When the compressor starts to work, the refrigerant pressure in the evaporator decreases. This causes the valve needle to open, allowing refrigerant to enter the evaporator. While the compressor is running, the automatic valve maintains a constant pressure in the evaporator.
When the compressor stops, the refrigerant pressure in the evaporator immediately starts to rise. This increase in pressure causes the valve needle to close.
As soon as the compressor stops working, it is important that the valve is closed, to prevent too much liquid refrigerant from entering the evaporator, as it could leak to the suction line. Therefore, it is necessary to regulate the pressure at which the valve must close, according to the temperature at which the compressor turns off. This is done by the adjustment screw. For this reason, every time the temperature control setting is changed, the automatic valve must also be adjusted.
This type of valve has its greatest use in systems where the loads are relatively constant and in systems with a single evaporator coil. One of the valve manufacturers uses the principle of capillary tubes with automatic expansion valves, causing the refrigerant to travel a long spiral path (similar to the screw thread) after passing the valve seat, thereby reducing erosion , deformation of the seat and the tendency to overfeed under reduced load conditions.
