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Receivers, which we sometimes call “liquid receivers,” are components that store refrigerant. You'll see it on everything from small self-contained refrigeration units to very large commercial and industrial systems.
Many new techs who are used to residential air conditioning confuse receivers with accumulators. It's an understandable error, as they both contain liquid refrigerant. However, you'll find the accumulator on the suction line before the compressor; it prevents liquid from entering the compressor. On the other hand, you'll find a receiver on the liquid line after the condenser. That's because it stores liquid refrigerant that doesn't need to be in circulation for the current heat load.
The liquid receiver stores refrigerant when the system is operating at less than its maximum heat load. In general, systems with receivers are designed so that the receiver can hold the entire system's charge and still be no more than 80% full. That design allows you to pump down the entire system charge into the receiver without the danger of creating hydrostatic pressure; that refers to very high pressures resulting from full liquid expansion. (That's the same reason why we should only fill recovery tanks up to 80% full.)
The multi-position service valve at the outlet of the receiver is called a “King valve.” It can be used for refrigerant circuit access, and it may be fully front seated (turned clockwise) for pump down.
Because a receiver has both liquid and vapor present inside, many techs argue that the refrigerant cannot be “subcooled” in the receiver. The truth is that while the refrigerant that interacts between the liquid and vapor at the top of the receiver is at saturation, the refrigerant below the liquid line can be and usually will and should be subcooled.