Clay oven size?

[Tboney]

First ever attempt at building a clay oven i would really appreciate some advice.Im planning on having an internal diameter of 34 inches. what internal height would be the best also what thickness would be ideal for each layer of clay? I plan to build two layers with glass bottles in between them. I am also after advice on the length height and width of the opening. Any advice will be much appreciated

[david s]

The usual design is a hemisphere which provides good structural integrity as well as excellent heat circulation and re-radiating for cooking. The low dome of the Neapolitan ovens gets more intense heat to the top of the pizza as it's closer to the food, but the hemisphere is a better all rounder. Don't go higher than a hemisphere though. If you are building it with cob, then probably make the dense internal layer no thinner than 3" thick. You should make the oven opening around 63% of internal height, but you can play around with its width. So for a 34" dial hemisphere internal height is 17" therefore oven mouth height 10.7" (give or take an inch). As you're in the UK check out UK wood fired oven forum.

[Tboney]

Thank you very much. What about the length of the opening is it as important to be specific with that?

[david s]

I presume you mean a flue gallery to exit the smoke. Many cob ovens have none, but if you want one I think it’s a good idea to make it as shallow as possible otherwise you end up with a deep tunnel to work past before getting to the oven proper. As long as you have a decent inverted funnel form above the oven opening to allow the smoke to collect, as well as an adequate cross section in the chimney or flue pipe, the oven will draw well with no smoke out the front. A 34” oven requires a 6” diam flue pipe or cross sectional equivalent chimney.

[Tboney]

Im bit pushed for space so if i can design the oven without the flue gallery then that would help. So it would be ok to just have a chimney on the top of the oven? I just thought needed one, and was hoping it didnt have to be long in length. Sorry im newbie

[david s]

No, I meant no gallery or chimney. The smoke simply comes out the oven mouth. You just have the problem of dealing with some smoke in your face at times. You could always build it without one, see how it goes and maybe fabricate something later if you find it a problem.
if you have a chimney coming out of the top of the oven, while it does work, because the chamber is empty the flame jumps straight to the flue opening resulting in inefficient combustion and poor circulation. This would be called an updraft system. An improvement is the cross draft arrangement of a traditional WFO with air entering the chamber from the oven mouth, combustingin the chamber and travelling across the roof before exiting out the top of the door. All the flame is captured this way, circulation is good andfuel consumption is lowered.

[Tboney]

So would it look something like this? Could you please explain what you meant by inverted funnel form and adequate cross section in flue please. Or if i do this design does that info not appy?. Thanks

[Tboney]

[david s]

Yes, that's correct. Your pic shows an oven without any flue gallery. Here's a pic of my design from castable refractory showing inverted funnel, which extracts smoke very efficiently, a bit like headers on an exhaust system on a car vs standard manifold. Same could be constructed from sheet stainless or other material