#1  
Old 01-19-2012, 09:24 PM
Serf
 
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Location: Idaho
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Default Help understanding air draw and venting?

I'm seeking knowledge about the way a chimney draws air from the oven chamber. I understand that hot air rises, therefore once it leaves the oven proper it will most likely rise up through the vent into the flue/chimney. Now, what are the physics that enhance that direction of air flow? Specifically, I want to know how does the size of the vent opening effect air flow? Also, what about flue size, i.e. diameter and length? What about the flue shape, e.g. straight wall from vent arch all the way up, vs larger at vent arch and tapering all the way up? Is there a specific formula that one should observe in regards to oven volume compared to flue size?

Thanks, Alex
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Old 01-20-2012, 05:23 PM
GianniFocaccia's Avatar
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Default Re: Help understanding air draw and venting?

Quote:
Is there a specific formula that one should observe in regards to oven volume compared to flue size?
Sharptail,

There are no formulas available specifically for flue size and configuration that I know of. There are many, many threads such as yours inquiring on these and other perameters of flue design. If you're gone through the FB plans and threads with the word 'flue' in them, take some time and look through the pictures to see what most builders have done. The telltale sign of an inadequately designed flue is soot marks on the face of the oven enclosure above the outer arch.

The only general guidelines I know of are that a 36"-42" oven requires an 8" ID minimum flue to draw sufficiently. Also, since rising smoke swirls as it rises, the most efficient shape for an oven flue is round. The next most efficient shape is a square, and lastly, a rectangle. You can Google pressure of a cylinder and get numbers for round flues based on their diameter and length. It is generally held that the longer of two flues of the same diameter will have the higher pressure and better draw. For a WFO application, I don't think it is that critical. The most important aspect, IMO, of flue design is to make it deep enough (front to back in the entryway) and wide enough to allow the smoke to collect in an inverted funnel shape. Otherwise it blows out the front and makes a mess.
John
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Old 01-20-2012, 05:39 PM
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Default Re: Help understanding air draw and venting?

Basically as you noted, hot air rises, creating draw, which innturn sucks more oxygen in to assist the combustion. More draw equals more oxygen for the fire to feed on. The larger the volume in the flue, the larger the draw. So this can be achieved by making the flue taller or a larger cross section (diam. if flue is round) or both.
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Old 01-20-2012, 06:56 PM
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Default Re: Help understanding air draw and venting?

I used 8" square terracotta flue with a funnel made from two pieces of flue cut on a diagonal. 14" wide at the bottom to 8" square on top. I have 6 feet of flue on top of that funnel piece. Even when I had only four feet of flue on top, the curing fires and first pizza fire, all the smoke went up the flue. None out of the front, outer arch area. This design, IMHO, is perfect for a 42" oven. I also cut the lower vent bricks to also funnel the smoke up to the funnel. I have had many fires, with all sorts of weather, wind direction and a clean outer arch.
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Old 01-21-2012, 01:51 PM
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Default Re: Help understanding air draw and venting?

I'm going to disagree with some of the above.

The design of the wood fired oven is such that the burning gasses are held within the top of the oven until they have given up their heat to the bricks. This is the reason behind the 62% ratio between the height of the opening and the interior height of the ceiling.

The oven will work at maximum efficiency with no vent/flue at all. No drafting is required or wanted. You will find many pizza ovens in Italy and Croatia with no vent.

Having said that, In my opinion, the only purpose of a vent/flu is to deflect outgoing gasses and smoke away from your face while you operate the oven. If you cook like I do, which is to use a large fire to come to temperature, push the coals to the back and not to add any wood while cooking pizza, and don't mind a bit of soot on the face of the oven, then a small vent is sufficient. If you add wood continuously while cooking, you may want a larger vent.

Last edited by Neil2; 01-21-2012 at 01:59 PM.
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Old 01-21-2012, 08:25 PM
Serf
 
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Default 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!
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Old 01-22-2012, 03:30 AM
Peasant
 
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Default Re: Help understanding air draw and venting?

Quote:
Originally Posted by GianniFocaccia View Post
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
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Old 01-22-2012, 01:46 PM
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Default 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.
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Old 01-22-2012, 02:51 PM
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Default 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
Attached Thumbnails
Help understanding air draw and venting?-short-flue.jpg   Help understanding air draw and venting?-short-flue-2.jpg  

Last edited by GianniFocaccia; 01-22-2012 at 03:28 PM.
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Old 01-22-2012, 03:35 PM
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Default 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.

Last edited by azatty; 01-22-2012 at 03:41 PM.
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