Thermodynamics

482 | Thermodynamicssteam. Neglecting any heat losses from the outer surfaces ofthe heater, determine (a) the ratio of the mass flow rates ofthe extracted steam and the feedwater heater and (b) thereversible work for this process per unit mass of the feedwater.Assume the surroundings to be at 25°C.Answers: (a) 0.247, (b) 63.5 kJ/kg8–122 Reconsider Prob. 8–121. Using EES (or other)software, investigate the effect of the state ofthe steam at the inlet of the feedwater heater on the ratio ofmass flow rates and the reversible power. Assume the entropyof the extracted steam is constant at the value for 1 MPa,200°C and decrease the extracted steam pressure from 1 MPato 100 kPa. Plot both the ratio of the mass flow rates of theextracted steam and the feedwater heater and the reversiblework for this process per unit mass of feedwater as functionsof the extraction pressure.8–123 In order to cool 1 ton of water at 20°C in an insulatedtank, a person pours 80 kg of ice at 5°C into the water.Determine (a) the final equilibrium temperature in the tankand (b) the exergy destroyed during this process. The meltingtemperature and the heat of fusion of ice at atmospheric pressureare 0°C and 333.7 kJ/kg, respectively. Take T 0 20°C.8–124 Consider a 12-L evacuated rigid bottle that is surroundedby the atmosphere at 100 kPa and 17°C. A valve atthe neck of the bottle is now opened and the atmospheric airis allowed to flow into the bottle. The air trapped in the bottleeventually reaches thermal equilibrium with the atmosphereas a result of heat transfer through the wall of the bottle. Thevalve remains open during the process so that the trapped airalso reaches mechanical equilibrium with the atmosphere.Determine the net heat transfer through the wall of the bottleand the exergy destroyed during this filling process.AIR30 kg900 KHEAIR30 kg300 KQ LQ HFIGURE P8–1258–126 Two constant-pressure devices, each filled with 30 kgof air, have temperatures of 900 K and 300 K. A heat engineplaced between the two devices extracts heat from the hightemperaturedevice, produces work, and rejects heat to the lowtemperaturedevice. Determine the maximum work that can beproduced by the heat engine and the final temperatures of thedevices. Assume constant specific heats at room temperature.8–127 A 4-L pressure cooker has an operating pressure of175 kPa. Initially, one-half of the volume is filled with liquidwater and the other half by water vapor. The cooker is nowplaced on top of a 750-W electrical heating unit that is kepton for 20 min. Assuming the surroundings to be at 25°C and100 kPa, determine (a) the amount of water that remained inthe cooker and (b) the exergy destruction associated with theW100 kPa17°C12 LEvacuatedFIGURE P8–1244 L175 kPa8–125 Two constant-volume tanks, each filled with 30 kgof air, have temperatures of 900 K and 300 K. A heat engineplaced between the two tanks extracts heat from the hightemperaturetank, produces work, and rejects heat to the lowtemperaturetank. Determine the maximum work that can beproduced by the heat engine and the final temperatures of thetanks. Assume constant specific heats at room temperature.750 WFIGURE P8–127