V/CDoes the ideal vapor-compression refrigerationcycle involve any internal irreversibilities?Yes; the throttling process is an internally irreversible process.Why is the throttling valve not replaced by an isentropicturbine in the ideal vapor-compression refrigerationcycle?To make the ideal vapor-compression refrigeration cycle more closely approximate the actualcycle.It is proposed to use water instead of refrigerant-134a as the working fluid in air-conditioning applicationswhere the minimum temperature never falls below the freezingpoint. Would you support this proposal? Explain.It is proposed to use water instead of refrigerant-134a as the working fluid in air-conditioning applicationswhere the minimum temperature never falls below the freezingpoint. Would you support this proposal? Explain.No. Assuming the water is maintained at 10C in the evaporator, the evaporator pressure will bethe saturation pressure corresponding to this pressure, which is 1.2 kPa. It is not practical to designrefrigeration or air-conditioning devices that involve such extremely low pressures.In a refrigeration system, would you recommend condensingthe refrigerant-134a at a pressure of 0.7 or 1.0 MPa ifheat is to be rejected to a cooling medium at 15C? Why?Allowing a temperature difference of 10C for effective heat transfer, the condensation temperatureof the refrigerant should be 25C. The saturation pressure corresponding to 25C is 0.67 MPa. Therefore,the recommended pressure would be 0.7 MPa.119C Consider two vapor-compression refrigeration cycles.The refrigerant enters the throttling valve as a saturated liquidat 30C in one cycle and as subcooled liquid at 30C in theother one. The evaporator pressure for both cycles is the same.Which cycle do you think will have a higher COP?The cycle that involves saturated liquid at 30C will have a higher COP because, judging from theT-s diagram, it will require a smaller work input for the same refrigeration capacity.The COP of vapor-compression refrigeration cyclesimproves when the refrigerant is subcooled before it enters thethrottling valve. Can the refrigerant be subcooled indefinitelyto maximize this effect, or is there a lower limit? Explain.11-10C The minimum temperature that the refrigerant can be cooled to before throttling is the temperatureof the sink (the cooling medium) since heat is transferred from the refrigerant to the cooling medium.cascadeWhat is cascade refrigeration? What are the advantagesand disadvantages of cascade refrigeration?Performing the refrigeration in stages is called cascade refrigeration. In cascade refrigeration, twoor more refrigeration cycles operate in series. Cascade refrigerators are more complex and expensive, butthey have higher COP's, they can incorporate two or more different refrigerants, and they can achievemuch lower temperatures.How does the COP of a cascade refrigeration systemcompare to the COP of a simple vapor-compressioncycle operating between the same pressure limits?Cascade refrigeration systems have higher COPs than the ordinary refrigeration systems operatingbetween the same pressure limits.1138C A certain application requires maintaining therefrigerated space at 32C. Would you recommend a simplerefrigeration cycle with refrigerant-134a or a two-stage cascaderefrigeration cycle with a different refrigerant at the bottomingcycle? Why?The saturation pressure of refrigerant-134a at -32C is 77 kPa, which is below the atmosphericpressure. In reality a pressure below this value should be used. Therefore, a cascade refrigeration systemwith a different refrigerant at the bottoming cycle is recommended in this case.Consider a two-stage cascade refrigeration cycle anda two-stage compression refrigeration cycle with a flash chamber.Both cycles operate between the same pressure limits anduse the same refrigerant. Which system would you favor?Why?11-39C We would favor the two-stage compression refrigeration system with a flash chamber since it issimpler, cheaper, and has better heat transfer characteristics.Can a vapor-compression refrigeration system witha single compressor handle several evaporators operating atdifferent pressures? How?Yes, by expanding the refrigerant in stages in several throttling devices.In the liquefaction process, why are gases compressedto very high pressures?To take advantage of the cooling effect by throttling from high pressures to low pressures.Gas RefHow does the ideal-gas refrigeration cycle differfrom the Brayton cycle?The ideal gas refrigeration cycle is identical to the Brayton cycle, except it operates in the reverseddirection.How does the ideal-gas refrigeration cycle differfrom the Carnot refrigeration cycle?In the ideal gas refrigeration cycle, the heat absorption and the heat rejection processes occur atconstant pressure instead of at constant temperature.How is the ideal-gas refrigeration cycle modifiedfor aircraft cooling?In aircraft cooling, the atmospheric air is compressed by a compressor, cooled by the surroundingair, and expanded in a turbine. The cool air leaving the turbine is then directly routed to the cabin.In gas refrigeration cycles, can we replace the turbineby an expansion valve as we did in vapor-compressionrefrigeration cycles? Why?No; because h = h(T) for ideal gases, and the temperature of air will not drop during a throttling(h1 = h2) process.How do we achieve very low temperatures withgas refrigeration cycles?with regeneration