The vessel shown in Fig. 10.15 should normally not be necessary, and in newly designed and constructed systems it usually is not found. In a two-stage liquid recirculation system the low-pressure receiver should be designed to prevent liquid from reaching the low-stage compressors, and the flash-tank/intercooler should prevent liquid from reaching the high-stage compressors.
The operators of many plants that employ a suction-line accumulator, however, are comforted by having one. In plants that through the years have been expanded, modified, and restructured there is sometimes liquid carryover and the suction-line accumulator is the final protector of the compressor from slugging with liquid. Some of the reasons that liquid carryover occurs are because of improper design of some other separating vessel, or occasional liquid carryover from a leaking expansion valve on a direct-expansion coil or the imposition of a sudden heat load on a flooded coil. The installation of a suction-line accumulator in a plant that is carefully designed could permit smaller surge drums on all the flooded coils and the occasional carryover of liquid could be accommodated by the suction-line accumulator.
Figure 10.15 shows the option of directing warm liquid from the high-pressure receiver to a coil in the bottom of the accumulator. When liquid does collect in the accumulator the vaporization by heat from the machine room is a slow process, and the supply of heat from the warm high-pressure liquid accelerates vaporization and recovers a small amount of refrigeration as well. The warm liquid coil in the accumulator will have the undesirable effect of super-heating the suction vapor when no liquid is present in the accumulator. Some plants equip the accumulator with a transfer system employing mechanical or gas pumping, which is initiated by a rise in the liquid level in the accumulator. These arrangements deliver the liquid to the high-pressure receiver.