Vessels in industrial refrigeration systems serve either or both of the following functions: (1) storage of liquid, and/or (2) separation of liquid from vapor. The major categories of vessels are:
• high-pressure receivers
• flash tank (or subcooler)/desuperheater
• low-pressure receiver for liquid recirculation
• surge drum on a flooded coil
• suction-line trap or accumulator
• thermosyphon receiver
Vessels are much more common in industrial refrigeration systems than in air-conditioning and there are several reasons for this difference:
Parallel refrigerant circuits. Air-conditioning systems usually are built with a single refrigerant circuit, while industrial systems incorporate parallel compressors, condensers, and evaporators. With a system of multiple components, liquid is likely to move from one condenser or evaporator to another. Also the liquid content in these components varies with time, so a vessel should be available to provide a reservoir for these changes in liquid content.
Liquid recirculation and flooded coils. Liquid along with vapor leaves both liquid overfeed and flooded coils, and this liquid must be separated for return to the evaporator. The vapor that passes on to the compressor must be free of liquid, so a vessel performs a process of separating the liquid from the vapor.
Defrosts. Industrial systems often refrigerate air at low temperatures which brings with it the need to periodically defrost evaporator coils. During hot-gas defrosts, refrigerant liquid shifts locations.
Frequent expansions. When air-conditioning systems require additional capacity, it is usually provided by the installation of an additional single-circuit package. The expansion of industrial systems, on the other hand, is usually accomplished by the installation of additional evaporators, compressors, and condensers. Additional refrigerant inventory is required for such enlargements, and generously sized storage vessels facilitate such expansions.