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Help to determine convection coeficent or temperatures

  1. Sep 12, 2017 #1
    1. The problem statement, all variables and given/known data

    This is more like a design problem, I'm to evaporate water at 20°C and 2000 psi(Tsat=335.472°C), I have the heat flux the water is going to absorb during heating, and If that flux remains constant during all the length then, how can I find the surface temperature for the part where the water change phases?

    Right now I have the conditions during heating; for the heat flux constant, but when steam appears I don't know how to proceed, what eqs should I use? Does the pipe reamins with a T=constant? Reasoning tells me that the pipe must always be at the same temperature along his lenght.

    See, The pipes abosrb heat through turbulent external forced convection, and the water absorbs the heat through internal forced convection. And ultimatly I need to find the temperatures of the hot gas that is going to serve as the heat source. Any Ideas? Was I clear? Im so confused right now...sorry.


    2. Relevant equations

    Q'=Cp*m'*ΔT
    Q'=h*As*ΔT
    q'=h*ΔT
    q=hfg


    3. The attempt at a solution

    Since I know all the heat transfer I need through all the pipe, I was thinking, that h=q'*ΔT, where I will force the Temperatures and see how much mass flow I need. I did a quick calculation and I got 8 kg/s of hot gas (combustion products) that starts at Ti=1227 °C and ends at To=350°C but i don't think this approach is correct, also it seems a lot ok kg/s of hot gas.

    Any Ideas?

    thanks!
     
  2. jcsd
  3. Sep 13, 2017 at 3:16 AM #2

    Nidum

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    What you are describing is a flash steam boiler . There is extensive literature available about design of these .

    Search on :

    Water tube boiler .
    Monotube steam generator .
    Flash steam boiler .
    Steam superheater .

    I've never heard of a flash steam boiler with a single straight pipe though - real ones always have the tubing coiled up in some way so as to make the boiler compact and efficient . A single straight pipe would have to be hundreds of metres long which is clearly impractical .

    Please confirm that this design work is just for your personal interest or some academic exercise . If you actually intend to manufacture steam generating equipment of any kind then I'll have to ask the mentors to close the thread on the grounds that it would be a far too dangerous a project to discuss on PF .
     
    Last edited: Sep 13, 2017 at 4:56 AM
  4. Sep 13, 2017 at 11:07 AM #3
    Well the amount of steam that will produce is low, but it uses 5 tubes, around 13 m long. The idea was to have water injected through them and have a heated and enclosed ambient in which flows combustion gases to heat the pipes and evaporate the water that flows inside it. The thing is I need to find up the temperatures and the mass flow of those combustion products to ensure that the process happens.

    It's no commercial or something similar, is a proposal of design for my thesis to recieve my degree as mechanical engineer. It should work this design, I just need to get a quality higher than 80% as a goal, but I need to ensure that is possible, to get that heat flux to the water and that the pipes arent going to melt during operation. Of course i'm simplifiying the design as it isn't adaptable to different sizes I give because it will mean a recalculation of all parameters I have already, it will not aboard external operating parameters, nor water threatment, nor instrumentation and control, just to ensure that it is possible to get this proposal to work.

    The thing about lierature that I have avaliable is that most things are correlations to boilers wich have drums, uses more tubes, have reheaters and many more accesories, and have lower pressure based operations while I just have 5 pipes that get water and one that throws Combustion gases around those pipes, all inclosed in another pipe, also I would have to propose a combustible in a fluid phase. Still, I'll do a search for information again.

    Thanks!
     
  5. Sep 14, 2017 at 2:45 AM #4

    Nidum

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    Please post a drawing of the system so that we can properly understand what we are dealing with .
     
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