A significant fraction of industrial refrigeration plants operate with a large difference between evaporating and condensing temperatures—perhaps 50° to 80° C (100° to 150°F). This large temperature lift imposes both problems and opportunities for the system. An opportunity is to use multistage compression, which although increasing the first cost over single-stage compression, also alleviates some problems and can save on total compressor power. In multistage compression, the refrigerant basically flows in series through two or more compressors with special processes performed on the refrigerant between stages. This chapter concentrates on two-stage compression, but the principles could be extended to three-stage compression which is an arrangement sometimes used in extremely low-temperature installations.
The use of two-stage compression opens up the opportunity to use two key processes which characterize multistage systems—flash gas removal and intercooling. It is instructive to examine these processes individually before combining them, as they exist in real systems. The number and temperature level of evaporators in the system imposes another variation on the configuration of multistage systems. The chapter concludes with an examination of cascade circuits, which are a form of multistage systems.