The two features made available by two stages of compression—flash- gas removal and intercooling—have been analyzed separately. A system4 with two stages of compression and one level of evaporating temperature usually incorporates both processes, as shown in Figure 3.15. The corresponding pressure-enthalpy diagram is shown in Figure 3.16. The flash-gas removal may be achieved either by direct flashing of liquid at the interstage pressure (Figure 3.2) or with a liquid subcooler (Figure 3.5). One of the variations of intercooling, Figures 3.9 or 3.10, may also be chosen.
Example 3.3. An R-507 two-stage system with flash-gas removal and intercooling provides 200 kW of refrigeration at an evaporating temperature of -40°C (-40°F) when operating with a condensing temperature of 35°C (95°F). The intermediate temperature is -5°C (23°F).
(a) What are the enthalpies of the refrigerant at all points in the system?
(b) Compute the flow rates through each compressor.
(c) What are the power requirements of the compressors?
(d) What would be the power required in a single-stage R-507 system with these evaporating and condensing temperatures, and the percentage saving in power through the use of a two-stage system?