Thermodynamics

412 | Thermodynamics7–136E In a production facility, 1.2-in.-thick, 2-ft 2-ftsquare brass plates (r 532.5 lbm/ft 3 and c p 0.091Btu/lbm · °F) that are initially at a uniform temperature of75°F are heated by passing them through an oven at 1300°Fat a rate of 450 per minute. If the plates remain in the ovenuntil their average temperature rises to 1000°F, determine(a) the rate of heat transfer to the plates in the furnace and(b) the rate of entropy generation associated with this heattransfer process.7–137 Long cylindrical steel rods (r 7833 kg/m 3 and c p 0.465 kJ/kg · °C) of 10-cm diameter are heat treated bydrawing them at a velocity of 3 m/min through a 7-m-longoven maintained at 900°C. If the rods enter the oven at 30°Cand leave at 700°C, determine (a) the rate of heat transfer tothe rods in the oven and (b) the rate of entropy generationassociated with this heat transfer process.Stainlesssteel, 30°COven7 m900°CFIGURE P7–1373 m/min7–138 The inner and outer surfaces of a 5-m 7-m brickwall of thickness 20 cm are maintained at temperatures of20°C and 5°C, respectively. If the rate of heat transferthrough the wall is 1515 W, determine the rate of entropygeneration within the wall.7–139 For heat transfer purposes, a standing man can bemodeled as a 30-cm-diameter, 170-cm-long vertical cylinderwith both the top and bottom surfaces insulated and with theside surface at an average temperature of 34°C. If the rate ofheat loss from this man to the environment at 20°C is 336 W,determine the rate of entropy transfer from the body of thisperson accompanying heat transfer, in W/K.7–140 A 1000-W iron is left on the ironing board with itsbase exposed to the air at 20°C. If the surface temperature is400°C, determine the rate of entropy generation during thisprocess in steady operation. How much of this entropy generationoccurs within the iron?7–141E A frictionless piston–cylinder device contains saturatedliquid water at 25-psia pressure. Now 400 Btu of heat istransferred to water from a source at 900°F, and part of theliquid vaporizes at constant pressure. Determine the totalentropy generated during this process, in Btu/R.7–142E Steam enters a diffuser at 20 psia and 240°F with avelocity of 900 ft/s and exits as saturated vapor at 240°F and100 ft/s. The exit area of the diffuser is 1 ft 2 . Determine(a) the mass flow rate of the steam and (b) the rate of entropygeneration during this process. Assume an ambient temperatureof 77°F.7–143 Steam expands in a turbine steadily at a rate of25,000 kg/h, entering at 6 MPa and 450°C and leaving at 20kPa as saturated vapor. If the power generated by the turbineis 4 MW, determine the rate of entropy generation for thisprocess. Assume the surrounding medium is at 25°C.Answer: 11.0 kW/K6 MPa450°CSTEAMTURBINE20 kPasat. vaporFIGURE P7–1434 MW7–144 A hot-water stream at 70°C enters an adiabatic mixingchamber with a mass flow rate of 3.6 kg/s, where it ismixed with a stream of cold water at 20°C. If the mixtureleaves the chamber at 42°C, determine (a) the mass flow rateof the cold water and (b) the rate of entropy generation duringthis adiabatic mixing process. Assume all the streams areat a pressure of 200 kPa.7–145 Liquid water at 200 kPa and 20°C is heated in achamber by mixing it with superheated steam at 200 kPa and150°C. Liquid water enters the mixing chamber at a rate of2.5 kg/s, and the chamber is estimated to lose heat to the surroundingair at 25°C at a rate of 1200 kJ/min. If the mixtureleaves the mixing chamber at 200 kPa and 60°C, determine(a) the mass flow rate of the superheated steam and (b) therate of entropy generation during this mixing process.Answers: (a) 0.166 kg/s, (b) 0.333 kW/K20°C2.5 kg/s150°CMIXINGCHAMBER200 kPaFIGURE P7–1451200 kJ/min60°C7–146 A 0.3-m 3 rigid tank is filled with saturated liquidwater at 150°C. A valve at the bottom of the tank is now