Any automotive air conditioning system employs four basic parts-a mechanical compressor, driven by the vehicle's engine; an expansion valve, which is a restriction the compressor pumps against; and two heat exchangers, the evaporator and the condenser. In addition, there is the refrigerant that flows through this system.
The belt-driven compressor uses engine power to compress and circulate the refrigerant gas throughout the system. The refrigerant passes through the condenser on its way from the compressor outlet to the expansion valve. The condenser is located outside the passenger compartment, usually in front of the vehicle's radiator. The refrigerant passes from the expansion valve to the evaporator, and after passing through the evaporator tubing, it is returned to the compressor through its inlet. The evaporator is located inside the vehicle's passenger compartment.
When the compressor starts running, it pulls refrigerant from the evaporator coil and forces it into the condenser coil, thus lowering the evaporator pressure and increasing the condenser pressure. When proper operating pressures have been established, the expansion valve will open and allow refrigerant to return to the evaporator as fast as the compressor is removing it. Under these conditions, the pressure at each point in the system will reach a constant level, but the condenser pressure will be much higher than the evaporator pressure.
The pressure in the evaporator is low enough for the boiling point of the refrigerant to be well below the temperature of the vehicle's interior. Therefore, the liquid will boil, remove heat from the interior, and pass from the evaporator as a gas. The heating effect produced as the refrigerant passes through the compressor keeps the gas from liquefying and causes it to be discharged from the compressor at very high temperatures. This hot gas passes into the condenser. The pressure on this side of the system is high enough so that the boiling point of the refrigerant is well beyond the outside temperature. The gas will cool until it reaches its boiling point, and then condense to a liquid as heat is absorbed by the outside air. The liquid refrigerant is then forced back through the expansion valve by the condenser pressure.