This is a short series of articles on cooling buildings in the Southwest, both for selection and proper installation in new construction and for operation and maintenance of existing systems. We will begin with a brief explanation of how an evaporative cooler system works, how it can be improved and how it should be operated and the environmental costs of an air refrigeration system in comparison to an evaporative cooling system. It is a fairly complicated issue and I hope this can shed some light on it.
The evaporative cooling system works on the principle of evaporating water by drawing air through a moistened media. The media is typically either fibers of aspen or pleated paper. Evaporating one gallon of water produces about 7,000 BTU’s of cooling. The amount of cooling you can achieve depends on how much air you can put through the cooler and how much water you can add to the air as it moves through it. The air coming into it with a lower humidity is going to absorb more water and will be cooled more thoroughly, so the temperature of the air coming out of the cooler is going to be lower than if you are working with outside air with a higher relative humidity. Evaporative coolers are much more effective in arid places like the Southwestern United States, but they also have limited use even in humid areas for industrial applications.
There are several ways to improve efficiency on an evaporative cooler – to move more air with the same amount of electricity or to add more moisture to the air that’s being moved through it. The more modern evaporative coolers, which are sometimes called a single-pad cooler, such as the Master Cool Unit, are much more effective than the older, three- or four-pad units. They have a single pad that’s 8- to 12-inches thick in comparison to the older coolers whose pads are typically only an inch and a half thick. The pad on the Master Cool Unit only has to be replaced once every three to ten years in comparison to once or even twice a year with the older style evaporative coolers.
Two ways to increase the coolers efficiency by moving more air are by having larger ductwork or shorter ductwork. A third way is to have a higher efficiency motor. The motors that are typically used on evaporative coolers are very low efficiency, shaded pole motors. It’s possible to put a higher efficiency motor on that will have up to three times the efficiency of the more typical motors. Grainger model #4K123 is recommended. The fan speed should always be adjusted by means of changing the diameter of the pulleys. Most coolers have an adjustable pulley to optimize the efficiency of the motor. This is done with an ammeter. The ammeter is connected to leads outside of the air conditioner, and the air conditioner is run in its normal condition with all panels closed, pads in place, and vents open. Then the pulley speed is adjusted if the amperage reading does not agree with the amperage rating on the motor rating plate.
A properly sized duct for the evaporative cooler should be at least as large as the outlet of the evaporative cooler. Most commonly, evaporative coolers are hooked up to duct works that are sized for refrigeration or forced air heating. Efficiency can be increased by a factor of two-fold. Combine that with an adjusted, high-efficiency motor, and it’s possible to move up to six-times as much air for the same amount of electricity. In addition, the high-efficiency motor has a much greater life expectancy and doesn’t require any maintenance. The payback period on the high-efficiency motor is less than two years.
Lots of ductwork is in the attic where it picks up a lot of heat, and it is often poorly sealed so there is a lot of leakage. If the duct is insulated and sealed, it will improve efficiency or if the ductwork is moved inside the house or just greatly reduced in length, you can improve the efficiency a lot. Opening the windows in the part of the house that requires the most cooling should do much to improve the air distribution in a house with an evaporative cooler. You can usually tell when you have enough windows open in the house because a piece of paper held up against a screen by the air pressure inside the house should barely stay on the screen. If it falls off, you have too many windows open and if it’s stuck on hard, you need to have some more windows open. You should open the windows in the part of the house that’s hottest. It is not unusual for the air in the ductwork to vary from 5 to ten degrees in temperature from one end to the other. That’s quite a bit when you’re talking about the relatively small comfort range that people tolerate.
Evaporative coolers should have a bleed-off system or a purging system that removes part of the water to remove excess salt from building up in the water, which will cause corrosion. It’s very easy to move part of this water to landscaping. With a typical bleed-off system, you need 50 feet of quarter-inch black plastic tubing and two or three drip emitters and then periodically move those drip emitters around. If there is supplemental irrigation applied periodically, you’re not likely to cause any damage from excess buildup in the soil around the plants.
At the end of each cooling season, it’s important that the coolers be maintained. The ductwork connected to the evaporative cooler needs to be closed off very tightly. It’s common practice to put a canvas cover over a cooler, but it’s much more effective to close off the ductwork. Some coolers will have their warranty voided if you put a canvas cover over it. The theory being that you will build up moisture there during the winter. With the single pad units, you should put a pint or so of vinegar in it and circulate it for a half hour and then drain it.
All coolers should be hosed down at the end of the season and drained and the belt that drives the pulley should be loosened or taken off for the winter. In the springtime, the pads need to be cleaned and replaced as needed, and all the bearings need to be lubricated. If there is any accumulated debris in the pans, they need to be washed out and the ductwork opened again. The air that comes out of the evaporative cooler eventually comes into the house. It retains the cool that has been produced by evaporating the water. Ductwork registers cause a lot of airflow restriction, and substantial improvements can be made by using more and larger ducts and registers, or by just removing registers.
In contrast, a refrigeration system works by pumping heat from the inside of the building to the outside and have a much higher environmental cost,. The temperature in Phoenix is now about 20 degrees F than it was a hundred years ago, and most of that change has occurred since the 1950’s when refrigerated A-C started being used there. When one building starts using refrigerated air it starts a positive feedback condition that requires the other buildings to need more cooling. Most of the new buildings in Las Cruces have refrigerated cooling now because few people have experienced a really good evaporative or radiant cooling system.
A refrigeration system’s efficiency (SEER) is based on how many BTU’s it can remove per watt of electricity it uses. A typical refrigeration system has a SEER rating of ten. The highest available right now is about sixteen, except for a radiant cooling system which is capable of providing a SEER rating of 40 in our climate . They are requiring an efficiency rating of twelve on new construction now. The maximum efficiency of a refrigeration system is going to be ultimately governed by how highly you can compress the gases that are used in it, and that is going to be limited by how high a temperature your systems can tolerate. I think we are probably reaching the limits of our efficiency. The higher efficiency refrigeration units are quite a bit more expensive than the standard units and have intrinsic problems with mold and indoor air quality.
A substantial amount of energy that goes into a refrigeration-cooling unit is used to remove moisture from the air. The evaporator coils in a refrigeration unit are typically -20 to -40 degrees; so even in an arid climate, much of the moisture from the air will be precipitated out. This will increase the operating cost because of the 7,000 BTU per gallon of water mentioned earlier, but in this case, there is energy that has to be supplied to the unit to remove water from the air even though typically our air is dryer than we want it to be.
In a more humid climate, the de-humidification is a useful thing, but around here, it usually isn’t. In addition, this moisture will remain on the capillary coils when the unit is shut off, which encourages air-borne fungus, which releases spores, which causes health problems. The water that is extracted from the air is put into the sanitary drain where it has to be disposed of by the city sewer system. It causes an incremental cost when it goes into a septic system. The water that is being used in an evaporative cooling system actually cools off the immediate environment, whereas the refrigeration system heats and dries the environment. The temperature in Phoenix is 20 degrees higher now than it was before the introduction of refrigerated air. While that temperature increase is due to several causes, the decision to cool buildings with refrigeration causes a positive feed back situation, with everybody pumping their heat out at the worst time and competing with everyone else’s heat.
Commercial electric generation stations only deliver about one-third of the power that they consume as electricity to the end user. Most of the rest of it is waste heat. Most of the waste heat has to be eliminated by evaporation of water, so electricity that is being used by refrigeration units requires the evaporation of a large amount of water. It requires about two gallons of water to generate one-kilowatt hour. It will require about a thousand gallons of water a month to generate the power to cool a typical house with refrigerative air conditioning.
From the El Paso Solar Energy Association:
The simplest way to keep cool is to first design and build your home for our unique climate.
For now we’ll concentrate on cooling an existing home.
If your goal is to keep a home cool in our desert climate, turning on the air conditioner or evaporative cooler is the LAST thing you should do. They’re expensive to operate, they use water and they’re noisy. There is no single answer to being comfortable but taking the shotgun approach is easier, and less expensive.
The three major sources of unwanted heat in your house during the summer are heat that conducts through your walls and ceiling from the outside, heat that is given off inside your house by lights and appliances, and sunlight that shines through windows.
Keeping the sun out of your home is obviously a major goal. Closing curtains and drapes makes your home darker but doesn’t effectively stop the heat. Stopping the sun’s energy from entering your home is best done on the outside of the window. Solar screens or shade cloth can stop as much as 80% of the sun’s heat before it enters your home. These materials are available at most home centers and window and screen suppliers. Windows on the west side are typically the biggest problem followed by the east and north facing windows. South facing windows are often protected by your roof overhang, which shades the glass when the sun is at its highest point. High-performance windows with “low-emissive” coatings (Low-E) and low “shading coefficients” will stop heat from the sun while allowing visible light to pass through the glass. These same windows help keep heat in the home during the winter.
Landscaping can play a large role in achieving comfort. Trees located on the west, east and north can not only put windows in shade, but also shade the walls of the home as well as the ground area. This shaded area keeps the home cooler and cools breezes as they reach the home. On the south side, you should choose low growing shrubs and plants so as not to block the winter sun from entering south facing windows. This vegetation will reduce the amount of solar energy, which is reflected into windows by lighter colors.
The colors of various materials around your home can have a dramatic effect on your comfort and wallet. A dark colored roof can reach temperature 40 degrees hotter than a light colored reflective roof. This heat not only increases the cooling load of your home but also decreases the life of your shingles. Ventilate your attic space to reduce heat build-up during the summer, which finds its way into your home. Darker colored walls especially brick and stone will actually absorb solar energy, which increases cooling problems and slows the nighttime cooling process. To reduce heat gains through walls and your ceiling, you can add insulation and seal up cracks to reduce air infiltration.
You can reduce heat from lights and appliances by purchasing energy-efficient products. A conventional, incandescent light bulb uses only 10% of its input energy to produce light and the other 90% is wasted heat. Compact Fluorescent lights can produce the same amount of light as an incandescent lamp but use about one-fifth the energy and produce about one-fifth the heat. Check out the EPA’s Energy Star web site (www.energystar.gov) for energy smart options for your home.
Natural ventilation by opening windows is only effective when outdoor temperatures are lower than interior temperatures. An indoor/outdoor thermometer is a useful tool to determine the optimum time to open your windows. Open your windows when this temperature difference is reached in the evening and then close your windows in the morning. Opening windows more on the downwind side will increase airflow.
Opening windows is very important when operating an evaporative cooler. A common mistake in the El Paso area is not opening windows enough. If we think of an evaporative cooler as providing a nice breeze, then the best way to kill that breeze and its cooling effect is to close windows. You can increase the amount of cooling in one particular room simply by opening those windows more. The amount of force from an evaporative cooler is limited and can’t compete with a strong summer breeze/wind. If you have a 100-degree breeze coming from the west, then close those windows. When checking the operation of your cooler, make sure that the entire pad(s) is wet. Hot, outside air will flow freely through dry openings and dry pads drastically reducing the cooler’s effectiveness.
It’s very important to supply fresh water to an evaporative cooler and flush out the salts etc. left behind in the evaporation process. Typically this is accomplished with a bleed-off line but there is better, water saving method available at most home centers. Sometimes referred to as a “power dump” this new pump is installed in addition to your regular pump. This new pump operates on a timer and is designed to flush all the water in the pan once every 8 to 12 hours of operation.
The more attention you pay to the sun’s impact and the way you operate your home, the less you’ll spend while being more comfortable.
From the US Department of Energy:
Don't forget your ducts!
Identify any leaks with diagnostic equipment. Seal your ducts with mastic, metal-backed tape, or aerosol sealant. Duct tape should not be used; it cannot withstand high temperatures and will not last. Test airflow after ducts are sealed. Your new or existing cooling and heating equipment is only as good as the system that carries its heated or cooled air. Central air conditioners, heat pumps and forced air furnaces rely on a system of ducts to circulate air throughout your home. To maintain comfort and good indoor air quality, it is important to have the proper balance between the air being supplied to each room and the air returning to your cooling and heating equipment. Leaky ducts can cause an unbalanced system that wastes energy. Sealing your ducts improves your system’s ability to consistently cool and heat every room in your home.
Duct Improvement Checklist
To improve your ducts, make sure to have your contractor: Insulate your ducts where it counts to keep the air at its desired temperature as it moves through the system. The contractor should use duct insulation material rated at R-6 to insulate ducts located in unconditioned spaces such as the attic. Conduct a combustion safety test after ducts are sealed to be sure all gas or oil-burning appliances are working properly.