New here

[Geezer]

Hi folks.
new to this forum
Is there some kind of measurement for length versus depth for good draw and exhaust?
Also, do you have to have a down slope of the inner chamber toward the chimney for the exhaust to work properly?
Cheers

[david s]

Efficient exhaust from the chamber.is a bit like efficient exhaust from an internal combustion engine. In particular:
1. Smooth flow
2. Chamber form
3. Flue draw
4. Flow design type
5. Chamber volume

1. Like a car engine, smooth flow can be improved by a smoother surface with reduced obstructions. Just as a smooth round stainless flue pipe is far more efficient than a square or rectangular cross section chimney. This is where a stainless or thin refractory cast galleries have the advantage over brick unit ones as well as being far lighter which reduces the heat sink effect drawing heat from the chamber’s thermal mass.
2. The best chamber form is the hemisphere and why it is used in high performance car engines.
3. Power of the draw of the flue is controlled largely by the diameter of the pipe and secondly by its height, creating the power from heat rising and expanding volume of the rising exhaust gases.
4.An updraft system relies on heat rising by convection. Great smoke free operation, but lots of heat lost up the flue.
A cross draft system far more efficient and used in most oven designs, but can be smoky at start up particularly if the chamber is too fuel rich (the flame is actually pulled slightly down)
In a downdraft system the flame is actually pulled down with the exit to the flue located in the bottom of the chamber. This is the most efficient and used by modern kilns and furnaces, but unsuitable for an oven with a large door and air intake.
5. The smaller the chamber the more critical the design. Larger oven chambers, while using much more fuel (consumption is proportional to volume) are more efficient with the energy produced.