Evaporative Cooling

Evaporative Coolers: An evaporative cooler (also called swamp coolers) is a completely different type of air-conditioner that works well in hot, dry climates. They are a popular and energy efficient cooling strategy in dry climates of the United States. Hot outside air enters the swamp cooler. The air passes over water-saturated pads, and the water evaporates into the air. The energy used removes heat from the air. The 15- to 40-degree-cooler air is then directed into the home, and pushes warmer air out through windows. Because this process also humidifies the air, swamp coolers are best used in areas with low summertime relative humidity. These units cool outdoor air by evaporation and blow it inside the building, causing a cooling effect much like the process when evaporating perspiration cools your body on a hot (but not overly humid) day. When operating an evaporative cooler, windows are opened part way to allow warm indoor air to escape as it is replaced by cooled air. Evaporative coolers use less than one-third the energy of air-conditioners, and cost about half as much to install. Unlike central air-conditioning systems that recirculate the same air, evaporative coolers provide a steady stream of fresh air into the house. However, they require more frequent maintenance than refrigerated air-conditioners and they're suitable only for areas with low humidity.

Sizing and Selection: Evaporative coolers are rated by the cfm of air that they deliver to the house. Most models range from 3,000 to 25,000 cfm. Manufacturers recommend providing enough air-moving capacity for 20 to 40 air changes per hour, depending on climate.

Installation: Evaporative coolers are installed in one of two ways - the cooler blows air into a central location, or the cooler connects to ductwork, which distributes the air to different rooms. Central-location installations work well for compact houses that are open from room to room. Ducted systems are required for larger houses with hallways and multiple bedrooms.

Most people install down-flow evaporative coolers on the roofs of their houses. However, many experts prefer to install ground-mounted horizontal units, which feature easier maintenance and less risk of roof leaks.

Small horizontal-flow coolers are installed in windows to cool a room or section of a home. These portable evaporative coolers work well in moderate climates, but may not be able to cool a room adequately in hot climates. Room evaporative coolers are becoming more popular in areas of the western United States with milder summer weather. They can reduce the temperature in a single room by 5 to 15 degrees.

Operation: An evaporative cooler should have at least two speeds and a vent-only option. During vent-only operation, the water pump does not operate and the outdoor air is not humidified. This lets you use the evaporative cooler as a whole-house fan during mild weather. Control the cooler's air movement through the house by adjusting window openings. Open the windows or vents on the leeward side of the house to provide 1 to 2 square feet of opening for each 1,000 cfm of cooling capacity. Experiment to find the right windows to open and the correct amount to open them. If the windows are open too far, hot air will enter. If the windows are not open far enough, humidity will build up in the home.

You can regulate both temperature and humidity by opening windows in the areas you want to cool, and closing windows in unoccupied areas. Where open windows create a security issue, install up-ducts in the ceiling. Up-ducts open to exhaust warm air as cooler air comes in from the evaporative cooler. Evaporative coolers installed with up-ducts will need additional attic ventilation.

Filters remove most of the dust from incoming air-an attractive option for homeowners concerned about allergies. Filters can also reduce the tendency of some coolers to pull water droplets from the pads into the blades of the fan. Most evaporative coolers do not have air filters as original equipment, but they can be fitted to the cooler during or after installation.

Evaporative Cooler Maintenance: Save yourself a lot of work and money by draining and cleaning your evaporative cooler regularly. Build-up of sediment and minerals should be regularly removed. Coolers need a major cleaning every season, and may need routine maintenance several times during the cooling season. The more a cooler runs, the more maintenance it will need. In hot climates where the cooler operates much of the time, look at the pads, filters, reservoir, and pump at least once a month. Replace the pads at least twice during the cooling season, or as often as once a month during continuous operation.

Some paper and synthetic cooler pads can be cleaned with soap and water or a weak acid according to manufacturer's instructions. Filters should be cleaned when the pads are changed or cleaned. Caution: Be sure to disconnect the electricity to the unit before servicing it.

Two-Stage Evaporative Coolers: Two-stage evaporative coolers are newer and even more efficient. They use a pre-cooler, more effective pads, and more efficient motors. They don't add as much humidity to the home as single-stage evaporative coolers, but are still much more efficient than air-conditioners.

Direct evaporative coolers have been used for many years in hot, arid parts of the country. These systems are typically roof-mounted. In direct evaporative cooling systems, cooling is provided as hot, dry outside air is blown through an evaporative media that is kept moist.

Indirect evaporative coolers can work in climates where moist air is not wanted in the building, though efficiency is lower.

On larger buildings in hot, dry climates, the benefits of evaporative cooling can be achieved through roof-spray technology. A modified spray-irrigation system can be used on the roof to drop daytime roof-surface temperatures from 135-160 degrees F to 85-90 degrees F (57-71 degrees C to 29-32 degrees C). With a typical (poorly insulated) roof system, this can reduce interior temperatures significantly.

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Figure 29: This cool tower at the Zion National Park Visitors Center uses evaporative cooling to keep the building comfortable.

A newer, more innovative use of evaporative cooling is night-sky radiant cooling. This approach works in climates with large diurnal temperature swings and generally clear nights (such as in the Southwest). Water is sprayed onto a low-slope roof surface at night, and the water is cooled through a combination of evaporation and radiation. This process typically cools the water to 5-10 degrees F (2.7-5.5 degrees C) below the night air temperature.

The water drains to a tank in the basement or circulates through tubing embedded in a concrete floor slab. Daytime cooling is accomplished either by circulating cooled water from the tank or through passive means from the concrete slab.

Drawbacks of Evaporative Coolers: Evaporative coolers should not be used in humid climates because they add humidity. Also, they cool your house down to a higher temperature than an air-conditioner would. They require maintenance (albeit easy) about once a month. If the cooler is installed on the roof, there is some roof deterioration caused by routine maintenance trips. A sunlit rooftop cooler will be about 1 degree Fahrenheit less effective than a shaded cooler. Rooftop maintenance also requires using a ladder, which may be an inconvenience.

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