William Brouillard on tue 16 feb 99
The following are notes and short articles from clayart
members that concern stacks for pottery kilns
kiln stacks, an irrelevant observation: Brouillard
Years ago I was helping with the construction of a kiln. Since
it was going to be a student fired kiln and all of the burners
were located in the back of the kiln, I suggested that we use
some soft brick to insulate the bottom of the stack. The idea
was to make the outside surface of the hard brick stack cooler
at the base to avoid accidental burns in the cramped quarters
around the kiln controls. I used some salvaged K-23s from
the previous construction and added 4.5 inches of insulation
to bottom of the stack . I insulated to a height of about three
feet or just past the damper and burners. It kept that area
cooler and reduced the risk of burns.
The additional layer of insulation also changed the draft of the
kiln. Kiln draft, As I understand it, is dependent on the
temperature difference between the top and bottom of the
stack. The greater that difference, the greater the draft. In
general terms the higher the stack the greater the difference in
temperature between the top and the bottom of the stack.
The volume of the flue and stack must be adequate to the
size and type of the kiln and the interior surface of the stack
can affect the amount of drag or loss of velocity of the gasses
moving though it.
The insulated stack kept the bottom of the stack hotter than
before and the draw was greater. The better draw allowed us
to use a shorter stack and perhaps we could have used a
smaller flue.The use of forced air burners makes the height of
the stack a question of getting the exhaust out of the building
or past the roof line. Since the burners act force the exhaust
out of the kiln stack height can be reduced. Atmospheric
burners require greater stack height and sometimes greater
volume as well.
Salt kiln stack construction. Brouillard
The height and interior cross sectional area of the stack will
be determined by at least three things.
1. The type of burner system you are using. Forced air
systems require less in terms of height than do natural draft
systems. Natural draft systems depend on stack height to
develop velocity in the draft.
2. Local building, zoning or fire codes which may specify
certain types of construct or construction materials. You
should check with your local codes or Fire Marshall. It may
be that your stack has to extend a certain height above your
roof line or that it can not be loose brick construction and
must be mortared.
3. Salt firing is a vapor process that is usually done at high
temperature. The exhaust gases will be very hot and very
corrosive. That may eliminate some materials that could be
used on other types of kilns. Chimney liners and red brick
are probable out of the question. A ten foot stack with a 9" x
9" flue area would use approx. 300 standard size firebricks.
Any low duty firebrick should do the job. Scrap brick could
be used in the upper third of the stack where the heat and
corrosive effects of the vapor will be less..
Many potter lay the brick for the stack dry or with a 50/50
mix of fireclay and sand. It fills the gaps but comes off easily
when rebuilding. The base of the stack should be two
courses thick if laid dry. It will be more stable and provide a
better heat differential for the stack. I usually put insulation
brick as the outside layer to protect students from being
burned on the hot firebrick and to increase the heat
differential which increases the velocity of the flue gasses. A
frame of angle iron will hold the loose bricks in place and
provide stability for the stack.
The rule of thumb for chimney height is: 3 times the linear
draw plus 1/3 the interior height. So, if your kiln is 36 inches
deep..., then you go through 9" of wall..., then another 9" to
the back inside of the chimney, you have a linear draw of 54".
If your kiln is 48" tall, 1/3 of your height is; 16". 54 x 3 = 162
inches.....add the 16" and you ge78" or 14.8 ft. You can
share the back wall (make sure every few courses is "tied"
into the wall) with the chimney or have it set back a few
bricks...Whatever works for you. Since you're salting, I would
go ahead and use hard brick. My apologizes to the rest of
the world that has enough sense to use the metric system.....
Subject: Flues and pipes
Sun, 25 Aug 1996 19:07:55 EDT
From: "Karl P. Platt" <71562.1555@CompuServe.COM>
Someone was wondering about flues and chimneys.........
Someone also mentioned that kilns with forced air
combustion didn't require much of a stack. They don't, but if
the flue is too small the kiln will run under pressure and more
likely than not all of the metal holding it together will get
fried...fried. Alternatively, if you're using dinky blowers, like
one sees on Alpine kilns, it's not too hard to find that the
pressure in the kiln caused by undersized (or choked off)
flues can actually exceed that of the gases (flame) issuing
from he burner, which sets the stage foost of combustion
difficulties -- and fried burner tips.
The height of a stack makes a the difference between whether
a kiln will work or not in a natural draft kiln -- one which
used "venturi" burners or solid fuel like coal or more typically
wood. In designing the flue/stack there are three things to
consider. 1. The size of the flue opening
2. The size/length of the breeching (the section between the
flue and the stack. 3. The stack diameter and height. In
general the nominal size of any of these depends on the
quantity of gas they must handle -- which in the case of
studio scale kilns is quite small relative to what one would
have needed for an old brickyard beehive of the early 20th
century. Most studio kilns operate at way less than 1 million
Taking the case of natural gas and a kiln that fires to 2,200 oF
it is noted that the volume of the cold air-gas mix input to the
flame expands about 5x. So, in a kiln using 400,000 BTU/Hr
the volume of flue gases evolved is 24,200 cubic feet/Hr. If
the kiln is, say, 80 cubic feet, this means that the gases inside
change every 10 seconds or so -- this sort of relation is also
the case in smaller kilns which, in general, require more heat
on a unit volume basis.
The numbers for LPG aren't all that different insofar as it still
requires roughly 10 cubic feet of air for every 1,000 BTU
released and the preponderance of gas being moved through
the kiln was introduced as air. The flue opening, while it is
intended to furnish an exit for the flue gas, also presents a
resistance. The breeching also presents a resistance and
there's resistance to the flow of flue gas up the stack --
regardless of the type of burner arrangement used. Since it's
much worse to build a stack that's too small than one that's
too large, it's always better to build them oversized -- but not
too much. A fleu that's too large wll probably cause a cold
spot in its' proximity.
There's actually a formula for working out generally how
many square inches of flue are desirable for a given volume
of flue gas, and the interested can go look at pg. 226 of the
North American Combustion Handbook for the gory details.
Generally speaking, however, considering that a studio kiln
will operate at less than 1 million BTU/Hr and seldom sees
much more temperature than 2,200 degrees F., 9" x 9" flues
are a handy -- and modular -- size for kilns operating between
500,000 and 1 million BTU/Hr. Smaller than that, say down
to 250,000 BTU/Hr., a 4.5" x 9" flue will do the trick quite
The size of the stack is usually determined by the setting of
the bricks making it up, but again, the 9" x 9" (81 in^2) is
usually more than adequate for Studio needs. There's
nothing lost by making it bigger as draft depends on height --
where area comes into play is in determining the resistance to
the flow of the flue gases.
Subject: Re: Source for Kaowool pipe liners
Date: Sun, 18 May 1997 10:37:03 EDT
From: Nils Lou
Kaowool chimney liners can be purchased from
Fire Brick Supply,
1407 Marshall av, St Paul, MN 55104
ask for Dona , They are about $10 each.
One company who manufactures them is Thermal Ceramics in
Augusta, GA. I have 2 numbers for their headquarters: (706)
555 1212 and (706) 796 4200.
They will probably refer you to a distributor. The one I went
through was the Philadelpha office at (610) 293-9750. They
will probably refer you to a a supply house. I actually
purchased my liners from P&H Sales in Hanover, Maryland at
(410) 859 8460.
Thermal refers to the liners as "riser sleeves". The 8-1/2" ID
variety come in 1 foot lengths in boxes of 8. The sleeves fit
quite nicely inside 10" galvanized stove pipe, which you can
get in 2 foot lengths at your local hardware store. BTW I
have Nils Lou to thank for this information. You can find it in
his excellent book, the Art of Firing.
2662 West 14th Street
Cleveland, Ohio 44113