- The thermostatic expansion valve consists of a needle valve, a seat, a metallic diaphragm, a spring, adjusting screw and a feeler bulb.
The opening or closing of valve depends upon following forces acting on the diaphragm:
i. Spring pressure acting on bottom of diaphragm
ii. Evaporator pressure acting on bottom of diaphragm
iii. Feeler bulb pressure acting on top of diaphragm
If load on evaporator increases, it causes the liquid refrigerant to boil faster in evaporator coil. Since feeler bulb is installed on the suction line, therefore it is at the same temperature as refrigerant at that point. So temperature of the bulb increases due to early vaporization of refrigerant.
- Thus the feeler bulb pressure increases and gets transmitted through the capillary tube to the diaphragm. The diaphragm moves downwards, opening the valve to admit more liquid refrigerant into the evaporator.
- This continues till pressure equilibrium on diaphragm is reached, at which feeler bulb pressure acting at top of diaphragm is balanced by spring and evaporator pressure acting at bottom of diaphragm.
- When evaporator load decreases, less liquid refrigerant evaporates in the coil, and the excess liquid flows towards the outlet. This cools the feeler bulb and its pressure and temperature decreases.
- This pressure makes the diaphragm move upward, reducing the valve opening and in turn decreasing refrigerant flow to evaporator. This causes decrease in evaporator pressure and again continues till diaphragm pressure equilibrium is reached.