Thermodynamics

350 kW power is produced by the turbine, determine the efficiencyof the turbine. Take the density of LNG to be 423.8kg/m 3 . Answer: 72.9 percentCryogenicturbine3 barLNG, 40 bar–160°C, 55 kg/sFIGURE P7–213Fundamentals of Engineering (FE) Exam Problems7–214 Steam is condensed at a constant temperature of30°C as it flows through the condensor of a power plant byrejecting heat at a rate of 55 MW. The rate of entropy changeof steam as it flows through the condenser is(a) 1.83 MW/K (b) 0.18 MW/K (c) 0 MW/K(d) 0.56 MW/K (e) 1.22 MW/K7–215 Steam is compressed from 6 MPa and 300°C to 10MPa isentropically. The final temperature of the steam is(a) 290°C (b) 300°C (c) 311°C(d) 371°C (e) 422°C7–216 An apple with an average mass of 0.15 kg and averagespecific heat of 3.65 kJ/kg · °C is cooled from 20°C to5°C. The entropy change of the apple is(a) 0.0288 kJ/K (b) 0.192 kJ/K (c) 0.526 kJ/K(d) 0 kJ/K(e) 0.657 kJ/K7–217 A piston–cylinder device contains 5 kg of saturatedwater vapor at 3 MPa. Now heat is rejected from the cylinderat constant pressure until the water vapor completely condensesso that the cylinder contains saturated liquid at 3 MPaat the end of the process. The entropy change of the systemduring this process is(a) 0 kJ/K (b) 3.5 kJ/K (c) 12.5 kJ/K(d) 17.7 kJ/K (e) 19.5 kJ/K7–218 Helium gas is compressed from 1 atm and 25°C to apressure of 10 atm adiabatically. The lowest temperature ofhelium after compression is(a) 25°C (b) 63°C (c) 250°C(d) 384°C (e) 476°C7–219 Steam expands in an adiabatic turbine from 8 MPaand 500°C to 0.1 MPa at a rate of 3 kg/s. If steam leavesthe turbine as saturated vapor, the power output of the turbineis(a) 2174 kW (b) 698 kW (c) 2881 kW(d) 1674 kW (e) 3240 kWChapter 7 | 4217–220 Argon gas expands in an adiabatic turbine from 3MPa and 750°C to 0.2 MPa at a rate of 5 kg/s. The maximumpower output of the turbine is(a) 1.06 MW (b) 1.29 MW (c) 1.43 MW(d) 1.76 MW (e) 2.08 MW7–221 A unit mass of a substance undergoes an irreversibleprocess from state 1 to state 2 while gaining heat from thesurroundings at temperature T in the amount of q. Ifthe entropy of the substance is s 1 at state 1, and s 2 at state 2,the entropy change of the substance s during this process is(a) s s 2 s 1 (b) s s 2 s 1(c) s s 2 s 1(d) s s 2 s 1 q/T(e) s s 2 s 1 q/T7–222 A unit mass of an ideal gas at temperature T undergoesa reversible isothermal process from pressure P 1 to pressureP 2 while losing heat to the surroundings at temperatureT in the amount of q. If the gas constant of the gas is R, theentropy change of the gas s during this process is(a) s R ln(P 2 /P 1 )(c) s R ln(P 1 /P 2 )(e) s 0(b) s R ln(P 2 /P 1 ) q/T(d) s R ln(P 1 /P 2 ) q/T7–223 Air is compressed from room conditions to aspecified pressure in a reversible manner by two compressors:one isothermal and the other adiabatic. If the entropychange of air s isot during the reversible isothermal compression,and s adia during the reversible adiabatic compression,the correct statement regarding entropy change of air per unitmass is(a) s isot s adia 0(b) s isot s adia 0(c) s adia 0(d) s isot 0 (e) s isot 07–224 Helium gas is compressed from 15°C and 5.40m 3 /kg to 0.775 m 3 /kg in a reversible and adiabatic manner.The temperature of helium after compression is(a) 105°C (b) 55°C (c) 1734°C(d) 1051°C (e) 778°C7–225 Heat is lost through a plane wall steadily at a rate of600 W. If the inner and outer surface temperatures of the wallare 20°C and 5°C, respectively, the rate of entropy generationwithin the wall is(a) 0.11 W/K (b) 4.21 W/K (c) 2.10 W/K(d) 42.1 W/K (e) 90.0 W/K7–226 Air is compressed steadily and adiabatically from17°C and 90 kPa to 200°C and 400 kPa. Assuming constantspecific heats for air at room temperature, the isentropic efficiencyof the compressor is(a) 0.76 (b) 0.94 (c) 0.86(d) 0.84 (e) 1.007–227 Argon gas expands in an adiabatic turbine steadilyfrom 500°C and 800 kPa to 80 kPa at a rate of 2.5 kg/s. For