Air-conditioning capacity 231
Air-conditioning systems 232
Integral system 232
Split system 232
Inspection procedure 233
Heat pump 240
Evaporative cooler 241
Refrigerant Low-pressure gas
High-pressure gas High-pressure liquid
Checkpoint summary 241
Before outlining the procedure for inspecting a central air-conditioning system, let me discuss the components of a typical residential system and their functions. Central air-conditioning systems provide comfort cooling by lowering the air temperature and removing excess moisture. This is achieved by recirculating air from the house across a cooling coil. As the air flows across the coil, its temperature drops, causing some of the moisture in the air to condense out on the coil. The cool, dehumidified air is then distributed throughout the house, and the moisture is disposed of through a drain.
The basic components of an air-conditioning system are the compressor, condenser, expansion device, and evaporator. The evaporator is the cooling coil mentioned above. The other components simply provide the means for the evaporator coil to cool the circulating air. This is done using a cooling medium, a refrigerant, that cycles between the components. (See FIG. 17-1.) The refrigerant is normally a gas at atmospheric pressure and temperature. By (1) applying pressure to the refrigerant and (2) removing absorbed heat, the refrigerant will change from a gas to a liquid. At this point, the refrigerant exists as a high-pressure liquid. When the pressure on the liquid refrigerant is released, the refrigerant (3) expands and changes back to a gas. In the process of changing from a liquid to a gas, the refrigerant (4) absorbs heat from its surroundings, thus cooling the air passing over the coil.
An air-conditioning system is a closed system, and theoretically there should never be a need for additional refrigerant. However, in practice the various fittings on the connecting pipes can loosen or develop hairline cracks that can allow some of the refrigerant gas to escape. Usually, when a system is low in refrigerant, the practice is to introduce additional refrigerant and not to look for the openings in the system that allowed the refrigerant to escape. However, if the air-conditioning system cannot hold a refrigerant charge for at least one season, the leaks in the pipes or fittings should be located and corrected.
Fig. 17-1. Basic air-conditioning cycle. Normally the compressor and condenser are contained in a single housing.