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Designating States of Water [Thermo]

  1. Sep 7, 2015 #1
    1. The problem statement, all variables and given/known data
    Case, P[bar], T[C], v[m3/kg], u[kJ/kg], x[quality], State
    a) 1.5, ?, ?, 3000, ?, ?
    b) ?, 120, .6, ?, ?, ?
    c) 3, ?, ?, 2543.6, ?, ?
    d) ?, 190, ?, ?, 20%, ?
    e) 25, 100, ?, ?, ?, ?
    f) ?, 240, ?, ?, 0, ?

    2. Relevant equations
    We are supposed to use the steam tables in the back of our book. Sorry I have no way of scanning them but I believe there are many online. Also please forgive me for the formatting but I had no idea how to make a table on here.

    3. The attempt at a solution
    I think I got a couple of them.
    c) 3, 133.55, .606, 2543.6, none, superheated water vapor
    e) 25, 100, 1042.3, 418.24, none, compressed liquid

    Now I believe I read online you can't find quality of a superheated vapor or a compressed liquid. As far as the others go, I really have no idea what I'm doing. Can someone help me figure out what direction to take? Do I have to use interpolation on some of them? I'm really lost. Thanks for your time.
     
  2. jcsd
  3. Sep 7, 2015 #2

    SteamKing

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    The quality of superheated vapor = 100% by definition.
    The quality of compressed liquid = 0%

    The rest, you should do your best to fill in the answers. Someone will check your work and tell you if you're on the right track.
     
  4. Sep 7, 2015 #3
    That last part helps me out too, thanks man. I'll look at the tables again and hopefully it'll click. What I'm having the biggest problem with is is this flowchart that's supposed to help. The first box says to start with the 2 phase tables and asks "at given T, is P=Psat? at given P is T = Tsat? at T or P is yf < y < yg?" How do I know what Tsat and Psat are?
     
  5. Sep 7, 2015 #4

    SteamKing

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    Your table should have a saturation line somewhere. (Hint: at Psat = 1 atm; Tsat = 100°C

    Your table might say "saturated water", and list either Psat or Tsat as the key datum.
     
  6. Sep 8, 2015 #5
    Here's what I have filled out so far. Does it look right?
    tl75ptF.jpg

    :oldlaugh: Here are the tables I have to use.

    @SteamKing I don't see saturation lines in these tables I don't really know what I'm looking for either. Please if anyone has any help on this let me know. Am I overthinking this? Thanks.
     
  7. Sep 8, 2015 #6

    SteamKing

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    The saturation line is right there on pages 891-892. The independent variable on those pages is the saturation temperature. The saturation pressure is listed in the next column. Remember, for Psat = 1.0 atm., Tsat = 100°C. (1 bar = 100 kPa; 1.0 atm. = 1.01325 bar)

    On pages 893-894, you have the same information, but Psat is the independent variable and Tsat is listed in the next column. The other columns list the thermo properties, like specific volume, internal energy, enthalpy, and entropy.
     
  8. Sep 8, 2015 #7

    SteamKing

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    The reason it's called a saturation line is that when the thermo properties of water are presented in graphical form, a heavy line usually separates the superheated vapor region from the saturated vapor region. The saturation line is shown in red below:

    mollier-diagram-water_2.png
    The heavy red line where x = 1.0 is the saturation line, and x is the quality in decimal form. The lighter lines plotted below represent other quality lines (x = 0.95, x = 0.90, etc.) Above the heavy red line is the superheated vapor region.
     
  9. Sep 8, 2015 #8

    SteamKing

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    The table you have filled out so far looks good. For a) and b), you may need to use your tables and interpolate for the missing information.
     
  10. Sep 8, 2015 #9
    Thank you SO MUCH. Your name is kind of relevant to this topic also haha. But seriously, thank you * 101325! :oldbiggrin::oldbiggrin::oldbiggrin::oldbiggrin::oldbiggrin:
     
  11. Sep 8, 2015 #10
    The two phase tables are headed Saturated Steam. The temperatures and pressures in this table are the temperature and pressure of liquid water in equilibrium with water vapor. This is definitely the place to start. For example, for question (a), they tell you that the pressure is 1.5 bars and the internal energy is 3000 kJ/kg. But in the table, the internal energy of the saturated vapor is only 2520 kJ/kg. This tells you that the internal energy is higher than that of the saturated vapor. So it must be superheated.

    Chet
     
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