William Brouillard on wed 27 mar 96
Kiln firing
My experience with firing kilns that have been equipped
with forced air burners is that the height of the exhaust
stack has very little to do with the temperature differential
of the inside of the kiln. I would agree with Mark Ward
that the height of the stack should be adequate to remove
the exhaust from the building or kiln shed. Any addition
height would have more to do with local fire or building
codes than with how the draft is affected. This would
only apply to forced air burner/blower powered kiln and
not to natural draft kilns. With a natural draft kiln the
height and capacity of the stack does have a great affect
the kiln firing.
With forced air burner you are creating a
pressurized atmosphere inside of the kiln. In effect you
are pushing the burning fuel/air mix through the kiln and
out the flue to the stack. In a natural draft kiln the stack
pulls the burn mix through the kiln. Larger taller stacks do
this with greater speed. With forced air burners, as long
as your stack is large enough in cross section to handle the
volume of spent gasses the height should make very little
difference. At CIA we have for gas kiln ranging in size
from 40 cu. ft to 120 cu. ft and they all have the same
height stack, 9.5 ft. They are only that high because we
have to reach the exhaust hood canopy. I have built
numerous forced air powered kilns worth stacks of 8 to 9
ft in height and have not had trouble with draft.
Controlling the heat differential inside of the kiln
can be approached in several ways. Someone has already
mentioned that the burner flame is at its hottest at the tip.
Adjustments to the length of the flame can be used to
move heat though out the kiln. A short, (12" to 16") flame
will release most of its heat at the bottom or bag wall level
of the kiln. A longer flame that goes up the bag wall to the
top of the kiln will release most of its heat there.
This is not as simple as it sounds. Short flames are
in general oxidizing flames and long flames, ( over three
feet), are fuel rich or reduction flames. The flames are
long because there is not enough air or oxygen at the
burner tip to completely burn the fuel. It travels through
the kiln using the oxygen inside of the kiln, pots and
glazes to complete its combustion. The rule of burner
flame thumb is that short, fast, noisy bushy blue flame are
oxidizing flames. and that long, lazy yellow flames are
reducing flames. If you are using long reducing flames in
an effort to move heat around in an oxidation firing you
may get some area of reduction.
The suggestion given about making changes in the
bag wall are right on the mark. Bag walls can be lowered
or the bricks that make up the bag wall can be re-spaced
to provide more openings. The additional opening would
allow more of the flame to disperse and burn at the bottom
of the kiln. I have some times used target bricks to
breakup the flame and release more heat at the bottom of
the kiln.
A target brick is a small brick placed directly in the
path of the flame to alter the direction or flow of the
flame. Thick of the flame as a stream of hot gas. It is like
a small stream of water and can be directed by putting
objects in its path. That is the function of bag walls and
target bricks. A small 2.5" x 2.5" x 4.5" brick can be
placed on end with an edge facing the burner, just inside
the kiln, ( about 12" or so). That brick will break up or
diffuse the flame pattern and cause some increase in heat
in that area. I some times use that setup if the part of the
kiln around the burner openings is cooler than the rest of
the kiln.
Flame retention tips can be added to the tips of your
burners to create a shorter flame pattern . there are made
of cast iron and are relatively inexpensive. They are
generally considered to be a safety feature on burners and
would be a good idea to install on most types of open
burners.
The speed of the firing can also effect the
distribution of heat with in the kiln. Much depends on the
shape of the kiln chamber and the design and placement of
the burners, but fast firings can lead to uneven
temperatures. The usual result of a fast firing in a tall kiln
is a hot top and a cold bottom. Slower firings allow the
heat to equalize through the processes of conduction and
radiation. A slower firing schedule may help even your
kiln temperature. Damper setting can affect the speed of
the firing.
AN open damper usually means that the burners are
wide open and that the kiln is being used at its highest
power settings. When the damper opening is restricted the
speed of the firing decreases. As long as the burner is kept
in an oxidizing setting the damper could be used to slow
the speed of the firing and help the kiln fire evenly. The
damper opening can not be too restricted while the
burners are running at full tilt but some adjustments
should be possible. Each kiln will have its own
requirements. A small brick may be placed in front of the
flue opening to restrict it and slow the pace of the firing.
Changing the way that the kiln is stacked can be the
easiest way to control temperature. If the bottom of the
kiln is running cold you might want to open up the
stacking. Put larger works on the bottom and leave some
extra room around them. Do not put the plate shelves with
the 2 inch posts at the bottom of the stack. Start with at
least a 9" post and save the plates for the hottest part of
the kiln. If the kiln is firing hot on the top and cold on the
bottom. Use open stacking at the bottom and tight
stacking at the top. Stagger the shelving height throughout
the kiln to improve convection heating or circulation.
Place addition cone packs throughout the kiln to see
exactly how it is firing. You might have a cold spot right
by the cone pack. All of that cold air from the outside
might be getting in through the spy hole for the cone pack.
Best wishes from the Forest City
--
william brouillard
1011 literary road
cleveland,oh.44113
ch151@cleveland.freenet.edu
Dave Eitel on thu 28 mar 96
Boy--did I goof! When I responded to the original post, the fact they are
firing with power burners completely escaped my notice! Got to be more
careful before I go shooting off my mouth. I have no experience firing
with blowers, but I certainly believe Mark Ward is correct about chimney
height not being a factor when they are used.
Later...Dave
Dave Eitel
Cedar Creek Pottery
Cedarburg, WI
daveitel@execpc.com
Bruce Gioia on fri 29 mar 02
>>Thank you Mel soooooo much for the tips on firing<<
______________________
I also appreciate the clarity on the subject.
After 30 firings with gas.
I was vaguely aware of the "action"
and response....
but this description has given a definition.
BTW thank god for cones...
I was stalling (for some time)
at 1180 and and bent cones further
than when attaining my usual 1200 degrees....
It was a windy day.
Bruce
ps others descriptions of hydrocarbon combustion
has given much insight into my perception
of what is happening in the kiln...thanx
Jami Jacobson on fri 29 mar 02
=20
Thank you Mel soooooo much for the tips on firing, I'm trying it =
again this weekend. I know I "turned it up" way too soon, and way too =
long....no surprise that couldn't reach temperature last weekend. Will =
try to take a calmer approach this time. =20
Thank you to Kurt too, your advice was extremely helpful!!
Jami=20
Bruce E Ross on tue 26 apr 05
Hello clayart especially kiln gurus. I've been lurking on the list for
about three years and finally joined the information age with a home
computer. I have been fascinated by the by-play that occurs. I've been
taking ceramics through my local community college for the last 5 years,
and having taken that as far as it would go I built my own kiln last
summer. I've now done 3 bisque and 2 glaze firings with moderate sucess
but some problems. Thus here I am. The kiln is about 28.5 cu.ft.
(3x3x39"at the top of the arch). I have four MR-100 burners from Ward
burner and fire with propane at 11" water column. The burner ports are
4.4x5 inches and the burners are mounted about 1.5 inches outside the
ports. The propane tank is 320 gallons and my gas man assures me that the
regulator can pass well over 400,000 BTU per hour. The kiln is single
layer IFB with an additional 3-4" of castable over the arch. The exit
flue is in the floor of the kiln not the back wall. Shelves are 14x28 and
are supported integral with the bag wall for 1/2 the interior height. The
chimney is attached to the back wall, two layers of brick, and is 81 in.
sq. (9x9) Transitioning to a 10 in diameter steel pipe just above the kiln
roof. Total chimney height is about 13 feet. Damper is a kiln shelf in a
slot about 1/2 way up the brick part of the chimney.
First bisque fire went fine, a little uneven top to bottom so I took a
brick out of the floor exit flue. Second bisque fire was excellent.
First glaze fire headed for cone 10 reduction never got to temperature
(pyrometer mounted in back wall) but sat at ~2200 F for 4 hours and after
17 hours total firing time dropped the cone 9 all the way and cone 10
mostly down throughout the kiln. Second glaze fire got to 2200 faster but
again stalled at ~2200F after 15 hours cone nine started and I got tired
and shut it down and went to bed. The propane tank exterior and exposed
piping froze (Dec air temp about 50F). Talked to Marc Ward and he thought
the Kiln was tight and suggested taking more bricks out of the exit flue
channel and out of the bagwall. I did this and did a third bisque a
couple of weeks ago. Kiln stalled at 1600F for 3 hours, total firing time
12 hours. Right at the end I shut down two burners and bricked the
ports. By the time I got to the other side the temp had gone up 30F. I
shut it down anyway.
I'm having the Gas company come out this week and change out the 320 gal
tank with a 500. I have pictures and temperature curves and firing log
notes for both glaze firings if anyone would like to see them.
I was at Mendocino with Mel and Steve et al. and am looking forward to
Portland NECEA nest year.
Thank you for your help.
Bruce
Vince Pitelka on wed 27 apr 05
> I built my own kiln last
> summer. I've now done 3 bisque and 2 glaze firings with moderate sucess
> but some problems.
Bruce -
That kiln should fire just fine with four MR-100s on 11 WCI propane. The
interior dimensions are appropriate for a downdraft kiln, but you don't say
anything about the flue dimensions. The fact that it is in the floor might
mean that it needs to be a bit larger than if it was in the back wall.
Also, you don't say anything about the specific firing practice. How many
WCI at the burners when you are firing? How are you gauging back pressure
in order to assure heatwork in the kiln? When are you reducing, and where
is your damper set in the various stages of the firing? The fact that the
temperature climbed quickly when you shut off too much burners tells me that
you might have your burners turned up too high and/or damper closed too far.
As you probably know, you get maximum heatwork through the kiln when you
have slight back pressure. At higher temperatures you can of course gauge
back pressure and atmosphere by the presence and color of flames at the
spyholes. At lower temperatures, adjust your burners, close the damper
until you have back pressure at the top spyhole (just wave your hand quickly
in front of the spy hole to gauge if there is back pressure), and then open
the damper a little at a time until the back pressure disappears. When that
happens, shut the damper slightly to just barely restore back pressure. As
you probably know, it is essential that you avoid any reducing atmosphere
before you get to body reduction temperature.
If you are doing reduction firing, at higher pressures you will need to
close the damper further to achieve the desired reduction periods, but be
cautious of excessive reduction. It accomplishes nothing for the clay or
glazes, and will stall your kiln.
Here's another good indicator. What is the shape of the flame inside the
burner ports? If the flame is pure blue, and tapers quickly as it enters
the burner port, then you have way too much secondary air being pulled in
around the burner tips. You only need enough secondary air to feed the
flames as they leave the burner - to allow the flames to finish combustion.
If the flame starts out blue and then spreads out to fill the burner port,
still blue but with flickers of yellow, then you likely have reasonable back
pressure. If the flame immediately fills the burner port and even backs up
out of it, with lots of orange or yellow, then you are reducing.
There are so many indicators and subtleties unique to every kiln, especially
when they are home-built. If you can give us a lot more information, then
we can give you a lot more help.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Marc Ward on fri 29 apr 05
> I built my own kiln last
> summer. I've now done 3 bisque and 2 glaze firings with moderate
sucess
> but some problems.
Bruce,
As Vince pointed out, there are a lot of variables involved... I tried
calling you but, the number I had was no longer in service. Give me a
call and we'll talk about this some more. What really helps is to have
some pictures. Many times the problem is apparent once I can see
the setup.
Marc Ward
Ward Burner Systems
PO Box 1086
Dandridge, TN 37725
865.397-2914 voice
865.397-1253 fax
wardburner@aol.com email
http://wardburner.com
Mike Gordon on wed 6 jul 05
Hi,
In the high school I taught at for 27yrs. I only left the gas kiln
unattended for the 30 min. I got for lunch. It was in a cement block
bld. with a hood and stack, and a chain link gate. This was around 900
to 1000 degree range. After lunch I was never farther than 100 feet
from the kiln till shut off. With the electric kilns, 3 of them in a
room next to the gas kiln, they shared the same roof. I fired them to
cone 05 every night, set the cone sitter and timer and left for the
day. Never had an accident due to a sitter malfunction. I did have a
kiln shelf crack in half with a lot of glazed Christmas projects in it.
At home it is a different story. Gas kiln is outside in a wood frame,
sheet metal bld. open on one side. It also has a hood and stack. I
NEVER leave the house while it is on. I put the pilots on over night
and sleep. My two electric kilns are inside my studio which is my
basement. I also never fire them unless I'm home, either. Mike Gordon
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