Cascade refrigeration systems
To achieve a temperature of about -60 to -90 °C it’s appropriate to use cascade refrigeration systems. This system consists of separate, interconnected refrigeration machines. The common element of each cascade machine is the condenser evaporator. For the upper cascade it’s an evaporator, for the lower it’s a condenser. Circulating refrigerant is located in each cascade.
A cascade refrigeration machine can consist of two or more cascades. Each cascade is a single-stage of multi-stage refrigeration machine. The simplest cascade machine consists of two single-stage machines:
The lower temperature cycle provides the desired refrigeration effect at a relatively low temperature. The condenser in the lower-temperature cycle is thermally coupled to the evaporator in the higher-temperature cycle. Thus, the evaporator in the higher cycle only serves to extract the heat released by the condenser in the lower cycle. Then this heat is rejected into the ambient air or a water stream in the condenser of the higher cycle.
A cascade refrigeration system can operate with a lower evaporating temperature, smaller compression ratio and higher compressor volumetric efficiency when compared with a single-stage refrigeration system.
However, it is more expensive to build and more complicated than a single-stage system. Moreover, the overlap of the condensing temperature of the lower cycle and the evaporating temperature of the higher cycle, which is caused by the heat transfer between the 2 cycles, reduces the efficiency of a cascade system.
Service and Maintenance
Daily maintenance and service of an ammonia/CO2 cascade system is almost similar in many aspects to a conventional ammonia system, but is typically quicker and easier. Like ammonia, the high operating pressure and fast-rising hydrostatic pressure of liquid trapped in isolated components must be managed carefully.
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