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Homework Help: Heat needed to evaporate water

  1. Sep 29, 2007 #1
    1. The problem statement, all variables and given/known data

    A vessel of 1m^3 volume is half filled with water. The pressure inside the vessel is 2bar.
    Steam tables are given
    2. Relevant equations

    How much heat is needed to completely evaporate the water?

    3. The attempt at a solution

    I can find the final enthalpy by finding h(g) at 2 bar. But how do you find the initial enthalpy?
  2. jcsd
  3. Sep 29, 2007 #2


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    Does the vessel have a pressure relief valeve that maintains the pressure at 2 bar? If not, the pressure is going to increase linearly with the volume of water boiled off. This part is easy to calculate. As for your steam tables, I'm not sure exactly what you are looking at (different steam tables have different sets of data and use different forms of presentation). There are 3 relevant enthalpies: the enthalpy of water at 100C, the enthalpy of saturated steam, and the enthalpy of vaporization (the difference between the previous 2 numbers).

    Have you written down the question completely, and exactly as it was given to you? If not, please post the question EXACTLY as it was given to you. What is the temperature of the water in the vessel?
    Last edited: Sep 29, 2007
  4. Sep 29, 2007 #3
    I quoted this question from a thermodynamics past paper. This is all that is given.
    It is a short answer question ie. given only couple of lines to answer.
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