Bruce Girrell on sat 17 apr 99
Dean McRaine wrote:
> I have often heard the prediction that updraft kilns
> fire hotter at the top but as usual my experience seems to run contrary to
> myth.
Same here. I would like to talk to someone who actually has an updraft that
is hotter at the top. If mine is hot at the bottom and theirs is hot at the
top, then there is some way to balance the thing out.
Personally, I think the hotter at top thing is a bunch of hooey. The exhaust
gases have to be cooler than the incoming flame temperature, otherwise
they've done no heat exchange work while in the kiln.
My kiln has about 31 cu. ft. internal volume and is fairly cube-shaped. I
usually see ^04 down at the bottom when ^08 is over at the top. Not exactly
an even firing.
I did manage to make it worse one time, though. I choked the air flow in the
kiln by using a kiln shelf too close to the interior dimension of the kiln.
Theoretically, I had sufficient cross-sectional area in the space around the
shelves for the gas volume from the burners plus secondary air (four times
the area of my burner ports), but it clearly was choked. I could see a
dramatic change in the color of the kiln wall bricks right at the level of
the offending shelf. I have no idea what temperature the bottom shelf hit
because the all cones were melted into a puddle. The pieces on the bottom
shelf had matured enough to be difficult to glaze.
I think the real key for evening out the temperature in (at least) an
updraft kiln is turbulence. Got to keep that hot air moving and bumping into
things.
Nils, Cameron, Marc, Others? Comments please?
Bruce "Honest, I really am going to build that downdraft one of these days"
Girrell
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