Thermodynamics

entropy generation during this process.69.5°C, (b) 0.258 kJ/kg KAnswers: (a) 442 kPa,7–166 Refrigerant-134a enters a compressor as a saturatedvapor at 200 kPa at a rate of 0.03 m 3 /s and leaves at 700 kPa.The power input to the compressor is 10 kW. If the surroundingsat 20°C experience an entropy increase of 0.008 kW/K,determine (a) the rate of heat loss from the compressor,(b) the exit temperature of the refrigerant, and (c) the rate ofentropy generation.7–167 Air at 500 kPa and 400 K enters an adiabatic nozzleat a velocity of 30 m/s and leaves at 300 kPa and 350 K.Using variable specific heats, determine (a) the isentropic efficiency,(b) the exit velocity, and (c) the entropy generation.Air500 kPa400 K30 m/sFIGURE P7–167300 kPa350 K7–168 Show that the difference between the reversiblesteady-flow work and reversible moving boundary work isequal to the flow energy.7–169 An insulated tank containing 0.4 m 3 of saturatedwater vapor at 500 kPa is connected to an initially evacuated,insulated piston–cylinder device. The mass of the piston issuch that a pressure of 150 kPa is required to raise it. Nowthe valve is opened slightly, and part of the steam flows tothe cylinder, raising the piston. This process continues untilthe pressure in the tank drops to 150 kPa. Assuming thesteam that remains in the tank to have undergone a reversibleadiabatic process, determine the final temperature (a) in therigid tank and (b) in the cylinder.4 m × 5 m × 7 mWater80°CROOM22°C100 kPaChapter 7 | 415HeatFIGURE P7–1707–171E A piston–cylinder device initially contains 15 ft 3 ofhelium gas at 25 psia and 70°F. Helium is now compressed ina polytropic process (PV n constant) to 70 psia and 300°F.Determine (a) the entropy change of helium, (b) the entropychange of the surroundings, and (c) whether this process isreversible, irreversible, or impossible. Assume the surroundingsare at 70°F. Answers: (a) 0.016 Btu/R, (b) 0.019 Btu/R,(c) irreversible7–172 Air is compressed steadily by a compressor from 100kPa and 17°C to 700 kPa at a rate of 5 kg/min. Determine theminimum power input required if the process is (a) adiabaticand (b) isothermal. Assume air to be an ideal gas with variablespecific heats, and neglect the changes in kinetic and potentialenergies. Answers: (a) 18.0 kW, (b) 13.5 kW7–173 Air enters a two-stage compressor at 100 kPa and27°C and is compressed to 900 kPa. The pressure ratio acrosseach stage is the same, and the air is cooled to the initial temperaturebetween the two stages. Assuming the compressionprocess to be isentropic, determine the power input to thecompressor for a mass flow rate of 0.02 kg/s. What wouldyour answer be if only one stage of compression were used?Answers: 4.44 kW, 5.26 kWHeat0.4 m 3sat. vapor500 kPa150 kPa900 kPa27°CP x P xWAIRCOMPRESSOR(1st stage)(2nd stage)FIGURE P7–1697–170 One ton of liquid water at 80°C is brought into awell-insulated and well-sealed 4-m 5-m 7-m room initiallyat 22°C and 100 kPa. Assuming constant specific heatsfor both air and water at room temperature, determine (a) thefinal equilibrium temperature in the room and (b) the totalentropy change during this process, in kJ/K.100 kPa27°CFIGURE P7–173