Thermodynamics

Consider a vortex tube that receives compressed air at 500kPa and 300 K and supplies 25 percent of it as cold air at 100kPa and 278 K. The ambient air is at 300 K and 100 kPa, andthe compressor has an isentropic efficiency of 80 percent.The air suffers a pressure drop of 35 kPa in the aftercoolerand the compressed air lines between the compressor and thevortex tube.(a) Without performing any calculations, explain how theCOP of the vortex tube would compare to the COP of anactual air refrigeration system based on the reversed Braytoncycle for the same pressure ratio. Also, compare the minimumtemperatures that can be obtained by the two systemsfor the same inlet temperature and pressure.(b) Assuming the vortex tube to be adiabatic and usingspecific heats at room temperature, determine the exit temperatureof the hot fluid stream.(c) Show, with calculations, that this process does not violatethe second law of thermodynamics.(d) Determine the coefficient of performance of thisrefrigeration system, and compare it to the COP of a Carnotrefrigerator.ColdairCompressedairFIGURE P11–106Warmair11–107 Repeat Prob. 11–106 for a pressure of 600 kPa atthe vortex tube intake.11–108 Using EES (or other) software, investigate theeffect of the evaporator pressure on the COPof an ideal vapor-compression refrigeration cycle with R-134aas the working fluid. Assume the condenser pressure is keptconstant at 1 MPa while the evaporator pressure is variedfrom 100 kPa to 500 kPa. Plot the COP of the refrigerationcycle against the evaporator pressure, and discuss the results.11–109 Using EES (or other) software, investigate theeffect of the condenser pressure on the COP ofan ideal vapor-compression refrigeration cycle with R-134a asthe working fluid. Assume the evaporator pressure is keptconstant at 120 kPa while the condenser pressure is variedfrom 400 to 1400 kPa. Plot the COP of the refrigeration cycleagainst the condenser pressure, and discuss the results.Chapter 11 | 647Fundamentals of Engineering (FE) Exam Problems11–110 Consider a heat pump that operates on the reversedCarnot cycle with R-134a as the working fluid executedunder the saturation dome between the pressure limits of 140and 800 kPa. R-134a changes from saturated vapor to saturatedliquid during the heat rejection process. The net workinput for this cycle is(a) 28 kJ/kg (b) 34 kJ/kg (c) 49 kJ/kg(d) 144 kJ/kg (e) 275 kJ/kg11–111 A refrigerator removes heat from a refrigeratedspace at 5°C at a rate of 0.35 kJ/s and rejects it to an environmentat 20°C. The minimum required power input is(a) 30 W (b) 33 W (c) 56 W(d) 124 W(e) 350 W11–112 A refrigerator operates on the ideal vapor compressionrefrigeration cycle with R-134a as the working fluidbetween the pressure limits of 120 and 800 kPa. If the rate ofheat removal from the refrigerated space is 32 kJ/s, the massflow rate of the refrigerant is(a) 0.19 kg/s (b) 0.15 kg/s (c) 0.23 kg/s(d) 0.28 kg/s (e) 0.81 kg/s11–113 A heat pump operates on the ideal vapor compressionrefrigeration cycle with R-134a as the working fluidbetween the pressure limits of 0.32 and 1.2 MPa. If the massflow rate of the refrigerant is 0.193 kg/s, the rate of heat supplyby the heat pump to the heated space is(a) 3.3 kW (b) 23 kW (c) 26 kW(d) 31 kW(e) 45 kW11–114 An ideal vapor compression refrigeration cycle withR-134a as the working fluid operates between the pressure limitsof 120 kPa and 1000 kPa. The mass fraction of the refrigerantthat is in the liquid phase at the inlet of the evaporator is(a) 0.65 (b) 0.60 (c) 0.40(d) 0.55 (e) 0.3511–115 Consider a heat pump that operates on the idealvapor compression refrigeration cycle with R-134a as theworking fluid between the pressure limits of 0.32 and1.2 MPa. The coefficient of performance of this heat pump is(a) 0.17 (b) 1.2 (c) 3.1(d) 4.9 (e) 5.911–116 An ideal gas refrigeration cycle using air as theworking fluid operates between the pressure limits of 80 and280 kPa. Air is cooled to 35°C before entering the turbine.The lowest temperature of this cycle is(a) 58°C (b) 26°C (c) 5°C(d) 11°C (e) 24°C11–117 Consider an ideal gas refrigeration cycle usinghelium as the working fluid. Helium enters the compressor at100 kPa and 10°C and compressed to 250 kPa. Helium is