The thermostatic expansion valves are essentially reducing valves between the highpressure side and the low-pressure side of the system. These valves, which are the most widely used devices, automatically control the liquid-refrigerant flow to the evaporator at a rate that matches the system capacity to the actual load. They operate by sensing the temperature of the superheated refrigerant vapor leaving the evaporator. For a given valve type and refrigerant, the associated orifice assembly is suitable for all versions of the valve body and in all evaporating temperature ranges.
When the thermostatic expansion valve is operating properly, the temperature at the outlet side of the valve is much lower than that at the inlet side. If this temperature difference does not exist when the system is in operation, the valve seat is probably dirty and clogged with foreign matter. Once a valve is properly adjusted, further adjustment should not be necessary. The major problem can usually be traced to moisture or dirt collecting at the valve seat and orifice. Figure 3.22 shows a common type of electrically driven expansion valve.