[Clayart] Kiln design

Hank Murrow hmurrow at efn.org
Sat Jan 7 22:01:46 EST 2017

> On Jan 7, 2017, at 9:48 AM, Douglas Fur <23drb50 at gmail.com> wrote:
> 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.

Dear Duff;

I have used Eclipse atmospheric mixers(burners) since 1966 in mostly downdraft kilns of my own design.
I found that i could double the capacity(BTU) by casting a burner port to fit into the brick pattern of the wall with a tapered inlet about ¾” larger in diameter than the Sticktite flame-retention tip, and expanding over the 9” wall to about 4.5” diameter. This allowed me to use two 1.5” Sticktites for each 2” Eclipse mixer, doubling the BYU capacity of the mixer and saving $$$ in the process.’The Eclipse mixers were cast and then accurately machined to yield a venturi that drew in the 60% combustion air you cite. i found i could more than double that with the method described above. The tip came from an Eclipse engineer in Denver CO while i was working at Anderson Ranch in Aspen in the early 70’s.

Cheers, Hank in snowy Eugene

> 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.
> 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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