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Lec 7: Property tables, ideal and real gases

2. For next time:Read: 3-8 to 3-12HW4 dueOutline:EESQuality, internal energy, enthalpyReal gasesImportant points:How to use the quality to find properties of mixturesHow to evaluate a given process in a property diagramHow to calculate and apply corrections to the IGL for real gases . 3.

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Lec 7: Property tables, ideal and real gases

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    1. 1 Lec 7: Property tables, ideal and real gases

    2. 2 For next time: Read: § 3-8 to 3-12 HW4 due Outline: EES Quality, internal energy, enthalpy Real gases Important points: How to use the quality to find properties of mixtures How to evaluate a given process in a property diagram How to calculate and apply corrections to the IGL for real gases

    3. 3 TEAMPLAY Find, for water, the following properties: the saturation pressure at a saturation temperature of 100 ?F. and find the saturation temperature at a pressure of 6 MPa. Make sure everyone in your group understands how to do this.

    4. 4 Quality We often represent the relative amount of vapor present by something called the quality x.

    5. 5 Quality is related to the horizontal differences of P-V and T-v diagrams

    6. 6 What is the new v?

    7. 7 So...

    8. 8 Obtaining u in the vapor dome What you do for v works for u (and for other things)

    9. 9 A new property: enthalpy, H Enthalpy is simply the sum of the internal energy, U, and the pressure volume product, pV H ? U + pV Now,

    10. 10 Enthalpy--the bottom line H = U + pV h = u + pv

    11. 11 The P-V or P-v plane For the next few lectures we will often look at the two dimensions P and v, or P and V. The P is always on the ordinate and the v is always on the abcissa, just opposite to the familiar x-y plane.

    12. 12

    13. 13

    14. 14 Superheat tables--compressed liquid tables are similar

    15. 15 TEAMPLAY

    16. 16 TEAMPLAY

    17. 17 Sample Problem

    18. 18 SOLUTION

    19. 19 What can we learn? Specific volume is approximately constant over large changes in pressure if T=C Liquid does not change specific volume significantly as pressure is changed…it can’t be “compressed” When compressed liquid tables are not available, estimate property data at sat. liquid conditions at the same temperature as the compressed liquid.

    20. 20 Consider R-134a (Refrigerant 134a) We can make a diagram for this as we did for water but there is no data in the compressed or subcooled liquid region.

    21. 21

    22. 22

    23. 23

    24. 24 TEAMPLAY

    25. 25 Ideal gas “law” is a simple equation of state

    26. 26 Molar mass or molecular weight is sometimes confusing

    27. 27

    28. 28

    29. 29

    30. 30 Real gases Pv = ZRT, or Pv = ZRuT, where v is volume per unit mole.

    31. 31 Principle of corresponding states

    32. 32 Compressibility factor What is it really doing? It accounts mainly for two things: Molecular structure Intermolecular attractive forces

    33. 33 Reduced properties

    34. 34 Where do you find critical properties? Look in the appendices of your text book. For the SI system they are on p. 930 in Table A-1, along with molar mass. For USCS system, they are on p. 976 in Table A-1E

    35. 35 Reduced properties This works great if you are given a gas, a P and a T and asked to find the v. However, if you are given P and v and asked to find T (or T and v and asked to find P), you can use the pseudoreduced volume.

    36. 36 Reduced properties In those cases use the pseudoreduced volume:

    37. 37 Compressibility factor It is shown in Figure 3-56 (p. 100) in terms of actual experimental data

    38. 38 Compressibility factor for ten substances

    39. 39 TEAMPLAY

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