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Homework Help: Another Thermo Question

  1. Aug 28, 2005 #1
    OK, I have a piston-cylinder with water.

    V = 50L = 0.05 m^3
    T = 25 C
    P = 300 kPa
    m = 49.85 kg
    specific volume @ 25 C = 0.001003 m^3/kg

    The water is heated to vaporization with pressure held constant. I need to determine the final temperature and the enthalpy change.

    I know what the answers need to be (133.55 C and 103,627 kJ). On the temperature, I tried doing T = (PV/mR), using R = 0.4615 (kPa*m^3)/(kg*K), but i didn't get anywhere close.

    On the enthalpy change part, I figured using m*h_fg, but that isn't right (I think).

    Any ideas? Thanks a lot!!!
  2. jcsd
  3. Aug 28, 2005 #2


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    1) Look at the water tables to see what enthalpy has the initial state,

    2)The thermodynamic trajectory is an isobaric one until you have reached a 100% of water vapor inside cylinder. Look at a Mollier diagram or go to tables to localize the enthalpy of water vapor at 300Kpa. Then you have the enthalpy difference.

    3) do not use the equation of state as you've done. It shows you didn't understand the problem at all. Imagine it physically: you are heating water to vaporize it with a free piston, do you really think volume is going to remain constant?
  4. Aug 28, 2005 #3
    OK, I got the enthalpy difference, no problems.

    I didn't realize my book had a pressure table, I found it there. Duh.

    I neglected to mention that I looked at the v_fg values as well, and that didn't work. Sorry about that.

    THANKS so much for your help. No wonder you are an engineer guru!
  5. Aug 29, 2005 #4


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    You're welcome!. Doubtless, there are another people who just wonder why the hell I am the PF eng. gurú.. :rofl: To those...maybe they're right. :biggrin: