Where the ratio of suction to discharge pressure is high enough to cause a serious drop in volumetric efficiency (see Chapter 4) or an unacceptably high discharge temperature, vapour compression must be carried out in two or more stages.
Two-stage systems use the same refrigerant throughout a common circuit, compressing in two stages. By using separate compressors for each stage, the second-stage displacement can be adjusted to accommodate an extra cooling load, side load. at the intermediate pressure. Compression in two stages within a single machine can be accomplished with multicylinder compressors. The first stage of compression takes place in, say, 4 cylinders and the second stage in 2 cylinders of a 6-cylinder machine. Hot discharge gas from the first compression stage passes via an intercooler to the high-stage compressor, and consists of a small evaporator supplied by refrigerant from the condenser. Alternatively a water-cooled heat exchanger could be used, or simple injection of a controlled amount of liquid refrigerant (from the condenser) to mix with the intermediate pressure gas.
A more energy efficient alternative is the arrangement shown in Figures 2.8 and 2.9. Part of the refrigerant liquid from the condenser is taken to a subcooler,
Figure 2.8 Two-stage cycle with subcooler
Figure 2.9 Mollier diagram for R404A showing two-stage vapour compression cycle with subcooler
Where the main liquid flow to the expansion valve is cooled from E to F, and this increases the duty of the evaporator (A-H). This cycle is more efficient than the single-stage cycle because the part of the mass flow is compressed only through the second stage. A flash intercooler may be used instead of a subcooler. All the liquid is then reduced to intermediate pressure via a suitable expansion valve. The intercooler acts as a separation vessel in which the flash gas formed in the expansion process is separated from the liquid. From the intercooler, the flash vapour is led to the high-stage compressor, whilst the liquid, which has been separated, is further expanded to the low pressure. A float valve of the type shown in Figure 8.10 can be used to control admission to the intercooler.
A version of the two-stage cycle, called an economizer cycle, can be applied with scroll and screw compressors. With these machines, access to the intermediate pressure within the compression process via an additional port on the casing allows vapour from the subcooler to be injected part way through the compression process. Only one compressor is needed, and it is almost identical to the single-stage version, requiring just the additional vapour injection port. The economizer cycle is a very cost-effective way of gaining improved performance.
The cascade cycle has two separate refrigeration systems, one acting as a condenser to the other (see Figure 2.10). This arrangement permits the use of different refrigerants in the two systems and high-pressure refrigerants such as R23 are used in the lower stage. The cycle is shown on one chart for convenience.
The Mollier diagrams for compound and cascade systems (Figures 2.9 and 2.10) indicate the enthalpy change per kilogram of circulated refrigerant, but it should be borne in mind that the mass flows rates in the low and high stages differ, and this must be accounted when calculating capacities.
Figure 2.10 Mollier diagram for R404A showing cascade cycle
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