Every automotive A/C system includes an expansion valve or expansion device of some sorts. The typical expansion valve, also referred to as a thermostatic expansion valve is... located at the inlet of the evaporator. The expansion valves function is to create just enough of a restriction in the A/C system so that hot, high pressure liquid refrigerant from the condenser can be reduced in pressure and temperature to become low temperature, low pressure liquid refrigerant before entering the evaporator.

Understand that refrigerant has a pressure temperature relationship. Whatever change you do to one will automatically effect the same change in the other. That means, just increasing the pressure of refrigerant will automatically increase the temperature of the refrigerant without adding any external heat source. The opposite is also true.

Therefore, in an automotive air conditioning system, it is very important that the liquid refrigerant entering the bottom of the evaporator be low pressure, cold refrigerant. As the refrigerant leaves the condenser, it is high temperature, high pressure refrigerant. By just restricting the flow of that refrigerant, it is changed from high pressure to low pressure refrigerant. Because of refrigerants pressure temperature relationship, when the pressure is reduced, the temperature is reduced automatically.

In operating within the auto A/C system, the expansion valve uses a temperature sensing bulb that is attached to the evaporator outlet and connected to the expansion valve on a capillary tube. That tube is a very thin tube that should never be kinked or bent. The temperature sensing bulb senses the evaporator outlet temperature and regulates the opening and closing of the expansion valve so that the evaporator always has an adequate amount of liquid refrigerant entering the bottom of the evaporator.

Other expansion valve designs like the H valve or block type expansion valves eliminate the capillary tube sensing bulb. That is because these types of valve attach to the evaporator in such a way that the cold vapor refrigerant leaving the evaporator passes back through the valve. The temperature sensing bulb in integral to the suction side of the expansion valve without the external capillary tube.

The expansion valve also creates one of the two divisions between high and low sides of the A/C system. The second division between high and low pressures is at the compressor, where compressor suction ends the low pressure side, and the compressor discharge, releasing compressed refrigerant to the condenser, is the start of the high pressure side of the A/C system.