Refrigeration & Equipment Engineering

Unit 3 - Parts of refrigeration systems

3.4.2.Automatic Expansion valve
The automatic expansion valve is also called constant pressure controlled expansion valve. Evaporation used with automatic expansion valve are called expansion evaporator
The automatic expansion valve consists of a needle valve, valve seat, diaphragm spring, strainer etc.
Here two types of forces (Pressures) on the diaphragms. One is the spring pressure and other evaporator pressure. The spring pressure exerted on the top of the diaphragm in the opening direction of the valve and the evaporator pressure acts beneath the diagram in the closing direction of he valve. Thus both the evaporator and spring pressure acts to counter balance each other.
The liquid refrigerant enters through the inlet strainer up to the orifice and needle. If the evaporator pressure is reduced to the setting pressure of the spring. The pressure of the spring increased and the valve opens more and more liquid refrigerant reaches in to the evaporator coils. Once the vaporization rate increases resulting in more pressure develop than the spring pressure setting the valve closes when by the flow of evaporator pressure equal to the spring pressure.
From the above point it is clear that this valve keeps the pressure constant in the evaporator coils so its also called constant pressure expansion valve this valve is used n domestic refrigeration systems.