If you've ever tested the wind by holding a wet finger in the air, you've used evaporative cooling. The same principle cools you off after a swim, and it also powers one of the oldest and simplest forms of air conditioning. Known in the U.S. as swamp coolers, modern evaporative coolers can trace their lineage to ancient Egypt. They're cheap, efficient and good for the environment, but they come with some limitations, so don't push your standard air conditioner out the window just yet.
The ancient Egyptians had a great need for air conditioning. They accomplished it by hanging wet blankets across the doors of their homes or, if they happened to be royalty, having servants fan them across jugs of water. When hot, dry air passes over water (or better yet, through it) the air cools off. Nowadays, we use electric fans instead of servants, but the principle of cooling the air by evaporation remains the same.
Unfortunately, evaporative air coolers don't work everywhere. Swamps, for instance, are lousy places for swamp coolers. It's not entirely clear where they got the nickname, but it probably refers to the humidity they add to the air or the swampy smell that can develop when they aren't cleaned often enough. In order to work, they need a hot, dry climate. In the U.S., swamp coolers work well in the arid southwest. There were more than 20 million evaporative coolers worldwide in 1998, with four million of those in the US. According to the Energy Information Administration's 2001 appliance report, only three percent of U.S. households had swamp coolers, but in the states along the Rocky Mountain Range, they could be found in 26 percent of the homes.
Swamp coolers are based on a simple, efficient technology that has been around a long time. The principles of evaporative cooling worked for the pharaohs, and they can still work for you. Next we'll take a look at how they work.
Evaporative Cooling Basics: from Ben Franklin's Underwear to Air Conditioning
July of 1750 was a scorcher in Philadelphia, Penn., with temperatures reaching 100 degrees Fahrenheit. As he later wrote in a letter, Benjamin Franklin was in his room, reading and writing with "no other cloaths on than a shirt, and a pair of long linen trousers, the windows all open and a brisk wind blowing through the house …." Even founding fathers sweat, and as he changed to a dry shirt, he noticed something -- it felt warm, like it had been near a fire, compared to the damp shirt he had just removed. He thought about it, and it brought to mind a century-old book that described the process of cooling water jugs in the desert by wrapping them with wet woolen cloths. Franklin theorized that he wasn't being cooled by the hot air blowing through his room, but by the perspiration evaporating off of his skin. Later, he tried some experiments -- wetting the bulb of a thermometer with spirits that evaporated quicker than water, and then blowing air across it. He managed to bring the temperature down so far that ice froze on the bulb [source: History Carper].
What he describes in his letter is evaporative cooling. Liquid evaporates by shedding molecules into the air, changing from a liquid state to a gas. As they become suspended in the air, the molecules draw some of the heat from the hotter air, cooling it down as the water and air find equilibrium. The process also cools the remaining liquid, as hotter, faster-moving molecules are the most likely to escape into the air.
Swamp coolers work by harnessing that cooling reaction -- you just need a way to circulate the now-cooler, more humid air through the house.
Picture an air conditioner -- just a sheet metal box on the outside of a window, really. In a standard air conditioner, there are some fairly complicated refrigerants inside, but with a swamp cooler, it's much simpler. The main thing inside is a blower -- a fan at one end of the box that brings air in from the outside and pushes it into the house at the other end. Before the air goes into the house, it passes through a set of damp pads, where the evaporation takes place. A small pump keeps the pads moist, so the water doesn't just evaporate away completely. It works just like the Egyptians' woolen blankets or Ben Franklin's sweaty shirt.
So how exactly does a swamp cooler compare to an air conditioner?
Swamp Coolers vs. Air Conditioners
So it's an air conditioner, right? Technically, yes -- it conditions the air by cooling it, but it works much differently than what we commonly think of as an air conditioner.
Standard AC units work by passing air over a set of coils filled with a refrigerant like Freon (a trade name for a variety of chemical blends), which heats and cools as it's compressed and expands. The air is cooled by the coils, sent into your house, and then re-circulated over and over through the machine, venting hot air generated in the process to the outside. (For more about standard air conditioners, see How Air Conditioners Work). It's a closed process -- leaving a window or door open allows the cool air to escape and makes the air conditioner work harder to supply cold air.
Swamp coolers are an open system. They rely on the flow of air through the building to direct the cool air, and since they always need hot, dry air to evaporate the water, it needs to displace the air already in the house. Both systems can use either a large central unit or small window units, but air from the swamp cooler needs a way out. Opening and closing windows and doors controls the air flow from the swamp cooler to different parts of the house, while central air conditioners use ducts to direct the flow. Swamp coolers can also use ducts in some cases, but they need to be larger than traditional air conditioner ducts to account for a greater flow of air from the swamp cooler.
Standard air conditioners also dry the air, condensing water vapor from the cooled room as it passes over the cold coils. The water drains outside -- that's the distinctive drip you'll feel if you stand under a window air conditioning unit long enough. The result is a dryer room, and in humid climates, that can be a good thing. Too much humidity can prevent perspiration, which is how we cool ourselves naturally. Since swamp coolers work by putting water into the dry air, they act as humidifiers. This is great in dryer climates, because humidity can also be too low for comfort. Under the right conditions the water-laden breeze also can have a secondary effect of helping the skin's perspiration, resulting in an even cooler feel than the swamp cooler would give on its own.
Because of the different ways they work, you can't run a swamp cooler and a standard air conditioner in the same house. They would cancel each other out, just like running a dehumidifier and a humidifier in the same room.
So, if you can only pick one, which is better? The swamp cooler or the air conditioner?
Swamp Cooler Benefits
Under the right conditions, swamp coolers look like they have lots of benefits. They're cheap to build and install. The only materials it takes to make them are a blower fan, a pump, a pad (either synthetic or made of wood shavings), some water and a box (usually made of sheet metal). Pumps and fans are widely available. The rest can be turned out in a local shop just about anywhere. In 1998, according to the Evaporative Cooling Institute, there were between 300 and 400 manufacturers in New Delhi turning out one million coolers a year, the smallest of which cost $35 USD [source: New Mexico State University]. The National Association of Home Builders (NAHB) estimates that an evaporative cooler, installed, costs between $700 and $1,000, compared to several thousand dollars for a central air conditioner [source: Toolbase].
The monthly operating costs are considerably lower as well -- about one third that of a standard air conditioner, depending on the costs of electricity and water, according to the NAHB. The savings is in the electricity -- a standard air conditioner takes as much as four times the energy to run than a comparable swamp cooler. That translates to savings on your energy bill, but also in the environment. The electricity saved by those 20 million evaporative coolers in 1998 meant power plants saved 60 million barrels of oil and produced 27 billion fewer pounds of carbon dioxide.
Swamp coolers have a further environmental benefit, since standard air conditioners have long relied on ozone-depleting chemicals to provide their cooling power. The use of CFCs (chlorofluorocarbons) has been discontinued in developed nations since 1996 by international treaty. The replacement byproducts, HCFCs (hydrochlorofluorocarbons), aren't as bad, but they still have some negative effects, and their use in new equipment will end in 2010. The Environmental Protection Agency (EPA) has compiled a list of approved substitutes for both CFCs and HCFCs in air conditioning systems [source: Environmental Protection Agency]. Also on the list: evaporative coolers.
Swamp Cooler Downsides
For all their benefits and cost savings, swamp coolers still only work in the right climate, and that unfortunately doesn't include areas like Philadelphia.
It needs to be not only hot but also dry, because as the dry bulb temperature approaches the wet bulb temperature, the difference between the two gets smaller, and the cooling effect of the evaporating water follows suit. A wet bulb temperature above 70 degrees Fahrenheit means the swamp cooler won't be able to adjust the temperature enough to keep it in the comfort zone. (This varies based on humidity, personal preference and activity, but it generally falls in the low 70s.) [source: Canadian Centre for Occupational Health and Safety].
If the air gets too saturated with water, the water condenses. 100 percent humidity outside the house means it might rain, and while a swamp cooler won't cause a downpour in your home, it won't have any cooling effect either -- that's the hot, sticky feeling you might associate with swamps. Your perspiration just doesn't evaporate into the saturated air. The Environmental Protection Agency recommends between 30 and 60 percent humidity in a house to keep down mold and mildew, and the humid air may keep your wooden furniture from drying out. Unfortunately, it can also cause metal to rust, even in the swamp cooler itself [source: Environmental Protection Agency]. The problem of excess humidity can be solved in evaporative coolers by using a heat exchanger to heat air inside the house while the humid air vents to the outside, but these systems are considerably less efficient than direct evaporative coolers.
Swamp coolers also require maintenance. The pads needs to be cleaned or changed regularly to avoid that swampy smell and associated problems with air quality. Minerals in the water can also build up as water evaporates, requiring a bleed-off of mineral-rich wastewater over time. The coolers also require a steady supply of water -- 3.5 to 10.5 gallons (13.25 to 39.75 liters) an hour, according to the National Association of Home Builders. That can be a tall order in the hot, dry climates where swamp coolers function best.
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More Great Links
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