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How do swamp coolers / evaporative coolers work?
Want to know why evaporation lowers temperature?
More on how evaporative cooling works ...
Benefits of evaporative cooling

How do swamp coolers / evaporative coolers work?

Swamp coolers are an efficient and effective machine for cooling. As a direct placement for air conditioning in dry climates, it is an example of how we can work with nature. Being so much less expensive than air conditioning, it almost seems that we are getting something for nothing.

The way a swamp cooler operates is very simple. There is a low horsepower motor which pumps the water from the floor of the cooler to the top of the cooler, where it proceeds to fall down the sides, along porous filter pads. A second motor drives a fan which pulls air from the outside, through the cooler, and then pushes it into the hot room. The significant cooling action is the water evaporating as the air passes through it. (Incidentally, the water level is kept constant with the help of a floating sphere functioning similar to the one in the toilet bowl.) The hot air enters the cooler, where two small motors power nothing more than a fan and a pump, in order to send cool air into the hot room.

The way the air is cooled in the cooler is similar to the way evaporating sweat cools the human body. When a substance is perceived at a certain energy heat level, measured in temperature; there is really a distribution of varying levels of temperature throughout the molecules of the substance. This assortment of temperatures average out to the measured value. Most of the molecules can be around the average, and the farther from the average, the less of them there are. For example, water at room temperature has most of its molecules at approximately that temperature. But it also contains molecules that are near the boiling point of water, and also near its freezing point; however small in quantity they are, they are an important presence. Because at the boiling point of water, there are molecules that are a gas and that are a liquid. The liquid molecules will absorb energy in the form of heat t o become a gas and escape the confines of its old form. As the molecules from the higher temperature evaporate, the remaining liquid averages less heat. But there will still be water at the higher temperatures because the remaining molecules redistribute themselves along the bell curve, which enables the next molecules to evaporate. Heat is siphoned off this way from the water. More importantly, heat is extracted from the air as the boiling point liquid water grabs the needed energy for its freedom into gas.

An evaporative cooler produces effective cooling 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 house or building by a large blower. As this happens, the temperature of the outside air can be lowered as much as 30 degrees.
Probably because evaporative coolers add moisture to the air and blow it around, they are sometimes knows as "swamp coolers." Evaporative coolers can work wonderfully well, provided the outside air they are drawing in is dry and desert-like. As the humidity increases, however, the ability for them to cool the air effectively decreases. Simply put, swamp coolers were not designed to work in swamp-like conditions.

Air conditioning, on the other hand, became popular because of its ability to cool the air, no matter what the humidity might be. Even on humid days, room and central air conditioners can lower the temperature to a thermostatically controlled temperature. They also use as much as four times as much electricity than swamp coolers do, and they are more expensive to install and maintain. Air conditioners can require ozone-damaging refrigerants, and they recirculate the same air over and over.

Fairly popular in desert areas, swamp coolers will work fine most of the time in California's more humid climates. Sacramento, for example, averages about 30 percent humidity on a typical hot summer afternoon, still dry enough for evaporative cooling to work effectively.

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Want to know why evaporation lowers temperature?

The process of evaporation happens all the time. Our bodies, for example, perspire in hot weather; through evaporation the sweat dries and drops our body temperature.

Whenever dry air passes over water, some of the water will be absorbed by the air. That's why evaporative cooling naturally occurs near waterfalls, at rivers, lakes and oceans. The hotter and drier the air, the more water that can be absorbed. This happens because the temperature and the vapor pressure of the water and the air attempt to equalize. Liquid water molecules become gas in the dry air, a process that uses energy to change the physical state. Heat moves from the higher temperature of the air to the lower temperature of the water. As a result, the air is cooler. Eventually the air becomes saturated, unable to hold more water, and evaporation ceases.

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More on how evaporative cooling works ...

An evaporative cooler is essentially a large fan with water-moistened pads in front of it. The fan draws warm outside air through the pads and blows the now-cooled air throughout the house.

The pads can be made of wood shavings - wood from aspen trees is a traditional choice - or other materials that absorb and hold moisture while resisting mildew.

Small distribution lines supply water to the top of the pads. Water soaks the pads and, thanks to gravity, trickles through them to collect in a sump at the bottom of the cooler. A small recalculating water pump sends the collected water back to the top of the pads.

Since water is continually lost through evaporation, a float valve - much like the one that controls the water in a toilet tank - adds water to the sump when the level gets low. Under normal conditions, a swamp cooler can use between 3 to 15 gallons of water a day.

A large fan draws air through the pads, where evaporation drops the temperature approximately 20 degrees. The fan then blows this cooled air into the house.

Small units can be installed in a window, blowing cooled air directly into a room. Larger units can blow air into a central location, or the air can travel through ductwork to individual rooms.

Normal air conditioning is a closed system, taking air from inside a house and recycling it. For air conditioning to function properly, doors and windows should be closed. Evaporative cooling, however, takes air from outside the house. For evaporative cooling to work properly, the cooled outside air must be allowed to escape. By choosing which doors or windows in your home you leave open, you can to help direct the flow of cooled air to areas where it is needed.

In addition to the dropping the temperature of the air, evaporative cooling offers an additional cooling benefit. The constant movement of the air created by the blower - the cooling breeze it creates, if you will - makes the occupants of a room feel 4 to 6 degrees cooler than the actual temperature. This is the same effect you feel when you turn on a ceiling fan or a simple window fan. For this reason, the "effective temperature" created by an evaporative cooler will feel 4 to 6 degrees cooler than temperatures shown on the chart.

An added benefit of evaporative cooling is that it works best in the hottest time of the day. As the temperature outside increases as the sun climbs, the humidity normally drops. In the early morning, for example, the temperature may be 70 degrees, with a relative humidity of 60 percent. By mid-afternoon, when the temperature has climbed to 90 degrees, the humidity may well have dropped to 30 percent - conditions that make evaporative cooling work more effectively.

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Benefits of evaporative cooling

• Thanks to a new awareness of energy efficiency, evaporative coolers are achieving a new popularity. Remember, swamp coolers use as much as 75 percent less electricity as air conditioning does.
• Because the technology is simpler, an evaporative cooler costs about half as much as an air conditioner that will cool the same sized area.
• Evaporative coolers operate on 120-volt electricity, which means they don't need special high-amperage circuits like many air conditioners do. A swamp cooler can be plugged into a nearby outlet.
• Many people appreciate the fact that evaporative cooling adds moisture to the air, which helps to keep wood furniture and fabrics from drying out. The moist pads through which the outside air flows are also fairly efficient air filters, trapping some dust and pollen. Since the pads are continually wetted, trapped particles are flushed out and trapped in the bottom of the cooler.
• Air conditioning works best when the windows are closed, since interior air is cooled and recalculated. Because swamp coolers cool outside air and blow it into the house, however, to work effectively they need at least one window open. The cooled outside air vents through open windows or doors, pushing out hot inside air and any smoke, odors and pollution that may be present. With evaporative cooling, a complete air change in a home occurs every one-to-three minutes. This flow of fresh air means that evaporative coolers can be operated without using the water pump to replace warm stale air with cooler nighttime air, much like a whole house fan does. That's an added benefit.
• Small evaporative coolers can be often placed in windows, much like a window air conditioner. This requires very little installation. Larger units usually require ducts to distribute the air, but these can be an existing forced air duct system in the house.
• For the most part, evaporative cooling doesn't require as much ductwork as air conditioning. For a newly installed system, a short duct can direct the cooled air to a central point in the house. From there, air can be directed through the various rooms by simply opening and closing doors and windows to allow the cooled air to flow.

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