Thermodynamics

enter at 180°C at a rate of 2.2 kg/s and leave at 95°C. Determine(a) the rate of heat transfer to the air, (b) the outlet temperatureof the air, and (c) the rate of entropy generation.7–129 A well-insulated, shell-and-tube heat exchanger isused to heat water (c p 4.18 kJ/kg · °C) in the tubes from20 to 70°C at a rate of 4.5 kg/s. Heat is supplied by hot oil(c p 2.30 kJ/kg · °C) that enters the shell side at 170°C at arate of 10 kg/s. Disregarding any heat loss from the heatexchanger, determine (a) the exit temperature of the oil and(b) the rate of entropy generation in the heat exchanger.70°CWater20°C4.5 kg/sOil170°C10 kg/sFIGURE P7–1297–130E Steam is to be condensed on the shell side of aheat exchanger at 120°F. Cooling water enters the tubes at60°F at a rate of 92 lbm/s and leaves at 73°F. Assuming theheat exchanger to be well-insulated, determine (a) the rate ofheat transfer in the heat exchanger and (b) the rate of entropygeneration in the heat exchanger.7–131 Chickens with an average mass of 2.2 kg and averagespecific heat of 3.54 kJ/kg · °C are to be cooled by chilledwater that enters a continuous-flow-type immersion chiller at0.5°C and leaves at 2.5°C. Chickens are dropped into thechiller at a uniform temperature of 15°C at a rate of 250chickens per hour and are cooled to an average temperature of3°C before they are taken out. The chiller gains heat from thesurroundings at 25°C at a rate of 150 kJ/h. Determine (a) therate of heat removal from the chickens, in kW, and (b) the rateof entropy generation during this chilling process.7–132 In a dairy plant, milk at 4°C is pasteurized continuouslyat 72°C at a rate of 12 L/s for 24 hours a day and 365days a year. The milk is heated to the pasteurizing temperatureby hot water heated in a natural-gas-fired boiler that has72°C72°CHot milkChapter 7 | 411an efficiency of 82 percent. The pasteurized milk is thencooled by cold water at 18°C before it is finally refrigeratedback to 4°C. To save energy and money, the plant installs aregenerator that has an effectiveness of 82 percent. If the costof natural gas is $1.04/therm (1 therm 105,500 kJ), determinehow much energy and money the regenerator will savethis company per year and the annual reduction in entropygeneration.7–133 Stainless-steel ball bearings (r 8085 kg/m 3 and c p 0.480 kJ/kg · °C) having a diameter of 1.2 cm are to bequenched in water at a rate of 1400 per minute. The ballsleave the oven at a uniform temperature of 900°C and areexposed to air at 30°C for a while before they are droppedinto the water. If the temperature of the balls drops to 850°Cprior to quenching, determine (a) the rate of heat transferfrom the balls to the air and (b) the rate of entropy generationdue to heat loss from the balls to the air.7–134 Carbon-steel balls (r 7833 kg/m 3 and c p 0.465kJ/kg · °C) 8 mm in diameter are annealed by heating themfirst to 900°C in a furnace and then allowing them to coolslowly to 100°C in ambient air at 35°C. If 2500 balls are tobe annealed per hour, determine (a) the rate of heat transferfrom the balls to the air and (b) the rate of entropy generationdue to heat loss from the balls to the air. Answers: (a) 542 W,(b) 0.986 W/KFurnaceAir, 35°C900°CSteel ball100°CFIGURE P7–1347–135 An ordinary egg can be approximated as a 5.5-cmdiametersphere. The egg is initially at a uniform temperatureof 8°C and is dropped into boiling water at 97°C. Taking theproperties of the egg to be r 1020 kg/m 3 and c p 3.32kJ/kg · °C, determine (a) how much heat is transferred to theegg by the time the average temperature of the egg rises to70°C and (b) the amount of entropy generation associatedwith this heat transfer process.BoilingwaterEGG97°CHeat(Pasteurizingsection)Regenerator4°CColdmilkT i = 8°CFIGURE P7–132FIGURE P7–135