Evaporative cooler

Evaporative coolers (also called air, swamp, or desert coolers) devices which use simple evaporation of water in air. 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; in Australia, evaporative air conditioning is popular in Perth. 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.

Additional recommended knowledge

Essential Laboratory Skills Guide

Daily Sensitivity Test

What is the Correct Way to Check Repeatability in Balances?

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.^[1] Nowadays Iranians have changed the windcatcher into an evaporative cooler (Coolere Âbi) and use it widely. There are 9,000,000 evaporative coolers in central Iran, and in just the first two months of year 1385 in the (Persian/Iranian calender) (April–May 2006) 130,000 evaporative coolers were sold in Iran.

Evaporative cooler designs

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, and the water too, 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 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.

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]), 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.

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.

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 reject heat to the environment from chillers, industrial processes, or the Rankine power cycle, for example.

Misting systems

  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 [1]. For patio systems, it is ideal to mount the mist line approximately 8 to 10 feet above the ground for optimum cooling. Misting is used for many different applications including orchids, pets, livestock, kennels, insect control, odor control, zoos, veterinary clinics, produce cooling, greenhouses, etc.

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. Under typical operating conditions, an evaporative cooler will nearly always deliver air cooler than 27°Celsius (80°Fahrenheit). A typical residential 'swamp cooler' in good working order should cool air to within 3°C–4°C (6°F–8°F) of the wet-bulb temperature.

In practice, it may be difficult to predict swamp cooler performance from standard weather report information, because weather reports usually contain the dewpoint and relative humidity, but not the wet bulb temperature. However, you may use either of two methods to estimate performance:

• Use a Psychrometric chart to calculate wet bulb temperature, and then add 6°F–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 two thirds of the difference between the ambient temperature and the dewpoint. As before, add 6°F–8°F as described above.

Some rough examples clarify this relationship.

• At 32°C (90°F) and 15% relative humidity, air may be cooled to nearly 16°C (60°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 (105°F) and 15% relative humidity, air may be cooled to nearly 21°C (70°F). The dew point for these conditions is 8°C (~47°F).

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.

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

Comparison to phase-change 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.
• Power consumption is limited 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.

Disadvantages

Performance

• 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.

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).

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. 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.

Miscellaneous

• 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.

Legionnaire's Disease

Evaporative coolers, like all equipment, require maintenance. Bacterium Legionella pneumophila and related bacteria ... "may be found in purpose built water systems such as cooling towers, evaporative condensers..." ^[2] As such, it is critical that evaporative coolers be properly installed and adequately maintained according to their manufacturers' recommendations.

See also

• Air conditioning
• Cooling tower
• Cooling tower system
• Pot-in-pot refrigerator
• Psychrometrics
• Dew point
• Evaporation
• Humidity
• HVAC
• Architectural engineering
• Building engineering

References