Effectiveness of Double Flue

[shuboyje]

Re: Effectiveness of Double Flue

I've got a spread sheet on my computer, but I also like the calculator at engineering toolbox:

Air Flow and Velocities due to Natural Draft

I swear they ha an imperial version, but I couldn't seem to find it. Luckily the conversion tools are right there on the side of the page.

That is also a great point made by Neil. The book also covers that very early on. It's not via a brick oven, but a type of reverbatory furnace that functions the exact same way. We are not using the draw in a traditional sense where it pressurizes the firebox, we are simply using it for the removal of gases.

[Neil2]

Re: Effectiveness of Double Flue

Remember that a wood fired oven will work perfectly well with no flue at all.

[GianniFocaccia]

Re: Effectiveness of Double Flue

This forum is awesome, so are the open source projects, questioning the questions and the answers till get great advances

Dmendo - You're right: this forum is a valued resource through individuals willingness to share knowledge, experience and ideas to evoke progress. Thanks for sharing the diagram of the Teplushka oven and how it it becomes 'tuned' via multiple damped intakes. I believe ovens are similarly tuned using a blast door during firing and and/or a damper on the flue during operation.

Thanks for the numbers, Shuboyje. Your explanation was fully understandable and it would be a bad day indeed if the ambient temperature ever did exceed the temperature of anyone's oven's outgoing flue gases.

Can you recommend a formula for calculating draw pressure a given pipe will yield given it's dimensions? I would like to chart a curve comparing my double-barrel flue's capacity to an 8" round flue at the various ambient temps we get here in southern california. Also, I would like to identify the optimum/minimum length the DB-flue should be. If there is a site with the formula you can direct me to it would be greatly appreciated.
John

[shuboyje]

Re: Effectiveness of Double Flue

I once again re-read the section on diverging flows, and still conclude it has no application in this situation. To understand what is going on here you need to understand draft. Hot gases have a lower density then cool gases, and hence they rise. The greater the temperature difference, the greater the force. Pretty simple really. Heat rises.

The issue described is a gas flow Q is divided into ascending flows Q1 and Q2 which then converge back into flow Q. For simplicity lets call the final flow Q'. So we have Q diverging into Q1 and Q2 and converging back into Q'. The issues arises when due to system dynamics one of the sub flows, Q1 or Q2, becomes cooler then Q'. The air in that subflow will no longer rise because it is no longer of a lower density then the air above it. Make sense?

The double barrel flue is a different situation. You simple have a flow Q diverging into Q1 and Q2 and not converging. In this situation Q' is essentially the outside ambient air. If your flue gases ever end up cooler then your ambient air temperature you have bigger issues at hand, lol. I think it is obvious to everyone that in the double barrel flue situation the flue gases in both Q1 and Q2 will always be hotter then the gases at Q' which is ambient outside air, hence the density will always be lower then the air above it and the flue gases will always ascend.

As for the 24" double barrel flue here are some numbers for you.

A 3' long 8" flue with 1000F flue gases and 70F ambient air would move 374 cfm.

A 2' long 6" flue with 1000F flue gases and 70F ambient air would move 166 cfm. Obviously 2 of those would be 332 cfm in an ideal situation, like I mentioned before not sure you would actually get that.

[Dmendo]

Re: Effectiveness of Double Flue

This forum is awesome, so are the open source projects, questioning the questions and the answers till get great advances ...

David s: you are right with the ventilating flue, it divides a flow of gas from common inlet, but appliying "the Flow of gases" theory again, this situation is a descending gas divided flow, and this is a good design practice as things happens here for self adapting to good performance. In an oven derived from this theory, the teplushka oven, the oven chamber have even 3 gas exits at the hearth level that re-joins in an under hearth chamber, or bell, before a common flue exit. This way everything works fine.

"Key
a: Fire box
b: Throat or in channel from fire box to bake chamber.
c: Out Channels, from the bake chamber to the lower chamber
d: Hearth support pillars
e: Hearth
f: Loading Opening
g: Shut off Damper from the avaloire
h: Shut off Damper from the under hearth smoke path
i: Ash box/ air intake door
J: Chimney
Red line-- smoke path during remote firing. Yellow line -- smoke path during direct firing. Blue line -- Primary Air.
Note: In this diagram the vault of the bake chamber is not shown"

Regarding the engines valves, they make dual, and even triple valves, because there is no space for make them bigger, the spark plug and the cilinder limits the radial size, so the only way to increase the valve area for quicker gas exit is to duplicate the number. And here, a tremendous presure and a pushing piston puts everybody in its place.
Regards

[Amac]

Re: Effectiveness of Double Flue

Hi John

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!

oops! - of course you (and David) are right - my math (or mind) is rusty - formula was OK (2/3*pi*r^3) but of course I fed the wrong radius in (18.5 instead of 19.5) - lucky I haven't fitted the flue yet

[brickie in oz]

Re: Effectiveness of Double Flue

with inlet from each room

I assume they mean vent the actual room?

[david s]

Re: Effectiveness of Double Flue

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

[david s]

Re: Effectiveness of Double Flue

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.

[brickie in oz]

Re: Effectiveness of Double Flue

It is common to see twin pots on the top of many chimneys. Looks good to me.

They usually service one flue each.

[david s]

Re: Effectiveness of Double Flue

A 39" is 92% of a 42" diameter
86% of area
And. 80% of volume

[GianniFocaccia]

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

[Dmendo]

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

[Amac]

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

[david s]

Re: Effectiveness of Double Flue

It is common to see twin pots on the top of many chimneys. Looks good to me.