BJ Clark | Stinking Desert Ceramics on sat 13 aug 05
Well, I guess all I'm saying is that it's kind of short sighted to
say, "Well, all gas kilns should just have a X sq in. flue."
And basing your guidelines on whether you bought crappy burners and
can't stack a kiln well is just silly. That's how people end up with
kilns that just plain won't fire.
My question is then, How does one compute "with the damper all the way open
or at least most of the way open how small can you make the flue and
still have the atmospheric control you need" before you even start on the k=
iln?
--=20
BJ Clark
Stinking Desert Ceramics
bjclark@stinkingdesert.com
www.stinkingdesert.com
On 8/13/05, Louis Katz wrote:
> You can think that if you want. It may have been good advice when the
> book was written but it was written on paper not stone.
> If you have no better information following Olsen seems smart.But Lots
> of people break all sorts of guidelines he gave with great success. How
> big is your chimney? Is the kiln forced air? Are your venturi burners
> good quality or poor? How tight is the kiln? All of these things have
> an impact on flue sizing. I would say "with the damper all the way open
> or at least most of the way open how small can you make the flue and
> still have the atmospheric control you need?"
>=20
> Louis
>=20
> On Aug 13, 2005, at 7:46 PM, BJ Clark | Stinking Desert Ceramics wrote:
>=20
> > I thought flue size was directly porportional to the size of your
> > primary air sources (buner ports). That's what the Olsen book says.
> >
> > Well, he really has 3 way.
> > 1. Total inlet flue size.
> > 2. 2 1/2sq.in./CF kiln space.
> > 3. For ever 7k BTU, 1 sq.in. exit flue. (1mil BTU =3D 143 sq.in exit
> > flue).
> >
> > I mean, If you have a 9 burner 100+Cu Ft. updraft (as we do at my
> > college), 40 sq inches might not do it.
> >
> > Book Fight! Book Fight!
> >
> >
> > --
> > BJ Clark
> > Stinking Desert Ceramics
> > bjclark@stinkingdesert.com
> > www.stinkingdesert.com
> >
> > _______________________________________________________________________
> > _______
> > Send postings to clayart@lsv.ceramics.org
> >
> > You may look at the archives for the list or change your subscription
> > settings from http://www.ceramics.org/clayart/
> >
> > Moderator of the list is Mel Jacobson who may be reached at
> > melpots@pclink.com.
> >
> >
> Louis Katz
> http://www.tamucc.edu/~lkatz
>=20
> _________________________________________________________________________=
_____
> Send postings to clayart@lsv.ceramics.org
>=20
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>=20
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclin=
k.com.
>
mel jacobson on sat 13 aug 05
the first thing you should do is get nils lou's `art of firing`.
he claims, and i agree...that most openings are far too large.
i see 81 square inch all the time.
heating sky. not pots. far too much wasted gas.
we have used the double venturi many times.
it works.
since 1960 i have had an opening into my flue box of 35 square inches.
then another opening to my stack of 35 square inches.
i had this long before nils wrote his book.
i just plain, in the early years did not want to bridge or corbel
the bricks for the opening..and i thought that one brick was big
enough.
it has worked just great for a long time.
if i had blowers, forced compressed air, i would re/evaluate.
maybe open it a bit more. but, never 81 square inches.
this is old stuff.
lots of talk around. some want huge openings. seem to think
it is safer.
it has a great deal to do with how much gas/volume/pressure you
have coming into the kiln. i run low pressure/low volume so i need
the small flue.
if you had 15 iwc and a blower on full, and had a small opening
...well you might have trouble.
of course the back pressure at the peep would be 30 inches long.
lots of dumb clucks out there.
i have seen that scenario. make me weep.
i still tell folks when i build a kiln...keep it at 40 sq..or
smaller. about 6.5x6.5 in.
then do it again when you add the stack.
nils`book has all the information you need.
mel
from mel/minnetonka.mn.usa
website: http://www.pclink.com/melpots
http://home.comcast.net/~figglywig/clayart.htm
for gail's year book.
Wayne Seidl on sat 13 aug 05
But Meeeelllll (he whines)...
How am I gonna roast my hot dogs and toast my marshmallows if I
don't have at least 30 inches of flame coming out the peep?
(You can get in line behind Dolita at NCECA )
Wayne Seidl
if you had 15 iwc and a blower on full, and had a small opening
...well you might have trouble.
of course the back pressure at the peep would be 30 inches long.
lots of dumb clucks out there.
i have seen that scenario. make me weep.
Hank Murrow on sat 13 aug 05
On Aug 13, 2005, at 9:37 AM, mel jacobson wrote:
> the first thing you should do is get nils lou's `art of firing`.
>
> he claims, and i agree...that most openings are far too large.
> i see 81 square inch all the time.
> heating sky. not pots. far too much wasted gas.
> since 1960 i have had an opening into my flue box of 35 square inches.
> then another opening to my stack of 35 square inches.
> i thought that one brick was big
> enough. it has worked just great for a long time.
Dear Mel;
I concur. My Doorless Fiberkiln has a flue outlet of 19.6 sq in. and
works fine, never having to open the damper more than two thirds, even
for an oxidizing fire. The kiln is 27 cu ft stacking volume.
Cheers, Hank
www.murrow.biz/hank
Gordon Ward on sat 13 aug 05
Hi Hank,
Perhaps stating the obvious here, but seems like efficiency is a big
factor. Reduced fuel requirement = smaller flue opening requirement?
No?
Cheers,
Gordo
On Aug 13, 2005, at 10:47 AM, Hank Murrow wrote:
> My Doorless Fiberkiln has a flue outlet of 19.6 sq in. and
> works fine, never having to open the damper more than two thirds, even
> for an oxidizing fire. The kiln is 27 cu ft stacking volume.
>
> Cheers, Hank
BJ Clark | Stinking Desert Ceramics on sat 13 aug 05
I thought flue size was directly porportional to the size of your
primary air sources (buner ports). That's what the Olsen book says.
Well, he really has 3 way.
1. Total inlet flue size.
2. 2 1/2sq.in./CF kiln space.
3. For ever 7k BTU, 1 sq.in. exit flue. (1mil BTU =3D 143 sq.in exit flue).
I mean, If you have a 9 burner 100+Cu Ft. updraft (as we do at my
college), 40 sq inches might not do it.
Book Fight! Book Fight!
--=20
BJ Clark
Stinking Desert Ceramics
bjclark@stinkingdesert.com
www.stinkingdesert.com
Louis Katz on sat 13 aug 05
You can think that if you want. It may have been good advice when the
book was written but it was written on paper not stone.
If you have no better information following Olsen seems smart.But Lots
of people break all sorts of guidelines he gave with great success. How
big is your chimney? Is the kiln forced air? Are your venturi burners
good quality or poor? How tight is the kiln? All of these things have
an impact on flue sizing. I would say "with the damper all the way open
or at least most of the way open how small can you make the flue and
still have the atmospheric control you need?"
Louis
On Aug 13, 2005, at 7:46 PM, BJ Clark | Stinking Desert Ceramics wrote:
> I thought flue size was directly porportional to the size of your
> primary air sources (buner ports). That's what the Olsen book says.
>
> Well, he really has 3 way.
> 1. Total inlet flue size.
> 2. 2 1/2sq.in./CF kiln space.
> 3. For ever 7k BTU, 1 sq.in. exit flue. (1mil BTU = 143 sq.in exit
> flue).
>
> I mean, If you have a 9 burner 100+Cu Ft. updraft (as we do at my
> college), 40 sq inches might not do it.
>
> Book Fight! Book Fight!
>
>
> --
> BJ Clark
> Stinking Desert Ceramics
> bjclark@stinkingdesert.com
> www.stinkingdesert.com
>
> _______________________________________________________________________
> _______
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
>
>
Louis Katz
http://www.tamucc.edu/~lkatz
Louis Katz on sun 14 aug 05
I f you have forced air burners that have some umph behind them then
you can get by with a flue thats real small. Most of my experience is
with kilns 25-50 cubic feet. Most of these kilns had roughly 4x5 "
burner ports. Four of them. Most had roughly 9" sq. exit flues. These
were always excessive except when the chimney was real short. The
Burner ports should have been round.
My suggestion would be to build an oversized flue and then constrict it
from firing to firing. In my soda kiln I use a piece of SiC in front of
the flue. I have an access hole that allows me to knock it over if I
find I need more air. I have never had to use this but it makes me feel
better.
Similarly if you are using atmospheric burners with adjustable orifices
the first few firings should be used to reduce the orifice size to a
minimum with the gas pressure at or near maximum. This gives you the
most kinetic energy in your gas stream allowing you to entrain more
primary air if you need it. It also makes your gas valve less sensitive
and easier to control ( smaller flue does the same thing to the
damper). Higher pressure and a smaller hole makes each little bit of
your gas move at a higher speed allowing it to drag more air along with
it.
If you are building a square chimney from brick and the brick is going
to go up more than six feet i would consider an 18" square on the
outside a minimum for structural reasons. My one try at a 13.5 x 18"
chimney was not as stable as I would like. (Not an issue with a big
kiln). If I was really going to frame it with angle this might not
matter. I often use low-duty fireplace brick above the first foot or
two of a chimney up two a foot over the arch on a six foot high kiln
and steel pipe at least 1/4 inch thick above that. It is important to
secure the steel pipe well. I inspect the pipe with a hammer to make
sure it is still solid enough at frequent intervals.
Despite my desire to build 18x18" brick chimneys the large surface area
at the top of a chimney makes it more sensitive to wind. If you know
you need much less use a smaller diameter steel pipe. There is probably
a good way to to cut down on wind sensitivity. If someone has a good
method I would like to know it. If the top of your chimney is well
protected from wind its probably not an issue. I frequently fire in 20
mph+ conditions.
If you have fired a similar kiln to the one you are designing in a
similar way and your damper has been closed down to 18 square inches
then using this as a starting point I see no reason to have an exit
flue of more than 36 square inches and probably less.
In high school 1974 I built a wood burning raku kiln with two boxes of
softbrick and a pile of red brick. I was taking calculus and gave
myself some design criteria and tried to maximize kiln volume with a
set number of brick. I ended up with a design requiring nearly every
brick to be cut. Instead I built a box, and made my firebox out of logs
below grade. I replace the firebox every few firings.
In building my flame throwing pipe organ i needed to size a manifold
pipe and went through a similar process. The manifold I was coming up
with was very complicated. Rather than eat up loads of cash buying
reduction fittings as I moved down a pipe with 60 taps in it, I just
built an obviously oversized manifold. The math was killing me. I would
not venture to tell anyone how to calculate a flue size. It is so much
easier to copy the best firing kiln you can find and try and tweak it
to make it fire better. I have told myself that I will never build a
kiln again with square burner ports unless there is a real compelling
reason to do so. I would let preferred kiln shelf sizes drive other
decisions. Flue size make it too big and then close it down. I love
good pyronics or eclipse mixers with adjustable orifices and flame
retention rings. I also love to fire junk, but would not want to do
this to make money on pots.
Also remember that a short restriction to 18 square inches has a
smaller effect than cutting the flue size over a long length of the
chimney.
There is a disadvantage to a large chimney. It takes longer to heat up
and in a downdraft this can lengthen the beginning of a firing
significantly especially if you are using pipe burners or other "junk"
or fire fast.
i guess my suggestion is 1. Do the calculations from Olsen or
elsewhere. 2. Look a similar kilns. 3 make sure your flue is big
enough. 4 close it down as you get to know the kiln.
Sorry if my last post seemed harsh or abrupt.
Louis Katz
http://www.tamucc.edu/~lkatz
http://www.LouisKatz.net
| |
|
