Inside a check valve there is a disc or ball pressed onto its seat by a light spring. When the flow comes from the right direction it pushes the disc and passes; if the flow stops or reverses, the spring and the back pressure close the disc. It has no electrical connection and no adjustment. The arrow on the body shows the direction of flow. The pressure difference needed to open it is between 0.05 and 0.3 bar depending on the type; this value is the minimum pressure drop the valve creates.
The main places it is used in refrigeration are these: the discharge line of each of the compressors running in parallel (so that gas does not come back to a stopped compressor), between the condenser and the receiver (so that in winter liquid does not migrate back to the cold condenser), hot gas defrost lines, the bypass around the expansion valve in heat pumps, the suction outlet of the colder evaporator in systems where several evaporators run at different temperatures, and oil return lines. In each of them the aim is the same: to prevent the refrigerant from migrating to the wrong place when the system stops.
In selection one looks not at the connection diameter but at the flow that will pass. In a check valve chosen too large the flow cannot open the disc fully; the disc keeps knocking against its seat, makes noise and wears early. A valve chosen too small, on the other hand, creates unnecessary pressure drop; in the suction line this is a direct loss of capacity. Discharge line check valves are chosen from damped types that withstand pulsating flow and are placed a little away from the compressor. The capacity tables in manufacturers’ catalogs are given separately by refrigerant and line type.
In installation attention is paid to the direction of the arrow and to the position permitted by the manufacturer (horizontal, vertical with upward flow). During brazing the body is cooled with a wet cloth; the seal inside is damaged by heat. Liquid must not be trapped between two check valves, or between a check valve and a closed solenoid valve: liquid that warms up expands and can raise the pressure to a level that splits the pipe. A check valve is not regarded as completely tight; for pump-down a solenoid valve is needed as well.