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Old 03-06-2012, 10:12 PM
GianniFocaccia's Avatar
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Default 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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Old 03-07-2012, 02:43 AM
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Default 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.
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Old 03-07-2012, 11:01 AM
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Default 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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Old 03-07-2012, 11:42 AM
Apprentice
 
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Default 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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Old 03-07-2012, 12:41 PM
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Default 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.
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Old 03-07-2012, 04:57 PM
Peasant
 
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Default 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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Old 03-08-2012, 12:16 PM
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Default 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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Old 03-08-2012, 02:53 PM
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Default 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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Old 03-08-2012, 05:22 PM
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Default Re: Effectiveness of Double Flue

Quote:
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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Old 03-08-2012, 05:43 PM
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Default 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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