John Baymore on sat 6 apr 02
John,
Hi. Interesting question / problem. Some thoughts from a longtime kiln
builder and wood firer.........
We're in the process of rebuilding the chimney on the wood-burning kiln a=
t
the ceramics department at the University of Oregon. We need to move the
chimney back a few feet, making the kiln longer, and we need to almost
double the height of the chimney, to about 26'. .....................
..............
The kiln fires great right now, but the chimney needs replacement. We
don't =
want to loose the quality of our fires! Right now there is a very readabl=
e
flame =
column out the top during reduction. With the increased height we're
worried =
that we may loose that key indicator. We're considering a secondary dampe=
r =
to allow fresh air in the chimney at the base so it draws less from the
firebox if =
we need to offset the greater draw.
Have a couple of questions. First of all, exactly WHY are you going to 2=
6
feet from about 13? Second.... is this kiln an anagama, noborigama, mor=
e
traditional single chamber downdraft, or .......... ????????
Without those answers.... I'll ramble on here anyway ...........
My assumption (yeah....dangerous ) is that this is to bring the height=
to what is being required by your current local "code" to have it termina=
te
X feet above the nearest structure for a distance of X feet diameter arou=
nd
the location of the chimney. My guess is that at the time the kiln was
built, either there WERE no codes in effect, no one was enforcing such
codes, or no one even asked. Now for some reason....... you are having t=
o
deal with it since the chimney has to be rebuilt for other reasons
(refractory deterioration, adding a chamber to a noborigama, etc.).
Or, it's possible that another possibility here is that the smoke from th=
e
kiln has become an issue over time.... either just in general now that th=
e
community has experienced the real "impacts" from the firings....or there=
has been some new construction put in since the kiln was originally built=
that has "changed" the nature of the general area.
First of all....... the passive dampers you mentioned are a GREAT idea. =
Put in more square inches of opening than you think you will ever need. =
Easy to close off if too much....... hard to open up MORE after the fact
.
So......... initial impression.......... 26 feet of potter-built hard bri=
ck
refractory chimney is getting into the realm of having some concerns for
structural stability. That's a pretty tall chimney. Pay close attention=
to level and plumb! Make sure the foundation is DEAD level to start with=
. =
Taper the chimney for structural if not draft flow reasons . You are
going to need to look at how that kind of chimney is braced and anchored
for sway issues....particularly DURING the construction phase.
Also look at possible base loading on the foundation you are putting in
for it. If the foundation tips....... that is a TALL chimney. A couple =
of
degrees off at the base is a LOT of angular displacement 26 feet up. =
Assuming an 13.5 x 13.5 internal XC (I don't know the size of your kiln=
),
a 4 1/2 wall thickness (thin for this height) and 8 lbs per brick for
hardbrick, the total weight of that chimney is about 8300 lbs, which is
about 2370 pounds per square foot dead loading if you construct it all of=
hard brick on a 3.5 sq. foot footprint. =
And ......... just another can of worms....... are you in an earthquake
zone ?
Another route for you to consider is to go to castable for the chimney
structure. If you do, follow the manufacturer's recommendations for
mixing, ramming, curing ,and firing exactly. The biggest problem for
forming 26 feet all in one shot is that there will be voids in the castin=
g
where the castable traps air and doesn't flow. You'll need a vibrating
system to get the castable to flow smoothly and densely into the tall for=
m.
It will be easier to cast in sections ..... but that approach opens up
bracing and joining the sections for structural stability. You'll need t=
o
look at how the sections interlock and seal, and hoe they are held togeth=
er
solidly.
Sonotube is great internal form material.... burns right out first firin=
g
. And the twisted corrugated galvanised culvert material is great for=
the outside for weather protection and some structural support. Make the=
walls thick enough to keep the cold face temp down and to provide
structural integrity! Problem with fired in place castable is that the
interior hot face is fired....and the exterior cold face is not. This se=
ts
up laminar stress cracks parallel to the hot face surface..... which can
contribute to the spalling (cracking off) of the hot face layers over tim=
e.
I always prefer to pre-fire castable casts in another kiln before using
them so that they are fired all the way through. In this case....pretty
hard to do . If you go this route, cap the chimney so that water
doesn't get to the unfired castable's outer layers. Many of the weight a=
nd
stability issues I mention relative to 26 feet of all-brick apply to this=
solution too.
I would recommend that you obtain a Dwyer Inclined Tube Manometer Mark II=
I
Model #25 that measures pressure differentials in .01 increments of an in=
ch
WC......... and measure and record the pressure differentials on your
existing chimney (referenced to atmospheric) somewhere near the base of t=
he
chimney during various parts of the firing. You'll need to make the
"probes" to insert into the chimney....metal will work but will deteriora=
te
over time....refractory is the way to go for longevity but a pain to make=
. =
That tool will give you very useful hard data for adjusting the NEW
chimney's performance so that the firings are somewhat similar. Put the
manometer back on the newly configured chimney.... in about the same
location and manner as before...... and match up the readings by adjustin=
g
your passive (and active) dampers. BTW..... the passive dampers should b=
e
located ABOVE the pressure measuring point.
The pressure differential between surrounding atmospheric and the interna=
l
pressure in the kiln is the driving force that creates flow into (and out=
of) the kiln. The larger the pressure differential... the greater the fl=
ow
through a given sized opening. Staying away from some possible friction
issues, the possible change in the temperature of the effluent at the EXI=
T
point of the system, and the effects of laminar flow issues along the
internal surfaces created in the new chimney.......... you can sort of sa=
y
that if the new chimney has the same internal XC and the measured pressur=
e
differential is the same..... the draft flow volume is about the same. =
Close enough place to start from anyway . U of Oregon......... state
school...... hey...... as they say....... "close enough for government
work" .
One approach to this problem that gets the effluent up higher but has les=
s
impact on the draft induced by the chimney is to utilize a non-contigious=
chimney. Put your new 13 foot tall refractory chimney in the location yo=
u
need it for other reasons than height. On top of this chimney BUT NOT
DIRECTLY CONNECETED TO IT add a 13 foot extension that is really a passiv=
e
hood system. You'll need to construct a flared "hood" that is larger tha=
n
the OUTER edge of the refractory chimney structure by a couple of inches,=
and this "hood" will need to smoothly taper down to the size of the pipe
that will act as the upper "chimney". The hood part is NOT sealed to the=
refractory chimney...... cold air is allowed to mix into the effluent
stream here. The outer edge of the bottom of the hood is mounted just a t=
ad
BELOW the top edge of the refractory chimney's termination point. This
will result in a gap around the refractory chimney of a couple of inches =
of
space on all four sides. air will flow into this gap during the firing. =
This tends to isolate the draft effect of the upper section a bit.
In this case, the upper chimney will be an all metal unit. You will need=
to look at how to structurally stabilize this extension..... bit of an
engineering problem . Steel braces, guy wires, and so on will probabl=
y
be necessary. But this will be FAR less weight than an all refractory
chimney. It will also limit the weight issues should the chimney ever
actually "topple" or the momentum/ intertia stuff should it be subjected =
to
inadvertant and unplanned sideways forces. The vertical component of the=
weight of the metal unit can be borne by the refractory structure
underneath. Tie the base to the steel strapping for the refractory
chimney. Watch for sway and lateral wind loading issues of the metal
structure even though they will be pretty minimal......... =
And, of course, keep in mind earthquakes, trucks backing up carelessly,
tired potters tripping in the dark, and so on no matter HOW you decide to=
do this whole new chimney bit . =
Given your description of "a column of flame out the chimney" for the way=
you tend to fire...... the lower part which starts with the metal hood
probably should be stainless steel for at LEAST 6 feet..... then you can
probably change to regular steel. However...... at that point you probab=
ly
should just stay with the stainless anyway....and extend the life of the
whole thing sitting up there 13 to 26 feet in the air . Won't cost a=
ll
that much more. The introduction of fresh air into the effluent stream a=
t
the hood to chimney junction will likely result in further combustion of
the unburned gases exiting the chimney as the firing progresses .... and
can result in pretty high stack temperatures just above that juncture. (=
In
effect....you have sort of created a huge aspirating burner right there
.) Stainless will be necessary if this happens...... which I expect i=
t
will. Any "stand offs" that act to mount the "hood" to the steel strappi=
ng
on the refractory chimney likely should be stainless also....IF they are
exposed to much in the way of heat or fumes near the connection point. A=
ll
other bracing/guys for the hood system can be common steel. Make sure th=
at
the internal square inches of the metal chimney section is at least about=
the same as the internal square inches of the refractory chimney's EXIT
point (if you taper the refractory chimney).... preferably a bit more.
As a side benefit........ this extra burning of the effluent also could
reduce the smoke emissions for a part of the firing for you.
Your chimney's "pillar of fire" as a stoking indicator will change with a=
ll
this no matter what you do . You'll need to learn to "read" a whole
new set of indicators, I guess. Don't you have blow holes that serve thi=
s
function down where the stokers can easily see them? On my noborigama
there are small blowholes on the top sides of each chamber that are ducte=
d
into a place that the person stoking the main firebox can easily see.....=
monitoring the conditions in each of the four chambers after the dogima a=
t
the same time. If you don't have something like that .....you might see =
if
they can be easily added.
Or...... put the passive damper opening on the FRONT side of the chimney
within view of the stokers. When it is open you will be able to see into=
it from the stoking area and learn to "read" the conditions in the chimne=
y
from that nature of the "picture" you see there. No heat and flame shoul=
d
EVER back out of that port. Some radiant heat....yes.... but not hot
gasses. If you get hot gases ...... you have OTHER problems going on =
.
Hope all of this is clearer than mud , and something is of use to you.=
=
Good luck with the project....... and let me know what you end up doing.
Best,
..............................john
PS: If the metal part of the above system needs to penetrate a roof
structure...... then there are a NUMBER of other considerations.... which=
I
won't go into here.
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752 (s)
800-900-1110 (s)
JohnBaymore.com
JBaymore@compuserve.com
"DATES SET: Earth, Water, and Fire Noborigama Woodfiring Workshop Augu=
st
16-25, 2002"
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