Thermodynamics

process. Assume the surroundings to be at 25°C. Answer:36.6 kJ/kg8–55 Air is compressed steadily by an 8-kW compressorfrom 100 kPa and 17°C to 600 kPa and167°C at a rate of 2.1 kg/min. Neglecting the changes inkinetic and potential energies, determine (a) the increase inthe exergy of the air and (b) the rate of exergy destroyed duringthis process. Assume the surroundings to be at 17°C.100 kPa17°CR-134a1.4 kg140 kPa20°CFIGURE P8–53AIR600 kPa167°CFIGURE P8–558 kW8–56 Reconsider Prob. 8–55. Using EES (or other)software, solve the problem and in additiondetermine the actual heat transfer, if any, and its direction, theminimum power input (the reversible power), and the compressorsecond-law efficiency. Then interpret the results whenthe outlet temperature is set to, say, 300°C. Explain the valuesof heat transfer, exergy destroyed, and efficiency whenthe outlet temperature is set to 209.31°C and mass flow rateto 2.466 kg/min.8–57 Refrigerant-134a at 1 MPa and 100°C is throttled to apressure of 0.8 MPa. Determine the reversible work andexergy destroyed during this throttling process. Assume thesurroundings to be at 30°C.8–58 Reconsider Prob. 8–57. Using EES (or other)software, investigate the effect of exit pressureon the reversible work and exergy destruction. Vary the throttleexit pressure from 1 to 0.1 MPa and plot the reversiblework and exergy destroyed as functions of the exit pressure.Discuss the results.QChapter 8 | 4758–59 Air enters a nozzle steadily at 300 kPa and 87°C witha velocity of 50 m/s and exits at 95 kPa and 300 m/s. Theheat loss from the nozzle to the surrounding medium at 17°Cis estimated to be 4 kJ/kg. Determine (a) the exit temperatureand (b) the exergy destroyed during this process. Answers:(a) 39.5°C, (b) 58.4 kJ/kg8–60 Reconsider Prob. 8–59. Using EES (or other) software,study the effect of varying the nozzle exitvelocity from 100 to 300 m/s on both the exit temperature andexergy destroyed, and plot the results.8–61 Steam enters a diffuser at 10 kPa and 50°C with avelocity of 300 m/s and exits as saturated vapor at 50°C and70 m/s. The exit area of the diffuser is 3 m 2 . Determine (a) themass flow rate of the steam and (b) the wasted work potentialduring this process. Assume the surroundings to be at 25°C.8–62E Air is compressed steadily by a compressor from14.7 psia and 60°F to 100 psia and 480°F at a rate of22 lbm/min. Assuming the surroundings to be at 60°F, determinethe minimum power input to the compressor. Assumeair to be an ideal gas with variable specific heats, and neglectthe changes in kinetic and potential energies.8–63 Steam enters an adiabatic turbine at 6 MPa, 600°C,and 80 m/s and leaves at 50 kPa, 100°C, and 140 m/s. If thepower output of the turbine is 5 MW, determine (a) thereversible power output and (b) the second-law efficiency ofthe turbine. Assume the surroundings to be at 25°C.Answers: (a) 5.84 MW, (b) 85.6 percent80 m/s6 MPa600°CSTEAM50 kPa100°C140 m/sFIGURE P8–635 MW8–64 Steam is throttled from 9 MPa and 500°C to a pressureof 7 MPa. Determine the decrease in exergy of the steamduring this process. Assume the surroundings to be at 25°C.Answer: 32.3 kJ/kg8–65 Combustion gases enter a gas turbine at 900°C, 800kPa, and 100 m/s and leave at 650°C, 400 kPa, and 220 m/s.Taking c p 1.15 kJ/kg · °C and k 1.3 for the combustiongases, determine (a) the exergy of the combustion gases atthe turbine inlet and (b) the work output of the turbine underreversible conditions. Assume the surroundings to be at 25°Cand 100 kPa. Can this turbine be adiabatic?