An AC compressor is a central component of the air conditioning (AC) system. The compressor is mounted to a bracket on the engine and driven by the serpentine belt. It is also filled with a special oil to lubricate its internal components.
The AC compressor takes in refrigerant in gaseous form and compresses it to a high pressure, as much as 300 psi, and then sends the refrigerant to the condenser. There, the refrigerant cools and condenses into a liquid, travels to the expansion valve where it drops in pressure, and makes its way to the evaporator where it turns back into a gas before heading back to the compressor once more.
The aim of this process is to remove warm air from the passenger compartment and allow cool air to enter.