The compressor is a key part of the air-conditioning system and is usually driven by the engine via a V-belt or ribbed V-belt. As low pressure liquid refrigerant from the evaporator flows into the compressor, the liquid is compressed into high pressure gas and sent to the condenser. These units compress liquid refrigerant into gas while keep the refrigerant flowing through the air conditioning system. Compressors are also filled with special lubricating oil, and some of this oil circulates with the refrigerant through the air-conditioning system keeping all of the parts lubricated.