David Woof on wed 13 dec 06
Good words Mel, and I will chime in here because i don't believe that the
importance of chimneys, and how to build and operate a well designed one in
relation to wood fire will be ever said strong enough or completly. and I
don't believe heard, accepted or understood well enough by a majority of
would be wood firers. Witness the energy put out building kilns that after a
few attempted fireings sit abandoned by frustrated folks. There are many
kiln books published that go into much detail about all aspects of wood kiln
building, primary/secondary air ratios to chimney volume bla bla bla as if
the thing were an automobile engine or an arbitrarily tuned forced air gas
kiln, but fall flat in presenting a comprehensive gathering of information
re the physics behind the chimney and why it is the heart and soul of a well
functioning wood kiln.
Dig in the archives folks, we have been writing about and demonstrating
proof of what we speak since the wood fire chimney was a clayart topic.
One can adjust all the primary and secondary air and flip and slide dampers
till hell freezes but if the chimney isn't built for the task nothing
happens. adequate air/oxygen will whistle thru some really tight spaces in
the primary settings if there is adequate draft from the chimney.
And there is no excuse for a kiln to stall if these physical needs are
recognized and provided for.
David
_________________________________
_________________________________
David Woof Studio
Clarkdale, Arizona
Ph. 928-821-3747 Fax. 866-881-3461
________________________________
________________________________
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David Woof on thu 14 dec 06
Part 2, I got distracted and hit send instead of save so wish to add a
finish to the other post.
Location is everything, is the kiln built on a hill side, in a valley, at
sea level or 5,000 feet? is it between taller buildings or natural features
that effect air flow or prevailing winds? what happens to the draw when the
wind shifts or dies? When a storm is coming in or leaving? We could go on.
if one can think of it, it will eventually become a condition. Our
atmosphere is a soup we swim with our kiln in. The chimney is a device whose
purpose is to create an artificial atmospheric condition in the kiln. It is
however subject to the natural physical atmospheric conditions present,
which can and usually change at least two times each day of the fireing and,
if unrecognized and no adjustment ability was built into the chimney, can
adversly effect the kilns performance.
Mel said it, "the capacity of the chimney must be more than adequate. make
sure it is big enough"
So what is big enough? here is where formulas and their adherents break
down. A formula can't accomodate all the variables of physical location and
atmospheric condition. So plan to build it bigger than you can imagine
you'll ever need and add 10%. Then build in so many passive damper pull out
bricks that everyone else thinks you are crazy. If you don't ever use all
of them, good! but you had them available. Case in point. Karen T. and I
were designing her kiln at UW LaCrosse some years ago and we had the kiln
site between buildings where the wind would on occasion down draft severely.
Plenty of passive damper holes, un-bricked, allowed the wind coming down
the chimney, an escape when it encountered the riseing heat from the kiln
and so passed harmlessly out the passive dampers instead of blowing a spew
of embers thru the kiln and out the firebox openings. The chimney had to be
big enough and tall enough to draw well with all these passive dampers open.
Same principal as the old parlor oil fired space heaters with that T in
the stove pipe with the counterbalanced flapper that danced and kept the
wind from blowing out the flame in the oil burner pot.
Our thanksgiving fireing here in Arizona, at 3500 feet, we had an uneven
load in the cross draft catenary and there was comment, concern and
speculation from a couple of newer to this kiln folks about even fireing.
The back half was loaded comfortably tight with 6 feet of shelved mugs,
bowls and vases etc. and the front had two large round 45 inch jars and
whatever else we could tumble stack around them set two shelves (12") off
the floor.
I had designed the chimney with a center divider serving an exit from each
side of the kiln. nothing new, It's been done before, I've done it before
also, but the saving feature was the more than adequate size of the chimney
and the passive damper system. after a slow warm up to even the beginning
heat, and the kiln was starting to go, the amount of smoke and the velocity
of the smoke from each side told me which bricks to pull to balance the draw
thru both sides of the kiln. Even fireing, small cones 12 all flat down
side to side, fronts to back. (oh let me crow a bit, it was a good
fireing.) made possible by years of recognizing the part the chimney plays
in successful kiln function.
The passive dampers allow one not only respond to changeing conditions and
fireing loads but also allow a variety of fireing techniques and results.
One may cause a languid caressing flame which deposits much ash and flame
wraps, or blast furnace the pots for other flame, color and ash effects. I
think of the passive dampers and chimney as a kiln flute allowing me many
notes to play.
David
_________________________________
_________________________________
David Woof Studio
Clarkdale, Arizona
Ph. 928-821-3747 Fax. 866-881-3461
________________________________
________________________________
peering over the edge, reverently taking an irreverent look at everything.
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Hank Murrow on thu 14 dec 06
On Dec 14, 2006, at 1:50 PM, John Dellow wrote:
>> David you and mel have missed the point. I may not have posed the
> question very well. I have 30 years experience and a dozen kilns behind
> me but I have always wondered if a short fat chimney of volume "X" will
> draw as well as a Tall narrow chimney of the same volume "X" ?
> John
John;
Let me take a stab at this question. Since draft is a result of the
containment of hot gases in a vertical column, a well-insulated column
will retain the buoyancy of the gases longer(and increase draft). So an
insulated chimney will create stronger draft. If a chimney were to be
built very tall, it could lose so much heat before the gases left, that
buoyancy would suffer. There is then, a limit on how tall chimney can
be and still work. So, a short well-insulated chimney will produce
stronger draft than a poorly insulated chimney of the same
cross-section. Up to a point, building that same chimney taller will
increase the draft. So your tall narrow chimney, PROVIDED IT CAN MOVE
ENOUGH GASES TO SATISFY THE DEMANDS OF THE FIREBOX, will produce the
greater draft. Let me know if you feel this amswers your question.
Cheers, Hank
www.murrow.biz/hank
Lee Love on fri 15 dec 06
On 12/15/06, Hank Murrow wrote:
> On Dec 14, 2006, at 1:50 PM, John Dellow wrote:
> will retain the buoyancy of the gases longer(and increase draft). So an
> insulated chimney will create stronger draft. If a chimney were to be
> built very tall, it could lose so much heat before the gases left, that
> buoyancy would suffer.
The single most common mistake that people make with fast fire
wood kilns, is thinking they behave like gas or other wood kilns.
With a gas or oil kilns with blowers, the correct chimney length is
not as important.
>There is then, a limit on how tall chimney can
> be and still work. So, a short well-insulated chimney will produce
> stronger draft than a poorly insulated chimney of the same
> cross-section.
Insulation is the key variable. And remember, John wants
to cut down on smoke. A short chimney will smoke more. Length gives
more time for the particles to burn. John also wants to put burners
in his chimney to reduce smoke. If the chimney is too short, it
will not contain the combustion. You will just blow the flame out
the top of the chimney. This is why I think a small cantenary
chamber between the Phoenix and the chimney is the most effective way
to go.
PLEASE get a copy of Olsen's kiln book. You need to
follow his specs for firebox size, height, grate position and chimney
length. Every one I know that has fudged on these ratios have to
fire much longer and some that never reach temperature.
Olsen on chimney height in the fast fire:
"The chimney height should be three times the height
of the kiln's chamber plus the height of the firebox plus 1 additional
foot for every 3 feet of horizontal travel of the flue gases. For
altitudes over 4,000 feet (not a problem in Oz?), an additional 3 feet
of chimney or more may be needed. Also increase chimney by 50% and
exit flue size (square inches of area) by 40%."
Also, related to reducing smoke, see Kusakabe's book. A
Bourry box is probably better in these regards. I put up photos of
his smokeless kiln, and also another kiln a student of his drew for
me. She says this design in Canada is totally smokeless, except when
the two ware chambers are stoked at the end of the firing. The
longer the gases travel, the fewer particles escape from the kiln
unburnt:
http://groups-beta.google.com/group/WoodKiln/web/kusakabe-smokeless-kiln?hl=en
--
Lee in Mashiko, Japan
Minneapolis, Minnesota USA
http://potters.blogspot.com/
"Let the beauty we love be what we do." - Rumi
"When we all do better. We ALL do better." -Paul Wellstone
John Dellow on fri 15 dec 06
Hank Murrow wrote:
> John;
>
> Let me take a stab at this question. Since draft is a result of the
> containment of hot gases in a vertical column, a well-insulated column
> will retain the buoyancy of the gases longer(and increase draft). So an
> insulated chimney will create stronger draft. If a chimney were to be
> built very tall, it could lose so much heat before the gases left, that
> buoyancy would suffer. There is then, a limit on how tall chimney can
> be and still work. So, a short well-insulated chimney will produce
> stronger draft than a poorly insulated chimney of the same
> cross-section. Up to a point, building that same chimney taller will
> increase the draft. So your tall narrow chimney, PROVIDED IT CAN MOVE
> ENOUGH GASES TO SATISFY THE DEMANDS OF THE FIREBOX, will produce the
> greater draft. Let me know if you feel this amswers your question.
-- Thank you hank,
between your post and Lees, I can now be confident that the chimney I
intend to use will draw. It will have an inner layer of insulating brick
backed up with diatoms.Looks like 12ft tall and 13&1/2 inches by 27
inches cross section should do the job,
John
p.s. Mel thanks also :)
John Dellow "the flower pot man"
From the land down under
Home Page http://www.welcome.to/jkdellow
John Dellow on fri 15 dec 06
David Woof wrote:
> Part 2, I got distracted and hit send instead of save so wish to add a
> finish to the other post.
>
> Location is everything, is the kiln built on a hill side, in a valley, at
> sea level or 5,000 feet? is it between taller buildings or natural features
> that effect air flow or prevailing winds? what happens to the draw when the
> wind shifts or dies? When a storm is coming in or leaving? We could go on.
> if one can think of it, it will eventually become a condition. Our
> atmosphere is a soup we swim with our kiln in. The chimney is a device
> whose
> David ,you and mel have missed the point. I may not have posed the
question very well. I have 30 years experience and a dozen kilns behind
me but I have always wondered if a short fat chimney of volume "X" will
draw as well as a Tall narrow chimney of the same volume "X" ?
John
John Dellow "the flower pot man"
From the land down under
Home Page http://www.welcome.to/jkdellow
Lee Love on fri 15 dec 06
On 12/15/06, John Dellow wrote:
> > David ,you and mel have missed the point. I may not have posed the
> question very well. I have 30 years experience and a dozen kilns behind
> me but I have always wondered if a short fat chimney of volume "X" will
> draw as well as a Tall narrow chimney of the same volume "X" ?
>
John, no it won't. The adding to height of a chimney
creates a difference in atmospheric pressure compared to ground level.
The taller the chimney the more pressure you have working for you.
You do not increase this pressure by making the diameter larger.
Check out Olsen's kiln book. He explains how to calculate
the required height of a chimney.
A taller chimney reduces smoke too.
--
Lee in Mashiko, Japan
Minneapolis, Minnesota USA
http://potters.blogspot.com/
"Let the beauty we love be what we do." - Rumi
"When we all do better. We ALL do better." -Paul Wellstone
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