Evaporative cooler

An evaporative cooler, photographed in Rocky Ford, Colorado, used in the drier parts of the American West to provide economical cooling

A misting fan

An evaporative cooler (also swamp cooler, desert cooler, and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from typical air conditioning systems which use vapor-compression or absorption refrigeration cycles. Evaporative cooling works by employing water's large enthalpy of vaporization. The temperature of dry air can be dropped significantly through the phase transition of liquid water to water vapor, which requires much less energy than refrigeration. In extremely dry climates, it also has the added benefit of conditioning the air with more moisture for the comfort of occupants. Unlike refrigeration, it requires a water source, and must continually consume water to operate.

In the United States, the use of the term swamp cooler may be due to the odor of algae produced by early units.^[1] Air washers and wet cooling towers use the same principles as evaporative coolers but are optimized for purposes other than cooling the air inside a building. For example, an evaporative cooler may be designed to cool coils of a large air conditioning system to increase its efficiency.

Evaporative cooling is especially well suited for climates where the air is hot and humidity is low. In the United States, the western/mountain states are good locations, with evaporative coolers prevalent in cities like Denver, Salt Lake City, Albuquerque, El Paso, Tucson, and Fresno where sufficient water is available. Evaporative air conditioning is also popular and well-suited to the southern (temperate) part of Australia. In dry, arid climates, the installation and operating cost of an evaporative cooler can be much lower than that of refrigerative air conditioning, often by 80% or so. However, evaporative cooling and vapor-compression air conditioning are sometimes used in combination to yield optimal cooling results. Some evaporative coolers may also serve as humidifiers in the heating season.

In locations with moderate humidity 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) 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.

History

The evaporative cooler was the subject of numerous U.S. patents in the twentieth century; many of these, starting in 1906,^[2] suggested or assumed the use of excelsior (wood wool) pads as the elements to bring a large volume of water in contact with moving air to allow evaporation to occur. A typical design, as shown in a 1945 patent, includes a water reservoir (usually with level controlled by a float valve), a pump to circulate water over the excelsior pads and a squirrel-cage fan to draw air through the pads and into the house.^[3] This design and this material remain dominant in evaporative coolers in the American Southwest, where they are also used to increase humidity.^[4]

Evaporative cooling was in vogue for aircraft engines in the 1930s, for example with the Beardmore Tornado airship engine. Here the system was used 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°C, as the actual boiling point is a function of the pressure. The superheated water was then sprayed through a nozzle into an open tube, where it flashed into steam, releasing its heat. The tubes could be placed under the skin of the aircraft, resulting in a zero-drag cooling system.^{[citation needed]} 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 added complexity and reliability issues. 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 U.S. developers used ethylene glycol instead, cooling the liquid in radiators. The Germans instead used streamlining and positioning of traditional radiators. Even the method's most ardent supporters, Heinkel's Günter brothers, eventually gave up on it in 1940.^{[citation needed]}

Externally-mounted evaporative^[5] cooling devices to cool interior air were used in some automobiles, often as aftermarket accessories, until modern vapor-compression air conditioning became widely available.

Physical principles

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, the amount of heat that is needed to evaporate the liquid, is drawn from the air. 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.

A simple example of natural evaporative cooling is perspiration, or sweat, which the body secretes in order to cool itself. The amount of heat transfer depends on the evaporation rate, however for each kilogram of water vaporized 2257 kJ of energy (about 890 BTU per pound of pure water, at 95°F) are transferred. The evaporation rate in turn depends on the humidity of the air and its temperature, which is why one's sweat accumulates more on hot, humid days: the perspiration cannot evaporate.

Evaporative cooling is not the same principle as that used by vapor-compression refrigeration units, although that process also requires evaporation (although the evaporation is contained within the system). In a vapor-compression cycle, after the refrigerant evaporates inside the evaporator coils, the refrigerant gas is compressed and cooled, causing it to return to its liquid state. In contrast an evaporative cooler's water is only evaporated once. In a space-cooling unit the evaporated water is introduced into the space along with the now-cooled air; in an evaporative tower the evaporated water is carried off in the airflow.

Applications

Evaporative cooling is a common form of cooling buildings for thermal comfort since it is relatively cheap and requires less energy than 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 cooling also raises the internal humidity level significantly, which can cause problems such as lumpy salt, swelling of wood paneling, doors and trim, pianos going out of tune or suffering internal rusting, etc.

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. 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.

An application of a similar principle to evaporative cooling is the "self-refrigerating" beverage can. A separate compartment inside the can contains a desiccant and a liquid. Just before drinking a tab is pulled so that the desiccant comes into contact with the liquid and dissolves. As it does so it absorbs an amount of heat energy called the latent heat of fusion. Evaporative cooling works with the phase change of liquid into vapour and the latent heat of vaporization but the self-cooling can uses the change from solid to liquid and the latent heat of fusion to achieve the same result.

On Earth, trees transpire large amounts of water through pores in their leaves called stomata, and through this process of evaporative cooling, forests interact with climate at local and global scales.^[6]

Evaporative cooling has been observed to operate on a planetary scale on Pluto, where it has been called 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

All designs take advantage of the fact that water has one of the highest known enthalpy of vaporization (latent heat of vaporization) values of any common substance.

Evaporative cooler illustration

Direct evaporative cooling (open circuit) is used to lower the temperature of air by using latent heat of evaporation, changing liquid water to water vapor. In this process, the energy in the air does not change. Warm dry air is changed to cool moist air. The heat of the outside 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. Traditional evaporative coolers use only a fraction of the energy of vapor-compression or absorption air conditioning systems. Unfortunately, except in very dry climates they 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 pre-cooled air passes through a water-soaked pad and picks up humidity as it cools. Since the air supply is pre-cooled in the first stage, less humidity is needed in the direct stage to reach the desired cooling temperatures. The result, according to manufacturers, is cooler air with a relative humidity between 50 and 70 percent, depending on the climate, compared to a traditional system that produces about 70–80 percent relative humidity air.

Typical

Typically, residential and industrial evaporative coolers use direct evaporation and can be described as an enclosed metal or plastic box with vented sides containing a centrifugal fan or 'blower', electric motor with pulleys (known as 'sheaves' in HVAC), (or a direct-driven axial fan), and a water pump to wet the evaporative cooling pads. The units can be mounted on the roof (down draft, or downflow), or exterior walls or windows (side draft, or horizontal flow) of buildings. To cool, the fan draws ambient air through vents on the unit's sides and through the damp pads. Heat in the air evaporates water from the pads which are constantly re-dampened to continue the cooling process. Thus cooled, moist air is then delivered to the building via a vent in the roof or wall.

Because the cooling air originates outside the building, one or more large vents must exist to allow air to move from inside to outside. Air should only be allowed to pass once through the system, or the cooling effect will decrease. This is due to the air reaching the saturation point. Often 15 or so air changes per hour (ACHs) occur in spaces served by evaporative coolers.

Didcot Power Station cooling towers

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 melamine 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.

Evaporative (wet) cooling towers

Main article: Cooling tower

Cooling towers are structures for cooling water or other working media to near-ambient wet-bulb temperature. Wet cooling towers operate on the evaporative cooling principle, but are optimized to cool the water rather than the air. Cooling towers can often be found on large buildings or on industrial sites. They transfer heat to the environment from chillers, industrial processes, or the Rankine power cycle, for example.

Misting systems

Mist spraying system with water pump beneath

Misting systems work by forcing water via a high pressure pump and tubing through a brass and stainless steel mist nozzle that has an orifice of about 5 micrometres, thereby producing a micro-fine mist. The water droplets that create the mist are so small that they instantly flash evaporate. Flash evaporation can reduce the surrounding air temperature by as much as 35 F° (20 C°) in just seconds.^[7] For patio systems, it is ideal to mount the mist line approximately 8 to 10 feet (2.4 to 3.0 m) above the ground for optimum cooling. Misting is used for applications such as flowerbeds, pets, livestock, kennels, insect control, odor control, zoos, veterinary clinics, produce cooling and greenhouses.

Misting fans

A misting fan is similar to a humidifier. A fan blows a fine mist of water into the air. If the air is not too humid, the water evaporates, absorbing heat from the air, allowing the misting fan to work as an air conditioner. A misting fan may be used outdoors, especially in a dry climate.

Performance

Understanding evaporative cooling performance requires an understanding of psychrometrics. Evaporative cooling performance is dynamic due to changes in external temperature and humidity level. A residential cooler should cool air to within 3–4 C° (5–7 F°) of the corresponding wet-bulb temperature.

It is simple to predict cooler performance from standard weather report information. Because weather reports usually contain the dewpoint and relative humidity, but not the wet-bulb temperature, a Psychrometric chart must be used to identify the wet bulb temperature. Once the wet bulb temperature and the dry bulb temperature are identified the cooling performance or leaving air temperature of the cooler may be determined:

T_LA = T_DB – ((T_DB – T_WB) x E)

T_LA = Leaving Air Temp

T_DB = Dry Bulb Temp

T_WB = Wet Bulb Temp

E = Efficiency of the evaporative media.

Evaporative media efficiency usually runs between 80% to 90% and the evaporation efficiency drops very little over time. Typical aspen pads used in residential evaporative coolers offer around 85% efficiency while CELdek type of evaporative media offer efficiencies of 90% + depending on air velocity. The celdec media is more often used in large commercial and industrial installations.

As an example, in Las Vegas, Nevada, with a typical summer design day of 108°F DB/66°F WB or about 8% relative humidity, the leaving air temperature of a residential cooler would be:

T_LA = 108° – ((108° – 66°) x 85% efficiency)

T_LA = 72.3°F

However, either of two methods can be used to estimate performance:

• Use a Psychrometric chart to calculate wet bulb temperature, and then add 6–8 F° as described above.
• Use a rule of thumb which estimates that the wet bulb temperature is approximately equal to the ambient temperature, minus one third of the difference between the ambient temperature and the dew point. As before, add 6–8 F° as described above.

Some examples clarify this relationship:

• At 32 °C (90 °F) and 15% relative humidity, air may be cooled to nearly 16 °C (61 °F). The dew point for these conditions is 2 °C (36 °F).
• At 32 °C (90 °F) and 50% relative humidity, air may be cooled to about 24 °C (75 °F). The dew point for these conditions is 20 °C (68 °F).
• At 40 °C (104 °F) and 15% relative humidity, air may be cooled to nearly 21 °C (70 °F). The dew point for these conditions is 8 °C (46 °F).

(Cooling examples extracted from the June 25, 2000 University of Idaho publication, "Homewise").

Because evaporative coolers perform best in dry conditions, they are widely used and most effective in arid, desert regions such as the southwestern USA and northern Mexico.

The same equation indicates why evaporative coolers are of limited use in highly humid environments: for example, a hot August day in Tokyo may be 30 °C (86 °F), 85% relative humidity, 1,005 hPa pressure. This gives dew point 27.2 °C (81.0 °F) and wet-bulb temperature 27.88 °C (82.18 °F). According to the formula above, at 85% efficiency air may be cooled only down to 28.2 °C (82.8 °F) which makes it quite impractical.

Comparison to air conditioning

Comparison of Evaporative cooling to phase-change air conditioning:

Advantages

Less expensive to install

• Estimated cost for installation is about half that of central refrigerated air conditioning.^[8]

Less expensive to operate

• 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.

Ease of Maintenance

• The only two mechanical parts in most basic evaporative coolers are the fan motor and the water pump, both of which can be repaired at low cost and often by a mechanically inclined homeowner.

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 the breathability of the air.
• The pad itself acts as a rather effective air filter when properly maintained; it is capable of removing a variety of contaminants in air, including urban ozone caused by pollution, regardless of very dry weather. Refrigeration-based cooling systems lose this ability whenever there is not enough humidity in the air to keep the evaporator wet while providing a constant trickle of condensate that washes out dissolved impurities removed from the air.

Disadvantages

Performance

• High dewpoint (humidity) conditions decrease the cooling capability of the evaporative cooler.
• No dehumidification. Traditional air conditioners remove moisture from the air, except in very dry locations where recirculation can lead to a buildup of humidity. Evaporative cooling adds moisture, and in dry climates, dryness may improve thermal comfort at higher temperatures.

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, particularly in the presence of dust. 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).

Water

• 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. Bleed-off and refill (purge pump) systems may reduce this problem. These deposits will include radioactive isotopes if the water is from a radioactive source such as deep wells in certain Texas aquifers. Such deposits would tend to concentrate the radioactivity, whether by crystallization of dissolved radioactive elements onto the pads or accumulation of radioactive particulates on the pads.
• The water supply line may need 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.

Miscellaneous

• Odors and other outdoor contaminants may be blown into the building unless sufficient filtering is in place.
• Asthma patients may need to avoid poorly maintained evaporatively cooled environments.
• A sacrificial anode may be required to prevent excessive evaporative cooler corrosion.
• Wood wool of dry cooler pads can catch fire even by small sparks.

See also

• Architectural engineering
• Building engineering
• Car cooler
• Pot-in-pot refrigerator

References

1. ^ Arthur William Gutenberg (1955). The Economics of the Evaporative Cooler Industry in the Southwestern United States. Stanford University Graduate School of Business. p. 167. http://books.google.com/books?id=uq1EAAAAIAAJ. 
2. ^ John Zellweger (1906). "Air filter and cooler". U.S. patent 838602. http://www.google.com/patents?=04pHAAAAEBAJ&printsec=abstract&zoom=4&dq=cooling+excelsior&as_drrb_is=b&as_minm_is=1&as_miny_is=1799&as_maxm_is=1&as_maxy_is=1910&num=30#PPA2,M1. 
3. ^ Bryant Essick (1945). "Pad for evaporative coolers". U.S. patent 2391558. http://www.google.com/patents?id=Z2BKAAAAEBAJ&pg=PA1&dq=excelsior+evaporative-cooler&as_drrb_is=b&as_minm_is=1&as_miny_is=1900&as_maxm_is=1&as_maxy_is=1950&num=30&rview=1&source=gbs_selected_pages&cad=0_1#PPA1,M1. 
4. ^ Scott Landis (1998). The Workshop Book. Taunton Press. p. 120. ISBN 9781561582716. http://books.google.com/books?id=bs7I7qf5cUQC&pg=PA120&dq=evaporative+cooler+%22squirrel+cage%22+southwest+popular. 
5. ^ Such units were mounted on the passenger-side window of the vehicle; the window was rolled nearly all the way up, leaving only enough space for the vent which carried the cool air into the vehicle.
6. ^ Gordon B. Bonan. Forests and Climate Change: Forcings, Feedbacks, and the Climate Benefits of Forests. 13 June 2008 Vol. 320 Science. [1]
7. ^ http://www.cool-off.com/faqs.html
8. ^ John Krigger and Chris Dorsi (2004). Residential Energy: Cost Savings and Comfort for Existing Buildings (4th ed.). Saturn Resource Management. p. 207. ISBN 9781880120125. http://books.google.com/books?id=7HlKF4trR-YC&pg=PA207&dq=evaporative+coolers+cost+install#v=onepage&q=evaporative%20coolers%20cost%20install&f=false. 

External links

• Holladay, April (2001). "A swamp cooler cools air by evaporation". WonderQuest Weekly Q&A science column. USAToday.com. http://www.wonderquest.com/swamp-coolers.htm. Retrieved 2006–07–14. 
• PATH Tech Inventory: Two Stage Evaporative Cooler
• PATH Tech Inventory: Evaporative Cooler
• Evaporative cooling simulation
• Coolerado indirect evaporative cooling