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Homework Help: Finding H for Water

  1. Dec 18, 2014 #1
    1. The problem statement, all variables and given/known data
    Determine H for a water stream with V=155.8828 cm^3/g at 1000 kPa.

    2. Relevant equations

    3. The attempt at a solution
    I was thinking of using a steam table and finding the H from there, but it would have to be a water steam, not stream. Is there any other way of doing it? I feel like I'm missing something important, but it's just not coming to me.

    Thanks in advance!
  2. jcsd
  3. Dec 18, 2014 #2
    If the question is talking about H, the implication is that you're going to be using the steam tables.
  4. Dec 18, 2014 #3
    That's what I thought, but we aren't given temperature so how do we know if it's superheated or or super-saturated or what-not? It seems like there is a lot of missing information that you need to solve with the steam table. I was thinking of using the equation h=u+pv and assuming u=0 because it is a steady state function. Would that work?
  5. Dec 18, 2014 #4
    What more do you need? You have the specific volume and the pressure. That's enough.
    No. What could possibly make you think that you could just assume u = 0? What is a "steady state function?"

  6. Dec 18, 2014 #5
    Here's a hint: I found it in the steam tables.

  7. Dec 18, 2014 #6
    Is it directly from the table? Do we use pv=nRT to get t and then h from the table?
  8. Dec 18, 2014 #7
    Yes, but two adjacent columns are involved.
  9. Dec 18, 2014 #8
    It's late and I'm about to go to bed. So I have one more hint for you. I hope that this doesn't reveal too much: Saturated.

  10. Dec 18, 2014 #9
    I don't know. I'm not getting what you're saying, and I've asked people. Can you post the number so that I can see where you got it from? I'm looking at the saturated table and I found where P is, but it doesn't coincide with the given v. How would you get h from there even if it did?
  11. Dec 19, 2014 #10
    From your saturated table, what did you get for the temperature? What does your table show for the specific volume of the saturated vapor? What does your table show for the specific volume of the saturated liquid? What is 155.8828 cc/g expressed in the same units as your saturated table?

    Don't worry for now about getting the enthalpy of the stream. I'll get you there. There are a couple of steps in between.

    Last edited: Dec 19, 2014
  12. Dec 22, 2014 #11
    Here is what I found in the steam tables at P = 10 Bars:

    Temperature = 179.9 C
    Specific volume of saturated liquid = 0.001127 m^3/kg
    Specific volume of saturated liquid = 0.194 m^3/kg
    Enthalpy of saturated liquid = 762.8 kJ/kg
    Enthalpy of saturate vapor = 2278 kJ/kg

    Nominal specific volume of your stream = 0.1558828 m^3/kg

    Does your stream consist of (a) pure saturated liquid, (b) a mixture of saturated liquid and saturated vapor, or (c) pure saturated vapor?

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