Expansion Valve: Function
When the heat load to the air conditioning system increases, the refrigerant temperature at the evaporator outlet rises and therefore the pressure P- 1 around the temperature sensing area increases! As this pressure P-1 becomes higher than the sum of the evaporator outlet (low-pressure side) pressure P2 and the spring force F (P-1 > P2 + F), the diaphragm is pressed down, moving the valve ball connected to the diaphragm clear of its seat. This increases the flow of the refrigerant.
When the heat load is small, the action of the valve's inner elements is contrary to the above; the valve ball closes and the flow of the refrigerant decreases.