Help understanding air draw and venting?

[Neil2]

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.

[Aegis]

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.

[david s]

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.

[GianniFocaccia]

Re: Help understanding air draw and venting?

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

[sharptailhunter]

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