Two Stage Cascade Refrigerating System

A two-stage cascade system employs two vapor-compression units working separately with different refrigerants, and interconnected in such a way that the evaporator of one system is used to serve as condenser to a lower temperature system (i.e. the evaporator from the first unit cools the condenser of the second unit). In practice, an alternative arrangement utilizes a common condenser with a booster circuit to provide two separate evaporator temperatures.

In fact, the cascade arrangement allows one of the units to be operated at a lower temperature and pressure than would otherwise be possible with the same type and size of single-stage system. It also allows two different refrigerants to be used, and it can produce temperatures below -150°C. Figure 3.38 shows a two-stage cascade refrigeration system, where condenser B of system 1 is being cooled by evaporator C of system 2. This arrangement enables to reach ultralow temperatures in evaporator A of the system.

A practical two-stage cascade refrigeration system.
A practical two-stage cascade refrigeration system.

For a schematic system shown in Figure 3.39, the condenser of system I, called the first or high-pressure stage, is usually fan cooled by the ambient air. In some cases a water supply may be used but air-cooling is much more common. The evaporator of system I is used to cool the condenser of system II called the second or low-pressure stage. The unit that makes up the evaporator of system I and the condenser of system II is often referred to as the inter-stage or cascade condenser. As stated earlier, cascade systems generally use two different refrigerants (i.e. one in  each stage). One type is used for the low stage and a different one for the high stage. The reason why two refrigeration systems are used is that a single system cannot economically achieve the high compression ratios necessary to obtain the proper evaporating and condensing temperatures. It is clear from the T-s diagram of the two-stage cascade refrigeration system as shown in Figure 3.39 that the compressor work decreases and the amount of refrigeration load (capacity) in the evaporator increases as a result of cascading (Cengel and Boles, 1998). Therefore, cascading improves the COP.


One Reply to “Two Stage Cascade Refrigerating System”

  1. Hey please give me the following
    Schematic diagram of Two stage system

    Schematic diagram of a casecade system

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