how air conditioning units work
1 Circuit diagram
The diagram shows that the system has high- and low-pressure sides. The former is between the output of the compressor and the input of the orifice tube or thermal expansion valve. It may be described as ‘hot’. The latter side is from the thermal expansion valve, or orifice tube, through to the input side of the compressor. It may be described as ‘cold’.
2 Thermostatic expansion valve
This offers a variable rate of flow of refrigerant to the evaporator. A bulb and capillary containing refrigerant acts as a temperature sensor. When the refrigerant being output to the evaporator is too cold, the diaphragm acts on the actuating pin, which in turn restricts flow, preventing fluid refrigerant from exiting the evaporator and damaging the compressor.