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  #11  
Old 03-08-2012, 10:28 PM
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Default Re: Effectiveness of Double Flue

Quote:
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  
Old 03-09-2012, 02:34 AM
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Default Re: Effectiveness of Double Flue

It is common to see twin pots on the top of many chimneys. Looks good to me.
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  #13  
Old 03-09-2012, 07:29 AM
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Default Re: Effectiveness of Double Flue

John
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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.

Quote:
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
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  #14  
Old 03-09-2012, 01:12 PM
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Default 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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Old 03-09-2012, 08:01 PM
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Default 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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  #16  
Old 03-09-2012, 09:12 PM
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Default Re: Effectiveness of Double Flue

A 39" is 92% of a 42" diameter
86% of area
And. 80% of volume
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  #17  
Old 03-10-2012, 12:06 AM
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Default Re: Effectiveness of Double Flue

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Originally Posted by david s View Post
It is common to see twin pots on the top of many chimneys. Looks good to me.
They usually service one flue each.
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Last edited by brickie in oz; 03-10-2012 at 12:09 AM. Reason: Adding pic
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  #18  
Old 03-10-2012, 12:16 AM
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Default Re: Effectiveness of Double Flue

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Originally Posted by david s View Post
A 39" is 92% of a 42" diameter
86% of area
And. 80% of volume
Interestingly if you plug in the numbers for a 36" and a 21" diam ovens you get the following

A 36" is 86% of a 42" diameter
............73% of area
.....and. 63% of volume

A 21" is 50% of a 42" diameter
............25% of area
......and 12.5% of volume
Assuming all domes are round in plan and hemispherical in form.
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  #19  
Old 03-10-2012, 12:28 AM
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Default Re: Effectiveness of Double Flue

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Originally Posted by brickie in oz View Post
They usually service one flue each.
Interesting old drawing Al. It also says "common ventilating flue with inlet from each room" Seems it's like the manifold of a petrol engine feeding into one pipe.
Modern engines work more efficiently with two exhaust valves for each combustion chamber rather than one. Shouldn't it work for an oven too?
Shuboyje where are you?
Dave

Last edited by david s; 03-10-2012 at 12:34 AM.
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  #20  
Old 03-10-2012, 12:36 AM
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Default Re: Effectiveness of Double Flue

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Originally Posted by david s View Post
with inlet from each room
I assume they mean vent the actual room?
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