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Effectiveness of Double Flue - Forno Bravo Forum: The Wood-Fired Oven Community

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Effectiveness of Double Flue

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  • Effectiveness of Double Flue

    My younger brother, who works on very large steel jobs (ie:bridges) called today to report he has a leftover 6' length of 6" round stainless steel that is 1/4" thick. (I forgot to ask him if it was 6" id or 6" od). I told him I would take it and he promised to cut it in half, weld the two 3' pieces together in parallel, and weld a base plate to the perimeter on one end.

    I do not anticipate any difficulty building a vent to accommodate this (heavy) double flue, but wanted to gather input from anyone with thoughts on this configuration and any suggestions/warnings in doing so. My oven is a 39" Pompeii.

    Thanks in advance.
    John
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  • #2
    Re: Effectiveness of Double Flue

    Sounds pretty indestructible. Only warning I can give is to make sure you don't surround it with anything that doesn't have some give in it. The second and third ovens I built both had cast refractory around the flue pipe. It was about 1.2 mm from memory. Both cracked the refractory as the thickish stainless expanded before the refractory. The same would go for stucco applied around the pipe. I now use a thinner gauge and surround it with vermicrete which is slightly elastic. No more problems.
    Kindled with zeal and fired with passion.

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    • #3
      Re: Effectiveness of Double Flue

      I also surrounded mine with a layer of vermicrete where it joins the vent opening. Not only does this minimize expansion cracking as Dave notes, but also improves the efficiency somewhat by insulating the flue.

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      • #4
        Re: Effectiveness of Double Flue

        It's all about airflow, as you know. I see double stacks on large smokers all the time. I don't think you'll have any problem with it, although I think your transition should be funnel shaped rather than have the two pipes directly over the oven entry. But you probably already thought of that.

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        • #5
          Re: Effectiveness of Double Flue

          two six inch flues have an exit area of (2*3^2*3.14) = 56.5 inches
          one eight inch flue has an exit area of 4^2*3.14 = 50.3 inches

          The two flues together are about the same as having a single 8.5 inch flue. So you don't gain much venting capacity from the double flue vs. a single larger flue. I'd say it makes sense only if you like the aesthetic, or just because the price is right.
          My build progress
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          • #6
            Re: Effectiveness of Double Flue

            According to the theory of flow of gases, by Groume-Grjimailo, is a bad design principle subdivide an ascending hot gas current (page 88 of the book): http://www.archive.org/download/TheF...InFurnaces.pdf
            It's proven many times, the flow of gas ends passing trough only one of the channels due to tiny differences in the temperature of the gas.
            All the ways the look of the design would be amazing.
            Regards

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            • #7
              Re: Effectiveness of Double Flue

              Thanks for the input, guys. Indeed, the plan is to construct a funnel-shaped vent with a level top to set the flue(s) base plate onto. My original thought was to mortar a course of firebrick around and on top of the base plate with a 1/8" gap all around (sides and top) to allow for plate and flue expansion. I'm now considering using insulating firebrick for this first course under a second/final course of standard firebrick. The entire length will be insulated and covered with cement board and cladding.

              I am not enamored with the look of a 3' tall flue but without the pressure data to indicate otherwise, think this length is needed to get the required draw from a 6" O.D. pipe. I'd love to hear someone tell me a 24" double-flue would work as well as a 36" one.

              Dmendo, thanks for the reference to the Theory of Flow Gasses in Furnaces.

              John

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              • #8
                Re: Effectiveness of Double Flue

                I've skimmed through a large portion of the book mentioned a few posts back, and it's a great read. I think it's age may yield a lot of useful information since it focuses mostly on solid fuel furnaces. That said it's section on divided flows does not in any way apply to the double barrel flue presented here. In this flue the gases entering at the bottom while in operation will always be VASTLY hotter then the ambient outside air at the top, and therefore during operation the gases will always flow upward through both chamber regardless of slight temperature differences.

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                • #9
                  Re: Effectiveness of Double Flue

                  That said it's section on divided flows does not in any way apply to the double barrel flue presented here. In this flue the gases entering at the bottom while in operation will always be VASTLY hotter then the ambient outside air at the top, and therefore during operation the gases will always flow upward through both chamber regardless of slight temperature differences.

                  Shuboyje, I was hoping you would chime in! For the life of me, I could not imagine how a flue like the one I'm proposing would not get very hot and literally suck the exhaust out of the oven. Thanks for your insight.
                  John

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                  • #10
                    Re: Effectiveness of Double Flue

                    Yeah, that section talks about flows which divide and then recombine. In that case the pressure difference between the two sides of the split is so small that slight changes and differences in temperature could actually cause the gases to flow backwards in one of the channels. It's really interesting, just not applicable to this situation. I'd bet if you figure 75% of 6" * 2 you would be fine and probable do better in the real world. I say 75% because I think you would lose a little bit do to turbulence where the two flows split at the entry and collide at the exit.

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                    • #11
                      Re: Effectiveness of Double Flue

                      In that case the pressure difference between the two sides of the split is so small that slight changes and differences in temperature could actually cause the gases to flow backwards in one of the channels.
                      Now that's really interesting! I would have loved discussing this phenomenon with my late father-in-law, the MIT-educated heat-transfer specialist.

                      I had calculated the I.D. of a 6" O.D. tube (1/4"-thick) as 5.5", which is 92% of 6". My 39" oven is coincidentally 92% of a 42" oven which an 8" flue accommodates. I hadn't even considered there are losses associated with splitting and re-joining turbulence, but it makes sense. Funny, the kinds of things one is exposed to on this forum.

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                      • #12
                        Re: Effectiveness of Double Flue

                        It is common to see twin pots on the top of many chimneys. Looks good to me.
                        Kindled with zeal and fired with passion.

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                        • #13
                          Re: Effectiveness of Double Flue

                          John
                          My 39" oven is coincidentally 92% of a 42" oven which an 8" flue accommodates.
                          If you compare the floor areas the percentage is 77% and maybe even more important the volume of the 39" is only 68% of the 42".

                          So a flue of 8" diameter has a crossectional area of 50.2 square inches and 5.5" has a crossectional are of 23.75.

                          That ratio is 47.2% - so two of them will get you 94% of an 8" flue.

                          If the dome volume in relation to flue diameter is the important item, then it would appear that you have way more than enough.

                          It is common to see twin pots on the top of many chimneys.
                          Not normally from the same fireplaces David I would say - whether that makes a difference in this case I couldn't guess

                          I'm coming to the same stage shortly myself - interested to see how it works out.
                          Aidan
                          Amac
                          Link to my WFO build

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                          • #14
                            Re: Effectiveness of Double Flue

                            GianniFoccacia... you have disturbed my all day thinking about this matter...
                            My intuition says that the shotgun style flue has to work nice. But, once readed your inital post came to my memory a dual air intake for a firebox someone designed in rocket stoves forum: two metal pipes emerged vertically from a common horizontal channel and rised separated vertically inside the fire chamber for preheat the combustion air. It doesn't worked at all. Separating the pipes circuits in two single air inputs solved the design and worked fine. So, then remembered that it was predicted in "The flow of gases in furnaces" book. The given explanation involves the energy loosed by the gas flow (kinetic and thermal) as slower becomes the jet the more heat it looses as more time is touching the walls, and as colder it gets, becomes slower... starting a vicious circle. In effect this situation is quite different as things happens in "open air" start and end conditions and only the thermal draft has to make the work... or maybe not and I can imagine half of the smoke escaping to your eyes... It has to be tested.
                            So my advice is: have ready a B plan in the case of Murphy's law affects the performance. I mean, make it in a way that if doesn't perform well you can arrenge an easy change to a single flue.
                            Regards

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                            • #15
                              Re: Effectiveness of Double Flue

                              Thanks for the calc's Aidan. Can you share with me the formula you used to derive 39" = 68% of 42" (volume)? All I could find for volume of a sphere was: V=4/3*Pi*r3, and the answer (halved) was 80%. I like your answer better!

                              Dmendo, sorry about the disturbed thought - I am exactly the same way and have spent more than one sleepless night thinking about elements of my oven construction during my build. Good advice, though: test the effectiveness of the flue before covering it up. This I plan to do, with insulation, since the pipe should heat up rather nicely and affect the draw.

                              Shuboyje, do you think a 24" double-barrel flue on top of a 12"-tall vent would operate sufficiently?

                              John

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