Wet-surface direct evaporative coolers typically use pumped recirculating water systems to keep the media wet. A fan blows air through the media, thereby cooling the air and increasing its humidity.

In a typical indirect evaporative air cooler, the essential element is a heat exchanger in which dry air contacts heat-exchange surfaces whose other sides are cooled evaporatively. These systems cool the air without adding humidity. (Illustrations copyrighted and provided courtesy E. Source Companies, 1965 N. 57th Court, Boulder, CO 80301, USA, 303-444-7788.)

Wall-mounted evaporative coolers are a good option for homes without attics or the space for a ground-mounted evaporative coole. (Photos courtesy Synertech Systems Corp.)

A Photo by Dave Emmitt

B Photo courtesy Synertech Systems Corp.

Ground-mounted evaporative coolers (B) are easier to maintain than attic coolers (A), which don't take up yard sale.

AdobeAir's SlimWall direct evaporative cooler mounts diminutively on an exterior wall and is capable of cooling up to 1,000 square feet of space. (Photo courtesy AdobeAir)

Cool Solutions

In Colorado 's dry climate, evaporative coolers can keep your home just as cool as AC, but at a quarter of the cost and with fewer environmental consequences.

by Dave Emmitt

In the early 1900s, people living in the Southwest often slept inside screened porches during summer. On particularly hot nights, they'd soak sheets in water and hang them on the inside of the screens, where whirling electric fans would pull the cooler night air through the moistened cloth to reduce the room's heat.

That concept became a key ingredient in evaporative coolers, which are several technological leaps ahead of the old “swamp coolers.” Today's modern coolers are a low-cost, lower-tech, less environmentally-polluting alternative to refrigerated air conditioning.

During Colorado summers, which average 18 percent relative humidity, evaporative coolers work by adding moisture to an airstream and allowing evaporation ( nature's way of removing heat) to do the heat removal. A simple analogy is to lick the back of your hand and blow on it; the spot you licked is cooler because the moisture evaporates as you blow on it.

Instead of “conditioning” the air, like an air conditioner does through the aid of a condenser and potentially ozone-depleting refrigerant, an evaporative cooler draws exterior air through special pads sprayed by water. These pads also filter out pollens and other pollutants, and are largely self-cleaning. As the water evaporates, the air is cooled and circulated through a home by means of a blower fan, and vented out through an open door or window.

For evaporative cooling to work properly, the cooled air must be allowed to escape. By choosing the doors or windows left open, the flow of cooled air is directed to areas where it's most needed, pushing out hot air, pollutants, smoke and odors in the process. Evaporative cooling won't work in a tightly shut house (unless you install attic dampers), which is the only way air conditioning works. Because of that, air-conditioned air is staler than the fresh air continually circulated by an evaporative cooler. The constant air movement also creates a cooling breeze that further drops the indoor temperature.

Evaporative cooling is more cost-effective than air conditioning, as it demands only a quarter of the electricity required to run an air conditioner. It's also cheaper to install and can be paired with photovoltaic panels that generate electricity to run the blower and small water pump, further reducing operating costs.

The lower the humidity and the higher the temperature, the better an evaporative cooler works. As the temperature climbs, humidity typically drops—conditions that make evaporative coolers work at
peak efficiency.

In humid climates, the water used by an evaporative cooler can be substantial, but that isn't a large issue in our dry climate (see “It Pays to Play It Cool” left).

Good Things Come in Threes

Three types of evaporative coolers are available in today's market: direct, indirect and two-stage (also known as direct/indirect).

Direct coolers use a pad through which air is blown and water is sprayed. The air that passes through the pad is filtered, cooled and pushed through the home. This raises humidity in the home to a degree that is usually welcome in our dry climate. This system is also cheaper and provides more airflow at less energy use than do the other systems.

Indirect coolers cool fresh air that flows through the home without adding any moisture. This is accomplished by using two airstreams, along with a heat exchanger. Air that's directly evaporatively cooled flows through one set of small ducts in the heat exchanger, cooling air that flows through another set of small ducts. This process results in dry, cool air that rivals that of air-conditioned air.

Two-stage systems precool air by an indirect process that doesn't add moisture, then directs the output through a direct-cooling process before it's brought into the home. The result is even cooler air than is possible with either process alone, albeit the conditioned air has a higher moisture content than that of indirect systems.

Most modern evaporative coolers use a polymer pad that requires less maintenance, cools more efficiently and lasts up to 10 times as long as conventional aspen pads, which can rot, compact and require frequent replacement.

Traditionally, evaporative coolers are placed on the roof and cooled air is distributed downward through ductwork. However, ground-mounted coolers are often more efficient at distributing the cooled air. That's because the air is not forced down through overheated attic ducts that can infuse heat into the cooled air. Instead, exterior air is cooled at ground level and the ground-mounted cooler directs the cooled air through “up-ducts”—foot-square back-draft dampers installed in the attic floor—that funnel it through the attic, where it's vented to the outside through existing
attic ventilation.

If these ducts are appropriately located, there's no need to leave a window or door open, thereby enhancing security. Additionally, cooling systems that use up-ducts lower attic temperatures, resulting in more efficient cooling.

Ground-level coolers are also much easier to maintain than attic or roof-mounted units. Some manufacturers even produce coolers that mount on a window or wall. AdobeAir's “SlimWall” direct evaporative cooler mounts on a wall and can be installed between the studs of a conventionally framed home to effectively cool up to 1,000 square feet of space.

Of course, all cooling strategies work more effectively if shades, awnings or exterior shutters prevent sunshine from directly entering the home in the first place.

Like many technologies, evaporative cooling has improved substantially through the years. It's sometimes difficult to get past the old swamp-cooler mentality to realize that if it's good enough for Mother Nature, it must be good enough for us. That's why evaporative cooling is called natural cooling. Here, in our semiarid desert, nature needs moisture to do her job. And we need to protect the environment by just saying no to AC.

Dave Emmitt is a Boulder Green Building Guild member and owner of Direct Drive Service Inc., a Boulder business that specializes in energy-efficient cooling technologies for residential and light commercial, and Go Tankless of Colorado, which specializes in tankless hot water systems.

For more details on evaporative cooling, log on to www.bgbg.org and select “More News,” then click on “Journal and Publications” and download the summer 2006 Boulder Green Building Journal.

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