The compressor is the heart of the cooling cycle. The cycle begins when the compressor draws in cool, low-pressure refrigerant gas from the indoors. The motor-driven compressor’s sole function is to "squeeze" the refrigerant, raising its temperature and pressure so that it exits the compressor as a hot, high-pressure gas.
The compressor pushes hot gas toward the finned condenser coil on the outside of the air conditioner, where fans blow cool outside air over the coil and through the fins, extracting heat from the refrigerant and transferring it to the air conditioner's outside air.
When sufficient heat has been removed from the refrigerant, it condenses into a warm liquid that flows under high pressure to an expansion valve that converts the refrigerant to a cold liquid at low pressure. The refrigerant flows from the expansion valve to the finned evaporator coil, which is located on the inside or room side of the air conditioner.
As the refrigerant enters the evaporator coil, where the pressure is much lower, it is chemically forced to vaporize into a gas. This process requires heat, which comes from warm room air being blown over the evaporator coil by another fan. As the heat in the room is transferred to the evaporating refrigerant, the air in the room cools down. The refrigerant, now again a cold, low-pressure gas, returns to the compressor to continue the cycle.
There are several ways to compress gas and many different types of compressors have been invented over the years to do this. Their ingenious designs allow for a method of pressurizing atmospheric air, and there are three common ways to do this. The three most common air compressors are reciprocating, screw, and centrifugal.
Piston compressors are considered positive displacement machines, ie they increase the pressure of the air by reducing its volume. Essentially, the machine takes successive amounts of air trapped in an enclosed space and pressurizes the air to a high level. A piston inside the cylinder helps accomplish this feat. These types of air compressors are available air-cooled or water-cooled in lubricated or non-lubricated configurations and are also offered in different pressures and capacities.
Another type of air compressor is the rotary screw compressor, which is a positive displacement compressor.The most common rotary air compressor is the single stage screw or helical oil flooded screw air compressor. This type of air compressor consists of two rotors placed in a housing and the rotors compress the air inside. These units are oil cooled, with the oil sealing the internal spaces, and have no valves.
Unlike the other two, the centrifugal air compressor is a dynamic compressor based on the transfer of energy from a rotating impeller to the air. This compressor is designed for higher capacity as the flow through the compressor is continuous.Centrifugal air compressors are oil-free and the oil-lubricated drive mechanism is isolated from the air by shaft seals and atmospheric vents.