To control the rate of flow of the liquefied refrigerant between the high- and low-side pressures of the system, some form of expansion device must be provided. This device is usually an expansion valve. Automatic expansion valves are pressure-controlled devices that operate to maintain a constant pressure in the evaporator. Such an expansion valve is usually applied to evaporators of the direct expansion type, but it is not satisfactory for fluctuating loads such as those encountered in air-conditioning installations.
Thermostatic expansion valves are similar in construction to automatic expansion valves but, in addition, incorporate a power element responsive to changes in the degree of superheat of the refrigerant gas leaving the evaporator coil. This power element consists of a bellows connected by means of a capillary tube to a feeler bulb fastened to the suction line from the evaporator.