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Steam Power Plants and the Second Law of Thermodynamics

  1. Jul 20, 2014 #1
    We know that a steam power plant is basically a heat engine with steam as the working fluid.

    According to the second law of thermodynamics a heat engine must operate between two thermal energy reservoirs one at a higher temperature(source) than the other(sink) to produce net work in a thermodynamic cycle.

    In a basic steam power cycle,heat received from burning fuel in a furnace is utilized to convert water into high temperature and high pressure steam,which when passes through blades of a turbine produces shaft work with low pressure low temperature steam leaving the turbine exit.The low temperature low pressure steam is then made to reject heat in a condenser before it is pumped back into the boiler for another cycle of operation.

    While it is understandable that the low temperature low pressure steam must be condensed to the state of liquid water initially fed into the boiler to complete a thermodynamic cycle in doing so we are basically wasting the latent heat condensation of water along with the sensible heat required to manufacture high temperature high pressure steam from liquid water again in the boiler!

    What if the low temperature low pressure steam from the turbine exit was directly pumped to the boiler such that less fuel would be required to restore high temperature high pressure steam at the turbine inlet thus recovering the waste heat as mentioned?
     
  2. jcsd
  3. Jul 20, 2014 #2

    SteamKing

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    The volume of steam exhausting from the turbine is about 1000 times the volume of condensate coming out of the condenser. This means a vapor pump must be this much bigger than a condensate pump, and require a proportionately larger amount of power to drive it. You might recover some of the latent heat which is lost condensing the steam, but this recovered energy could not be turned into useful output work from the plant as a whole.
     
  4. Jul 21, 2014 #3
    You don't need us to help you address this question. Just do some calculations to see how the scenario you are describing would play out. Do it on a sample system, by defining the operating conditions at the end of each step. This is how modeling can help you resolve uncertainties and doubts. (First, of course, to get your feet wet, do the calculations for a standard steam engine cycle).

    Chet
     
  5. Jul 22, 2014 #4
  6. Aug 21, 2014 #5
    This is the answer I was looking for and I am glad I came across it :smile:
     

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  7. Aug 21, 2014 #6

    Jano L.

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    Which book is it from?
     
  8. Aug 23, 2014 #7
    "Thermodynamics:An Engineering Approach" by Yunus A. Cengel and Michael A. Boles
     
  9. Aug 23, 2014 #8

    jim hardy

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    How to make it intuitive:....

    if you've ever pumped up a bicycle or automobile tire with an old fashioned hand pump you know how much work it takes to pump a compressible fluid.
     
  10. Aug 23, 2014 #9
    Very well said!
     
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