Earl Krueger on wed 14 apr 04
I've been thinking about building a small, < 1 m3,
gas kiln in order to try reduction effects and learn
about firing a gas kiln.
Intuitively it seems that downdraft or crossdraft
designs would provide more uniform temperatures
than does an updraft. However, these designs
usually require a rather tall chimney in order to
achieve the required draft. Since I would like
to build the kiln on a cart that could be rolled
out of my garage for firing and then back in for
protection from weather, a tall chimney wouldn't
work.
So, my idea is to build a chimney only as tall as
the kiln but extend it down below where the flue
from the chamber enters the chimney. Then
arrange a squirrel cage blower to force air up
the chimney from below the flue. By creating
a venturi in the chimney you could create a
partial vacuum at the flue that would provide the
required draft through the chamber. The draft
could be controlled by adjusting the amount
(velocity) of air forced through the chimney.
Would this work? (Assume venturi burner.)
Crude schematic, not to scale:
(use courier font)
-----------| C |
| H |
Firing | I |
Chamber | M |
| N |
|\ E /
- --- ----| Y |
Flue ---> |
------------| / / | | | <--- damper
| |__|_____
\ |
\ <----- Forced Air
\_______________
Thanks for your comments.
Earl K...
Bothell, WA, USA
Earl Krueger on thu 15 apr 04
Thanks everybody for your comments, but they only raise more questions
On Thursday, Apr 15, 2004, at 08:37 US/Pacific, Bruce Girrell wrote:
> You would only need to be sure that the forced flow was vertical and
> mostly laminar as it passed the outlet of the kiln.
Bruce, You are correct. Might be a trick to get this just right. If
the venturi was not correctly designed and well built then Taylor's
concern about air flowing the wrong way might come true.
Dave, it seems to me from what I've read on Clayart that updrafts are
much more sensitive and harder to control. Is this real or just my
interpretation?
I've read Olson's book but it seems a little dated. I will pick up
Nil's book on Saturday.
On Thursday, Apr 15, 2004, at 09:16 US/Pacific, Dave Finkelnburg wrote:
> I do like a chimney extension, just to get the hot kiln gas up above
> your head, though, for safety on a windy day. That could be as simple
> as a piece of culvert pipe you set on top of the chimney.
By "culvert" pipe are you referring to something like concrete or clay?
I was thinking that by pumping a lot of cool air in at the base of the
chimney if I wanted a taller chimney I would be able to use light
weight PVC or regular stove pipe.
Paul, For some reason I was avoiding forced air burners but now that
you mention it some thoughts do come to mind. Wouldn't using a forced
air burner slightly pressurize the kiln, whereas using a draft chimney
put the kiln under slight vacuum? Would this have an effect in terms
of producing even reduction? I guess it might depend on how tight the
kiln was built.
On Thursday, Apr 15, 2004, at 15:26 US/Pacific, Hendrix, Taylor J.
wrote:
> What would keep the forced air from entering the ware
> chamber and exiting the burner ports?
Taylor, With a properly designed venturi Bournelli's law from physics
says there would be a lower pressure in the chimney than in the ware
chamber. Of course if the back pressure of the chimney above the
venturi were too high air could be forced backwards through the flue.
So, it is definitely something to watch out for.
Regards.
Earl K...
Bothell, WA, USA
Bruce Girrell on thu 15 apr 04
> By creating
> a venturi in the chimney you could create a
> partial vacuum at the flue that would provide the
> required draft through the chamber. The draft
> could be controlled by adjusting the amount
> (velocity) of air forced through the chimney.
Earl,
I'll wait for others who are more knowledgeable than I in this department,
but to me it sounds like an excellent idea. You have the damper in the right
place. I don't see why this wouldn't work. You would only need to be sure
that the forced flow was vertical and mostly laminar as it passed the outlet
of the kiln. I would also guess that you would have much better control over
the draft this way than with a "normal" damper, or what we were calling an
"obstruction" damper in a thread a little ways back.
Bruce "just guessing though" Girrell
Dave Finkelnburg on thu 15 apr 04
Earl,
A small gas kiln would be great. You could just make it an updraft, and
it might be easier. :-)
Or, use a forced air burner, of the design shown by Nils Lou in his
book, "The Art of Firing," and you won't need a chimney any taller than the
kiln. I do like a chimney extension, just to get the hot kiln gas up above
your head, though, for safety on a windy day. That could be as simple as a
piece of culvert pipe you set on top of the chimney.
Regards,
Dave Finkelnburg
----- Original Message -----
From: "Earl Krueger"
To:
Sent: Wednesday, April 14, 2004 11:47 PM
Subject: Downdraft kiln design variation
> I've been thinking about building a small, < 1 m3,
> gas kiln in order to try reduction effects and learn
> about firing a gas kiln.
Paul Herman on thu 15 apr 04
Hi Earl,
The question comes to mind, why not just use the blower on the burners
instead? It seems like it might be simpler, and a short chimney would be
adequate in that case.
good firings,
Paul Herman
Great Basin Pottery
Doyle, California US
http://www.greatbasinpottery.com/
----------
>From: Earl Krueger
> I've been thinking about building a small, < 1 m3,
> gas kiln in order to try reduction effects and learn
> about firing a gas kiln.
>
> Intuitively it seems that downdraft or crossdraft
> designs would provide more uniform temperatures
> than does an updraft. However, these designs
> usually require a rather tall chimney in order to
> achieve the required draft. Since I would like
> to build the kiln on a cart that could be rolled
> out of my garage for firing and then back in for
> protection from weather, a tall chimney wouldn't
> work.
>
> So, my idea is to build a chimney only as tall as
> the kiln but extend it down below where the flue
> from the chamber enters the chimney. Then
> arrange a squirrel cage blower to force air up
> the chimney from below the flue. By creating
> a venturi in the chimney you could create a
> partial vacuum at the flue that would provide the
> required draft through the chamber. The draft
> could be controlled by adjusting the amount
> (velocity) of air forced through the chimney.
>
> Would this work? (Assume venturi burner.)
>
> Crude schematic, not to scale:
> (use courier font)
>
> -----------| C |
> | H |
> Firing | I |
> Chamber | M |
> | N |
> |\ E /
> - --- ----| Y |
> Flue ---> |
> ------------| > / > / > | | | <--- damper
> | |__|_____
> \ |
> \ <----- Forced Air
> \_______________
>
>
> Thanks for your comments.
>
> Earl K...
> Bothell, WA, USA
>
Hendrix, Taylor J. on thu 15 apr 04
Earl,
A quick thought: What would keep the forced air from entering the ware
chamber and exiting the burner ports?
Taylor, in Waco
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Earl
Krueger
Sent: Thursday, April 15, 2004 12:48 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Downdraft kiln design variation
...
Crude schematic, not to scale:
(use courier font)
-----------| C |
| H |
Firing | I |
Chamber | M |
| N |
|\ E /
- --- ----| Y |
Flue ---> |
------------| / / | | | <--- damper
| |__|_____
\ |
\ <----- Forced Air
\_______________
...
John Baymore on fri 16 apr 04
So, my idea is to build a chimney only as tall as
the kiln but extend it down below where the flue
from the chamber enters the chimney. Then
arrange a squirrel cage blower to force air up
the chimney from below the flue. By creating
a venturi in the chimney you could create a
partial vacuum at the flue that would provide the
required draft through the chamber. The draft
could be controlled by adjusting the amount
(velocity) of air forced through the chimney.
Would this work? (Assume venturi burner.)
Earl,
Hi. Absolutely it will work. What you have "invented" is the draft
inducer. They exist commercially. I've designed and built a couple
"homebrew" versions of them over the years to solve specific weird design=
issues for a particular kiln site. Usually constructed out of metal work=
placed onto the ducting that connects to the refractory chimney....not
right at the flue exit point. Not common for craft pottery........ more=
common in industrial situations. But still not all that common. =
Commercial units for larger kilns can be a bit "pricey"....so they are
probably not an option here.
Considerations to keep in mind if you go this route of building one
yourself:
The ducting (stack) located past the throat area where the air from the
blower combines with the effluent coming from the kiln needs to be sized =
so
that it will handle the combined total of gases that the TWO together
supply.
Second.... designing an effective venturi is a lot more complicated than
just restricting or narrowing down a pipe. One factor that is important =
is
the "friction" induced by the exact shape of the constriction and also th=
e
matrerial it is made out of. This is evident if you look at the equation=
for venturi function. So you might get about the same effect if you just=
do a "T" connection rather than going to the trouble of trying to create =
a
home-made venturi that actually DOES much. Just use a slightly larger
blower .
Third....the temperature of the effluent gases traveling thru the "chimne=
y"
after the point where they combine with the air from the blower will
typically be lower than those experienced in a kiln stack without this ty=
pe
of system, due to the dillution effect. One caveat here though.....
if you are dumping unburned fuel into the chimney
and at the point the air from the blower is introduced the gases are
sustainably at the ignition point....... you can create much HIGHER
temperatures in the area just past that point due to reigniting the
partially burned fuel which then gives off the heat in the lower part of
the chimney.
But an even more important thought on all this.........
You may be making this more complicated than it needs to be. Yes, you ca=
n
solve this propblem this way. But I am reminded of the scene in the
Indiana Jones movie where the huge hulking "bad guy" is making this reall=
y
fantastic and intimidating display of skill with his wicked, long curved
scimitar type sword. Indy shrugs his shoulders to the camera, and simply=
takes out his sidearm and shoots him.
If you just switch you basic concept from using a VENTURI burner to
utilizing a good quality FORCED AIR BURNER....... you don't need to futz=
around with solving the draft inducer design problems. A good forced air=
burner(s) will have the mechanical ability to drive the flow within the
system. So the chimney is completely "out of it" as far as circulation
goes. In fact, you could just leave the exit flue down below floor
level....and the hot gaqses and flames would spew out down there. Not go=
od
design for safety in most cases..... but it'd work that way. Just add a
"chimney" to duct the gases up to a "safe" height.
If you want to really solve this ....... used a forced air burner and go
with a closed port system. This means that the burner is mounted onto th=
e
kiln with no open "burner port" like you see on about 99.9% of craft
potter's kilns. The burner fires into a refractoy block mounted in the
wall of the kiln. Since the burner is sized to be able to provide a slig=
ht
excess of primary air........ there is no need for that "messy,
non-precise" secondary air. So no exterior opening. This is a more
industrial type solution....and is not likely necessary for what you want=
to accomplish.
Hope this stuff helps with your thinking.
best,
.......................john
PS: Nice ASCII "schematic" .
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086-5812 USA
JBaymore@compuserve.com
http://www.JohnBaymore.com
603-654-2752 (studio)
800-900-1110 (studio)
"Earth, Water, and Fire Noborigama Woodfiring Workshop: August 2004 Date=
s
TBA"
Dave Finkelnburg on fri 16 apr 04
Earl,
You ask about updraft kilns and temporary flues.
Every kiln design has its good points...and its limitations. In both
updrafts and downdrafts, getting the right ratio of height to width, and the
right distribution of burners, is critical to even firing. Having the walls
and door tight is essential to achieving a uniform atmosphere everywhere in
the kiln.
I happen to have a 30-cubic foot (stacking space) downdraft car kiln,
and a very small (<2-cubic foot stacking space) updraft. I use the latter
for all my glaze tests and for short turnaround batches. I can ruin pots
equally well in either kiln...and have...in the process of learning to fire
them. But as we have gotten better acquainted, and I've learned to do what
each kiln likes, my success rate has become very acceptable.
A lot of pottery has been fired in Olsen updraft kilns over the past
three decades. That alone should be evidence that the updraft is not a
design to reject out of hand.
I was thinking of corrugated steel culvert pipe. Do not try PVC for a
temporary flue. IF you lose power to your fan, it will first melt, then
ignite. :-( You need to consider how much heat a stovepipe will take.
Assuming you can reasonably operate it at 800-degrees F, then for a cone 10
kiln your blower, at full volume, will have to push roughly 3 times as much
air as is coming from the kiln. That's a very crude, back of the envelope,
kind of estimate. You can and should do a more precise heat balance if you
decide to pursue your very interesting design. Just do the arithmetic, so
you won't have the stovepipe glowing cherry red.
Good luck with your kiln project!
Dave Finkelnburg, done firing for today
----- Original Message -----
From: "Earl Krueger"
Sent: Thursday, April 15, 2004 11:22 PM
> Dave, it seems to me from what I've read on Clayart that updrafts are
> much more sensitive and harder to control. Is this real or just my
> interpretation?
>
> By "culvert" pipe are you referring to something like concrete or clay?
> I was thinking that by pumping a lot of cool air in at the base of the
> chimney if I wanted a taller chimney I would be able to use light
> weight PVC or regular stove pipe.
Logan Oplinger on fri 16 apr 04
On Wed, 14 Apr 2004 22:47:45 -0700, Earl Krueger
wrote:
>So, my idea is to build a chimney only as tall as
>the kiln but extend it down below where the flue
>from the chamber enters the chimney. Then
>arrange a squirrel cage blower to force air up
>the chimney from below the flue. By creating
>a venturi in the chimney you could create a
>partial vacuum at the flue that would provide the
>required draft through the chamber. The draft
>could be controlled by adjusting the amount
>(velocity) of air forced through the chimney.
>
>Would this work? (Assume venturi burner.)
>
>Crude schematic, not to scale:
>(use courier font)
>
>-----------| C |
> | H |
> Firing | I |
> Chamber | M |
> | N |
> |\ E /
>- --- ----| Y |
>Flue ---> |
>------------| > / > / > | | | <--- damper
> | |__|_____
> \ |
> \ <----- Forced Air
> \_______________
>
>
>Thanks for your comments.
>
>Earl K...
>Bothell, WA, USA
Hello Earl,
Your concept works in principle, the basis for air injection/inspiration/
exaust systems.
I will suggest a couple of modifications if I may:
1. In the space where the direction of flow changes at the flue opening
into the chimney, enlarge this volume, and shape it so that it is similar
in shape to that of the primary air opening of a venturi burner.
2. Make the cross section of the chimney larger than the cross section of
the opening at the base of the chimney where the forced air enters.
In my diagram below the "**" represents the zone around the forced air
entry opening where a vacuum is created (look up Bernoulli effect).
The "|||||" is an air-flow straightener to help counteract the turbulence
that will be created where the air flow changes direction just before
entering the flue chamber at the base of the chimney.
Firing |_|_| |_|
Chamber |_|_| |_|
___________|_|_/ \__|_|_|_|_|_|_|/ \_Flue Channel ** ** |_||< damper
________________|||||__|_||_______
_|_|_|_|_|_|_|_|| | _|_|_|_|_|_|_|_|\_________________ ===========================
Logan Oplinger
Another Pacific Island
Where we finsished another anagama this last week and will open the kiln on
Sunday the 18th
Bruce Girrell on fri 16 apr 04
John Baymore wrote:
> If you just switch you basic concept from using a VENTURI burner to
> utilizing a good quality FORCED AIR BURNER....... you don't need to futz
> around with solving the draft inducer design problems.
Of course. But those puppies are EXPENSIVE! There's a big difference between
a $50 Grainger squirrel cage blower and a couple of $750-$1500 power
burners.
Bruce "just buy the Geil and be done with it" Girrell
John Baymore on sun 18 apr 04
Of course. But those puppies are EXPENSIVE! There's a big difference
between
a $50 Grainger squirrel cage blower and a couple of $750-$1500 power
burners.
Bruce,
If he has the design and fabrication competency to design a somewhat
complex item like a draft inducer....... then designing and building a
power burner would be a "piece of cake". A home made power burner is
simple and CHEAP to make if you have the tools and skills. Likely about
the same cost as coming up with a draft inducer that actually works plus =
a
venturi burner. It'll take a bit more than ONLY a Grainger blower .
=46rom the general subject and intent... it is pretty clear that "permitt=
ing"
and such is not an issue.... the "roll away kiln" idea seems to indicate
that.
So not having "commercial" burner units would not seem to be an issue. =
If a homemade draft inducer would pass muster.... so would a home made
power burner. And the power burner is more likely to "work" on the first=
try. As to flame safety.... since there are likely no "regs" bneing
looked at....... use a BASO thermocouple switch wired to a 110V SOV (not =
a
baso valve).... which will give both flame proof and electriclal
interuption protection for pretty cheap. =
best,
...........................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086-5812 USA
JBaymore@compuserve.com
http://www.JohnBaymore.com
603-654-2752 (studio)
800-900-1110 (studio)
"Off to Japan again in June!"
"Earth, Water, and Fire Noborigama Woodfiring Workshop: August 2004 Date=
s:
TBA"
John Baymore on sun 18 apr 04
For some reason I was avoiding forced air burners but now that
you mention it some thoughts do come to mind. Wouldn't using a forced
air burner slightly pressurize the kiln, whereas using a draft chimney
put the kiln under slight vacuum? Would this have an effect in terms
of producing even reduction? I guess it might depend on how tight the
kiln was built.
The function of "draft" (as I think that it is being defined here) on a
fuel fired kiln serves basically two functions:
One function is to remove the byproducts of combustion. If this were not=
happening and the kiln were made of rubber the kiln would inflate like a
balloon . Since it isn't made of rubber (hopefully)...... what would
happen is that the internal pressure within the rigid walled chamber woul=
d
quickly rise due to the volume of gases being mechanically "pumped in" by=
the burner....... and also due to the expansion of those gases due to the=
heating of them. (based on the standard Gas Law PV =3D nRT
)
At some point........ and in practical terms, pretty quickly..... the
total pressure in the chamber would exceed that of the surrounding air an=
d
the burner's mechanical abilty to "push" more gases into the chamber. At=
this point the gases coming out of the burner would no longer enter the
chamber (assuming it was not a sealed port setup) and would "back out" of=
the burner port. Bit of a mess . Flame everywhere.....ugly.
The other main function of draft is to circulate the byproducts of
combustion and burning gases throughout the chamber to distribute the hea=
t
energy and reducing agent(s) evenly. The effectiveness of this circulati=
on
depends a lot on the physical design of the interior structure of the kil=
n
as well as the location and sizing of the various openings thru which the=
gases flow. It is also dependent on the settings of stuff like the dampe=
r,
fuel input, and primary air ratios.
So looked at like this...... draft itself is not important.... gas FLOW
thru the kiln unit is. You can develop the necessary gas flow thru the
unit by other means than naturally induced "draft". One is the draft
inducer already mentioned. Another is the use of a "blower" to induce fl=
ow
on the "front end"....... ie. power burners.
A lightly "pressurized" chamber (relative to ambient atmospheric pressure=
)
actually can have a positive effect on kiln operation. If the structure =
of
the kiln or the door is not "tight"....... and if the kiln is operating
under a negative pressure, tiny streams of air infiltration can occurr th=
ru
cracks and crevices. This uncontrolled secondary air supply can sometime=
s
cause unevenness in reducing atmosphere, among other things. A high
negative chamber pressure also makes the kiln very sensitive to the exact=
damper setting. On the down side....... this positive chamber pressure
will cause gases to leak out of the kiln if there are any structural
"leaks" This requires that you pay particular attention to things like
hoods and ventilation to make sure that various gases and fumes are not
becoming an issue in the kiln room.
In fact, most "natural draft" kilns are actually operated with a slight
positive pressure in the upper reaches of the chamber as the firing
progresses. The pressure however at the open "burner port" typically is
eventually adjusted (mainly via the damper setting) to be only slightly
negative (relative to atmospheric) or even very close to equilibrium. =
(pressure drop across the port.....outside to inside ratio) This results=
in only a small amount of secondary air being entrained thru the open
"hole" .... and therefore a more homogeneous mixture flowing into the
chamber. However, the less efficient the burners are in entraining prima=
ry
air with the fuel gas...... the larger the negative pressure needs to be =
so
that enough secondary air is supplied to make up for the missing primary.=
=
This approach can cause uneven mixing within the chamber.... and often
spotty temperature and atmosphere issues.
This is one of the ressons that industry most often utilizes closed port
type systems....... all chamber mixture situations are controlled by
setting burner primary air ratios, and there is no possibility of
uncontrolled secondary air. This helps keep the atmosphere and temperatu=
re
distribution very even. (not to mention the use of high velocity, nozzl=
e
mix burners and precision controls .) This sealed approach also allow=
s
a wide variation of chamber pressure control.... since ther are no "open
holes" into/out of the chamber.
If you have the skills and are able to build the whole draft inducer
idea........ you'd likely have a much easier "go" of it by simply buildin=
g
yourself a forced air burner that will serve the same desired gas flow in=
the chamber.
The forced air burner is FAR easier to design and construct than an
effective draft inducer. =
best,
.....................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086-5812 USA
JBaymore@compuserve.com
http://www.JohnBaymore.com
603-654-2752 (studio)
800-900-1110 (studio)
"Off to Japan again in June!"
"Earth, Water, and Fire Noborigama Woodfiring Workshop: August 2004 Date=
s:
TBA"
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