Ray Carlton on wed 5 nov 97
thanks to those who reponded to my hysterical posting last week
i was able to save a lot of the work in second firings in my trusty old
fibre kiln [now almost dead] and thankfully three of the studio glazes work ok
i do have questions which have arisen from the replies I have receievd
how high should the bagwall be ??
what pressure should be used as a top?? [3/4" copper pipe to kiln 1/2" to
burners}
what is the best way of judging the amount of reduction?
[it's obviously different in the fibre kiln]
thanks to john pettyjohn for very detailed response
i can fire the kiln confidently with three of the glazes so that is what i
will do and fill up the holes with tests of the copper red.
thanks to all
cheers
raycarlt@valylink.net.au
Ray Carlton
McMahons Creek Victoria Australia 3799
Craig Martell on thu 6 nov 97
Hello Ray:
I have a 66 cu ft sprung arch IFB kiln. My bagwall is 13.5 in high and 18
in high at the front of the kiln where the flames impact. I have two
500,000 BTU burners that fire forward from the back of the kiln so it's a
bit different from your setup. But, with the bagwalls set at that height,
the kiln fires very evenly.
I have the same type of piping that you have. My burners use 3psi max
pressure. They have a 7/64 orifice, which is just shy of 1/8th inch. If
your burners function efficiently at high pressure, you can do at least 3psi
if not more. Maybe up to 5 or 6?? If your burners are designed to operate
best at low pressure, you wouldn't want to exceed 1psi. It sounded, from
your earlier post, that you were getting plenty of flame in the kiln. In my
wildest moments, I would be hard pressed to get ANY flame out of my stack,
which is 15 ft high. You can get info from your propane co. about BTUs
produced at X pressure through X sized orifices, using X number of gallons
per hour. Information on flow rates per hour will give you the correct
orifice size, pressure at the orifice etc. to obtain the needed BTUs. Did
you say your kiln is about 65 cu ft? I think a low but sufficient estimate
of required BTUs per hr would be 10,000 per cu ft. or 650,000 total per
hour. Might as well go up to a million for a bit of extra muscle. So,
you'd need about 166,666 BTUs per burner if you have six burners. How are
your flues sized? With flame coming out of your stack the way it was, you
may have too big an exit flue. The exit flue should be slightly larger in
area than the total area of all your inlet flues.
Judging reduction is tricky and varies from kiln to kiln as I'm sure you
already know. When I'm at moderate to slightly heavy reduction at cone 8
and above, my burners are at 3psi with full primary air and the damper is
about 65% open. I have a foot long translucent orange-yellow flame out the
top spy and a very lazy licking flame about 4-5 inches long out the bottom
spy, which is 10 inches above the floor. The bottom flame is very
translucent and yellow in color. With propane, an oxidising flame is blue.
I think you were getting a green flame from volatile copper, as another
clayarter mentioned in an earlier post.
If nothing else pans out well, get some input on your flue sizes. That's a
real critical point in downdraft design and REAL hard to get good solid
information about. I hope some of this stuff is helpful to you.
regards, Craig Martell-Oregon
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