Thermodynamics

Each stage operates on the ideal vapor-compression refrigerationcycle with refrigerant-134a as the working fluid. Heatrejection from the lower cycle to the upper cycle takes placein an adiabatic counterflow heat exchanger where bothstreams enter at about 0.4 MPa. If the mass flow rate of therefrigerant through the upper cycle is 0.24 kg/s, determine (a)the mass flow rate of the refrigerant through the lower cycle,(b) the rate of heat removal from the refrigerated space andthe power input to the compressor, and (c) the coefficient ofperformance of this cascade refrigerator.Answers: (a) 0.195 kg/s, (b) 34.2 kW, 7.63 kW, (c) 4.4911–43 Repeat Prob. 11–42 for a heat exchanger pressure of0.55 MPa.11–44 A two-stage compression refrigeration systemoperates with refrigerant-134a between thepressure limits of 1 and 0.14 MPa. The refrigerant leaves thecondenser as a saturated liquid and is throttled to a flashchamber operating at 0.5 MPa. The refrigerant leaving thelow-pressure compressor at 0.5 MPa is also routed to theflash chamber. The vapor in the flash chamber is then compressedto the condenser pressure by the high-pressure compressor,and the liquid is throttled to the evaporator pressure.Assuming the refrigerant leaves the evaporator as saturatedvapor and both compressors are isentropic, determine (a) thefraction of the refrigerant that evaporates as it is throttled tothe flash chamber, (b) the rate of heat removed from therefrigerated space for a mass flow rate of 0.25 kg/s throughthe condenser, and (c) the coefficient of performance.11–45 Reconsider Prob. 11–44. Using EES (or other)software, investigate the effect of the variousrefrigerants for compressor efficiencies of 80, 90, and 100percent. Compare the performance of the refrigeration systemwith different refrigerants.11–46 Repeat Prob. 11–44 for a flash chamber pressureof 0.32 MPa.11–47 Consider a two-stage cascade refrigeration systemoperating between the pressure limits of 1.2 MPa and 200kPa with refrigerant-134a as the working fluid. Heat rejectionfrom the lower cycle to the upper cycle takes place in an adiabaticcounterflow heat exchanger where the pressure in theupper and lower cycles are 0.4 and 0.5 MPa, respectively. Inboth cycles, the refrigerant is a saturated liquid at the condenserexit and a saturated vapor at the compressor inlet, andthe isentropic efficiency of the compressor is 80 percent. Ifthe mass flow rate of the refrigerant through the lower cycleis 0.15 kg/s, determine (a) the mass flow rate of the refrigerantthrough the upper cycle, (b) the rate of heat removal fromthe refrigerated space, and (c) the COP of this refrigerator.Answers: (a) 0.212 kg/s, (b) 25.7 kW, (c) 2.687ExpansionvalveEvaporator4 1··Q HCondenser8 Evaporator 53ExpansionvalveHeatCondenserQ LChapter 11 | 641Compressor11–48 Consider a two-stage cascade refrigeration systemoperating between the pressure limits of 1.2 MPa and 200 kPawith refrigerant-134a as the working fluid. The refrigerantleaves the condenser as a saturated liquid and is throttled to aflash chamber operating at 0.45 MPa. Part of the refrigerantevaporates during this flashing process, and this vapor is mixedwith the refrigerant leaving the low-pressure compressor. Themixture is then compressed to the condenser pressure by thehigh-pressure compressor. The liquid in the flash chamber isthrottled to the evaporator pressure and cools the refrigeratedspace as it vaporizes in the evaporator. The mass flow rate ofthe refrigerant through the low-pressure compressor is 0.15kg/s. Assuming the refrigerant leaves the evaporator as a saturatedvapor and the isentropic efficiency is 80 percent for bothcompressors, determine (a) the mass flow rate of the refrigerantthrough the high-pressure compressor, (b) the rate of heatremoval from the refrigerated space, and (c) the COP of thisrefrigerator. Also, determine (d) the rate of heat removal andthe COP if this refrigerator operated on a single-stage cyclebetween the same pressure limits with the same compressorefficiency and the same flow rate as in part (a).62CompressorFIGURE P11–47·W in·W in