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Homework Help: Two questions on retrieving thermodynamic properties

  1. Apr 17, 2014 #1
    1. The problem statement, all variables and given/known data

    ''A rigid insulated container fitted with a paddle wheel contains 5lb of water, initially at 260F and a quality of 60%. The water is stirred until the temperature is 350F. For the water, determine the work.''

    How do we know whether the state of the water at 350F is still saturated water or superheated vapor ?

    "Water, contained in a closed rigid tank initially at 100lbf/in^2, 800F is cooled to a final state where the pressure is 20lbf/in^2. Determine the change in specific entropy."

    likewise for this question, how do I know if the water is still superheated vapor or not at the final state?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 17, 2014 #2
    You can determine the specific volume of the water in the initial state of each of these problems. The chambers are rigid, so the specific volume (even averaged over two phases if two phases are present at the end) does not change.

  4. Apr 17, 2014 #3

    for the second question I found the specific volume at the initial state to be 7.07
    using this, I find that the final state is a super heated vapor

    But the super heated vapor table gives data by pressure and I don't have any value for the pressure at the final state
  5. Apr 17, 2014 #4
    The problem statement says that the final pressure is 20 psi.

  6. Apr 17, 2014 #5
    oh sorry, that post was meant for the first question. Only the temperature at the final state is given
  7. Apr 17, 2014 #6
    If you have steam tables, you know the final temperature and the final specific volume. That should be enough to determine the change in internal energy. The tank was insulated, so no heat was transferred.

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