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Homework Help: Thermodynamics problem - internal energy of a saturated steam?

  1. Feb 13, 2013 #1
    1. The problem statement, all variables and given/known data

    For those who are not interested in reading through the different values I get for each stage then please answer this question:

    What is the internal energy at state 1? When water is a saturated steam... Is it U_vap + U_liq since saturated steam has both liquid and gas phases in it? Or is it just U_vap?

    If you are confused it may be worthwhile to see what each state is in the following text I wrote:


    In this question there are 3 states. The question also has three parts to it.

    First Part:
    Find the initial pressure, temperature and specific volume values at state 1 from the steam tables and appendices. I ended up getting these values (these are correct as I have checked the answer key):
    P1 = 500kPa
    T1 = 425.65K
    v(specific volume)1 = 0.37297

    Second Part:
    Find the work done by the steam. I calculated this and got the same answer as the answer key which is:
    W(state 1->3) = 232.3 kJ/kg Note they want the answer in specific energy

    Third Part:
    This is where I am stuck. This one asks for the total heat transferred to the steam.

    I know that:
    H = ΔU + W

    Where ΔU is the change in internal energy from state 1 to state 3. My main problem concerns solving for the internal energy in state 1 using the steam tables. First off state 1 is saturated steam, so from the Thermodynamics tables I get several internal energy values. I get U_vap, U_liq and U_evap. From what I have learned a saturated steam includes water in both liquid and gas phases? Thus U(state 1) = U_vap + U_liq...?

    This does not work and the answer suggests that U(state 1) = U_vap only.
  2. jcsd
  3. Feb 14, 2013 #2
    I can not see the image. Can you re-upload it please ? Also absolute internal energy of the system is undefined. (Think why ?)
  4. Feb 14, 2013 #3
    Dear beast,

    The term "saturated steam" refers only to the vapor.

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