Thermodynamics

262 | Thermodynamicsby the incoming streams be equal to the energy transportedout of it by the outgoing stream?5–74C Consider a steady-flow heat exchanger involvingtwo different fluid streams. Under what conditions will theamount of heat lost by one fluid be equal to the amount ofheat gained by the other?5–75 A hot-water stream at 80°C enters a mixing chamberwith a mass flow rate of 0.5 kg/s where it is mixed with astream of cold water at 20°C. If it is desired that the mixtureleave the chamber at 42°C, determine the mass flow rate of thecold-water stream. Assume all the streams are at a pressure of250 kPa. Answer: 0.865 kg/sT 1 = 80 °Cṁ 1.= 0.5 kg/sT 2 = 20°Cm 2H 2O(P = 250 kPa)FIGURE P5–75T 3 = 42°C5–76 Liquid water at 300 kPa and 20°C is heated in achamber by mixing it with superheated steam at 300 kPa and300°C. Cold water enters the chamber at a rate of 1.8 kg/s. Ifthe mixture leaves the mixing chamber at 60°C, determinethe mass flow rate of the superheated steam required.Answer: 0.107 kg/s5–77 In steam power plants, open feedwater heaters are frequentlyutilized to heat the feedwater by mixing it with steambled off the turbine at some intermediate stage. Consider anopen feedwater heater that operates at a pressure of 1000kPa. Feedwater at 50°C and 1000 kPa is to be heated withsuperheated steam at 200°C and 1000 kPa. In an ideal feedwaterheater, the mixture leaves the heater as saturated liquidat the feedwater pressure. Determine the ratio of the massflow rates of the feedwater and the superheated vapor for thiscase. Answer: 3.73T 1 = 50°Cṁ 1.T 2 = 200°Cm 2H 2 O(P = 1000 kPa)FIGURE P5–77Sat.liquid5–78E Water at 50°F and 50 psia is heated in a chamber bymixing it with saturated water vapor at 50 psia. If bothstreams enter the mixing chamber at the same mass flow rate,determine the temperature and the quality of the exitingstream. Answers: 281°F, 0.3745–79 A stream of refrigerant-134a at 1 MPa and 12°C ismixed with another stream at 1 MPa and 60°C. If the massflow rate of the cold stream is twice that of the hot one,determine the temperature and the quality of the exit stream.5–80 Reconsider Prob. 5–79. Using EES (or other)software, investigate the effect of the mass flowrate of the cold stream of R-134a on the temperature and thequality of the exit stream. Let the ratio of the mass flow rateof the cold stream to that of the hot stream vary from 1 to 4.Plot the mixture temperature and quality against the cold-tohotmass flow rate ratio, and discuss the results.5–81 Refrigerant-134a at 1 MPa and 90°C is to be cooledto 1 MPa and 30°C in a condenser by air. The air enters at100 kPa and 27°C with a volume flow rate of 600 m 3 /minand leaves at 95 kPa and 60°C. Determine the mass flow rateof the refrigerant. Answer: 100 kg/minR-134aP 1 = 1 MPaT 1 = 90°CAIR⋅V 3 = 600 m 3 /minP 3 = 100 kPaT 3 = 27°CP 4 = 95 kPaT 4 = 60°CFIGURE P5–81P 2 = 1 MPaT 2 = 30°C5–82E Air enters the evaporator section of a window airconditioner at 14.7 psia and 90°F with a volume flow rate of200 ft 3 /min. Refrigerant-134a at 20 psia with a quality of 30percent enters the evaporator at a rate of 4 lbm/min andleaves as saturated vapor at the same pressure. Determine(a) the exit temperature of the air and (b) the rate of heattransfer from the air.5–83 Refrigerant-134a at 700 kPa, 70°C, and 8 kg/min iscooled by water in a condenser until it exists as a saturatedliquid at the same pressure. The cooling water enters the condenserat 300 kPa and 15°C and leaves at 25°C at the samepressure. Determine the mass flow rate of the cooling waterrequired to cool the refrigerant. Answer: 42.0 kg/min5–84E In a steam heating system, air is heated bybeing passed over some tubes through whichsteam flows steadily. Steam enters the heat exchanger at 30psia and 400°F at a rate of 15 lbm/min and leaves at 25 psia