The compressor is a pump that changes the air conditioner's refrigerant or coolant (now commonly Puron, though in years past Freon was used) from a low-pressure gas to a high-pressure gas.
The high pressure created by the compressor induces condensation (the process of changing a gas into a liquid), and condensation produces heat. The refrigerant is pushed through a series of coils known as the condenser (or condenser coil) and the refrigerant changes from a gas to a liquid, releasing heat through the coils into the outside air.
The (now liquid) refrigerant leaves the condenser and passes through an expansion valve, which relieves the pressure on the refrigerant.
The refrigerant now passes into a second set of coils, which are typically located inside the space to be cooled. The new low-pressure environment induces evaporation, and the process of evaporation requires a transfer of heat from the surrounding area into the refrigerant. While the refrigerant is in the evaporation coil, it draws heat from the surrounding air and absorbs it within the refrigerant.
This process of raising and lowering the pressure on the refrigerant is how an air conditioner is able to cool a house.
Single-Stage vs Two-Stage Compressors
The main difference between a two-stage air conditioning unit and a standard, single-stage cooling system is the compressor. Because single-stage air conditioners run only on one speed, high, the unit provides a blast of cold air at one speed: high, whether you need that much cool air or not. This results in the potential for lost energy and money. Two-stage air conditioning units have a compressor unit that enables them to cycle from low to high, and can therefore adjust the amount of cool air that cycles through the house.
A two-stage air conditioner runs at its lowest speed about 80% of the time, and this low speed is usually sufficient to keep a home comfortable. As temperatures and humidity soar, the unit adjusts to its higher setting immediately. During the low speed, however, the unit runs nearly continuously and hardly ever shuts on and off. This allows it to dehumidify the air better; the cold blast of high speed is too quick to remove water vapor from the air. At low speed, water vapor has time to move into the coils and evaporate.
Continuous operation in the two-stage air conditioner also offers the benefit of consistent temperature. In single-stage air conditioners, the blast of chilly air leaves the room very cold, but when the system stops, the air becomes hot and stuffy. Two-stage air conditioners keep the air circulating and more consistently comfortable.
Finally, the compressor in a single-stage air conditioner is subject to more wear and tear as the unit is frequently turned on and off. The compressor in a two-stage compressor tends to have a longer life and save the homeowner repair and replacement.
Types Of A/C Compressors
Compressors come in many different types, but the two common to air conditioning units are the centrifugal compressor and the scroll compressor. Scroll compressing technology has been in use for a long time. In this setup, the compressor uses a pair of interleaved scrolls to compress, pressurize or pump liquids and gases. This design also boasts a superior capacity to handle liquids, enabling small amounts of dirt and liquid to pass through without damaging the compressor, while at the same time increasing reliability by eliminating stress on the motor. Scroll compressors are considered very efficient and used in examples such as Lennox's HS-22, which includes a double-row condenser coil. Centrifugal compressors are considered best suited to continuous-duty applications, such as cooling units, ventilation fans and air movers because they give a high airflow at an efficient level but can't achieve a high compression ratio, meaning they can't handle large variations in pressure. Centrifugal force drives the compression of the refrigerant, rendering these compressors ideal for larger refrigerant volumes and low-pressure differentials.