Evaporative Cooling
From Swikipedia
Introduction
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.
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, which 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.
Brief Description
Compared to Air-Conditioning, Evaporative Cooling has the following advantages and disadvantages:
Advantages
1. Less expensive to install
• Estimated cost for installation is about half that of central refrigerated air conditioning.
2. 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.
3. 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 for very little and often by a mechanically able homeowner.
4. 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
1. 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.
2. 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).
3. 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.
• 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.
4. 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.
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