace the pads
Miscellaneous
- Pollen, odours and other outdoor contaminants may be blown into the building unless sufficient filtering is in place
- The vents that let air exit the building may pose a physical security risk
- Asthma patients may need to avoid environments using evaporative coolers