[Clayart] Kiln design

Douglas Fur 23drb50 at gmail.com
Sat Jan 7 12:48:49 EST 2017


We've had a recent discussion of gas kiln design and now a discussion of
wood fired kilns. I'd like to link Mel's comment about the need for a
substantial chimney for a successful wood kiln back to gas kiln design.

Something I picked up from my gas burner experiments is how the energy of
pressurized gas induces air flow. This may seem obvious but I find it hard
to conceptualize things which I can't see or easily touch. And sometimes
stating the obvious can be a place for learning.

I read somewhere that about 60% of the combustion air for a gas burner is
the primary air which flows through burner. This air flow is induced by the
energy of the pressurized gas injected into the burner. (The effect of the
chimney here is negligible. Most burners are set back from the fire-mouths
to avoid heat damage. Draft induced air flow into a kiln propagates
spherically and follows the path of least resistance, therefore it's easier
for it to flow around the burner than through it.) My arithmetic says that
the chimney on a gas kiln only draws in 40% of a the required combustion
air.

In a wood fuel kiln the gasses which are burned are distilled out of the
wood.* Their gas pressure is low. As they burn they are heated and expand
they rise and induce air flow. These hot gasses rising through the kiln and
chimney are proportional to the height of the chimney. Which is to say a
the chimney on a wood kiln draws in 100% of the combustion air.

This difference between 40% and 100% can be seen when lighting a kiln. With
a gas kiln a burner could be fully turned on and produce a clean flame.
While a wood kiln needs to be coaxed along with a tiny fire until the
chimney starts to draw. If you tried to "turn the burner on full" by piling
on the wood you wouldn't get clean combustion but the opposite, a kiln shed
full of smoke.

DRB
Seola Creek

* Another basic fact of potter's physics: you can only burn gasses, not
liquids or solids. Using liquid or solid fuels requires both a place and
process to gasify the fuel.
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