kurt wild on fri 17 apr 98
I have fired my "new" kiln for 3 years and put up with the lack of
reduction near the exit flue. I periodically tried this and that to get
reduction there to no avail. The kiln is a 20 cubic foot kiln with the
burners and stack (exit flue) on the same side. Other than this
problem my kiln fired wonderfully.
Any suggestions??
Vince Pitelka on sat 18 apr 98
>I have fired my "new" kiln for 3 years and put up with the lack of
>reduction near the exit flue. I periodically tried this and that to get
>reduction there to no avail. The kiln is a 20 cubic foot kiln with the
>burners and stack (exit flue) on the same side. Other than this
>problem my kiln fired wonderfully.
Kurt -
Assuming that this is a downdraft kiln, and that the stack is softbrick, you
might try the following. Use a masonry drill to bore a 3/4" hole in the
stack just outside the kiln wall, before the damper. Make a little
softbrick plug to close this hole. When putting the kiln in reduction,
check that hole, and increase back pressure and reduce air until you can see
visible flames inside the stack through this hole. When you see them, you
know you have reduction throughout the kiln. Check the hole when not in
reduction to get a sense of what it looks like with no flames. In reduction
you will not be able to see the difinitive shape or appearance of flames -
more like a flickering glow. You will recognize it when you see it. Good luck.
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
John Baymore on mon 20 apr 98
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I have fired my =22new=22 kiln for 3 years and put up with the lack of
reduction near the exit flue. I periodically tried this and that to get
reduction there to no avail. The kiln is a 20 cubic foot kiln with the
burners and stack (exit flue) on the same side. Other than this
problem my kiln fired wonderfully. Any suggestions??
(snip)
If there is no reduction near the flue, then the flow of gases through this
area is markedly different than through other areas, or the temperature of
that area is markedly different from the other areas. This means that one
of a number of things is happening:
1.) The flame front is burning off within the chamber before it gets to the
area in question, leaving that ware bathed only in CO2, H2O vapor, inert N,
and trace volitiles coming from the glazes.
2.) Because of flow design of the kiln and combustion system (paths the
flames take in the kiln), the burning flame front is bypassing the area in
question and getting pulled out the exit flue without flowing through the
area at all.
3.) There is poor mixing of the secondary air flowing into the kiln with
the mixture from the burners, and a pretty undiluted stream of this air is
flowing through the area in question.
4.) Air is leaking into the area in question through some other source.
5.) At the point that you are putting the kiln into reduction the area in
question is already hot enough that the clay body in that area has become
so tight that it is no longer permeable enough to be reduced an accpetable
amount.
Taking these one at a time:
1.) To assure that you have reduction in the lowest area of the kiln, you
need to see into the flue at the exit point from the chamber. If you are
getting good mixing of the gas/air in the fireboxes, then you just have to
get the flame front to be burning off somewhere downstream of the bottom
back.
Hopefully you left a cleanout port at the base of the chimney. If there is
no port, try carefully tapping out a brick and replacing a whole brick you
remove with a half to leave an opening that is closed up with another half
when not being used to look in. Look at the courses above where you will
be removing a brick to see the brick joints.... so that you pick a good
spot=21 Or..... if the base of the chimney is softbrick, get a 1=22 wood
paddle bit and bore a port thru the softbrick (will ruin the drill). Make
a plug to fit this hole for when you are not looking in. This port should
be right at the bottom area.
Use this port to adjust the damper when setting reduction so that you just
see the flame front exiting the chamber into the chimney. Remember that
when you replace the plug into this opening, it is the same as opening the
damper a bit. The air flowing into the port into the chimney is acting
like a passive damper when you are looking. Adjust the slab damper a
little furthur closed then you want with the port open .... then it'll be
correct.
Or you could adjust the kiln damper to JUST back a little flame out this
port at the base of the chimney. This will be a little =22dirtier=22 than =
you
need, but it will work OK too.
2.) It is possible that you will see flame coming out the exit flue into
the chimney, but still have no flame passing through the area in question.
If there are paths that provide for exceptionally unrestricted flow, then
the gases will tend to go there rather than through areas of more
restricted flow.
If the space between the bottom rear shelf stack and the back wall is very
generous, then the flames might be tending to run through that space
directly from the firebox/bagwall area and not being drawn through the
lower back bottom shelf. If this spacing is more than say..... 1 1/2 to 2
inches, try tightening up the spacing between the rear stack of shelves and
the back wall surface.... and leave a bigger space down the middle between
the front stack of shelves and the back stack than you normally do.
Also.... if the floor shelf is too high from the actual floor, then flow
under the shelves can be excessive too. Usual spacing is 2-2 1/2 inches or
so. If more than this... try lowering the floor shelves a bit.
Also look for a large hole in the bagwall at the back bottom right by the
rear wall surface. This can be the =22short circuit=22 point for flame
shooting along the surface of the back wall and right out the exit flue
without going thru the rear bottom shelf..... fooling you when you look at
the apparent flame coming out the exit flue into the chimney.
3.) Picture two different water flow situations. First, picture a smooth
concrete culvert. Get a big bucket of food coloring. Using a small pump
inject the food coloring steadily into the water flow at one point. Can
you picture how long it would take for the color to mix into the water
evenly? Now, picture a small stream with lots of rocks in it. Again
picture how the color would disburse.
Now substitute fuel gas (or a partially areated mixture of gas/air) for the
water and secondary air for the dye.
The flow of secondary air through a burner port can sort of act like this.
While the burner's fully or partially aerated flame had some pretty
turbulent flow and a strong directionality, the secondary air coming in
around the flame is a little less well mixed or directed. Secondary mixing
is highly dependant on the kiln design.
Intimate contact between the O2 and the fuel is necessary for good
combustion. If the mixing is not good.... some areas are =22rich=22 (lots =
of
dye) and some areas are =22lean=22 (little dye). The goal in good even
reduction is to get the gases passing throug the wares evenly mixed but
deficient of oxygen. If a pot is being hit by a =22rich=22 mixture in one
area, that area is exposed to a strong reducing condition. If the other
side is getting hit with a =22lean=22 mixture than that area is getting less
reduction conditions.... or even oxidising conditions.
This situation is common on kilns that have burners that do not pull in
significant amounts of primary air. These kilns depend on a lot of
secondary air...... and the mixing of this pure air with the partially
areated gas stream. If the kiln doesn't have a good design that promotes
turbulent flow before the gas and air streams hit the wares, unven firing
effects result.
If the secondary air doesn't mix well with the burner flame, then it can be
pulled in pretty pure streams through the kiln by the draft (like the dye
in the smoothly flowing water). It is possible that the secondary air
coming thru the burner ports in this design kiln is =22short curcuiting=22 =
and
turning toward the exit flue without mixing much with the burning gases.
This would result in a lower level (or NO) reduction in the problem area,
and probably also a strong tendency to be either really cold or really hot
(=2Amore on this later below).
This situation is exacerbated by a kiln with excessively large burner ports
or with excessive draft for the size of the kiln. (Or running the damper
excessively open and compensating to get reduction =5Bin most areas=5D by =
large
amounts of fuel input.) If your kiln has burner ports that are overly
large compared to others you have seen with similar style and size burners,
try reducing the square area with temporary pieces of brick. If this fixes
it, get some good refractory materials and permanantly change the ports.
(You can cast new inserts ...... burner blocks.......out of something like
Mizzou castable.) If the chimney cross section and/or height is greater
than you typically see on kilns of this size, try lowering the chimney
height a bit.
A target brick ot two in the fireboxes can also facilitate mixing
turbulence..... but watch baffling flow into an area that you don't want,
causing new problems=21
=2AAs to the comment about either really hot or really cold above......
If the secondary air stream is somewhat undiluted and unwarmed... then the
cold comment is pretty obvious.
But why hot? This is because this unmixed secondary air COULD be joining
into the flame path in this area and resulting in the burning of the
unburned gase in the flame at this point. So the heat energy is suddenly
realized in this area, as the flame front burns off. It sort of becomes a
little =22firebox=22 area and has a higher temperature than the average
surrounding it.
4.) An additional factor that could be causing this lack of reduction is
unwanted, and unaccounted for, outside air leaking into the chamber in this
area. The lower part of a downdraft kiln of this type typically operates
at a slightly lower than atmospheric pressure. So there is a =22driving
force=22 that will cause air from outside the kiln to leak into the kiln
through any openings it can find.
Look around carefully in the lower floor area, in the connection between
back wall and the chimney, and in the lower back side walls for one or more
cracks, gaps, or other =22air inlets=22. A 1/4 inch crack 4 inches long is =
an
open =22port=22 of 1 square inch. Sometimes it is the sum total of a lot =
of
small cracks that causes the problem. If you find cracks, fill them
in...... Greenpatch is a good choice. Or the ITC stuff. Or even a stiff
mixture of fireclay, water, and silica sand, in a pinch.
If it is a sitebuilt kiln and you followed one of the common (but in my
estimation ...... exceptionally poor) practices of only using two, 2 1/2
inch layers of hard firebrick for the floor courses, and they weren't laid
VERY tightly (or with mortar), there is great opportunity for air
infiltration in the whole floor area. Particularly as the floor =22opens =
up=22
over time due to refractory shrinkage, and the effects of repeated
expansion and contraction. If this is the case, consider adding a 1/2 inch
layer of fiber board to the interior floor (will get beat up fast due to
mechanical wear), and coat it with ITC.... sealing all the cracks. Leave
holes for the stilt points down to the original hardbrick.... don't stilt
the shelves on the fiber board. This will upgrade the floor insulating
factor a little as well as seal the air infiltration.
At the least... seal all the cracks.
If it is a fiber kiln, remember that fiber is somewhat gas permeable. If
any spots in that particular area are only thinly layered, it is possible
air is leaking significantly through the intact fiber. Increase the
thickness of fiber, and / or seal the outer skin with a welded sheet metal
skin. Or just coat the interior surface with ITC.
5.) Uneven heating. This is a real possibility in the design you describe
(burners at back firing toward front..... single large flue at back
bottom). Often in this design a target brick is placed in the firebox
floor area to splatter some of the flame so that it goes thru the back
bottom. This is done because the directionality the burners give the flow
in the chamber favors the top front getting the most flow. If this target
is splattering too much flame and the bag wall is too open at the back
bottom, a lot of heat could be finding its way there early on in the
firing.
Also... the bagwall and the wall refractories in this area are the first to
become radiant sources and are hotter than the other area all along. This
radiant soiurce of transfer (major factor in heating in a kiln) can also
cause the bottom rear to fire hotter in this particular design kiln.
If the clay is too mature when reduction starts, it cannot be penetrated
very effectively by the reducing agents (H or CO gases....or god forbid
.......huge C particles). It is no longer permeable to the gases. This
causes a look that is commonly refered to as =22no reduction=22. In fact =
there
may have been reduction in that area.... just not at the right time=21
Different clay bodies tighten up at different points. If you are putting
the kiln into reduction at a point quite near your body's =22critical =
point=22
(the point it becomes too tight for the effects you want by the level of
reducing agent you expose it to), then a slightly hotter temperature in the
problem area could actually be the culprit in your =22not reduced=22 =
problem.
Look at the bag wall in the back bottom. If it is very open, and/or if
there is a target brick midway along the firebox floor (or closer).... this
might be the cause. If you can't get a cone pack into this area that you
could see during the firing to monitor temp/time,....... for the next
firing get a thermocouple probe into this area by hook or by crook. At the
point you are looking at your typical indicator cones to put the kiln into
reduction, also look at the pyro. Check the temp according to the same
pyro in the area of the indicator cones for calibration. If it is
significantly hotter than what it should be (taking into account the
inaccuracies of the type of pyrometer you use), then you can adjust the
bagwall and traget brick to even out the flow in this area.
Hope all of this helps you trouble shoot the problem.
Best,
..................john
PS: There are health hazards associated with fiber dust and with brick
dust. See appropriate references if you don't already have great
familiarity with this issue. (Just had to say that =3Cg=3E.)
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
Call for info on the =22Earth, Water, and Fire Workshop '98=22 ....... =
started
in 1981
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