Thermodynamics

404 | Thermodynamicsthis problem. Let the surrounding temperature vary from 0 to100°C. Discuss your results.7–37E A piston–cylinder device contains 2 lbm of refrigerant-134aat 120 psia and 100°F. The refrigerant is now cooledat constant pressure until it exists as a liquid at 50°F. Determinethe entropy change of the refrigerant during this process.7–38 An insulated piston–cylinder device contains 5 L ofsaturated liquid water at a constant pressure of 150 kPa. Anelectric resistance heater inside the cylinder is now turned on,and 2200 kJ of energy is transferred to the steam. Determinethe entropy change of the water during this process.Answer: 5.72 kJ/K7–39 An insulated piston–cylinder device contains 0.05 m 3of saturated refrigerant-134a vapor at 0.8-MPa pressure. Therefrigerant is now allowed to expand in a reversible manneruntil the pressure drops to 0.4 MPa. Determine (a) the finaltemperature in the cylinder and (b) the work done by therefrigerant.R-134a0.05 m 30.8 MPaFIGURE P7–397–40 Reconsider Prob. 7–39. Using EES (or other)software, evaluate and plot the work done by therefrigerant as a function of final pressure as it varies from 0.8to 0.4 MPa. Compare the work done for this process to onefor which the temperature is constant over the same pressurerange. Discuss your results.7–41 Refrigerant-134a enters an adiabatic compressor assaturated vapor at 160 kPa at a rate of 2 m 3 /min and is compressedto a pressure of 900 kPa. Determine the minimumpower that must be supplied to the compressor.7–42E Steam enters an adiabatic turbine at 800 psiaand 900°F and leaves at a pressure of 40 psia. Determinethe maximum amount of work that can be delivered by thisturbine.7–43E Reconsider Prob. 7–42E. Using EES (orother) software, evaluate and plot the workdone by the steam as a function of final pressure as it variesfrom 800 to 40 psia. Also investigate the effect of varying theturbine inlet temperature from the saturation temperature at800 psia to 900°F on the turbine work.7–44 A heavily insulated piston–cylinder device contains0.05 m 3 of steam at 300 kPa and 150°C. Steam is now compressedin a reversible manner to a pressure of 1 MPa. Determinethe work done on the steam during this process.7–45 Reconsider Prob. 7–44. Using EES (or other)software, evaluate and plot the work done on thesteam as a function of final pressure as the pressure variesfrom 300 kPa to 1 MPa.7–46 A piston–cylinder device contains 1.2 kg of saturatedwater vapor at 200°C. Heat is now transferred to steam, andsteam expands reversibly and isothermally to a final pressureof 800 kPa. Determine the heat transferred and the work doneduring this process.7–47 Reconsider Prob. 7–46. Using EES (or other)software, evaluate and plot the heat transferredto the steam and the work done as a function of final pressureas the pressure varies from the initial value to the final valueof 800 kPa.7–48 A piston–cylinder device contains 5 kg of steam at100°C with a quality of 50 percent. This steam undergoestwo processes as follows:1-2 Heat is transferred to the steam in a reversible mannerwhile the temperature is held constant until the steam existsas a saturated vapor.2-3 The steam expands in an adiabatic, reversible processuntil the pressure is 15 kPa.(a) Sketch these processes with respect to the saturation lineson a single T-s diagram.(b) Determine the heat added to the steam in process 1-2, in kJ.(c) Determine the work done by the steam in process 2-3, in kJ.7–49 A rigid tank contains 5 kg of saturated vapor steam at100°C. The steam is cooled to the ambient temperature of 25°C.(a) Sketch the process with respect to the saturation lines ona T-v diagram.(b) Determine the entropy change of the steam, in kJ/K.(c) For the steam and its surroundings, determine the totalentropy change or S gen associated with this process, in kJ/K.7–50 Steam at 6000 kPa and 500°C enters a steady-flowturbine. The steam expands in the turbine while doing workuntil the pressure is 1000 kPa. When the pressure is 1000kPa, 10 percent of the steam is removed from the turbine forother uses. The remaining 90 percent of the steam continuesto expand through the turbine while doing work and leavesthe turbine at 10 kPa. The entire expansion process by thesteam through the turbine is reversible and adiabatic.(a) Sketch the process on a T-s diagram with respect to thesaturation lines. Be sure to label the data states and the linesof constant pressure.(b) If the turbine has an isentropic efficiency of 85 percent,what is the work done by the steam as it flows through the turbineper unit mass of steam flowing into the turbine, in kJ/kg?