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stacks for pottery kilns

updated wed 17 feb 99


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.

Marc Ward

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.....

Marc Ward

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

BTU/Hr. input.

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

the kiln

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.

William Brouillard
2662 West 14th Street
Cleveland, Ohio 44113