A Guide to Evaporative Coolers

Splash some water on your face on a hot day in the summer and you will immediately feel cooler. That's evaporative cooling working. An evaporative cooler works on the same principle - evaporating water provides cooling - but they combine water evaporation with air movement to cool our homes.

Evaporative coolers are sometimes called "swamp coolers," probably because they add humidity to the air, making it more "swamp like." The concept of evaporative cooling has been around for years, when people living in arid climates used to hang damp clothes in their windows at night, knowing the breeze blowing through the window would help cool the room.

How Do They Work?

While evaporative coolers have moved beyond the "hang a cloth in the window" stage, the principle remains the same. Nowadays an evaporative cooler usually consists of a large box containing a fan surrounded by a wet pad, mounted on the roof or side of a house. A pump circulates water to keep the pad wet and the fan blows air through the pad. The damp (but cooler) air is blown into the house driving the hotter inside air out through open windows or vents.

According the US Department of Energy, the difference between the inside air temperature and the cooled air can be expected to be in the range of 15 to 20 degrees, but the difference can be as high as 40 degrees in certain conditions. As an added bonus, evaporative coolers provide incremental cooling effect, since the constant air movement caused by the cooler's fan actually makes a room's temperature feel even cooler.

Coolers versus Air Conditioners

While both evaporative coolers and air conditioners aim to do the same thing - cool our homes - evaporative coolers are definitely low tech and unlike air conditioners in many ways. To start with, evaporative coolers add humidity to the air, while air conditioners remove humidity. They don't use any refrigerants (like Freon) so they aren't potentially damaging to the atmosphere. Evaporative coolers provide a constant supply of fresh air, while an air conditioner operates best in a closed environment and re-circulates the same air over and over. Air conditioners also cost substantially more to install and maintain than evaporative coolers, and use three to four more times electricity to operate.

Sizing an Evaporative Cooler

The size of an evaporative cooler is measured in cubic feet per minute (cfm), or the number of cubic feet of air a cooler can move in a minute. Household units are commonly available in sizes from 3000 cfm to 25000 cfm. Manufacturers recommend that a properly sized unit should be able to change the air in a house 20 to 40 times in an hour. You can determine the proper size unit for your home by calculating the cubic feet in your home (i.e., 1500 square foot house with 8 foot ceilings is 12,000 cubic feet) and dividing that number by 2. So in this case, the 12,000 cubic foot house will require a 6000 cfm evaporative cooler.

Benefits of Evaporative Cooling

There are a number of economic and environmental benefits to using an evaporative cooler. For example:

• Evaporative coolers use substantially less electricity than an air conditioner for the same sized house (some estimates are as much as 75 percent less).
• Evaporative coolers use simple technology and as a result are much cheaper to buy, install and maintain. Plus, they operate on regular household current so they don't require a dedicated circuit.
• In dry climates, adding moisture to the air can help keep both furniture and fabrics from drying out.
• The moist pads in the cooler act as air filters, helping keep dust and pollen out of the house while providing a constant supply of fresh air into the home.
• Evaporative coolers are even available in small sized window and portable units that can be used to cool an individual room.

Sounds great! Why don't we use them everywhere?

• Unfortunately, evaporative coolers work best when the air they are pulling in is dry. As the humidity in the outside air increases, the effectiveness of evaporative cooling goes down. In reality, evaporative coolers work best only in certain areas such as the southwest or where the climate is arid and the air is dry. In areas with higher humidity, evaporative coolers aren't really an option since they'd actually be adding moisture to the air and there would be no cooling effect.
• Another drawback to evaporative coolers is they use a substantial amount of water (up to 15 gallons per day) to provide their cooling effect. In dry areas, this consumption can put a real strain on water supplies.

In spite of the fact that evaporative coolers may not be a cooling option for all of us, there is no doubt that they do provide energy savings and reduce the risk of damage to the environment. So even if we all can't use an evaporative cooler, we all do benefit from the energy conservation and environmental protection they provide.

The Physical Principles and Designs of Evaporative Cooling

Evaporative cooling is a physical phenomenon in which evaporatin 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.

Evaporative cooler designs
All designs take advantage of the fact that water has the highest known enthalpy of vaporization (latent heat of vaporization) of any 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.

2011-7-17How does the evaporative air cooler work?

     It used to be that people who cared about the environment were looked down upon. People called them derogatory names like tree hugger, eagle freaks and greenies. Nowadays, it's considered a smart thing to care about the world we all share and the air we all breathe. We also care about keeping cool on a warm day. How do we merge these two? Evaporative air coolers.

    Evaporative air  Coolers like the Swamp Cooler are coolers that use the cooling effect of water evaporation to lower air temperature. Think about when you've just exerted your body by playing a sport or running to catch some sort of public transportation. You feel hot, sweaty and out of breath. Once your sweat evaporates, you feel cooler though. That is the idea behind evaporative cooling.

   Evaporative coolers use 75% less energy than air conditioners. That's not the only way they are good for the environment though. They don't use refrigerants like freon. Freon destroys the ozone layer. They also curb noise pollution by using a blower instead of an axial fan like an air conditioner.

    Some people would think that with less energy and without freon, evaporative coolers aren't that powerful, but they can cool up to 350 square feet. They also help maintain the durability and lifespan of furniture by keeping furniture and fabric moisturized. Evaporative coolers also channel fresh air into the area they are cooling. As well as the hot air in the area being pushed out by the cool air through evaporation, bad odor, dust and smoke are also eliminated. Air circulation occurs every two to three minutes giving you a continuous supply of fresh air. The risk of bacteria being trapped in the air is also reduced. The moisture pad also acts as a filter which helps trap dust.

     Evaporative coolers are also easy to take care of. They have refillable water tanks. Each refill should last up to ten hours. Sometimes they can have a hose connection which will continuously supply fresh water to the cooler. The filters in evaporative coolers can often be removed and rinsed.

     It's finally easy being green.