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Does the Rankine Cycle really need a liquid phase?

  1. Jan 15, 2012 #1
    A question about Thermodynamics, if someone would help with that...

    I understand a steam engine that follows the Rankine cycle has basically 4 steps: (a) heating in a boiler; (b) expansion on a piston or turbine; (c) condensation; (d) pump back into the boiler.

    For many years I have been curious why step (c) involves condensation. Couldn't I change (c) to be cooldown of the steam to a temperature just above boiling, and therefore avoid the waste of heat due to evaporating the water again? The usual explanation I find about this is that "condensation avoids the problem of controlling a two-phase mixture, and decreases the energy spent in pumping back to boiler".

    That explanation is fine, but it leaves me very curious why I never heard about any single steam engine that used a more efficient cycle by avoiding the liquid phase. I mean, if (c) = cooldown then that's closer to Carnot cycle, isn't it?

    Any help with that?
  2. jcsd
  3. Jan 15, 2012 #2


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    Staff: Mentor

    Why in your last paragraph do you ignore the pumping energy? Why do you think that not condensing would be more efficient when you know (you said earlier, at least) that the pumping energy is greater for a gas than a liquid?
  4. Jan 16, 2012 #3

    My thinking around that is from my poorly drawn diagram below. Red is the Carnot cycle, blue is the Rankine cycle and pink is the hypothetical cycle I'm asking about.

    So, if I take a Rankine engine and stop the cooling short of full condensation, then my curve becomes closer to that of a Carnot cycle, therefore it should be more efficient.

    I mean, the Carnot cycle has a compression stage, and yet it yields maximum efficiency, right? So to use the condensator as a means of avoiding a compressor is a nice trick of doing that with a much simpler equipment, so that was very cool for the 1800s, but nowadays we can just put a pump in there?


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  5. Jan 16, 2012 #4


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    Staff: Mentor

    Wait - your diagrams don't match your description in the OP. You said "cooldown of the steam to a temperature just above boiling". That isn't what is happening in any of the cycles you drew. All cool down to boiling and partly, mostly or completely condense.
  6. Jan 16, 2012 #5

    Oh... good point. So there's no way of avoiding condensation

    So a Carnot cycle with saturated steam will also involve partial condensation. Got it now - thank you, Russ, appreciate the explanation.
  7. Apr 28, 2012 #6
    Hi. I'm still learning about this. Wouldn't there be need for some sort of check valves used to keep the heated fluid from expanding back into the condenser? Also, wouldn't the fluid pump have to work hard to pump the cold liquid into the boiler...because there is pressure being produced in there?
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