HVAC Expansion Valves

An expansion valve is a piece of HVAC equipment that reduces pressure in a working HVAC system. There are two main type of expansion valves used in heating and air conditioning equipment; automatic and thermostatic expansion valves. Thermostatic expansion valves are used to control the amount of refrigerant used to control the temperature of an air conditioner.  Automatic expansion valves are used to control pressures of the evaporator.  Here the two types of expansion valves are described in more detail:

Automatic Expansion Valves
An automatic expansion valve is a pressure-actuated diaphragm valve used to maintain a constant pressure in the evaporator of a direct-expansion mechanical refrigeration system.  The automatic valve works by regulating the flow of a refrigerant from the liquid line into the evaporator. With this method, the evaporator is always supplied with the proper amount of refrigerant to meet conditions. An automatic expansion valve does not respond well to fluctuations in the load.  Because of this automatic expansion valves are not used in air conditioning systems.

Thermostatic Expansion Valves
The thermostatic expansion valve is designed to automatically control the flow of liquid refrigerant entering the evaporator coil. This valve must operate freely without restriction in order to allow the proper amount of refrigerant to enter the evaporator.