search  current discussion  categories  kilns & firing - burners 

raku burner change and soot on fiber

updated sat 31 may 97


John Baymore on tue 6 may 97 wrote, in part:
I've been firing with a ceramic fiber kiln. I started out using a weed
burner, but upgraded to a venturi burner. Since the change in burners, I
have black soot covering the outside of my kiln. .........

There have been a lot of good possibilities for the soot on the outside
pointed out already, and without seeing the specifics of the installation
it is hard to exactly pinpoint the cause. (Like being a doctor and working
via letter .) However another possibility is...........

The kiln chamber is a bit of a "system" that is dependant on the total
interior volume, the size of the inlet flue, the type of burner supplying
the combustion products, and most particularly, the size of the exit flue.

Simplified, you can say that for a given interior pressure in the chamber,
the exit flue will exhaust a certain volume of effluent. This works
exactly like the concept of the orifice in a gas burner. Generally
speaking, the larger the hole, the larger the volume of gases that it will
pass at a given pressure. (Other factors involved in this passage of gas
include the temperature, density, and composition of the gasses, and the
exact shape and material composition of the "hole" they are being forced

At any given instant of time, a volume of byproducts of combustion created
by the burning of a fuel /air mixture need to get out of the kiln (in short
order) in order to make room for the newly burning stuff coming in. If
you put less total volume in than the kiln is exhausting, some secondary
air flows into the burner port and makes up the difference (to attempt
maintain an equilibrium with atmospheric pressure). If you put MORE total
volume into the chamber than the kiln is exhausting, the pressure in the
chamber increases.

(In the practical real world, if you increase this enough, you will start
to see flame coming out of the burner port near (and flashing back onto)
the burner nozzle.......not desireable .)

Unlike brick kilns, ceramic fiber is quite porous. Gases leak through it,
given sufficient driving pressure differential. Generally speaking, the
thicker the fiber layers, the less permeable the "wall" is. That is why in
total fiber construction sealing the surrounding steelwork of the frame is
important if you want maximum efficiency and slow cooling cycles in
periodic kilns.

So another possible question in this raku kiln case is what is the pressure
differential between the inside of the kiln chamber and the outside? How
thick is the fiber? If you get this differential between the inside and
surrounding atmospheric high enough and if the fiber is thin enough, a
significant amount of partially burned (or unburned) gases will leak right
through the walls to the outside.
As these gases near the outside they are dropping in temperature, and an
effect called "quenching" comes into play. The gases, even though
partially or even fully aerated, drop below ignition temperature....... and
they stop burning. At this point the fuel is often already
"cracked"....... partially broken down into constituent parts ...... like
carbon. Often leaving sooty "carbon" deposits. On fiber, this is often
very diffuse and general in character...... not specific dark areas near
seams or overlaps or in "curtaiin patterns" above gaskets. It is general
overall greying or blackening.

So another possibility here is that the new burner may be producing more
total effluents than the kiln chamber/flue system can handle in a given
amount of time, even though the new burner is better areated than the weed
burner (which is probably certain!). In effect, the burner is oversized
for the system. Try turning it DOWN overall, yet running the maximum
primary air possible for efficient combustion.

Venturi type burners use the kinetic energy supplied by the gas stream
coming out of the orifice to pull in air (enhanced by the "venturi effect")
and mix it with the fuel. Generally speaking, the higher the available
kinetic energy (gas pressure), the higher the % primary air that the burner
can pull in. Another factor here is certainly the quality of the casting
of the venturi tube. The better the original engineering design and the
higher quality the machining of the actual burner.... the higher the % of
primary air entrained. (Yes...... expensive venturi burners are worth the
money! )

One problem with all venturi burners is that at a certain point the
"mechanical linkage" of air entrained to gas expelled through the orfice
(as a ratio) reaches a lower design limit. At that point, as the gas
pressure decreases, the percentage (not just the total volume) of primary
air that the burner entrains goes DOWN dramatically.

So a burner that can entrain 72% primary air at 11" W.C. (with the air
shutter wide open) and 68% at 8" W.C., and 66% at 6" W.C., may only be able
to entrain 40% at 4" W.C. {numbers are just as an example....not actuals}.
The designers for XYZ Burners may have then decided that the acceptable
published working range for this particular burner is 6-11" W.C., because
that 66-72% range of primary air is an acceptable range to them. The
sudden rolloff of % of primary air as the gas pressure gets down below 6"
W.C. is unacceptable to them.

On a raku kiln that has little "draft" with which to pull in secondary air,
the efficiency of the setup is quite dependant on the capacity of the
burner system itself. If the venturi mixer is not aerating the fuel well,
then the combustion will not be very efficient.

If you have to turn down your new burner TOO FAR, you may be heading into
this range of decreased potential to entrain primary air. At some point,
the "venturi" will not work much better than the straight pipe "weed
burner" . The manufacturers of venturi burners have these figures
plotted for their burners (part of the engineer's design work). You can
obtain this information form the suppliers.

If the raku kiln is a commercial unit, the manufacturer can tell you the
required BTU's for optimum design performance, based on insulating value,
heat storage, interior volume, and inlet and exit flue sizes. Then get the
specs on the new burner. It should put out the necessary BTU's (as
stipulated by the kiln mfgr.) at a pressure rating that is in the burner
mfgr.'s accepted "working range" for the % of primary air it is designed
to entrain.

Note the pressure setting for the desired target BTU's through the orifice.
Have a pressure guage on your control setup........... You shouldn't
exceed this pressure by much in practice.

Another $0.02 to throw into the "possibilities pot" .



John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA