154 Thermodynamics REFERENCES AND SUGGESTED READINGS PROBLEMS* Pure Substances, Phase-Change Processes, Property Diagrams.

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1 cen84959_ch03.qxd 4/1/05 12:31 PM Page Thermodynamics REFERENCES AND SUGGESTED READINGS 1. ASHRAE Handbook of Fundamentals. SI version. Atlanta, GA: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc., ASHRAE Handbook of Refrigeration. SI version. Atlanta, GA: American Society of Heating, Refrigerating, and Air- Conditioning Engineers, Inc., A. Bejan. Advanced Engineering Thermodynamics. 2nd ed. New York: Wiley, M. Kostic. Analysis of Enthalpy Approximation for Compressed Liquid Water. IMECE 2004, ASME Proceedings, ASME, New York, PROBLEMS* Pure Substances, Phase-Change Processes, Property Diagrams 3 1C Is iced water a pure substance? Why? 3 2C What is the difference between saturated liquid and compressed liquid? 3 3C What is the difference between saturated vapor and superheated vapor? 3 4C Is there any difference between the intensive properties of saturated vapor at a given temperature and the vapor of a saturated mixture at the same temperature? 3 5C Is there any difference between the intensive properties of saturated liquid at a given temperature and the liquid of a saturated mixture at the same temperature? 3 6C Is it true that water boils at higher temperatures at higher pressures? Explain. 3 7C If the pressure of a substance is increased during a boiling process, will the temperature also increase or will it remain constant? Why? 3 8C Why are the temperature and pressure dependent properties in the saturated mixture region? 3 9C What is the difference between the critical point and the triple point? 3 10C Is it possible to have water vapor at 10 C? 3 11C A househusband is cooking beef stew for his family in a pan that is (a) uncovered, (b) covered with a light lid, and (c) covered with a heavy lid. For which case will the cooking time be the shortest? Why? *Problems designated by a C are concept questions, and students are encouraged to answer them all. Problems designated by an E are in English units, and the SI users can ignore them. Problems with a CD-EES icon are solved using EES, and complete solutions together with parametric studies are included on the enclosed DVD. Problems with a computer-ees icon are comprehensive in nature, and are intended to be solved with a computer, preferably using the EES software that accompanies this text. 3 12C How does the boiling process at supercritical pressures differ from the boiling process at subcritical pressures? Property Tables 3 13C In what kind of pot will a given volume of water boil at a higher temperature: a tall and narrow one or a short and wide one? Explain. 3 14C A perfectly fitting pot and its lid often stick after cooking, and it becomes very difficult to open the lid when the pot cools down. Explain why this happens and what you would do to open the lid. 3 15C It is well known that warm air in a cooler environment rises. Now consider a warm mixture of air and gasoline on top of an open gasoline can. Do you think this gas mixture will rise in a cooler environment? 3 16C In 1775, Dr. William Cullen made ice in Scotland by evacuating the air in a water tank. Explain how that device works, and discuss how the process can be made more efficient. 3 17C Does the amount of heat absorbed as 1 kg of saturated liquid water boils at 100 C have to be equal to the amount of heat released as 1 kg of saturated water vapor condenses at 100 C? 3 18C Does the reference point selected for the properties of a substance have any effect on thermodynamic analysis? Why? 3 19C What is the physical significance of h fg? Can it be obtained from a knowledge of h f and h g? How? 3 20C Is it true that it takes more energy to vaporize 1 kg of saturated liquid water at 100 C than it would at 120 C? 3 21C What is quality? Does it have any meaning in the superheated vapor region? 3 22C Which process requires more energy: completely vaporizing 1 kg of saturated liquid water at 1 atm pressure or completely vaporizing 1 kg of saturated liquid water at 8 atm pressure? 3 23C Does h fg change with pressure? How?

2 cen84959_ch03.qxd 4/1/05 12:31 PM Page C Can quality be expressed as the ratio of the volume occupied by the vapor phase to the total volume? Explain. 3 25C In the absence of compressed liquid tables, how is the specific volume of a compressed liquid at a given P and T determined? 3 26 Complete this table for H 2 O: T, C P, kpa v, m 3 /kg Phase description Saturated vapor Reconsider Prob Using EES (or other) software, determine the missing properties of water. Repeat the solution for refrigerant-134a, refrigerant- 22, and ammonia. 3 28E Complete this table for H 2 O: T, F P, psia u, Btu/lbm Phase description Saturated liquid E Reconsider Prob. 3 28E. Using EES (or other) software, determine the missing properties of water. Repeat the solution for refrigerant-134a, refrigerant- 22, and ammonia Complete this table for H 2 O: T, C P, kpa h, kj/kg x Phase description Complete this table for refrigerant-134a: T, C P, kpa v, m 3 /kg Phase description Saturated vapor Complete this table for refrigerant-134a: T, C P, kpa u, kj/kg Phase description Saturated liquid Chapter E Complete this table for refrigerant-134a: T, F P, psia h, Btu/lbm x Phase description Complete this table for H 2 O: T, C P, kpa v, m 3 /kg Phase description Saturated liquid Complete this table for H 2 O: T, C P, kpa u, kj/kg Phase description Saturated vapor A 1.8-m 3 rigid tank contains steam at 220 C. Onethird of the volume is in the liquid phase and the rest is in the vapor form. Determine (a) the pressure of the steam, (b) the quality of the saturated mixture, and (c) the density of the mixture. Steam 1.8 m C FIGURE P A piston cylinder device contains 0.85 kg of refrigerant- 134a at 10 C. The piston that is free to move has a mass of 12 kg and a diameter of 25 cm. The local atmospheric pressure is 88 kpa. Now, heat is transferred to refrigerant-134a R-134a 0.85 kg 10 C FIGURE P3 37 Q

3 cen84959_ch03.qxd 4/1/05 12:31 PM Page Thermodynamics until the temperature is 15 C. Determine (a) the final pressure, (b) the change in the volume of the cylinder, and (c) the change in the enthalpy of the refrigerant-134a. 3 38E The temperature in a pressure cooker during cooking at sea level is measured to be 250 F. Determine the absolute pressure inside the cooker in psia and in atm. Would you modify your answer if the place were at a higher elevation? Pressure cooker 250 F FIGURE P3 38E 3 39E The atmospheric pressure at a location is usually specified at standard conditions, but it changes with the weather conditions. As the weather forecasters frequently state, the atmospheric pressure drops during stormy weather and it rises during clear and sunny days. If the pressure difference between the two extreme conditions is given to be 0.3 in of mercury, determine how much the boiling temperatures of water will vary as the weather changes from one extreme to the other A person cooks a meal in a 30-cm-diameter pot that is covered with a well-fitting lid and lets the food cool to the room temperature of 20 C. The total mass of the food and the pot is 8 kg. Now the person tries to open the pan by lifting the lid up. Assuming no air has leaked into the pan during cooling, determine if the lid will open or the pan will move up together with the lid Water is to be boiled at sea level in a 30-cm-diameter stainless steel pan placed on top of a 3 kw electric burner. If 60 percent of the heat generated by the burner is transferred to the water during boiling, determine the rate of evaporation of water Repeat Prob for a location at an elevation of 1500 m where the atmospheric pressure is 84.5 kpa and thus the boiling temperature of water is 95 C Water is boiled at 1 atm pressure in a 25-cm-internaldiameter stainless steel pan on an electric range. If it is observed that the water level in the pan drops by 10 cm in 45 min, determine the rate of heat transfer to the pan Repeat Prob for a location at 2000-m elevation where the standard atmospheric pressure is 79.5 kpa Saturated steam coming off the turbine of a steam power plant at 30 C condenses on the outside of a 3-cmouter-diameter, 35-m-long tube at a rate of 45 kg/h. Determine the rate of heat transfer from the steam to the cooling water flowing through the pipe The average atmospheric pressure in Denver (elevation 1610 m) is 83.4 kpa. Determine the temperature at which water in an uncovered pan boils in Denver. Answer: 94.6 C Water in a 5-cm-deep pan is observed to boil at 98 C. At what temperature will the water in a 40-cm-deep pan boil? Assume both pans are full of water A cooking pan whose inner diameter is 20 cm is filled with water and covered with a 4-kg lid. If the local atmospheric pressure is 101 kpa, determine the temperature at which the water starts boiling when it is heated. Answer: C P atm = 101 kpa m lid = 4 kg H 2 O Vapor FIGURE P % 3 kw FIGURE P % 3 49 Reconsider Prob Using EES (or other) software, investigate the effect of the mass of the lid on the boiling temperature of water in the pan. Let the mass vary from 1 kg to 10 kg. Plot the boiling temperature against the mass of the lid, and discuss the results Water is being heated in a vertical piston cylinder device. The piston has a mass of 20 kg and a cross-sectional area of 100 cm 2. If the local atmospheric pressure is 100 kpa, determine the temperature at which the water starts boiling.

4 cen84959_ch03.qxd 4/1/05 12:31 PM Page A rigid tank with a volume of 2.5 m 3 contains 15 kg of saturated liquid vapor mixture of water at 75 C. Now the water is slowly heated. Determine the temperature at which the liquid in the tank is completely vaporized. Also, show the process on a T-v diagram with respect to saturation lines. Answer: C 3 52 A rigid vessel contains 2 kg of refrigerant-134a at 800 kpa and 120 C. Determine the volume of the vessel and the total internal energy. Answers: m 3, kj 3 53E A 5-ft 3 rigid tank contains 5 lbm of water at 20 psia. Determine (a) the temperature, (b) the total enthalpy, and (c) the mass of each phase of water A 0.5-m 3 vessel contains 10 kg of refrigerant-134a at 20 C. Determine (a) the pressure, (b) the total internal energy, and (c) the volume occupied by the liquid phase. Answers: (a) kpa, (b) kj, (c) m A piston cylinder device contains 0.1 m 3 of liquid water and 0.9 m 3 of water vapor in equilibrium at 800 kpa. Heat is transferred at constant pressure until the temperature reaches 350 C. (a) (b) (c) (d) What is the initial temperature of the water? Determine the total mass of the water. Calculate the final volume. Show the process on a P-v diagram with respect to saturation lines. H 2 O P = 800 kpa FIGURE P Reconsider Prob Using EES (or other) software, investigate the effect of pressure on the total mass of water in the tank. Let the pressure vary from 0.1 MPa to 1 MPa. Plot the total mass of water against pressure, and discuss the results. Also, show the process in Prob on a P-v diagram using the property plot feature of EES. 3 57E Superheated water vapor at 180 psia and 500 F is allowed to cool at constant volume until the temperature drops to 250 F. At the final state, determine (a) the pressure, (b) the quality, and (c) the enthalpy. Also, show the process on a T-v diagram with respect to saturation lines. Answers: (a) psia, (b) 0.219, (c) Btu/lbm Chapter E Reconsider Prob. 3 57E. Using EES (or other) software, investigate the effect of initial pressure on the quality of water at the final state. Let the pressure vary from 100 psi to 300 psi. Plot the quality against initial pressure, and discuss the results. Also, show the process in Prob. 3 57E on a T-v diagram using the property plot feature of EES A piston cylinder device initially contains 50 L of liquid water at 40 C and 200 kpa. Heat is transferred to the water at constant pressure until the entire liquid is vaporized. (a) What is the mass of the water? (b) What is the final temperature? (c) Determine the total enthalpy change. (d) Show the process on a T-v diagram with respect to saturation lines. Answers: (a) kg, (b) C, (c) 125,943 kj 3 60 A 0.3-m 3 rigid vessel initially contains saturated liquid vapor mixture of water at 150 C. The water is now heated until it reaches the critical state. Determine the mass of the liquid water and the volume occupied by the liquid at the initial state. Answers: kg, m Determine the specific volume, internal energy, and enthalpy of compressed liquid water at 100 C and 15 MPa using the saturated liquid approximation. Compare these values to the ones obtained from the compressed liquid tables Reconsider Prob Using EES (or other) software, determine the indicated properties of compressed liquid, and compare them to those obtained using the saturated liquid approximation. 3 63E A 15-ft 3 rigid tank contains a saturated mixture of refrigerant-134a at 50 psia. If the saturated liquid occupies 20 percent of the volume, determine the quality and the total mass of the refrigerant in the tank A piston cylinder device contains 0.8 kg of steam at 300 C and 1 MPa. Steam is cooled at constant pressure until one-half of the mass condenses. (a) (b) (c) Show the process on a T-v diagram. Find the final temperature. Determine the volume change A rigid tank contains water vapor at 250 C and an unknown pressure. When the tank is cooled to 150 C, the vapor starts condensing. Estimate the initial pressure in the tank. Answer: 0.60 MPa 3 66 Water is boiled in a pan covered with a poorly fitting lid at a specified location. Heat is supplied to the pan by a 2-kW resistance heater. The amount of water in the pan is observed to decrease by 1.19 kg in 30 minutes. If it is estimated that 75 percent of electricity consumed by the heater is transferred to the water as heat, determine the local atmospheric pressure in that location. Answer: 85.4 kpa

5 Problems: Developing Engineering Skills 115 apply the incompressible substance model. use the generalized compressibility chart to relate p v T data of gases. apply the ideal gas model for thermodynamic analysis, including determining when use of the ideal gas model is warranted, and appropriately using ideal gas table data or constant specific heat data to determine u and h. Key Engineering Concepts state principle p. 69 simple compressible system p. 69 p v T surface p. 70 phase diagram p. 72 saturation temperature p. 73 saturation pressure p. 73 p v diagram p. 73 T v diagram p. 73 two-phase, liquid vapor mixture p. 75 quality p. 75 superheated vapor p. 75 enthalpy p. 83 specific heats p. 91 ideal gas model p. 100 Exercises: Things Engineers Think About 1. Why does food cook more quickly in a pressure cooker than in water boiling in an open container? 2. If water contracted on freezing, what implications might this have for aquatic life? 3. Why do frozen water pipes tend to burst? 4. Referring to a phase diagram, explain why a film of liquid water forms under the blade of an ice skate. 5. Can water at 40 C exist as a vapor? As a liquid? 6. What would be the general appearance of constant-volume lines in the vapor and liquid regions of the phase diagram? 7. Are the pressures listed in the tables in the Appendix absolute pressures or gage pressures? 8. The specific internal energy is arbitrarily set to zero in Table A-2 for saturated liquid water at 0.01 C. If the reference value for u at this reference state were specified differently, would there be any significant effect on thermodynamic analyses using u and h? 9. For liquid water at 20 C and 1.0 MPa, what percent difference would there be if its specific enthalpy were evaluated using Eq instead of Eq. 3.13? 10. For a system consisting of 1 kg of a two-phase, liquid vapor mixture in equilibrium at a known temperature T and specific volume v, can the mass, in kg, of each phase be determined? Repeat for a three-phase, solid liquid vapor mixture in equilibrium at T, v. 11. By inspection of Fig. 3.9, what are the values of c p for water at 500 C and pressures equal to 40 MPa, 20 MPa, 10 MPa, and 1 MPa? Is the ideal gas model appropriate at any of these states? 12. Devise a simple experiment to determine the specific heat, c p,of liquid water at atmospheric pressure and room temperature. 13. If a block of aluminum and a block of steel having equal volumes each received the same energy input by heat transfer, which block would experience the greater temperature increase? 14. Under what circumstances is the following statement correct? Equal molar amounts of two different gases at the same temperature, placed in containers of equal volume, have the same pressure. 15. Estimate the mass of air contained in a bicycle tire. 16. Specific internal energy and enthalpy data for water vapor are provided in two tables: Tables A-4 and A-23. When would Table A-23 be used? Problems: Developing Engineering Skills Using p v T Data 3.1 Determine the phase or phases in a system consisting of H 2 O at the following conditions and sketch p v and T v diagrams showing the location of each state. (a) p 5 bar, T C. (b) p 5 bar, T 200 C. (c) T 200 C, p 2.5 MPa. (d) T 160 C, p 4.8 bar. (e) T 12 C, p 1 bar. 3.2 Plot the pressure temperature relationship for two-phase liquid vapor mixtures of water from the triple point temperature to the critical point temperature. Use a logarithmic scale for pressure, in bar, and a linear scale for temperature, in C.

6 116 Chapter 3 Evaluating Properties 3.3 For H 2 O, plot the following on a p v diagram drawn to scale on log log coordinates: (a) the saturated liquid and saturated vapor lines from the triple point to the critical point, with pressure in MPa and specific volume in m 3 /kg. (b) lines of constant temperature at 100 and 300 C. 3.4 Plot the pressure temperature relationship for two-phase liquid vapor mixtures of (a) Refrigerant 134a, (b) ammonia, (c) Refrigerant 22 from a temperature of 40 to 100 C, with pressure in kpa and temperature in C. Use a logarithmic scale for pressure and a linear scale for temperature. 3.5 Determine the quality of a two-phase liquid vapor mixture of (a) H 2 O at 20 C with a specific volume of 20 m 3 /kg. (b) Propane at 15 bar with a specific volume of m 3 /kg. (c) Refrigerant 134a at 60 C with a specific volume of m 3 /kg. (d) Ammonia at 1 MPa with a specific volume of 0.1 m 3 /kg. 3.6 For H 2 O, plot the following on a p v diagram drawn to scale on log log coordinates: (a) the saturated liquid and saturated vapor lines from the triple point to the critical point, with pressure in KPa and specific volume in m 3 /kg 150 C (b) lines of constant temperature at 300 and 560 C. 3.7 Two kg of a two-phase, liquid vapor mixture of carbon dioxide (CO 2 ) exists at 40 C in a 0.05 m 3 tank. Determine the quality of the mixture, if the values of specific volume for saturated liquid and saturated vapor CO 2 at 40 C are v f m 3 /kg and v g m 3 /kg, respectively. 3.8 Determine the mass, in kg, of 0.1 m 3 of Refrigerant 134a at 4 bar, 100 C. 3.9 A closed vessel with a volume of m 3 contains 1.2 kg of Refrigerant 22 at 10 bar. Determine the temperature, in C Calculate the mass, in kg, of 1 m 3 of a two-phase liquid vapor mixture of Refrigerant 22 at 1 bar with a quality of 75% A two-phase liquid vapor mixture of a substance has a pressure of 150 bar and occupies a volume of 0.2 m 3. The masses of saturated liquid and vapor present are 3.8 kg and 4.2 kg, respectively. Determine the mixture specific volume in m 3 /kg Ammonia is stored in a tank with a volume of 0.21 m 3. Determine the mass, in kg, assuming saturated liquid at 20 C. What is the pressure, in kpa? 3.13 A storage tank in a refrigeration system has a volume of m 3 and contains a two-phase liquid vapor mixture of Refrigerant 134a at 180 kpa. Plot the total mass of refrigerant, in kg, contained in the tank and the corresponding fractions of the total volume occupied by saturated liquid and saturated vapor, respectively, as functions of quality Water is contained in a closed, rigid, 0.2 m 3 tank at an initial pressure of 5 bar and a quality of 50%. Heat transfer occurs until the tank contains only saturated vapor. Determine the final mass of vapor in the tank, in kg, and the final pressure, in bar Two thousand kg of water, initially a saturated liquid at 150 C, is heated in a closed, rigid tank to a final state where the pressure is 2.5 MPa. Determine the final temperature, in C, the volume of the tank, in m 3, and sketch the process on T v and p v diagrams Steam is contained in a closed rigid container with a volume of 1 m 3. Initially, the pressure and temperature of the steam are 7 bar and 500 C, respectively. The temperature drops as a result of heat transfer to the surroundings. Determine the temperature at which condensation first occurs, in C, and the fraction of the total mass that has condensed when the pressure reaches 0.5 bar. What is the volume, in m 3, occupied by saturated liquid at the final state? 3.17 Water vapor is heated in a closed, rigid tank from saturated vapor at 160 C to a final temperature of 400 C. Determine the initial and final pressures, in bar, and sketch the process on T v and p v diagrams Ammonia undergoes an isothermal process from an initial state at T 1 80 F and v 1 10 ft 3 /lb to saturated vapor. Determine the initial and final pressures, in lbf/in. 2, and sketch the process on T v and p v diagrams A two-phase liquid vapor mixture of H 2 O is initially at a pressure of 30 bar. If on heating at fixed volume, the critical point is attained, determine the quality at the initial state Ammonia undergoes a constant-pressure process at 2.5 bar from T 1 30 C to saturated vapor. Determine the work for the process, in kj per kg of refrigerant Water vapor in a piston cylinder assembly is heated at a constant temperature of 204 C from saturated vapor to a pressure of.7 MPa. Determine the work, in kj per kg of water vapor, by using IT kg mass of ammonia, initially at p 1 7 bars and T C, undergo a constant-pressure process to a final state where the quality is 85%. Determine the work for the process, kj Water vapor initially at 10 bar and 400 C is contained within a piston cylinder assembly. The water is cooled at constant volume until its temperature is 150 C. The water is then condensed isothermally to saturated liquid. For the water as the system, evaluate the work, in kj/kg Two kilograms of Refrigerant 22 undergo a process for which the pressure volume relation is pv 1.05 constant. The initial state of the refrigerant is fixed by p 1 2 bar, T 1 20 C, and the final pressure is p 2 10 bar. Calculate the work for the process, in kj Refrigerant 134a in a piston cylinder assembly undergoes a process for which the pressure volume relation is pv constant. At the initial state, p kpa, T 1 10 C. The final temperature is T 2 50 C. Determine the final pressure, in kpa, and the work for the process, in kj per kg of refrigerant.

7 Problems: Developing Engineering Skills 117 Using u h Data 3.26 Using the tables for water, determine the specified property data at the indicated states. Check the results using IT. In each case, locate the state by hand on sketches of the p v and T v diagrams. (a) At p 3 bar, T 240 C, find v in m 3 /kg and u in kj/kg. (b) At p 3 bar, v 0.5 m 3 /kg, find T in C and u in kj/kg. (c) At T 400 C, p 10 bar, find v in m 3 /kg and h in kj/kg. (d) At T 320 C, v 0.03 m 3 /kg, find p in MPa and u in kj/kg. (e) At p 28 MPa, T 520 C, find v in m 3 /kg and h in kj/kg. (f) At T 100 C, x 60%, find p in bar and v in m 3 /kg. (g) At T 10 C, v 100 m 3 /kg, find p in kpa and h in kj/kg. (h) At p 4 MPa, T 160 C, find v in m 3 /kg and u in kj/kg Determine the values of the specified properties at each of the following conditions. (a) For Refrigerant 134a at T 60 C and v m 3 /kg, determine p in kpa and h in kj/kg. (b) For ammonia at p 8 bar and v m 3 /kg, determine T in C and u in kj/kg. (c) For Refrigerant 22 at T 10 C and u 200 kj/kg, determine p in bar and v in m 3 /kg A quantity of water is at 15 MPa and 100 C. Evaluate the specific volume, in m 3 /kg, and the specific enthalpy, in kj/kg, using (a) data from Table A-5. (b) saturated liquid data from Table A Plot versus pressure the percent changes in specific volume, specific internal energy, and specific enthalpy for water at 20 C from the saturated liquid state to the state where the pressure is 300 bar. Based on the resulting plots, discuss the implications regarding approximating compressed liquid properties using saturated liquid properties at 20 C, as discussed in Sec Evaluate the specific volume, in m 3 /kg, and the specific enthalpy, in kj/kg, of ammonia at 20 C and 1.0 MPa Evaluate the specific volume, in m 3 /kg, and the specific enthalpy, in kj/kg, of propane at 800 kpa and 0 C. Applying the Energy Balance 3.32 A closed, rigid tank contains 2 kg of water initially at 80 C and a quality of 0.6. Heat transfer occurs until the tank contains only saturated vapor. Kinetic and potential energy effects are negligible. For the water as the system, determine the amount of energy transfer by heat, in kj A two-phase liquid vapor mixture of H 2 O, initially at 1.0 MPa with a quality of 90%, is contained in a rigid, wellinsulated tank. The mass of H 2 O is 2 kg. An electric resistance heater in the tank transfers energy to the water at a constant rate of 60 W for 1.95 h. Determine the final temperature of the water in the tank, in C Refrigerant 134a vapor in a piston cylinder assembly undergoes a constant-pressure process from saturated vapor at 8 bar to 50 C. For the refrigerant, determine the work and heat transfer, per unit mass, each in kj/kg. Changes in kinetic and potential energy are negligible Saturated liquid water contained in a closed, rigid tank is cooled to a final state where the temperature is 50 C and the masses of saturated vapor and liquid present are 0.03 and kg, respectively. Determine the heat transfer for the process, in kj Refrigerant 134a undergoes a process for which the pressure volume relation is pv n constant. The initial and final states of the refrigerant are fixed by p kpa, T 1 10 C and p kpa, T 2 50 C, respectively. Calculate the work and heat transfer for the process, each in kj per kg of refrigerant A piston cylinder assembly contains a two-phase liquid vapor mixture of Refrigerant 22 initially at 24 C with a quality of 95%. Expansion occurs to a state where the pressure is 1 bar. During the process the pressure and specific volume are related by pv constant. For the refrigerant, determine the work and heat transfer per unit mass, each in kj/kg Five kilograms of water, initially a saturated vapor at 100 kpa, are cooled to saturated liquid while the pressure is maintained constant. Determine the work and heat transfer for the process, each in kj. Show that the heat transfer equals the change in enthalpy of the water in this case One kilogram of saturated solid water at the triple point is heated to saturated liquid while the pressure is maintained constant. Determine the work and the heat transfer for the process, each in kj. Show that the heat transfer equals the change in enthalpy of the water in this case A two-phase liquid vapor mixture of H 2 O with an initial quality of 25% is contained in a piston cylinder assembly as shown in Fig. P3.40. The mass of the piston is 40 kg, and its diameter is 10 cm. The atmospheric pressure of the surroundings is 1 bar. The initial and final positions of the piston are shown on the diagram. As the water is heated, the pressure inside the cylinder remains constant until the piston hits the stops. Heat transfer to the water continues until its pressure is 4.5 cm 1 cm Diameter = 10 cm Mass = 40 kg p atm = 100 kpa Q Initial quality x 1 = 25% Figure P3.40

8 Answer to Selected Problems , , (a) 1.33, (a) 0.2, (vacuum) 1.28 decreases 1.29 A: 2.68, B: Mpa, , , no (a) 56,520, 489,000 (b) kj 2.7 (b) , 91.4 (c) 0.5 m, , 100 k, , kj (a) Kpa (b) J (c) J 2.49 (b) 8.28 kj (c) kj (d) 36.9 kj kj, 30% kj, 200 kj kw 3.1 (a) two-phase, liquid vapor mixture, (b) superheated vapor, (c) subcooled (compressed) liquid, (d) superheated vapor, (e) solid , , , , , m 3 /kg, kj/kg kj/kg, , , , (a) 0.79 (b) , (a) (b) , , 24.1, , (a) 252.4, (b) , tf , , , (a) 664.1, (b) 17, kj/kg K mc1dt dt2 ha1t T1t2 T 0 1W # 0 T 2 W #, ha2 51 exp3 1hA mc2t , bars , kw (a) kg/min (b) kj/min (a) 47.4 kg , decrease T C 5.16 (a) (T H T C ) 2, (b) (T H T C ) (a) (1 max ) 1, (b) 1 max 5.20 (a) T C T H T 0 T H T 0 T C 5.21 T C (a) (b) 6 T H T H T H T C T H T H T C, T C T H (a) 6.7% 5.30 possible but uneconomical 5.34 no , no 5.46 (b) 1 2: 2914, 330, 2 3: 0, 983.0, 3 4: , 167.1, 4 1: 0, 373.0, (c) (a) 0.667, imp (b) 0, internally reversible (c) 0.667, possible, and irreversible 6.2 (a): W R W Q # s11 T 0 T s 2 11 T 0 T u (a): W mp(v g v f ), Q m(h g h f ) 6.12 F, T, F, F T C 822