Thermo - static Expansion Valve:

The name 'thermostatic-expansion valve' can give the impression that this is a temperature control device. This is not a temperature control device and this cannot be adjusted and utilized to vary evaporator temperature. In fact TEV is a throttling device that works automatically, maintaining correct and proper liquid flow as per the dictates of the load on the evaporator. Due to its adaptability to any kinds of dry expansion application, high efficiency, automatic operation and ability to stop liquid flood backs, this valve is widely used.

The functions of the thermostatic-expansion valve are following:

 (a) To decrease the pressure of the liquid from the condenser pressure to evaporator pressure,

 (b) To remain the evaporator fully active and

 (c) To vary the flow of liquid to the evaporator according to the load needs of the evaporator so as to stop flood back of liquid refrigerant to the compressor.

It does the last two functions through maintaining a stable superheat of the refrigerant at the outlet of the evaporator. This would be more suitable to call it a 'constant superheat valve'.

The important parts of the valve are following:

Power element along with feeler bulb, valve seat &needle, and a superheat adjustment spring.