The function of a thermostatic expansion valve is determined by three fundamental pressures:
P1: Bulb pressure which acts on the upper surface of the diaphragm, in the valve opening direction.
P2: Evaporating pressure which acts on the underside of the diaphragm, in the valve closing direction.
P3: Spring pressure which also acts on the underside of the diaphragm, in the valve closing direction.
When the expansion valve regulates, balance is created between bulb pressure on one side of the diaphragm and evaporating pressure plus spring force on the other side.
The spring is used to set superheat.
What if the Electronic Expansion Valve orifice is too big?
Oversized valve: The expansion valve may be oversized for the application or operating condition of the system. Valve capacity significantly exceeds the requirements of the system and when the valve attempts to adjust to system load, it overcompensates because it is oversized.