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Thermodynamics and steam, temperature etc tables

  1. Dec 26, 2011 #1
    1. The problem statement, all variables and given/known data

    2000 kg of water, initially in a saturated liquid state and at 150 ºC, are heated in a closed container of constant volume, untill the pressure is 2,5 MPa.
    Find the final temperature and the volume of the container.

    2. Relevant equations

    I believe it is a matter of consulting the tables... but I am missing something because I don't know how to do it!

    3. The attempt at a solution

    I believe it is necessary to know if the water, in the final state, is superheated vapor ou saturated liquid/ vapor.... is it? How do we know that?

    Many thanks!!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Dec 26, 2011 #2
    I think it's safe to assume that all of the water becomes vapor, because it asks to find the volume of the container, which might mean you will have to find the volume of the gaseous H2O.
     
  4. Dec 26, 2011 #3
    The initial state is saturated liquid at T=150 C.

    (1) Can you use the steam tables to find the specific volume of this state? While you're at it, what is the initial pressure?

    (2) If you know the mass and specific volume, can you find the volume?

    The liquid is heated at constant volume. Think about what the saturation curve looks like on a P-v diagram. Which direction are you headed if you start with a saturated liquid and add heat? What state is the fluid in when it reaches 2.5 MPa? (Hint: Does the saturation specific volume of pure liquid increase or decrease as temperature and pressure are increased? Is 2.5 MPa above or below the critical pressure?) Which table do we use for water in this state?

    BBB
    p.s. cryora's speculation is inapt.
     
  5. Dec 26, 2011 #4
    Thank you very much bbbeard!

    Regarding to (2)... I've done that, but can we assume that the volume of the liquid is equal to the volume of the container?
     
  6. Dec 26, 2011 #5
    Well, if you don't assume that that liquid takes up the entire container, then the problem does not have a unique solution. My take, as someone who has taught thermo many times, is that they intend for you to assume that the process is a constant volume process, i.e. that the liquid takes up the entire container and that the container is perfectly rigid.

    BBB
     
  7. Dec 27, 2011 #6
    oh... I'll assume that then! I didn't know we could do it because nothing in the problem tells us that.

    Thank you very much for your previous hint on the P-v diagram!
    I was making the huge mistake of thinking that the saturated liquid would get into vapor and that was driving me mad!! Thanks to you, now I understand that it becomes a compressed liquid ;)

    Thank you!
     
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