The Care and Feeding of a Swamp Cooler


W. E. Lopez

"What the heck is a swamp cooler?" As soon as they hear the term mentioned, that's usually the first question asked by folks who have never been out west and never seen an evaporative cooler in operation.

For a while, I taught air conditioning and refrigeration at Riverside Community College, in Riverside, California. In many western locations, the swamp cooler serves the same purpose in cooling a home as an air conditioner does. Operation and maintenance of the swamp cooler is so simple that it was not even discussed as part of our class. The principle involved is simple physics: Dry air can absorb moisture through evaporation. When water evaporates, it absorbs heat. A special type of fan is used to increase the rate of evaporation and make the air cooler, as well as distributing the cool air throughout the home.

You can demonstrate this principle to yourself by putting on a wet T-shirt and standing in front of a fan. If you live in a dry climate where humidity is low, you will feel an immediate cooling, even chilling effect. If you live where humidity is high, you will simply feel damp. Cooling by evaporation works very efficiently in dry areas, but isn't worth spit in the tropics, or any other humid climate. Unlike air conditioning, where people are accustomed to closing all doors and windows, "to keep the cool air in", an evaporative cooler requires adequate ventilation and air flow to distribute the cool air through the home and expel warm, moist air through window openings.

This picture shows a typical window mounted evaporative cooler, also called a swamp cooler. Inside this unit, a centrifugal blower pulls air through the pads of the housing, and discharges it into the space to be cooled through the box-like opening in the front. Switches on the front panel usually control blower speed (HI-MED-LO) and a small pump motor inside the housing. A water line supplies water to a shallow reservoir in the bottom of this unit. As water evaporates, addition of make-up water is controlled by a simple float-valve.

This larger capacity evaporative cooler is designed to be mounted on the roof of the home. It also has a shallow water reservoir in the bottom, a small water pump to provide water to the distributor or manifold at the top of the housing, and a float valve to maintain water level. A roof mounted unit usually is more effective at cooling a home because the cooling pads cover all areas of the exterior housing, instead of only three sides as on the window mounted, side-discharge cooler. The roof mounted cooler normally includes a thermostat located somewhere in the home, which turns the unit on when the indoor temperature becomes uncomfortably warm, and off when the home is sufficiently cooled.WiringThermostatsMaintenanceAutomobile

The interior working of all evaporative coolers is very similar, as shown in this illustration, although they may vary according to size and orientation of discharge. A small table mounted unit may have only a single motor to power both the water pump and fan. Portable units are usually filled and refilled by pouring water into the side as needed. They produce only limited cooling effects.

For real efficiency, the side- or bottom-discharge units are used. The fan blower resembles a squirrel cage, hence its name, squirrel cage fan. Centrifugal force expels air from the blower housing while fresh air flows in to fill the area of lower pressure. The in-rushing air is pulled through the damp cooler pads and is cooled by evaporation.

The capacity of an evaporative cooler is measured in CFM, or cubic feet per minute. A small unit may be 2,500 CFM, while a whole house unit will be 6,000 CFM or larger. For example, a modest home with three bedrooms, living room and kitchen, may have 850 square feet of floor space with ceilings 7 feet high. The resulting volume is 5,950 cubic feet. A 6,000 CFM cooler would replace 100% of the air in this home each minute.

With 6,000 cubic feet of air being continually forced into the home, several windows will have to be left partially open to provide an exit path. Opening a window 2 to 3 inches in each room will provide ample circulation.

The key to efficient evaporative cooling lays with the design of the cooler pads and relative humidity of the ambient air. While little can be done to control humidity outside the home, much research has gone into the design of cooler pads and materials. Aspen pads are effective and inexpensive, easy for the home owner to change, and are biodegradable when disposed of properly. Synthetic fibers cost a little more and may last longer, but they are seldom biodegradable and don't really cool much better than natural pads.

Evaporative coolers are energy efficient since the blower motor only has to move the air and not compress a refrigerant gas as required in an air conditioner. The blower motor in a residential unit is typically 1/3 to 3/4 horsepower, while water pumps may be smaller than 1/20th horsepower. When compared to 3 and 5 horsepower compressors needed for refrigerated air, plus the fans for cooling and condensing, evaporative coolers require only about one-tenth as much energy for home cooling, a large annual saving on your electric bill.

As described above, the evaporative cooler can easily be serviced by any home owner with a minimum of mechanical knowledge, a screwdriver and an adjustable wrench. On the other hand, repair and maintenance of air conditioning systems should be left to professional service technicians.

Replacement parts for evaporative coolers are inexpensive and readily available at most hardware stores in areas where they are used extensively. To be on the safe side, always use extreme care when working on the roof of your home, and be sure that circuit breakers for your cooler are turned OFF when working with the electrical wiring. The combination of water and electricity is nothing to fool around with, and thermostatically controlled blowers and pumps can turn on when you least expect them to.

How do I know if a swamper will work for me? The easiest, rule-of-thumb, way to tell if you will be satisfied with a swamp cooler, is to visit your local building supply and home improvement center or hardware store. If they do not stock evaporative coolers and the parts to maintain them, it's most likely because there is not a big demand in your area.

Is a swamp cooler right for you? That depends upon the temperature and relative humidity normally experienced in your locale. To calculate the lowest possible temperature (dewpoint) which can be achieved through evaporation of water, click on one of the links below to download a spreadsheet. Play around by entering different values for temperature and relative humidity (shaded blocks only) to determine your dewpoint.

Evaporative coolers are also made for RV's and campers, and are designed to utilize 12 volt DC power.  Because of the low power required for a swamper, they present a minimal load to RV batteries.  You can expect to pay $300 or more for a unit designed for RV's.

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