A liquid receiver will be required if it is necessary to temporarily store refrigerant charge within the system, or to accommodate the excess refrigerant arising from changing operating conditions. The total refrigerant charge required in a circuit will vary with different operating loads and ambients, and must be sufficient at all times so that only liquid enters the expansion valve.
A receiver requires a minimum operating charge which adds to overall charge and cost, and also increases system complexity. Hence receivers are avoided on many smaller systems.
A typical receiver suitable for a large system is shown in Figure 6.10 .
Figure 6.10 Liquid receiver
Receivers also act as pump-down reservoirs, and should be capable of holding enough of the total refrigerant charge to permit evacuation of any one vessel for maintenance, inspection or repair. They should never be more than 85% full, to allow for expansion and safety.
Receivers are commonly made of steel tube with welded dished ends, and are located horizontally. Small receivers may be vertical, for convenience of location. The liquid drain pipe from the condenser to the receiver should be amply sized, and any horizontal runs sloped to promote easy drainage. Shut-off valves in this line should not be in a horizontal outlet from the condenser, since their slight frictional resistance will cause liquid back-up in the condenser. Outlet pipes from the receiver may be from the bottom or, by means of an internal standpipe, may leave at the top. A valve is invariably fitted at this point.
Ammonia receivers may have an oil drum pot, and the receiver will slope slightly down towards this.
Receivers are pressure vessels covered by the provisions of EN378 and require appropriate safety pressure relief devices. In cases where there is no shut-off valve between the condenser and receiver, such protection may be fitted to one or the other, providing the total volume is considered.
In practice, receivers will operate about one-sixth full during normal running. Some means are usually provided to indicate the liquid level inside. For example:
1. An external, vertical sight glass, of suitable pattern, having self-closing shut-off valves.
2. A number of sight glasses arranged at different heights in the shell.
3. A pair of sight glasses, arranged on the same cross-section and some 45° up from the horizontal diameter. A light is shone through one and the observer looks through the other.
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