How Thermostatic Expansion Valve Works?

There are three pressures, P1, P2 and P3 working inside the thermostatic expansion valve. P1 is the pressure at the top of the thermostatic expansion valve acting inside the power element above the diaphragm. The pressure P2 is the pressure acting on the lower side of the diaphragm due to the refrigerant pressure inside the evaporator. The pressure P3 is the spring pressure that tends to close the opening of the valve. This pressure remains constant. The pressures P2 and P3 act against the pressure P1. The pressure P1 tends to open the valve while the pressure P2 and P3 tend to close the thermostatic expansion valve. Thus if the valve has to open P1 should be greater than combined forces of P2 and P3.