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Thermodynamic Properties; Isentropic Expansion

  1. Feb 3, 2013 #1
    1. The problem statement, all variables and given/known data

    An amount of saturated steam at a pressure of 10 Bar, undergoes an isentropic (constant entropy) expansion. The final temperature is 30C.

    Find the final specific enthalpy and specific volume.

    2. Relevant equations

    Saturated Steam tables, Linear Interpolation methods?
    No real equations of sort.

    3. The attempt at a solution

    By using the tables;

    at 10 bar;

    sf ( entropy) = 2.138 kJ/kg K
    v (specific volume) = 0.1944 m3/kg

    at 30C;

    sf ( entropy) = 0.436 kJ/kg K
    v (specific volume) = 32.93 m3/kg



    I've been staring into space with the question, not sure what to do.

    The final answers are given as 1990 kJ/kg for specific enthalpy and 25.28m3/kg for specific volume.

    Thanks for any help!
     
  2. jcsd
  3. Feb 3, 2013 #2
    Based on constant entropy, you can determine the quality of the steam at 30 C. Knowing the quality, you then determine the specific volume and enthalpy.
     
  4. Feb 3, 2013 #3

    rude man

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    Homework Helper
    Gold Member

    At 30C you are certainly in the superheated (unsaturated) region. You need to use your charts to find the state at which your gas is at 30C with the same entropy you started out with.

    Also, for saturated steam at 10 Bar (~1 MPa) I got s = 6.59 kJ/kg-K. Looks like you looked up s for saturated water, not steam.
     
    Last edited: Feb 3, 2013
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