The thermal expansion valve can automatically adjust the opening of the valve according to the superheat of the refrigerant vapor at the outlet of the evaporator, so as to achieve the purpose of adjusting the refrigerant flow and make the refrigerant flow match the load of the evaporator. this working process can not only make full use of the heat transfer area of the evaporator, but also prevent the compressor from "liquid shock" phenomenon. In the form of balance structure, the thermal expansion valve can be divided into inner balance type and outer balance type. The external balance thermal expansion valve is used in the system with long evaporator pipeline, high refrigerant flow resistance and liquid separator..