Dave Finkelnburg on fri 22 mar 02
Paul,
You asked,
>Does anyone know if there are any standard rules when determining how large
>the inside dimensions of a chimney should be?
The answer is, in my opinion, in two parts.
First, it should be a practical size, one you can build from available
materials without too much cutting, etc.
Second, it depends entirely on what your kiln is made from--how thick,
what kind of materials, total volume, amount of area to lose heat out of,
and amount of ware and shelves and kiln furniture that will be inside the
kiln.
A British Thermal Unit (BTU) is a unit of heat. You get roughly 1,000
BTU's out of burning one standard cubic foot of natural gas. Marc Ward,
Ward Burner Systems, Dandridge, TN, recommends supplying 10 to 16,000
BTU's/hour per cubic foot at cone 10, 8-13,000 BTUH at cone 6. You say you
have a 90-cubic foot kiln. I'm sure others will have data on that size
kiln.
Once you decide how many BTUH you will need in your kiln at full fire,
then you can convert that to SCFH of gas in, add enough air to get the
amount of oxygen needed to burn the fuel, factor in the increase in gas
volume due to higher temperature, assume a reasonable gas velocity up the
chimney, and thus arrive at the proper size for your flue.
This is not all that difficult, just time-consuming. Calculating the
heat loss from information on the kiln dimensions and materials of
construction is the most difficult part. I do think it's worth the effort.
It's what I did for my own kiln.
I have argued this before on the list. Size your flue correctly, don't
just make it "big enough." An oversized flue creates problems controlling
kiln atmosphere. Neither passive nor active dampers perform well in a
grossly oversize flue.
I hope your kiln project goes well!
Dave Finkelnburg in snow-melting Idaho
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