Thermodynamics

642 | Thermodynamics6Gas Refrigeration Cycle5Expansionvalve·Q HCondenserHigh-pressurecompressor9Flash·chamber327ExpansionLow-pressurevalvecompressorEvaporator8 1·Q LFIGURE P11–4811–49C How does the ideal-gas refrigeration cycle differfrom the Brayton cycle?11–50C Devise a refrigeration cycle that works on thereversed Stirling cycle. Also, determine the COP for thiscycle.11–51C How does the ideal-gas refrigeration cycle differfrom the Carnot refrigeration cycle?11–52C How is the ideal-gas refrigeration cycle modifiedfor aircraft cooling?11–53C In gas refrigeration cycles, can we replace the turbineby an expansion valve as we did in vapor-compressionrefrigeration cycles? Why?11–54C How do we achieve very low temperatures withgas refrigeration cycles?11–55 An ideal gas refrigeration cycle using air as theworking fluid is to maintain a refrigerated space at 23°Cwhile rejecting heat to the surrounding medium at 27°C. Ifthe pressure ratio of the compressor is 3, determine (a) themaximum and minimum temperatures in the cycle, (b) thecoefficient of performance, and (c) the rate of refrigerationfor a mass flow rate of 0.08 kg/s.11–56 Air enters the compressor of an ideal gasrefrigeration cycle at 12°C and 50 kPa and theturbine at 47°C and 250 kPa. The mass flow rate of airthrough the cycle is 0.08 kg/s. Assuming variable specific4heats for air, determine (a) the rate of refrigeration, (b) thenet power input, and (c) the coefficient of performance.Answers: (a) 6.67 kW, (b) 3.88 kW, (c) 1.7211–57 Reconsider Prob. 11–56. Using EES (or other)software, study the effects of compressor andturbine isentropic efficiencies as they are varied from 70 to100 percent on the rate of refrigeration, the net power input,and the COP. Plot the T-s diagram of the cycle for the isentropiccase.11–58E Air enters the compressor of an ideal gas refrigerationcycle at 40°F and 10 psia and the turbine at 120°F and30 psia. The mass flow rate of air through the cycle is 0.5lbm/s. Determine (a) the rate of refrigeration, (b) the netpower input, and (c) the coefficient of performance.11–59 Repeat Prob. 11–56 for a compressor isentropicefficiency of 80 percent and a turbineisentropic efficiency of 85 percent.11–60 A gas refrigeration cycle with a pressure ratio of 3uses helium as the working fluid. The temperature of thehelium is 10°C at the compressor inlet and 50°C at the turbineinlet. Assuming adiabatic efficiencies of 80 percent forboth the turbine and the compressor, determine (a) the minimumtemperature in the cycle, (b) the coefficient of performance,and (c) the mass flow rate of the helium for arefrigeration rate of 18 kW.11–61 A gas refrigeration system using air as the workingfluid has a pressure ratio of 4. Air enters the compressor at7°C. The high-pressure air is cooled to 27°C by rejectingheat to the surroundings. It is further cooled to 15°C byregenerative cooling before it enters the turbine. Assumingboth the turbine and the compressor to be isentropic andusing constant specific heats at room temperature, determine(a) the lowest temperature that can be obtained by this cycle,(b) the coefficient of performance of the cycle, and (c) themass flow rate of air for a refrigeration rate of 12 kW.Answers: (a) 99.4°C, (b) 1.12, (c) 0.237 kg/s11–62 Repeat Prob. 11–61 assuming isentropic efficienciesof 75 percent for the compressor and 80 percent for theturbine.11–63 A gas refrigeration system using air as the workingfluid has a pressure ratio of 5. Air enters the compressor at0°C. The high-pressure air is cooled to 35°C by rejecting heatto the surroundings. The refrigerant leaves the turbine at80°C and then it absorbs heat from the refrigerated spacebefore entering the regenerator. The mass flow rate of air is0.4 kg/s. Assuming isentropic efficiencies of 80 percent forthe compressor and 85 percent for the turbine and using constantspecific heats at room temperature, determine (a) theeffectiveness of the regenerator, (b) the rate of heat removalfrom the refrigerated space, and (c) the COP of the cycle.Also, determine (d) the refrigeration load and the COP if thissystem operated on the simple gas refrigeration cycle. Use the