What is the function of a thermostatic expansion valve in a refrigeration system?

The thermostatic expansion valve is used to regulate the flow of refrigerant to ensure that it evaporates completely in the coil to ensure the reduction of system pressure and to maintain constant superheat of the steam left by the coil. They can be external equalization type and internal equalization. Thermostatic expansion valves with external pressure equalization are used when, when flowing through the evaporator, the fluid undergoes a high pressure drop due to friction. In this way, its saturation temperature is always lower in the output than in the input. As an example, consider the valve with external pressure equalization, shown in Figure 3, mounted on a system with pressure drop in the evaporator of 62kPa.