The automatic-expansion valve controls liquid flow by responding to the suction pressure of the unit acting on its diaphragm or bellows (see Fig. 11-2). When the valve opens, liquid refrigerant passes into the evaporator. The resulting increase in pressure in the evaporator closes the valve. Meanwhile, the compressor is pulling the gas away from the coils, reducing the pressure. This pressure reduction allows the expansion valve to open again. In operation, the valve never quite closes. The needle floats just off the seat and opens wide when the unit calls for refrigeration. When the machine is shut down, the pressure building up in the coils closes the expansion valve until the unit starts up.
