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Homework Help: Steam turbine

  1. Apr 12, 2006 #1
    A steam turbine has an inlet of 2kg/s water at 1000kPa and 350C and velocity of 15m/s. The exit is at 100kPa, 100% quality and very low velocity. Find specific work and power produced.

    I have done a lot of these problems but it usually says adiabatic or the inlet and exit temp is the same. How do i solve with the energy equation when I have 2 unknowns? (q and w)

    thanks
     
  2. jcsd
  3. Apr 12, 2006 #2

    Andrew Mason

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    What does 100% quality mean? Check the question carefully. One has to have a temperature of the exit steam/water in order to determine the loss of energy of the steam.

    AM
     
  4. Apr 12, 2006 #3
    I understand that that gives me the exit temp, I just do not understand how to solve the energy equation for work when I have heat loss as well. Is the heat loss just the difference in internal energy between those two temperatures and pressures?
     
  5. Apr 12, 2006 #4

    Andrew Mason

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    The first law of thermodynamics:

    [tex]dQ = dU + dW = nC_vdT + PdV[/tex]

    where dQ is the heat removed from the hot reservoir, applies only to systems in thermodynamic equilbrium. Here you have a dynamic element: moving steam, so you have to take into account the kinetic energy of the steam as well.

    I still don't understand how you get the exit temperature. What does "100% quality" mean?

    AM
     
  6. Apr 12, 2006 #5
    The heat flow is zero. Turbines under steady state are adiabatic unless otherwise specified. You have one unknown, solve for it.

    AM, 100% quality means the liquid is a saturated vapor, x=1.0, and the specific enthalpy is equal to the value of Hg at said temperature.

    Beckton and his never ending slew of thermo questions, aye caramba. :wink:
     
    Last edited: Apr 12, 2006
  7. Apr 12, 2006 #6
    Thanks guys, sorry about all of the thermo questions, just studying for an exam and doing a bunch of problems that were not assigned so I sometimes come across something that was not covered and haven't seen that situation before, but that helps me the more situations I see, the more I understand the underlying concepts.
     
  8. Apr 12, 2006 #7
    I figured it out and found that without taking into account the velocity, the anwser is 482.19 kj/kg, and 482.303 kJ,kg with the velocity considered (the right anwser in the back of the book). Is this not negligible?
     
  9. Apr 12, 2006 #8
    What do you think?
     
  10. Apr 12, 2006 #9
    It seems to me that we should almost always neglect kinetic energy. Should there be a rule of thumb or something?
     
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