Help understanding air draw and venting?

[Tscarborough]

Re: Help understanding air draw and venting?

That is the way it works. I thought I had the charts electronically but only have them on paper. The only thing that is not common sense is that square and rectangular flues are measured by outside diameter and round flues are measured by interior diameter. Don't ask me why.

[shuboyje]

Re: Help understanding air draw and venting?

Actually I believe q is based on the hydraulic diameter, which ends up being the diameter when it is a circle but is calculated via 4A/P where A is the cross sectional area and P is the perimeter. Hydraulic diameter essentially converts any shape to the round equivalent.

[azatty]

Re: Help understanding air draw and venting?

It applies to airflow no matter what the purpose. I have the information at work and will post the tables and charts tomorrow.

The formula to calculate "q" (volume of air moved) uses the diameter of the flue and pi, so it's treating everything as a circle anyway. Then with the square flue, you probably have added air friction from the dead space in the corners.

[brickie in oz]

Re: Help understanding air draw and venting?

That is as close as anyone on this sight has come to cosigning the "old folks method" of preheating a chimney/flu before lighting the fire

Man I can just hear the banjos starting up.......

[Tscarborough]

Re: Help understanding air draw and venting?

It applies to airflow no matter what the purpose. I have the information at work and will post the tables and charts tomorrow.

[GianniFocaccia]

Re: Help understanding air draw and venting?

Flue shape. A 6" round flue is better than a square 8" flue, and a (clean) steel flue is better than a (clean) terra-cotta flue.

Tom- this is the first time I have heard of this. Does this primarily apply to fireplaces, or any word burning device? Is there a minimum length for this to be true?

John

[Tscarborough]

Re: Help understanding air draw and venting?

Part of my job involves diagnosing issues with new fireplaces. At least 50% of the time it is simply a matter of how they build a fire.

[Gulf]

Re: Help understanding air draw and venting?

How you build your fire. You want a small fire putting out lots of heat to get the stack warmed up first, then it is game on.

T,
That is as close as anyone on this sight has come to cosigning the "old folks method" of preheating a chimney/flu before lighting the fire

http://www.fornobravo.com/forum/9/flue-has-trouble-drawing-when-its-17222-2.html#post125619

[Tscarborough]

Re: Help understanding air draw and venting?

There are tables to determine the optimal flue size VS stack height, but they are for fireplaces not ovens. The important factors for good draw are:

Smooth transition. The better shaped your transition from the entry to the stack, the better a given size will draw.

Flue shape. A 6" round flue is better than a square 8" flue, and a (clean) steel flue is better than a (clean) terra-cotta flue.

How you build your fire. You want a small fire putting out lots of heat to get the stack warmed up first, then it is game on.


If you do not want the chimney stack at the front of the oven, consider building a squirrel tail design.

[sharptailhunter]

Re: Help understanding air draw and venting?

I most definitely plan on having a vent landing area and chimney. I just don't care for the look of a big chimney flue sticking up from a beautiful oven housing. As such, I just wanna know how small/short I can make it.

Side firebox smoker? Now you're speaking my language! I have one too. That's what got me interested in WFOs, I was thinking back then about making a better smoker, a brick smoker. I stumbled onto this site and the budget has been destroyed ever since.

[azatty]

Re: Help understanding air draw and venting?

Flue diameter affects both air velocity through the flue and the volume of air drafted. Chimney height affects air velocity. The volume of air moved is greatly dependent upon the air velocity. Therefore, if your flue is narrow, make the chimney taller to increase air velocity, which then translates to greater volume moved through the chimney.

However, you may have to double (or more) the height of the chimney to make up for the smaller diameter.

[GianniFocaccia]

Re: Help understanding air draw and venting?

Sharptail,

Like Neil says, it is not mandatory to have a flue at all, but there are multiple benefits from including an entryway into your oven build. You are able to incorporate a vent and flue, shield your fire somewhat from any wind, and have a warming/staging area for food going into/out of your oven. The prior benefit of having a flue is to direct heat and smoke upwards out of the chef's face.

I went through the exact same outdoor kitchen decisions you are going through. It is not my preference to have to build, house or look at an extended (36"+) flue, although I do have a 48" (removable) vertical stovepipe on my side-firebox smoker and it draws really well. I still can't decide on a 24" vs a 36" flue length, but I feel either of them will suffice.

The attached photos are of two of the ovens I photographed at the first ever Forno Bravo Expo in 2010. Each of the four ovens (24"-44") operating had flues less than 24". There were soot stains on the front of all four, but I think this can be minimized through employing a vent of proper design (wide and tall). The other option is to employ a removable flue extension.

Hope this helps,

John

[david s]

Re: Help understanding air draw and venting?

If you don't like the look of a pipe and want to keep it short you could leave out the pipe and just have a brick chimney.

[scottsummers]

Re: Help understanding air draw and venting?

The only general guidelines I know of are that a 36"-42" oven requires an 8" ID minimum flue to draw sufficiently.

Mate, is that really that important? I have just built a 36" and am about to fit a 6 inch x 6 foot stainless round flu onto my opening.. Will I get away with a 6 inch or should I go back to the shop now and change up to an 8".
thanks in advance,

Scotty

[sharptailhunter]

Re: Help understanding air draw and venting?

I'm asking because I'm in the planning stage for an outside kitchen area. I want to know just how tall I need to make the chimney. IMO, the chimney is a sore spot for the oven. So, for me, I would like to make it as short as possible but still have adequate draw or flow.

John, I have read many threads about building ovens, to the point my wife thinks I'm obsessed about it. Granted, I haven't searched for flue because I was afraid I would get a billion threads. Maybe I need to search it out anyway. But, correct me if I'm wrong, doesn't the flue induce an area of LOW pressure, not high pressure? I understand Bernoulli's law well. I just was trying to see how it fits in with our ovens, i.e. straight walled flue or tapered, etc. Nonetheless, I'm certain one could get carried away with the ideal gas law and all sorts of laws that pertain to our ovens. But all I really need to know is how small and inconspicuous of a chimney I can make and still have the oven work properly without a sooty arch.

Thanks guys for your replies thus far. This is a great forum, I read it all the time!