Dave Finkelnburg on thu 23 aug 07
Duff,
IF you estimate the heat loss/hour to the
surroundings through the kiln floor, walls and roof
and the heat/hour necessary to raise the temperature
of the kiln furniture and ware (this depends on the
rate of temperature rise), you can estimate the amount
of fuel you need. From stoichiometry you can then
arrive at the amount of air you will need to burn the
amount of fuel you calculated, remembering air is only
21% oxygen. Then you can determine the number of
moles/hour of stack gas leaving the kiln. Finally, if
you select a target velocity for flue gas up the stack
at peak firing temperature, and adjust the moles of
gas to the conditions of air pressure at your
elevation and peak firing temperature, you can
calculate the maximum size stack that will work for
your kiln. Adjustments for lower temperature or
reduction are necessarily made using active or passive
damper(s).
Fred Olsen's suggestions appear way oversized for
an IFB kiln. Nils Lou's are much better.
BTW, to prevent stalling due to the velocity
pressure of local winds, a good rule of thumb is to
make any chimney at least 24-inches taller than the
highest building or roofline within 10-feet of the
stack. Since kilns are a special case...the inside is
MUCH hotter than the outside...one can get away with
less, but even with nothing near the kiln I prefer the
stack to be 2-feet taller than the downdraft kiln.
This is really only an issue in windy weather...but
some folks do fire where wind is a four-letter word!
:-(
Good firing,
Dave Finkelnburg
From: Duff bogen
All the references I've seen have been re-hashing the
"rules of thumb" from B.L.'s Potter's Book. These
were based on massive hard brick kilns. Has anyone
done research on sizing stacks for IFB kilns?
A friend followed the reasonning "Hard brick kilns
need a lot of BTU's => a lot of fuel => a lot of air
=> alot of chimney but IFB kilkns need less BTU's less
fuel=> less air => less chimney. Following this logic
he built his chimney only as high as the top of the
arch. "I fired this kiln several times and it was as
normal as could be."
As this worked should this be the new rule of thumb
for IFB kiln chimnies
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