Thermodynamics

214 | Thermodynamicsthe entire air remains at 30°C at all times. Determine the heattransfer for this process.AIRT = const.N 21 m 3500 kPa80°CHe1 m 3500 kPa25°CQFIGURE P4–138FIGURE P4–1364–137 A well-insulated 4-m 4-m 5-m room initially at10°C is heated by the radiator of a steam heating system. Theradiator has a volume of 15 L and is filled with superheatedvapor at 200 kPa and 200°C. At this moment both the inletand the exit valves to the radiator are closed. A 120-W fan isused to distribute the air in the room. The pressure of thesteam is observed to drop to 100 kPa after 30 min as a resultof heat transfer to the room. Assuming constant specific heatsfor air at room temperature, determine the average temperatureof air in 30 min. Assume the air pressure in the roomremains constant at 100 kPa.4–139 Repeat Prob. 4–138 by assuming the piston is madeof 5 kg of copper initially at the average temperature of thetwo gases on both sides. Answer: 56°C4–140 Reconsider Prob. 4–139. Using EES (or other)software, investigate the effect of the mass ofthe copper piston on the final equilibrium temperature. Let themass of piston vary from 1 to 10 kg. Plot the final temperatureagainst the mass of piston, and discuss the results.4–141 An insulated rigid tank initially contains 1.4-kg saturatedliquid water and water vapor at 200°C. At this state, 25percent of the volume is occupied by liquid water and the restby vapor. Now an electric resistor placed in the tank is turnedon, and the tank is observed to contain saturated water vaporafter 20 min. Determine (a) the volume of the tank, (b) thefinal temperature, and (c) the electric power rating of theresistor. Answers: (a ) 0.00648 m 3 , (b) 371°C, (c) 1.58 kW10°C4 m × 4 m × 5 mFanSteamradiatorWater1.4 kg, 200°CW eFIGURE P4–141FIGURE P4–1374–138 Consider a well-insulated horizontal rigid cylinderthat is divided into two compartments by a piston that is free tomove but does not allow either gas to leak into the other side.Initially, one side of the piston contains 1 m 3 of N 2 gas at 500kPa and 80°C while the other side contains 1 m 3 of He gas at500 kPa and 25°C. Now thermal equilibrium is established inthe cylinder as a result of heat transfer through the piston.Using constant specific heats at room temperature, determinethe final equilibrium temperature in the cylinder. What wouldyour answer be if the piston were not free to move?4–142 A vertical 12-cm diameter piston–cylinder devicecontains an ideal gas at the ambient conditons of 1 bar and24°C. Initially, the inner face of the piston is 20 cm from thebase of the cylinder. Now an external shaft connected to thepiston exerts a force corresponding to a boundary work inputof 0.1 kJ. The temperature of the gas remains constant duringthe process. Determine (a) the amount of heat transfer,(b) the final pressure in the cylinder, and (c) the distance thatthe piston is displaced.4–143 A piston–cylinder device initially contains 0.15-kgsteam at 3.5 MPa, superheated by 5°C. Now the steam losesheat to the surroundings and the piston moves down, hitting a setof stops at which point the cylinder contains saturated liquidwater. The cooling continues until the cylinder contains water at200°C. Determine (a) the final pressure and the quality (if mix-