Multistage refrigeration systems are widely used where ultra low temperatures are required, but cannot be obtained economically through the use of a single-stage system. This is due to the fact that the compression ratios are too large to attain the temperatures required to evaporate and condense the vapor. There are two general types of such systems: cascade and multistage. The multistage system uses two or more compressors connected in series in the same refrigeration system. The refrigerant becomes more dense vapor whilst it passes through each compressor. Note that a two-stage system (Figure 3.36) can attain a temperature of approximately -65°C and a three-stage about – 100°C.
Single-stage vapor-compression refrigerators are used by cold storage facilities with a range of +10°C to -30°C. In this system, the evaporator installed within the refrigeration system and the ice making unit, as the source of low temperature, absorbs heat. Heat is released by the condenser at the high-pressure side.
In cases where large temperature and pressure differences exist between the evaporator and the condenser, multistage vapor-compression systems are employed accordingly. For example, if the desired temperature of a refrigerator (i.e. freezer) is below -30°C, a severalstage compression system is required in order to prevent the occurrence of high compression ratios. Some disadvantages of a high compression ratio are:
• decrease in the compression efficiency,
• increase in the temperature of the refrigerant vapor from the compressor, and
• increase in energy consumption per unit of refrigeration production.
Figure 3.36a shows a schematic diagram of a two-stage vapor-compression refrigeration unit that can provide temperatures below -30°C (approximately to — 50°C), and its T-s and log P-h diagrams are shown in Figures 3.36b and 3.36c. This system also uses an intercooler with air.
As an example, three-stage refrigeration systems can provide an evaporator temperature of – 100°C. In the two-stage unit shown, the refrigerant is compressed in the first stage and, after being desuperheated by an intercooler, is further compressed in the second stage. An intercooler is used between the two compression stages for reducing the compression work. In other words, a booster (first-stage) compressor and a gas-liquid intercooler are attached to the single-stage cycle. The intercooler subcools the refrigerant liquid supplied to the evaporator by vaporizing a portion of the refrigerant after the first throttling stage. The flash gas returns at an intermediate point in the compression process in order to improve the compression efficiency by cooling the superheated gas. Not only a compressor, but a set of compressors are required to use in each stage, depending on the capacity and temperature. In large systems with a number of evaporators and large compression (temperature) ratios, the number of intercoolers and compression stages yields increased system efficiency and hence increased COP.
