Thermodynamics

478 | Thermodynamics8–87 Ambient air at 100 kPa and 300 K is compressedisentropically in a steady-flow device to 1 MPa. Determine(a) the work input to the compressor, (b) the exergy of the airat the compressor exit, and (c) the exergy of compressed airafter it is cooled to 300 K at 1 MPa pressure.8–88 Cold water (c p 4.18 kJ/kg · °C) leading to a showerenters a well-insulated, thin-walled, double-pipe, counter-flowheat exchanger at 15°C at a rate of 0.25 kg/s and is heated to45°C by hot water (c p 4.19 kJ/kg · °C) that enters at 100°Cat a rate of 3 kg/s. Determine (a) the rate of heat transfer and(b) the rate of exergy destruction in the heat exchanger. TakeT 0 25°C.Hotwater3 kg/s100°C8–89 Outdoor air (c p 1.005 kJ/kg · °C) is to be preheatedby hot exhaust gases in a cross-flow heat exchanger before itenters the furnace. Air enters the heat exchanger at 95 kPa and20°C at a rate of 0.8 m 3 /s. The combustion gases (c p 1.10kJ/kg · °C) enter at 180°C at a rate of 1.1 kg/s and leave at95°C. Determine the rate of heat transfer to the air and the rateof exergy destruction in the heat exchanger.Air95 kPa20°C0.8 m 3 /s45°CFIGURE P8–88FIGURE P8–89Cold water0.25 kg/s 15°CExhaust gases1.1 kg/s95°C8–90 A well-insulated shell-and-tube heat exchanger isused to heat water (c p 4.18 kJ/kg · °C) in the tubes from 20to 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 a rate of10 kg/s. Disregarding any heat loss from the heat exchanger,determine (a) the exit temperature of oil and (b) the rate ofexergy destruction in the heat exchanger. Take T 0 25°C.8–91E Steam is to be condensed on the shell side of a heatexchanger at 120°F. Cooling water enters the tubes at 60°F ata rate of 115.3 lbm/s and leaves at 73°F. Assuming the heatexchanger to be well-insulated, determine (a) the rate of heattransfer in the heat exchanger and (b) the rate of exergydestruction in the heat exchanger. Take T 0 77°F.8–92 Steam enters a turbine at 12 MPa, 550°C, and 60 m/sand leaves at 20 kPa and 130 m/s with a moisture content of5 percent. The turbine is not adequately insulated and it estimatedthat heat is lost from the turbine at a rate of 150 kW.The power output of the turbine is 2.5 MW. Assuming thesurroundings to be at 25°C, determine (a) the reversiblepower output of the turbine, (b) the exergy destroyed withinthe turbine, and (c) the second-law efficiency of the turbine.(d) Also, estimate the possible increase in the power outputof the turbine if the turbine were perfectly insulated.Steam12 MPa550°C, 60 m/sTURBINE20 kPa130 m/sx = 0.95FIGURE P8–928–93 Air enters a compressor at ambient conditions of100 kPa and 20°C at a rate of 4.5 m 3 /s with a low velocity,and exits at 900 kPa, 60°C, and 80 m/s. The compressor iscooled by cooling water that experiences a temperature riseof 10°C. The isothermal efficiency of the compressor is 70percent. Determine (a) the actual and reversible power inputs,(b) the second-law efficiency, and (c) the mass flow rate ofthe cooling water.8–94 Liquid water at 15°C is heated in a chamber by mixingit with saturated steam. Liquid water enters the chamberat the steam pressure at a rate of 4.6 kg/s and the saturatedsteam enters at a rate of 0.23 kg/s. The mixture leaves themixing chamber as a liquid at 45°C. If the surroundings areat 15°C, determine (a) the temperature of saturated steamentering the chamber, (b) the exergy destruction during thismixing process, and (c) the second-law efficiency of the mixingchamber. Answers: (a) 114.3°C, (b) 114.7 kW, (c) 0.207Q