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The cascade system consists of two separate refrigeration circuits connected only by an intermediate cascade heat exchanger.
As shown in Figure 10.18, the high-temperature circuit is cooled by an air condenser (2) at ambient temperature, and uses the cascade heat exchanger (1) as the system evaporator. The low-temperature system produces the low-temperature cooling in the cold evaporator (3), and uses the cascade exchanger as a condenser. The corresponding outline in a log P/h diagram is shown in Figure 10.19.
The cascade heat exchanger connects the two refrigerant circuits thermally by acting simultaneously as an evaporator and a condenser. The primary advantage of a cascade system is that the two stages do not necessarily contain the same refrigerants. A refrigerant with a higher vapor pressure can be used in the low-temperature system, while a refrigerant with a lower vapor pressure is suitable for the high-temperature system.
Multi-stage refrigeration cycles can also achieve very low temperatures efficiently, but there are some major disadvantages compared with the cascade cycle. In multi-stage refrigeration, the same refrigerant must work at the highest and the lowest pressure levels.
The selection of refrigerant to avoid excessively large pressures in the ambient condenser, and evaporation pressures below one atmosphere in the cold evaporator, can be difficult. Vacuum should always be avoided, because this increases the risk of air and moisture leaking into the system, leading to reduced system performance and increased wear on components. Because the refrigerant oil has a higher solubility in the refrigerant at higher temperatures, a multi-stage system also has a higher risk of uneven oil distribution, giving lubrication problems in the low-stage compressor.
By contrast, refrigerant selection and oil distribution for a cascade system can be dealt with separately for each circuit. It is important to note that the cascade heat exchanger will be exposed to temperature and pressure fluctuations. In the cascade unit, the evaporating side typically operates at -10 to -20°C. The discharge gas from the low-temperature compressor may very well be 80°C or higher.
To avoid the risk of thermal fatigue inside the cascade unit due to the very high temperature differences, the installation of a desuperheater (5) is recommended before the inlet on the condensing side, i.e. in the "cold" circuit.
The desuperheater reduces the inlet gas temperature of the condensing side of the cascade unit, while utilizing the superheating energy to produce high-temperature water (6). The main duty, however, is to reduce the gas temperature. The desuperheater must therefore not be bypassed.
If you would like to learn more about Cascade applications, download the free digital handbook "A Technical Handbook for Refrigerant Applications". Go to download page.
