Refrigeration & Equipment Engineering

Unit 3 - Parts of refrigeration systems

3.4.3.Thermostatic expansion valve
This valve controlling the liquid refrigerant is very prevalent. This works with the temperature and pressure. This valve is a precision device that it regulates the rate of flow of liquid refrigerant into the evaporator in exact proportion to the rate of evaporation of liquid refrigerant which is according to the heat load. This also regulates the quantity of refrigerant vapour exhauled by evaporator. This works according to the temperature of the refrigerant vapour leaving the evaporator and pressure in the evaporator.
In its construction it consists of needle valve, diaphragm, remote feeler bulb, controlling spring, adjusting screw etc.
Here three pressures are acting one is remote bulb pressure on the top pf diaphragm, second and third is spring pressure and evaporator pressure acting beneath the diaphragm.
The spring exerts its force beneath the diaphragm through the needle valve in the closing direction of the valve. This pressure can be made or less by adjusting screw. The evaporator pressure is also act against the beneath diaphragm in the closing direction of the valve. The pressure of the remote bulb is equal to the sum of pressure of the spring and evaporator. The valve opens due to remote bulb pressure but the valve starts to closing due to spring and evaporator pressure.
Spring pressure has got to be adjusting to balance the super heat quantity.
It has got the following three functions in the refrigeration system.

• Throttling action : It reduces the pressure
• Modulating action: It sends liquid refrigerant in the evaporator in proper quantity
• Keeps the necessity of load change in order

This valve not only set to keep a constant superheat and temperature pressure in the evaporator but it also used for variable heat load of the system so this valve is most commonly used in commercial refrigeration and air- conditioning system.