Michael McDowell on sat 11 apr 98
Dear Ellen =26 Vince,
This is getting to be one of the Rites of Spring here on Clayart. Vince =26 =
Ellen
going after each other in a just barely civil disagreement over some =
technical
issue. Last year at this time the topic was =22compression=22, this year =
it's
chimneys on natural draft kilns. It seemed to me that last year's big debate=
was
more about alternate terminologies, definitions or usage than it was about
substance. I'm wondering whether this year's discussion might be also more =
about
word usage and personal style than it is a difference in practice.
I don't consider myself expert in kiln construction by any means. I have my
hands =26 mind full trying to just get my kiln fired well here at nearly sea
level. I fire an updraft kiln anyway, so I'm not going to offer myself up as=
any
kind of authority on proper stack heighth. But I do carry my burden of
=22impressions=22 around with me. One of these impressions is that for any =
given
kiln design using natural draft burners, a taller stack will be required at =
say
5,000 ft. altitude than at sea level.
My =22impression=22 is that this is due to two factors. One is that, since I=
believe
chimney draft is driven at least in part by differential atmospheric =
pressure,
and that the amount of differential per foot of chimney heighth is less at
higher altitudes, a taller chimney is required at the higher altitude to =
achieve
the same volume thoughput of gases into and out of the kiln. The second =
factor
is that since at higher altitude there are fewer molecules of oxygen in a =
given
volume of air, a larger volume of air needs to be drawn into the kiln at =
higher
altitude in order to combust the same amount of fuel.
I have no intention of defending my usage, or terminology here. What I'm
wondering is if either Vince or Ellen feel that the substance of the above
discussion is faulty. I'm certainly ready to change my impression if either =
or
both of you would care to explain to me where I've gone wrong.
Already dreading having stepped into a crossfire...
Michael McDowell
Whatcom County, WA USA
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