Evaporative Cooling

April 25, 2007
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Back in the 1920s, people who lived in places like Arizona would often sleep outside on screened-in porches during the summer.1 On hot nights, they’d hang bed sheets or blankets soaked in water inside the screens. Whirling electric fans would pull the night air through the moist cloth to cool the room.

That’s the concept — albeit slightly less refined — behind what we now know as evaporative cooling, a lower-cost, lower-technology alternative to refrigerated air conditioning. Evaporative cooling, or thermal dynamics, effectively cools the environment by combining a natural process — water evaporation — with a simple, reliable air-moving system. Fresh outside air is pulled through moist pads where it is cooled by evaporation and circulated through a building by a large blower. As this happens, the temperature of the outside air can be lowered as much as 30°F.1

Combating heat stress

With winter behind us and warmer weather on its way, safety personnel once again need to consider effective ways to keep workers from overheating. In many parts of the country, and in virtually any industrial workplace regardless of outside temperature, hot conditions can make for uncomfortable — and even dangerous — work environments, both indoors and outdoors.

In addition to such serious and potentially deadly heat-related illnesses as heat stroke and heat exhaustion, hot work conditions can cause problems like nausea, cramping and fatigue, all of which not only adversely affect workers, but can also slow down productivity. Employers need to find ways to combat heat stress, and evaporative cooling can be a cost-effective method of producing cooler indoor temperatures.2

While most companies would love to just simply install air conditioning, this is usually not a viable option at your average large industrial facility. Air conditioners use at least four times the amount of electricity than evaporative cooling systems do,1 and they are more expensive to install and maintain. In addition, air conditioners can require ozone-damaging refrigerants, plus they recirculate the same air over and over — a potential health issue.

Evaporative cooling systems lower air temperature while increasing relative humidity (RH) using mists, sprays, wetted pads or through the use of industrial fog fans. For evaporative cooling to work properly, the cooled outside air must be allowed to escape. By choosing which doors or windows in your facility to leave open, you can help direct the flow of cooled air to areas where it is needed.

Out of the fog

Fog systems can provide cooler work conditions through the process of evaporative cooling.3 Water requires energy to evaporate (600 calories of heat per gram of water). As the fog evaporates, the energy exchange results in reduced temperature. This cooling process becomes more effective as the temperature rises (85°F to 115°F is ideal) and the humidity level drops (the lower the humidity, the better). The temperature reduction using fogging systems will depend on the climate conditions, but cooling up to 40°F is achievable under ideal circumstances.3

Fogging systems include tubing attached to or suspended from a structure or incorporated into a fan, impeller-driven fog nozzles, and a high-pressure pump. In an enclosed environment the system will initially create cooling. Over time, the humidity level increases and high levels of humidity can be artificially created and maintained. Not only are cooler temperatures achieved, but the process is also useful for removing static electricity in electronics production facilities, or removing dust and odors in many industrial and agricultural applications.

Humid climates

Since fog fans introduce moisture into the air, they create humidity. Yet, despite this, they can be effective in humid environments as well as drier environments, as long as the water fog droplets are air-aspirated.3 Cooling of air by evaporating water can only occur when the RH is less than 100%, or unsaturated. As the RH drops from 100%, more water can evaporate into the air to achieve temperature reductions. This can be done without wetting people or surfaces by pressurizing the water to over 1,000 pounds per square inch (psi) and forcing that water through a 0.008” orifice nozzle with an impeller behind the orifice, generating practically invisible 10-micron water droplets. The smaller the water droplet the greater the exposed surface area, for that given amount of water, to flash-evaporation and temperature reduction.

Maximum cooling without wetting is achieved by staging the amount of water fog-injected into the fan’s air stream. This way only the proper amount of water is present to assure all available water is evaporated. As the temperature rises and/or the RH declines, more water is added to the air stream to gain even larger reductions in temperature for people in the air from the fan. Circulation of air is the key to providing comfort in humid areas.

FOOTNOTES

1 Evaporative Cooling – How an Evaporative Cooler Works, www.consumerenergycenter.org

2 Ohio State University Fact Sheet, AEX-127-99

3 Cool Zone USA Web site

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