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Choice of steam Turbine/Expander?

  1. Sep 5, 2010 #1
    Hi!

    It's a real problem where we today have a steam throttle and would like to change it for a steam turbine/expander to provide some electricity.

    The problem is that the steam is at condensationline:
    16 [bar] – T=201,37 [C] – h=2793,65 [KJ/kg]– s=6,4216 [KJ/kg*K]

    with an isentropic process over the turbine/expander the steam would get to:
    p = 6.6673, v = 0.2693, t = 163, x = 0.9421, h = 2.6395e+003, s = 6.4216
    you see that the steam will be wet!

    the above gives the Power:
    P= (efficiency) * 12 [kg/s] * 154.15 [kJ/kg]= 1849.8 kW
    the efficiency will lower the power alot.

    Now my question is if someone have solved something similar or know what to choose? Is it possible to use a steam turbine when the steam is this wet? Can a double screw expander be an alternative? We have to regulate the outlet to 163C, will this influence oour choice? What retailers exists?

    Thank you very much for your help,
    Best regards,
    Martin
     
  2. jcsd
  3. Sep 6, 2010 #2
    The steam table I used gives me a different answer:

    Inlet conditions:
    16 bar(abs) - 201.4 C (sat) - h = 2792 kj/kg

    Exhaust
    6.667 bar (abs) - 163 C (sat) - h = 2760 kj/kg
    Normally, when finding the exhaust state point, we have to use the enthalpy at the saturation line, quality x = 1, not after condensing some of the steam.

    This gives a delta h of only 32 kj/kg. Or a theoretical steam rate of 21.88 kg/kw-hr.

    With a flow of 12 kg/s or 720 kg/hr, the available energy I calculate is 720 kg/hr / 21.88 kg/kw-hr = 33 kw.

    Please correct if someone finds a mistake...

    My experience is with steam turbines. You may be hard pressed to get a turbine to do what you want at such a low flow.

    As you know, a turbine doesn't actually follow isentropic expansion. A very small turbine might only be 30-55% efficient. The efficiency deficit is realized by the turbine creating less than theoretical power, and this results in a higher than theoretical exhaust temperature. One way to think of it is that all the heat that could be extracted from the steam was not. So if the turbine was exhausting steam at 6.667 bar(abs) at 170 C, it may be necessary to cool it down to saturation temperature somehow.

    Wet steam coming into a turbine is not preferred; it would be considered severe duty. Exhausting saturated steam is not unusual. It is best to keep the moisture below 2-3% with these small single stage turbines or else you can start to have bucket erosion problems.
     
  4. Sep 7, 2010 #3
    thank you for your answer!

    What I have learned is that you can say that the turbine acts like an isentropic process and then in reality like you say it doesnt. If it is like you say that you should fallow the saturation line and delta h is 32 kj/kg. The effect would be 32 kj/kg*12 kg/s=384kW.

    after the turbine I would realy preffer wet steam than overheated. What the turbine should regulate against is the temperature so if the exhaust steam gets warmer with the losses it might be intresting to lower the pressure even more.

    Do you know any company that makes solutions to this kinds of problems?
     
  5. Sep 7, 2010 #4
    oops, I made a simple mistake with the steam flow. 12 kg/s = 43200 kg/hr. Which is a big difference in my mind. Sorry for that.

    And yes, you're right, the theortical power would be as much as 384 kw as you said. Thanks for the correction, so the theoretical steam rate would be more like 112 kg/kw-hr.

    Ok, it seems with this, you may be interested in a desuperheater. I am nowhere near an expert on these devices, but the first link I found on the subject may be a place to start.
    http://www.documentation.emersonpro...uments/articles_articlesreprints/ag365652.pdf
     
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