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.
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.