Air Conditioning in Southern New Mexico
For the next issue, I would like to start an article on air conditioning systems that are commonly used in Southern New Mexico. 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.
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. It requires about 7,000 BTU’s to evaporate one gallon of water. 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 places like the Southwestern United States. But they also have limited use even in more 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 have 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 make the coolers more efficient 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 there that will have up to three times the efficiency of the more typical motors. The fan speed should always be adjusted by means of changing the diameter of the pulleys. They typically have an adjustable pulley to optimize the efficiency of the motor. This is done by using 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 scope, you can improve the efficiency a lot. Much of the air distribution in a house by an evaporative cooler should be done by opening the windows in the part of the house that requires the most cooling. 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 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 out of the house. It retains the entire cool that has been generated by evaporating the water. Ductwork registers cause a lot of airflow restriction, and substantial improvements can be made by using more and larger ducts, or by just eliminating them.
In contrast, a refrigeration system works by pumping heat from the inside of the building to the outside. A refrigeration system’s efficiency is based on how many BTU’s it can remove per watt of electricity uses. A typical refrigeration system has an efficiency rating of ten. The highest available right now is about sixteen. They are requiring an efficiency rating of twelve on new construction now. The maximum by how highly you can compress the cages 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 already. The higher efficiency units are quite a bit more expensive than the standard units.
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 you will have the same 7,000 BTU per gallon of water, 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 there before 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.
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 be electrical generating units. It requires a gallon of water to generate one-kilowatt
hour. For the typical generating station that makes up most of our power supply, (here we will insert some figures about how much water is required for each unit of cooling the building requires, and how much power is in how many units of fossil fuel required for each cooling unit in the building.
Beating the Heat
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, but that’s another article. 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 on 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 the 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.
Don’t Forget your Ducts!
From the US Department of Energy:
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 Improvment 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.