John Baymore on tue 13 jul 99
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Can you talk a bit more about how one can determine where the flame front
is burning off? I think you mentioned the best place is right before the
exit flue. Does this give you better mixing, more even temperature, etc.?
And, would these assumptions be the same for venturi vs. forced air
burners?
(snip)
Hi Shelly,
You can generally SEE where the flame is burning off if you have visual
access to that area. That is the =22end=22 of the flame. Seeing flame =
usually
indicates that the fuel gases are still combusting....so they are still
capable of causing reduction in elements from the clay and glaze. The
presence of flame is not always the actual end of the excess fuel......
sometimes the (relatively) cold chimney refractories do something called
=22quench=22 the flame....it goes out....and there is still unburned fuel in
the transparent gases after the flame stops burning...which still do go up
the chimney. But in most cases at the normal reduction point the chimney
is hot enough to support reasonable combustion at least in the lower part.
Hopefully when a kiln is constructed, you placed a closeable port in the
lower chimney area at the floor level of the chimney. This serves the
double function of a clean out for any junk that ends up falling down there
(like pieces of a cracked damper slab that breaks and falls off or a dead
sparrow =3Cg=3E) and also for viewing the flame as it enters the chimney. =
Or,
depending on design....... if there are flue collection channels or a flue
collection box or whatever....there is a port into that space for viewing
conditions inside. Flue collection areas outside the ware chamber can be
treated as if they were part of the chimney in this discussion.
Lacking these viewing ports, sometimes you can sort of step back a bit from
the chimney and =22see=22 through the fine cracks in the hardbrick (if that =
is
what the chimney base area is made of). Or if you can locate one or two
strategic cracks between bricks that are in teh correct area. BTW......if
you can REALLY see into the chimney through such cracks.......it is time to
fix the chimney =3Cg=3E........ it is leaking cold air in and decreasing =
draw.
Sometimes the damper slab fits so poorly in the damper slot that this is a
space that you can see into the chimney too. This is generally not great
design..... cause a lot of air gets sucked into the hole around the damper
which decreases draft too. But it can be used if you need to see in =
=3Cg=3E.
When not looking, block the open space with wet clay or bricks or something
to restrict air flow (NOT lose fiber blanket, please).
Lacking visual access to the chimney you can sometimes use auditory cues
too. LISTEN to the chimney for the sound of flame. Try different
settings...and listen to the changes in sound not only at the burners bu in
the chimney. If it sounds like an F-15 warming up for takeoff....you've
got a lot of flame in there =3Cg=3E.
Actually, you want the flame front burning off just to the chimney side of
the flow path...... not just before exiting the ware chamber. In most
typical potter-type kilns this point is at or just below the damper slab.
Because there are SO many different designs of kilns in use by studio
potters it is difficult to describe the exact details in a totally generic
sense. Some kilns have the damper almost AT the exit from the
chamber..............and others have the damper about 20 FEET from the exit
from the chamber.
This practice is mainly for fuel efficiency ........... if the flame is
burning way up somewhere in the chimney you are wasting fuel (and creating
unnecessary pollution). If the =22massive flame in the chimney=22 condition=
is
necessary in order to get adequate results in the ware chamber of your
kiln....you might want to look at making some design modifications to the
kiln/burner system.
Burning the flam front off just as it enters the chimney all assumes that
the kiln/burner system actually works well. If there is poor mixing of the
gases (not meaning fuel here) in the kiln chamber....then in order to get
adequate reduction in the places that have the LEAST reducing conditions,
you often have to effecticvely over-reduce the other parts. (By
over-reduce here I mean reduce more than the minimum that is necessary to
get desired results....not screw-up the pots.) This usually results in the
flame buring off somewhere well up in the chimney. But to do other than
that, usually results in spotty reduction in such kilns. This is typical
of most wood kilns....... by modern industrial standards...they don't mix
well. (And by God...I love em' =3Cg=3E.) There are a lot of marginal kiln
designs relative to mixing in use by studio potters....so this need to get
the less reduced areas adequately reduced is the basis for a lot of the
=22seat of he pants=22 firing methods you find in use.
I mentioned above the =22kiln/burner system=22. It is important to think =
this
way. Put a =22good=22 burner on the wrong kiln design and you can get poor
results. Put a poor burner on a kiln design that compensates for the
burner's shortcomings....and you can get performance that is far better
than the burner's =22reputation=22 by itself. The burners and the kiln
together form the combustion system. They have to be designed to work
together. If the kiln is hooked to an =22external=22 ventilation =
system....you
also often have to add this into the =22system=22 considerations. This is
mostly the stuff that determines mixing and heat distribution (evenness).
In theory, it matters not whether you use any different type of burner.
However, low pressure (4-11inches WC) venturi burners placed on a kiln with
marginal design will often cause poor mixing in the chamber since even the
best of these burners entrains very little primary air ..........and often
gets the results mentioned in the paragraph above on spotty reduction. Or
even with burners that can supply into the excess air realm...... if
operated in certain ways, the same thing can happen. On a well designed
and operated system it would not really make any difference.
Technology can help you in learning here. Having an oxygen probe (AIC or
Storm Eq.) or a withdrawal CO2 analyzer (like the Fyrite) makes this much
easierto get the balance right and optimal. Having such accurate and
reproduable data can help you learn a lot about firing and increase your
understanding of the visual, auditory, and olefactory sensors you have
built into YOUR systems =3Cg=3E. When firing..... you want to use all your
senses. Keep accurate resords. Having more scientific data to compare to
the more intuitive stuff helps you make cause and effect relationships
clear in your mind. You can learn a lot about firing from this. It will
help speed your aquiring real firing skills.
(Of course, if you blindly just =22fire by the numbers=22....... and not =
make
any attempt at learning about what the numbers mean and how that relates to
what it is you want to DO, it can sort of become a crutch too. But let's
not get into that one again =3Cg=3E.)
So..... synopsized...... the visual presence of flame usually (but not
always) indicates that the gasses are still combusting.... and there is the
possibility of reduction happening in the areas where the flame is. If you
put flame up the chimney very far..... you are wasting fuel...cause you
don't need it there. You tell where the flame is by looking and listening.
It shouldn't make a diference about burner type... but it often does due
to kiln design variations. Having some instrumentation is an aid in
learning about firing.
Hope this makes the thoughts from the earlier post clearer than mud =3Cg=3E.
BEst,
.....................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40compuserve.com
John.Baymore=40GSD-CO.COM
=22Earth, Water, and Fire climbing kiln firing workshop Aug. 20-29,1999
-one space available=22
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