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Ammonia Refrigeration
Liquid recirculation systems, often called liquid overfeed systems, were introduced to the industry in 1925 when a patent was issued to the York Corporation. As with many great ideas, it took between 35 and 40 years before it became common practice in the industrial refrigeration area. Even though there has been some very astute research regarding liquid recirculation, there seems to be as many designs as there are designers.
One of the major concerns in designing a liquid recirculation system is cavitation of the mechanical pumps. Liquid recirculation systems create cavitation in several ways:
* By an irregular refrigerant feed to the recirculator vessel causing pressure fluctuations that create bubbles in the refrigerant, which find their way to the pump suction.
* By improper pump selection, where the net positive suction head (NPSH) required by the pump is higher than the NPSH provided by the recirculator designer.
* By vapor bubbles migrating from the standby pump.
* By improper pump selection, where the pump total dynamic head (TDH) capability is not sufficient to satisfy the system demand.
* By using piping practices that have been outdated by definitive research and operating experience.
* By allowing compressors in the system to load too rapidly.
* By not "cold soaking" the ammonia pumps prior to starting.
* By improper recirculation vessel design and internal arrangement.
Most pump cavitations problems can be...