Refrigerating Systems. Two-stage cascade systems
A two-stage cascade system uses two pairs of compressor plants, working individually with different refrigerants, connected among themselves so that evaporator one system is used to serve as the capacitor to a lower temperature of the system (i.e. the evaporator with the first unit cools the condenser of the second unit). In practice, an alternative approach utilizes a common capacitor with a booster circuit to provide two separate temperature of the evaporator.
In fact, stacking allows one of the divisions to be operated at lower temperatures and pressures than would be possible with the same type and size of the single-stage system. It also permits two different refrigerants used, and it can produce temperatures below-150VC. Fig. 3.38 shows two stages : cascade-cooling system, where B capacitor system 1 is cooled in the evaporator system C 2. This location allows you to reach very low temperatures of the evaporator system.
For a diagram of the system is shown on Fig.
3.39, condenser system I, called the first or high pressure stage, as a rule, the fan cools the surrounding air.
In some cases, water can be used for air cooling is much more common. Evaporator system I used for cooling the condenser system II called the second or low pressure stage. Unit that is evaporator system I and capacitor II system is often referred to as inter-stage or cascade capacitor. As noted earlier, cascade systems typically use two different refrigerants (i.e., one INB each stage). One type is used for the low degree, and the other for higher stage. The reason why two refrigeration systems used that one system can not economically provide a high degree of compression of the data, necessary for obtaining the required temperature of evaporation and condensation. Clearly, T-s diagram in two stages of the cascade refrigerating system, as shown in Fig. 3.39 that the compressor is reduced and the amount of cooling load (capacity) in the evaporator increases as a result of the cascade (Cengel and boles, 1998). Therefore, cascading improves the COP.