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Air Conditioning

Central Air Conditioner
All air conditioning systems reduce the temperature and adjust the humidity of the air in the home to levels that provide a level of comfort. The most common type of system used in the U.S. is the basic, air-cooled central air conditioning system. Another type of system, found primarily in the hot and arid areas such as the southwestern U.S., is known as an evaporator cooler or “swamp cooler”. Other, less common types include the water-cooled systems, gas chillers and geothermal systems.

How They Work
The common, air-cooled central air conditioner removes heat from the air in the home and moves the heat to the air on the exterior of the home.  Air conditioners accomplish this by taking advantage of the basic physical law that heat moves to areas that are cooler.  Heat from the interior of the home is transferred to a refrigerant that carries the heat to the exterior of the home.  Currently the most effective refrigerant is freon.  The warm air in the home is blown over an evaporator coil that contains cold (approximately 20 degrees F) freon liquid. The freon absorbs heat from the air in the house, which cools the air. The warmer (approximately 50 degrees F) freon, which has boiled and turned into a gas,  is then moved to the exterior of the home where it disperses the heat.

The question is, “How is the heat dispersed when it is hot outside?”  A gas or liquid, when compressed, will have a higher temperature.  A compressor is used to compress the freon gas, increasing its temperature by approximately 100 degrees F and causing the freon to become much hotter than the air outside. The air outside (approximately 85 degrees F) can then be blown over the condensing coil that contains the hot freon (approximately 150 degrees F) . The air absorbs the heat from the compressed freon, in effect cooling it. The cooled, but still compressed freon (approximately 100 degrees F), is then returned to the house.  To lower the temperature of the freon liquid even more, the freon is expanded or decompressed (to approximately 20 degrees F). The freon is then once again ready to absorb the heat from the air in the home.

Main Components

Evaporator Coil
The coil containing the cold freon liquid over which the warm air from the interior of the home is passed. The heat from the air is absorbed by the freon liquid in the coil cooling the air but causes the freon to boil and turn into a gas. The coil gets its name because the freon in the coil evaporates or turns into a gas when it absorbs the heat from the air passing over the coil. The location and use of the coil may indicate the type of air conditioning system. An evaporator coil that is incorporated into a warm air furnace is called a split system. An evaporator coil that is used to warm air is called a heat pump. An evaporator coil that is not part of a heating system is an independent system.

Compresses the freon gas to increase its temperature. The compressor is needed to make the temperature of the gas much higher than the temperature of the air outside the home. The compressor is the main device that moves the freon through the system and is usually located next to the condensing coil.

Condensing Coil
The coil containing the high pressure and high temperature freon over which air on the exterior of the home is passed. The air on the exterior of the home absorbs the heat from the freon in the coil. As the high pressure freon gas loses heat, it turns back into a liquid. The coil gets its name from the freon condensing or turning back into a liquid. Condensing coils are generally cooled by air, however, some systems use water to absorb the heat from the coil.

Freon Expansion Device
A device designed to reduce the pressure of the freon liquid coming from the condensing coil. Reducing the pressure greatly reduces the temperature of the freon. (Opposite action of the compressor.) The device reduces the pressure of the freon by regulating the amount leaving the device. The device is usually located before the evaporator coil.

Fan used to move the warm air from the interior of the home, over the evaporator coil to be cooled, then back into the home.

Condensate Tray and Line
Captures the moisture that develops on the evaporator coil as the air in the home is cooled. The line directs the water to a plumbing drain.

Common Concerns

Size of System
Oversized System: Bigger is not definitely not better. Oversized systems are prone to short cycling or turn on and off frequently and will reduce the life expectancy of the equipment. In addition, the ideal comfort level of the home will not be obtained because of dramatic changes in the temperature of the house as the house cools and heats rapidly. Finally, an oversized system will not properly dehumidify the air due to the short cycling, which can cause swamp-like conditions (cool but humid air).

Air conditioning systems should be serviced every year at the beginning of the cooling season. The exterior compressor/condenser units should be kept clean and free of all leaves and debris; they should never be covered or otherwise boxed-in, and shrubs and hedges should be cut back. All of the above restricts the free flow of air, which reduces the gas-to-liquid process and overall efficiency of the system. In extreme cases, this may cause damage to the compressor. Window units should be removed during the heating system, because they allow an excessive amount of heat loss when they are left in.

Evaporator Coil
Though usually made of copper, these coils can corrode as the system ages.

The compressor is the main mechanical component of a central air conditioning system, which both compresses the freon and moves the freon through the system. In general, air conditioning compressors should not be operated in temperatures below 60 degrees F, or damage may result.


Removing Moisture from the Air
In addition to removing the heat from the air in the home, the basic central air condition system also lowers the moisture level in the air in the home. When warm, moist air comes in contact with a cooler surface, drops of water start to form on the cooler surface. A simple example of this is illustrated by a glass filled with an ice cold drink on a hot summer day. Water drops develop on the exterior of the cold cup. This is moisture in the air that is condensing on the cooler surface. The same occurs when the warm air from the house is passed over the evaporator coil. Moisture in the air turns to water on the evaporator coil. As the moisture accumulates on the coil, it will drop into a pan called the condensate tray, which is located under the evaporator coil. The water will collect here and eventually flow down a pipe into a drain. The old adage, “It’s not the heat, it’s the humidity” rings true here. Dryer air allows the sweat produced by a human body to be evaporated from the skin, cooling the body and making the body more comfortable.