The amount of refrigerant in the system is fixed but the need changes: in summer at high condensing pressure the condenser holds less liquid, in winter more; as the evaporator load changes, the circulating quantity changes. The receiver is the buffer that makes up this difference. Liquid from the condenser fills the vessel and leaves from the bottom through the filter drier and sight glass to the expansion valve.
Its second job is service: with pump-down the whole refrigerant charge of the system is collected in the receiver, the inlet and outlet valves are closed and work can be done on the evaporator or pipework without releasing refrigerant to the atmosphere. For this reason the receiver is sized to hold the entire system charge without exceeding 80% full.
The receiver carries a safety valve or fusible plug, a level indicator and shut-off valves. On our housed and open condensing units the receiver comes mounted on the frame; its capacity is stated in litres in the catalog. Very small hermetic units and critically charged systems (for example some R290 appliances) do not use a receiver; the charge quantity is calculated exactly.