Pizza oven vent geometry

[gronbeck]

Hello! I've built earthen ovens in the past, and am endeavoring a brick oven, for a change. I have a question about the vent geometry that I haven't been able to find an answer for, perusing the forums. I'm wondering about what vent geometry might be most effective, considering the interaction of the oven interior, chimney gallery, and front arch opening.

Fig. 1 is a schematic diagram of the oven, a front view of the dome and front arch.

Fig. 2 is a front view showing the vent gallery & chimney (the chimney will probably be 8" diameter and the arch-chimney interface will be custom fabricated metal and sloped to aid in exhaust collection).

Fig. 3 is a side view of the oven with the front arch and vent system.

Fig. 4 is a side view cross-section through the exact middle. The opening will be about 65% of the height of the oven interior.

My question is this: to optimize exhaust / venting, is there a more effective geometry for the front arch and vent system? For instance, a more narrow or deeper front arch? Or a slightly smaller front opening that might induce exhaust to exit via the chimney? Something like Fig. 5, for instance, where another layer of the arch has been added at the very front?

I assume that once you get a good draw going, exhaust will be pulled up the chimney and not exit via the front arch, but are there specific geometric design details / features that will contribute toward funneling the effluent into the chimney? I'm wondering how sensitive exhaust performance is to the specific geometry of the arch & vent.

Thank you for any thoughts, and please feel free to ask for clarification / further details.

Happy cooking,

Christopher
Bainbridge Island, WA, USA

[david s]

Hi Christopher,

You are on the right track. Just as high performance cars and motorbikes have more efficient exhaust extraction, so should wood fired ovens. The most important factor is probably flue pipe diameter. Not sure of your proposed internal diameter, but generally a 6” stainless pipe is ok for ovens up to 36”, over that they should be 8”.

Fabricating in sheet steel works very well because you are not adding a heavy thermal mass which acts like a heat sink sucking heat from the inner chamber. But if you go with sheet steel use at least 304 stainless and thickness should be 0.7 mm which is sturdy enough, yet still easy enough to shape.

Decent funnelling to the base of the pipe will produce a nice path for exit. Unfortunately this creates a deep tunnel entry to the oven mouth which makes working the oven difficult and requires long handled tools.

My rough drawing shows your proposed set up in 1.
A better arrangement in 2.
and better again with a shallow entry in 3.
I hope this explains it clear enough.

You will also need to consider either buttressing or steel bracing (or both) particularly if you are building in brick, because verticals sides of the oven chamber combined with a low dome is far less stable than a hemisphere due to the expansion/contraction cycling. Cast builds not relying on lots of small unit sizes are less susceptible to this problem and are better suited to deviations from the hemisphere.

Dave

[gronbeck]

Dave,

Hi, there...thanks so much for the helpful reply. Much appreciated.

Chimney

• I was originally planning on a chimney internal diameter of 8", but might change to 6" if the resulting 2" shallower front arch would produce a tangible improvement in oven access.
• I had been considering 22-gauge 304 stainless for fabricating the vent scoop, which looks to be about 0.75mm...sounds like that's in line with your suggestion.

Oven Geometry

• I understand what you're saying about the advantages of the flatter dome. I'll give that some more thought, and just need to balance that with the brick construction. I have a moderately experienced bricklayer helping out with construction, so I'll see what he's comfortable with.

Vent Geometry

• I'm going to shoot for a large exhaust scoop with good slope, and it looks like backing that up into the dome a bit more would help minimize the depth of the front arch (which oven users will greatly appreciate), but I wonder if that makes the dome-vent interface more complicated or structurally weaker? Perhaps not...I'll put a little more thought into that. I'd like to better understand the dome-vent interaction, since that seems like an obvious place for problems to occur with differential thermal expansion.

Thanks again for your feedback and illustrations!

Christopher

[david s]

Here's a large Valiorani oven I built which shows such a flue gallery.

[gronbeck]

Thanks for sharing, Dave. That front arch looks like a pretty reasonable depth for access.

Christopher