Thermodynamics

476 | Thermodynamics8–66E Refrigerant-134a enters an adiabatic compressor assaturated vapor at 30 psia at a rate of 20 ft 3 /min and exits at70 psia pressure. If the isentropic efficiency of the compressoris 80 percent, determine (a) the actual power input and (b) thesecond-law efficiency of the compressor. Assume the surroundingsto be at 75°F. Answers: (a) 2.85 hp, (b) 79.8 percent8–67 Refrigerant-134a at 140 kPa and 10°C is compressedby an adiabatic 0.5-kW compressor to an exit state of 700 kPaand 60°C. Neglecting the changes in kinetic and potentialenergies and assuming the surroundings to be at 27°C, determine(a) the isentropic efficiency and (b) the second-lawefficiency of the compressor.air at 60°F at a rate of 1500 Btu/min. The power input to thecompressor is 400 hp. Determine (a) the mass flow rate of airand (b) the portion of the power input that is used just toovercome the irreversibilities.8–73 Hot combustion gases enter the nozzle of a turbojetengine at 260 kPa, 747°C, and 80 m/s and exit at 70 kPa and500°C. Assuming the nozzle to be adiabatic and the surroundingsto be at 20°C, determine (a) the exit velocity and(b) the decrease in the exergy of the gases. Take k 1.3 andc p 1.15 kJ/kg · °C for the combustion gases.700 kPa60°C260 kPa747°C80 m/sCombustiongases70 kPa500°CR-134a140 kPa–10°CFIGURE P8–670.5 kW8–68 Air is compressed by a compressor from 95 kPa and27°C to 600 kPa and 277°C at a rate of 0.06 kg/s. Neglectingthe changes in kinetic and potential energies and assumingthe surroundings to be at 25°C, determine the reversiblepower input for this process. Answer: 13.7 kW8–69 Reconsider Prob. 8–68. Using EES (or other)software, investigate the effect of compressorexit pressure on reversible power. Vary the compressor exitpressure from 200 to 600 kPa while keeping the exit temperatureat 277°C. Plot the reversible power input for thisprocess as a function of the compressor exit pressure.8–70 Argon gas enters an adiabatic compressor at 120 kPaand 30°C with a velocity of 20 m/s and exits at 1.2 MPa,530°C, and 80 m/s. The inlet area of the compressor is130 cm 2 . Assuming the surroundings to be at 25°C, determinethe reversible power input and exergy destroyed.Answers: 126 kW, 4.12 kW8–71 Steam expands in a turbine steadily at a rate of15,000 kg/h, entering at 8 MPa and 450°C and leaving at50 kPa as saturated vapor. Assuming the surroundings to beat 100 kPa and 25°C, determine (a) the power potential of thesteam at the inlet conditions and (b) the power output of theturbine if there were no irreversibilities present. Answers:(a) 5515 kW, (b) 3902 kW8–72E Air enters a compressor at ambient conditions of15 psia and 60°F with a low velocity and exits at 150 psia,620°F, and 350 ft/s. The compressor is cooled by the ambientFIGURE P8–738–74 Steam is usually accelerated in the nozzle of a turbinebefore it strikes the turbine blades. Steam enters an adiabaticnozzle at 7 MPa and 500°C with a velocity of 70 m/s andexits at 5 MPa and 450°C. Assuming the surroundings to beat 25°C, determine (a) the exit velocity of the steam, (b) theisentropic efficiency, and (c) the exergy destroyed within thenozzle.8–75 Carbon dioxide enters a compressor at 100 kPa and300 K at a rate of 0.2 kg/s and exits at 600 kPa and 450 K.Determine the power input to the compressor if the processinvolved no irreversibilities. Assume the surroundings to be at25°C. Answer: 25.5 kW8–76E A hot-water stream at 160°F enters an adiabaticmixing chamber with a mass flow rate of 4 lbm/s, where it ismixed with a stream of cold water at 70°F. If the mixtureleaves the chamber at 110°F, determine (a) the mass flow rateof the cold water and (b) the exergy destroyed during thisadiabatic mixing process. Assume all the streams are at apressure of 50 psia and the surroundings are at 75°F.Answers: (a) 5.0 lbm/s, (b) 14.6 Btu/s8–77 Liquid water at 200 kPa and 20°C is heated in achamber by mixing it with superheated steam at 200 kPa and20°C2.5 kg/s Mixingchamber300°C 200 kPa600 kJ/minFIGURE P8–7760°C