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Chimney transition - Forno Bravo Forum: The Wood-Fired Oven Community


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Chimney transition

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  • Chimney transition

    Hi All, I'm at the final design-build stage of my chimney transition. I've a thread "Steel Dome Oven" but thought I might get more opinions with a separate posting here under chimneys.

    So here goes, I'm hoping someone will jump in and tell me where I'm missing something or misunderstanding what is happening. Or on the off chance, that I have it right.

    It seems most of the ovens here have an area between the outside of the entrance and the actual entrance to the dome with an enlarged roof area. The area I'm talking about is where the arch of the entrance opens into the bottom of the chimney, usually it's a few missing or shortened bricks and the chimney sets upon a brickwork/ cast base/ steel hood assembly above these missing bricks. The cross section of this space seems larger than the cross section of the chimney itself. If this is a correct observation then this area is serving as a "hood" to collect the smoke before it exits the chimney.

    Ok, now for the tricky bit: isn't this in effect a funnel upside down? I mean we are all familiar with a funnel and how it works. As long as one pours something into a funnel at a rate slower than or equal to the rate it exits the funnel, the funnel doesn't over fill. With a fluid and a funnel if one pours faster than it can exit it slops over the side and makes a mess, with a chimney hood if the smoke enters it faster than it can exit it exits via the front of the entrance making for the soot stains on the brickwork.

    So I know that gas dynamics and fluid dynamics aren't exactly the same but doesn't it seem reasonable that if the opening into the chimney was slightly smaller in cross section than the cross section of the chimney the hot gasses would have a place to expand into creating a draft rather than being crowded and possibly backing up like they would be in the funnel analogy?

    In the oven I'm building it's easy to cut a slot into the top of the tunnel and it is as complex one way or the other to make the transition piece a funnel with a larger cross section than the chimney or a slot with a slightly smaller cross section. It just seems to me the volume of this funnel would be so small that it would be readily filled and spill out the front and that it would be better to have the draft the smaller opening would create.

    For dimensions I'm talking about a 6 inch chimney with a cross section of 28.27 sq inches and a 3x9 inch slot (27 sq inches).

    Thoughts, opinions welcomed.

  • #2
    Re: Chimney transition

    not 100% sure of what you are talking about but a chimney works that as it heats up it actually pumps the smoke up the hotter it gets the more volume it can move.
    if you want to stop smoke spill out the front of an oven light a paper fire up the chimney first.
    as for the funnle idea it does not work so well for smoke as surface area is increaced and that ceates longer heating time thus longer drawing time.
    the chimney needs to heat to draw.
    thats my thoughts anyway



    • #3
      Re: Chimney transition



      is a flow rate calculator for chimneys. However, we have not yet identified the lowest rate which will draw well. It should give you a little information to work with.

      Sharpei Diem.....Seize the wrinkle dog


      • #4
        Re: Chimney transition

        The inverted funnel is traditional. We had a builder use a rather thin slot at the top of the entry, but I don't remember if s/he reported back on it's effectiveness. I think the most important variable in the height of the chimney, but then I have a really tall chimney.
        My geodesic oven project: part 1, part 2


        • #5
          Re: Chimney transition

          Thanks Brokencookie,
          I saw your calculator and actually posted on that thread. Lots of variables on this chimney thing and I wish that more people would respond with details on their ovens. I'm really close on "design proportions" of interior height to entry height for theoretical best air/hot gas flow. And certainly once the chimney heats and starts to draw and the smoke turns to more fully conbusted products most chimneys should draw well. It's that first bit of the firing where the air/gases haven't established their path where I expect most of the sooting problems occur.

          Hagrid, thanks for replying,
          I've heard the burning paper stuffed up the chimney thing before and actually saw an amazing bit of ingenuity once in Ireland with a full sheet of newspaper across a "standard" fireplace to get it to draw and "blow" on the fire at the same time. That actually might be worthwhile with these ovens to establish flow. Simply start the fire and spread a sheet of newspaper across the hearth opening side to side top to bottom, grabbing the fold at the bottom center with one hand and with the other holding the top edge of the paper against the front top of the hearth. Gently pull up and a bit out with the hand holding the fold and the draft will blow quite strongly on the fire. The outer edges of the paper will suck down tightly around the hearth and the paper will turn brown as the fire develops and the paper bakes. Once the edges of the paper are sucked tight against the front edges of the hearth you can stop holding the top. Then after a few minutes and the fire is going well, in one smooth action, you simply pull the paper outward until the edges clear the sides and top of the hearth and let go. The paper is sucked into the fire and instantly bursts into flame and vanishes leaving a nicely drawing and burning fire. Quite dramatic and very effective.


          • #6
            Re: Chimney transition

            Two observations:
            1. Making the entry into the chimney from the funnel smaller than the chimney creates turbulence. From a laminar airflow perspective, turbulence is a bad thing. Turbulence is why a square or rectangular chimney cross-section is much less efficient than a round one of same area.
            With your slot idea, the little bit of extra velocity (if any) from expansion of the exhaust gases into the larger chimney would be lost to turbulance. As your chimney got hotter and created higher velocity, turbulence would increase logrithmically. In effect, the chimney would draw slightly less well at higher temps than it would if a smooth transition.
            2. Having said that, Rumsford chimneys actually have something similiar to your slot idea at the top of the fire-box...then expands into the chimney. On most of them, there is really isn't that big of a difference in the area of the slot vs. the chimney but more of difference in the configuration of the space. A good Runford draws well because there is the extra oomph from the height of the chimney .... a bad one will smoke like my ole gran-pappy.
            Paradise is where you make it.