Car A/C Anti-Frosting Devices
The evaporator pressure regulator (Figs 2.43 and 2.44) is mounted between the outlet of the evaporator and the compressor inlet (suction side).The valve regulates the pressure inside the evaporator to prevent icing. If the pressure drops below a certain threshold (196 kPa (28.4 psi)) then the valve closes to restrict the flow of refrigerant and increase the pressure inside the evaporator.This is to stop the evaporator temperature from reaching 0°C due to the relationship between temperature and pressure.
When the cooling load is high the vapour pressure of the refrigerant in the evaporator is high. The valve fully opens and the refrigerant flows unobstructed to the compressor.The valve operation is based on a spring bellows which expands and contracts with changes in refrigerant pressure. This device virtually eliminates the need for the compressor to cycle on and off to regulate the temperature of the evaporator.
The only problem with this type of valve is the possibility of oil starvation due to the reduced flow of refrigerant.