Problem 6.37:
Figure 6-20 shows a two-stage refrigeration cycle with two evaporating temperatures, flash gas removal, and intercooling. The cycle operates using carbon dioxide as the refrigerant. The condenser (HE1) uses cooling water at 25 °C to produce saturated liquid (stream 2). One third of the mass flow rate at 2 is diverted to stream 3 with the remainder going into stream 5. Both evaporators (HE2 and HE3) produce saturated vapor. The flash gas separator (S1) removes all the vapor at the top (stream 8) and the liquid at the bottom (stream 7). The second stage HE3 removes heat at the rate of 100 W at - 40
°C. Both compressors increase the pressure of their respective incoming streams by the same factor and they both operate with an efficiency of 80%. This system must be capable of removing 100 kW of heat in evaporator HE2 at - 40 °C. Determine the flow rate of the refrigerant, the amount of work in the compressors, and the heat loads in all heat exchangers. Perform a second-law analysis and identify the unit that contributes most to the irreversible operation of this cycle. Finally, sketch a PH graph and show the states of all streams.
