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Does steam rising in an insulated tube lose thermal energy

  1. Jan 21, 2012 #1
    I want to propose an interesting thought experiment, but don't want to
    A/ make a bigger fool of myself than I normally do
    B/ well that's about it really

    The premise behind my experiment is that rising steam does not lose thermal energy if we ignore thermal losses both radiative and conductive.

    I know that's an odd thing to say but its important in this context.

    So my pre thought experiment question is this.

    If I heated steam in a perfectly thermally insulated container which led to a perfectly thermally insulated riser (tube, pipe, chimney or whatever you want to call it) does the water vapour lose thermal energy as it rises?

    Assume the following...
    1/ riser is open at the top
    2/ sufficient time has passed such as the tube only contains water vapour in the form of steam. (the walls of the riser are at the same temp as the steam and therefore no condensation).
    3/ perfect thermal insulation of all components (not possible, but just humour me)
    4/ steam that exits the riser is ignored.

    In the above scenario, does the water vapour lose thermal energy as it rises. if so why and how?


    The source of steam is water boiling in the container with energy supplied by an external source. As the water boils, the steam generated displaces the contents of the riser, thus water molecules as steam vapour rise up the tube.

    Last edited: Jan 21, 2012
  2. jcsd
  3. Jan 21, 2012 #2
    It seems to me that the steam is rising, and the energy for that has to come from someplace.
  4. Jan 21, 2012 #3
    The steam comes from the heating of water in the container. as the water turns to steam it expands thus filling the riser and displacing the previous contents of the riser (out the top).

    Sorry I should have said heated a container of water till it boiled, thus emitting steam.

  5. Jan 21, 2012 #4
    The pressure at the top of the tube is atmospheric pressure. The pressure inside the container where the water is heated is above atmospheric pressure (which is what causes the steam to be pushed through and out of the tube.) As the steam rises through the tube its pressure drops, and the steam expands. Assuming the tube is perfectly insulated and thermal conduction though the steam along the tube is negligible, the expansion of the steam lowers the temperature of the steam.

    The thermal energy isn't lost, it is converted into work, and it is that work which pushes the steam through the tube.
  6. Jan 22, 2012 #5

    So as the steam rises (against atmospheric pressure and gravity) thermal energy is not lost. Of course the temperature drops due to the larger volume the vapour occupies. This would also happen within the tube itself as the vapour expands to fill the tube.

    That's exactly as I assumed it would be, but I am looking for losses of energy other than thermal loss due to imperfect insulation. Of course I realise once the steam exits the tube it will mix with the atmosphere and losses would then start via both convective and radiative.

    Any more comments on energy losses anyone? I am particularly concerned about the vapour as it rises UP the tube. I have had an astounding thought and I want to be 100% sure of it before I potentially make a fool of myself.

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