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ENTC 370: Announcements

ENTC 370: Announcements

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ENTC 370: Announcements

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  1. ENTC 370: Announcements • Homework assignments No.2: • Assigned Problems: • 2.1, 2.19, 2.20, 2.24, 2.45, 2.53, 2.58, 2.80, 2.82, 2.83, 2.95. • Due Tuesday, September 23rd before 10:50 am • For more information, go to: • http://etidweb.tamu.edu/classes/entc370

  2. Phase-Change Processes of Pure Substances P V

  3. State 1 P = 1 atm T = 20° C Phases and Phase Change Processes • Phases: Solid, Liquid or Gas • Liquid • Compressed Liquid (CL) or subcooled liquid: Is not ready to vaporize (P > Psat, T < Tsat) • Saturated liquid (SL): Is about to vaporize Compressed Liquid Saturated Liquid State 2 P = 1 atm T = 100° C

  4. State 3 Saturated Vapor P = 1 atm T = 100° C Saturated Liquid Phase Change • Vapor • Saturated: Is about to condense • Superheated: Is not ready to condense (T > Tsat) • Saturated liquid-vapor mixture • Liquid and vapor phases coexist Superheated Vapor Saturated Vapor State 4 P = 1 atm T = 100° C P = 1 atm T = 300° C

  5. Heating Process of Water at Constant Pressure SHV CL T All vapor → ← all liquid SV SL MX States 2,3 & 4: Same T & P Sat liquid n ← vapor Sat vapor → ← liquid

  6. Phase Change: Liquid to Vapor • The energy required for solid-liquid or liquid-gas (vapor) transformations is called latent heat • For melting: Latent Heat of Fusion (LHF) • For vaporization: Latent Heat of Vaporization (LHV) • Examples: • Water: LHF is about 333.7 kJ/kg LHV is about 2,257.1 kJ/kg

  7. Latent Heats Latent Heat of Vaporization (LHV): Amount of energy required to allow phase change (Liquid ←→ Vapor) Latent Heat of Fusion (LHF): Amount of energy required to allow phase change (Solid ←→Liquid)

  8. Saturation Temperature and Saturation Pressure • The temperature at which water starts boiling depends on pressure. If pressure is fixed, so is the boiling temperature. • Tsat @ P • Psat @ T • During phase change, temperature and pressure are dependent properties Saturation Temperature ←→ Saturation Pressure

  9. Water: Liquid-Vapor Saturation Temperature and Pressure Liquid Gas or Vapor

  10. Liquid Gas or Vapor

  11. Property Diagrams for Phase-Change Process Critical Point: Saturated Liquid and Vapor States are indistinguishable LHV decreases with Pressure Each curve is represented in Slide No. 5

  12. Property Diagrams for Phase-Change Process (T-v Diagram)

  13. P-v Diagram

  14. P-T Diagram Note: Each substances has its own Triple Point (Table 3-3)

  15. Questions • What is LHV and LHF? • What is saturation temperature, pressure? • Does saturation temperature increase, decrease or remain the same when saturation pressure goes up? • Does LHV increase, decrease or remain the same when saturation pressure increases?

  16. Applications of Phase Change Process • Refrigeration: Pressure of liquid or vapor is controlled to induce phase change • During phase change process energy is released or absorbed • Examples: • Vacuum Freezing • Typical refrigeration and air conditioning systems • Power Generation: Water undergoes phase change process • Steam is used to power steam turbine

  17. Property Tables • Enthalpy – A combination property • H = U + PV (kJ) • h = u + Pv (kJ/kg) • Thanks to Prof. Mollier who combined u + Pv to represent heat content or total heat in steam turbines h1 = u1 + P1n1 h2 = u2 + P2n2

  18. Saturated Liquid and Saturated Vapor • Table A-4: water properties as a function of saturation temperature • Table A-5: water properties as a function of saturation pressure • Subscripts: • f: Saturated Liquid • g: Saturated Vapor • fg: Difference between vapor and liquid

  19. Table A-4 See eCampus folder with tables

  20. Difference between vapor and liquid Changes in properties due to complete phase change between liquid and vapor states, internal energy, enthalpy, and entropy, respectively hfgis called the enthalpy of vaporization (or latent heat of vaporization, LHV). Represent the amount of energy a unit of mass at constant pressure. hfg = LHV

  21. Example • A rigid tank contains 50 kg of saturated liquid water at 90° C. Determine the pressure in the tank and the volume. Facts and Assumptions:

  22. Example • A piston-cylinder device contains 2 ft3 of saturated water vapor at 50 psia. • Determine the temperature and the mass of the vapor inside the cylinder. Facts and Assumptions:

  23. What should I do if I do not find the exact values in the Tables? • Example: • Psat = 115 kPa, what is Tsat? • Interpolation is required: Property 1Property 2 A X B Y←Unknown value C Z

  24. Interpolation Then, solve for Y: You will use interpolation many times in this course!