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Heat Transfer and the 2nd Law of Thermodynamics

  1. Jan 31, 2009 #1
    1. The problem statement, all variables and given/known data

    Heat is transferred into saturated liquid water until it is saturated vapor. The water remains at 300 C throughout the process.

    a. What is the initial pressure?

    b. What is the final pressure?

    c. How much heat is transferred per kg of water?

    I'm having trouble with a and b. I assume i need to use the steam and compressed liquid tables, but I am not sure how to use this given info to get it.

    2. Relevant equations

    part c: energy balance gives Q/m = u1 - u2

    Q = heat transfer
    u = specific internal energy

    I can simply use tables to find the u values once parts a and b are found.

    3. The attempt at a solution
     
  2. jcsd
  3. Feb 1, 2009 #2

    Astronuc

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    Staff: Mentor

    What is the significance of "saturated liquid water until it is saturated vapor", i.e. this is just a change of phase from pure liquid to pure vapor.

    Take a look at saturated steam tables.
     
  4. Feb 1, 2009 #3
    Wouldnt i also need to know the pressure?
     
  5. Feb 2, 2009 #4

    stewartcs

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    Science Advisor

    What must the pressure be if the water is at a saturation point?

    CS
     
  6. Feb 2, 2009 #5
    looking at the saturated liquid-vapor table, is it 85.81 bar?

    If so, it also gives values for entropy, enthalpy... for saturated liquid and vapor. I could just use those values right?
     
  7. Feb 3, 2009 #6

    stewartcs

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    Since I don't have your table I can't say. However, if the value listed for 300 C saturated water is 85.81 bar then I would say yes. Essentially you will just read it off the table since the problem tells you it is saturated. You'll need to use the super-heated vapor table for part b.

    CS
     
  8. Feb 3, 2009 #7
    It says saturated liquid vapor mixture.

    When I am looking at the sat. vapor tables, there are a wide variety of pressures. How do i know which to use?
     
  9. Feb 3, 2009 #8

    stewartcs

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    Most superheated vapor tables list the saturation temperature as well. So just find the one with the pressure that corresponds to 300 C and that will be the saturation pressure of the superheated vapor. Note: you may have to interpolate if 300 C is not listed.

    The pressure should be around 8.5 MPa for 300 C superheated vapor based on my tables.

    T= 295.01 C, P = 8.0 MPa

    and

    T = 303.35 C, P = 9.0 MPa

    So just interpolate to find 300 C exactly.

    Hope this helps.

    CS
     
  10. Feb 5, 2009 #9
    I have found similiar temperatures: 295.06 and 311.06, but we cant interpolate directly between sets of tables, right?

    So should I, say, find specific volume v in one set (at 295.09 C for example), do the same for the other, then interpolate these v and T values to find the actual v at 300C so that I can use the ideal gas equation?
     
  11. Feb 5, 2009 #10

    stewartcs

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    Sure you can.

    CS
     
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