The high-pressure receivers in small plants that operate on a seasonal schedule are typically sized to contain all the refrigerant existing in the system. During pump-down operation, the king valve (Fig. 10.11) in the liquid line from the receiver is closed and the refrigerant delivered by the compressor(s) condenses and drains into the receiver. When all liquid has been withdrawn from the low side, the valves between the condenser(s) and receiver are closed to confine all liquid refrigerant in the receiver. In these plants, the receiver must accommodate all the liquid of the system in the storage volume between the high and low liquid levels, as shown in Fig. 10.11. Some vapor volume is always required above the highest liquid level, and a reserve of liquid should always prevail in
the receiver, even when the remainder of the system is fully supplied. The piping of the receiver illustrated in Fig. 10.11 is top inlet, and the other concept in piping, as was shown in Fig. 7.27, is the bottom inlet.
Many large plants operate all year, and receivers for these plants are never expected to contain all the liquid in the system. Two of several bases used by designers to select the size of receivers are:
• storage volume to pump down the largest refrigerated room or unit served by the plant
• store full refrigerant flow for a specified duration of time, 30 minutes, for example.
The rationale behind the first basis is that any of the refrigerated spaces, including the largest, may need to be taken out of service. The second basis is predicated on being able to interrupt the liquid supply to the plant for a short period of time, meanwhile continuing compressor operation. Standard shell diameters in North America are 0.51 m (20 in); 0.56 m (22 in); 0.61 m (24 in); 0.76 m (30 in);…1.22 m (0.48 in); 1.37 m (54 in), etc. The cost of vessels larger in diameter than 1.52 m (60 in) increases abruptly.