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Enthalpy & Steam Tables Help Required!

  1. Jun 23, 2012 #1
    1. The problem statement, all variables and given/known data
    Find the enthalpy of steam at 0.4 MPa and 0.7 dryness


    2. Relevant equations



    3. The attempt at a solution
    At a pressure of 0.4MPa dry saturated steam has a total enthalpy of hg = 2,739 kj/kg.

    ∴ 0.7 kg of steam has an enthalpy of (0.7 x 2,739) = 1917.3 kj

    The total heat of 0.3kg of water is 0.3hf (at 0.4MPa), so 0.3 x 604.8 = 181.44 kj

    ∴ Total heat for wet steam = 181.44 + 1917.3 kj/kg
    = 2,098.74 kj/kg


    This is my first time using steam tables so am struggling to know if I am gripping it or not. Any help you can offer in order for me to answer this question is VERY much appreciated.

    Regards
     
  2. jcsd
  3. Jun 23, 2012 #2

    rock.freak667

    User Avatar
    Homework Helper

    Yes, your answer is correct.

    Your method is correct but it might take a little longer to do if you are in an exam. (the same formula with less lines would be)

    h = hf+ xhfg= hf+x(hg-hf)

    (Your steam table might have values for hfg sometimes)


    Formula is the same for specific entropy and specific volume in that

    s=sf+xsfg

    v = vf+xvfg
     
  4. Jun 24, 2012 #3
    Thank you rock.freak667
     
  5. Jun 24, 2012 #4
    Hi rock.freak667,

    Sorry, can you just clarify how my calculations would fit into that equation?

    I can follow it through but I am unsure what your X stands for? Can you clarify.

    hf + x (hg - hf)

    ∴ 604.8 + x (2739-604.8)

    ∴ 604.8 + x (2134.2)

    Can you clarify for me?

    thanks a lot
     
  6. Jun 24, 2012 #5

    rude man

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    Homework Helper
    Gold Member

    x = dryness = 70% = 0.7.
     
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