Richard Aerni on mon 5 apr 04
OK, gang, here's one for the kiln gurus. I've got a new downdraft, natural
gas fired cone 10 (supposedly) kiln. Gas pressure is seven WCI, and I've
got a 630,000 BTU meter, which the utility people assure me is good for
over 900,000 BTU/hr. I sized my orifices to provide me with 600,000 BTU/hr
maximum, which in relation to the interior size of the kiln has always been
able to get me to cone 10 during a long firing (suitable for single firing
with ash glazes, which work best if the temp rise is slow at the end). The
kiln, while it has a hard brick interior, is backed by K-23 softbrick and
on the sides has a further inch of 1900 F hardboard, and the arch has four
layers of kaowool over the softbrick second layer. It's well insulated and
tight. Problem is, it only got me to cone 8. At cone 8, I had a beautiful
flame licking through the interior, and the oxyprobe readings were spot on
for a good temperature rise. Except, no matter what I did, whether it be
with the damper, secondary air, or primary air, I couldn't budge it above
cone 8. And, lest I forget, the kiln has good draft.
Now here is where I am looking for input. I've two manifolds of three
burners each. At full gas, the first burner (first off the gas line) has a
pressure of 5 1/4 WCI at the gas cock on the burner. Second is a bit less
the 5 WCI, third is 4 1/2 WCI. This is identical on the other side. I
understand about pressure drop with open orifices and burners (the gas
coming into the manifold was 6 1/2 WCI), but I'm thinking that it just
hasn't got enough ooomph to get it to cone 10.
I've arranged with the utility company to double my gas pressure, to 14
WCI. This way, I can either use a stepdown regulator on the main line so
that the burners when full open will give me 7 WCI (and the BTUs I figured
I needed to get to cone 10), or else I can drill myself new orifices on the
burners which when operating at 14 WCI will still give me the 100,000 BTUs
I figure I need per burner (assuming 1 3/4-2 1/2 WCI drop from the main gas
line).
Questions: Is my logic sound concerning the higher gas pressure? If so,
what would those of you in the know recommend...smaller orifices/higher
pressure, or stepdown regulator with the original orifices (say set the
regulator to 9 WCI or so)?
Any sensible input will be appreciated.
Richard Aerni
Rochester, NY
Jim Tabor on tue 6 apr 04
Richard Aerni wrote:
>Any sensible input will be appreciated.
>
>
>Greetings Richard-
>
The 2 kilns I've built that sound like yours fires with 4 alfred
burners. One with a low arch top and the other using circle brick. They
were designed after the kiln I fired at school which was the only one of
the three with problems hovering under temp. I was told barometric
pressure could have been a problem but don't know.
Stack cross-section and height could be an issue. Also, I used 3 orifice
sizes that are switched out during firing. My first kiln had an 1 1/2"
line from the meter because of the distance and pressure drop. You could
also consider bag wall placement and height for an adequate fire box.
I wanted one of your large platters at KC NCECA but the show was down
when I returned so I didn't hesitate this time and bought one at The
Gallery in Bloomington. Beautiful work.
Jim Tabor
Tulsa
http://www.members.cox.net/taborj/index.html
Richard Aerni on tue 6 apr 04
Hi Jim,
Thanks for the input and financial support!
It was raining, so barometric pressure was down, but somehow I don't think
that was the issue. My stack cross section is 13 1/2 x 13 1/2 for the
first 9 feet, then it tapers in to 12" x 12" for the last six feet. The
draw was good during the firing. Gas line is 2 1/2 inches for the first 89
feet, then 2 inches for the last 80 feet. There are 14 elbows and tees in
that distance, but even so, the Olsen figures in his book show I should be
getting plenty of gas.
I was talking with Freddy Frederickson at Alfred over the weekend, and he
was telling me about the orifice switching trick. He mentioned that Robert
Turner used to switch orifices five times during his firings. But, that
seemed to be mainly to make sure they entrained enough air during the early
stages of the firing. I didn't seem to have that problem. Also, as for
bagwall placement and heighth, well, there is plenty of room for combustion
(6 inches) between bagwall and interior walls of kiln (at least it's been
plenty in the past) and the height is the same as in my other kilns. I
figured I'd be playing with the bagwall to even out temperature and
reduction in future firings, but didn't figure it would be a problem in
getting it to temp. It was an even cone 8 front to back, top to bottom
.
I don't know. I'm kind of stumped. I didn't want to fire again until I'd
come up with a new plan. That's the reason for the pressure boost in the
gas. There just doesn't seem to be any reason not to get to temperature
unless the kiln is underpowered.
Best,
Richard Aerni
Rochester, NY
On Tue, 6 Apr 2004 07:25:14 -0600, Jim Tabor wrote:
>>Greetings Richard-
>>
>The 2 kilns I've built that sound like yours fires with 4 alfred
>burners. One with a low arch top and the other using circle brick. They
>were designed after the kiln I fired at school which was the only one of
>the three with problems hovering under temp. I was told barometric
>pressure could have been a problem but don't know.
>
>Stack cross-section and height could be an issue. Also, I used 3 orifice
>sizes that are switched out during firing. My first kiln had an 1 1/2"
>line from the meter because of the distance and pressure drop. You could
>also consider bag wall placement and height for an adequate fire box.
>Jim Tabor
Hank Murrow on tue 6 apr 04
Dear Richard;
The different gas pressures that you report from front to back of your
manifold may be evened out by stepping down the supply pipe in size
from larger to smaller along each manifold. You can see this in action
at any group shower facility where the pipes are in the open. They
always step down in size from the main supply end to the last nozzle.
This is how even pressure is maintained at each nozzle.
As to pressure vs. larger orifii, once you get the manifold sizing
correct, turn up the main regulator to give you around 10 " w.c. Fire
it this way and see if that does the trick. I am betting that it will.
One question: does the kiln fire cleanly at maximum pressure when you
are stalled at cone 8?
Cheers, Hank in Eugene
On Apr 5, 2004, at 7:29 PM, Richard Aerni wrote:
> OK, gang, here's one for the kiln gurus. I've got a new downdraft,
> natural
> gas fired cone 10 (supposedly) kiln. Gas pressure is seven WCI, and
> I've
> got a 630,000 BTU meter, which the utility people assure me is good for
> over 900,000 BTU/hr. I sized my orifices to provide me with 600,000
> BTU/hr
> maximum, which in relation to the interior size of the kiln has always
> been
> able to get me to cone 10 during a long firing (suitable for single
> firing
> with ash glazes, which work best if the temp rise is slow at the end).
> The
> kiln, while it has a hard brick interior, is backed by K-23 softbrick
> and
> on the sides has a further inch of 1900 F hardboard, and the arch has
> four
> layers of kaowool over the softbrick second layer. It's well
> insulated and
> tight. Problem is, it only got me to cone 8. At cone 8, I had a
> beautiful
> flame licking through the interior, and the oxyprobe readings were
> spot on
> for a good temperature rise. Except, no matter what I did, whether it
> be
> with the damper, secondary air, or primary air, I couldn't budge it
> above
> cone 8. And, lest I forget, the kiln has good draft.
>
> Now here is where I am looking for input. I've two manifolds of three
> burners each. At full gas, the first burner (first off the gas line)
> has a
> pressure of 5 1/4 WCI at the gas cock on the burner. Second is a bit
> less
> the 5 WCI, third is 4 1/2 WCI. This is identical on the other side. I
> understand about pressure drop with open orifices and burners (the gas
> coming into the manifold was 6 1/2 WCI), but I'm thinking that it just
> hasn't got enough ooomph to get it to cone 10.
>
> I've arranged with the utility company to double my gas pressure, to 14
> WCI. This way, I can either use a stepdown regulator on the main line
> so
> that the burners when full open will give me 7 WCI (and the BTUs I
> figured
> I needed to get to cone 10), or else I can drill myself new orifices
> on the
> burners which when operating at 14 WCI will still give me the 100,000
> BTUs
> I figure I need per burner (assuming 1 3/4-2 1/2 WCI drop from the
> main gas
> line).
>
> Questions: Is my logic sound concerning the higher gas pressure? If
> so,
> what would those of you in the know recommend...smaller orifices/higher
> pressure, or stepdown regulator with the original orifices (say set the
> regulator to 9 WCI or so)?
Craig Martell on tue 6 apr 04
Hi Richard:
From the description of your kiln, it sounds like it's built very well and
you've covered all the bases in terms of insulation. The thing that I'm
wondering about is the size of the kiln (total cubic feet) and the number
of btus calculated per cubic foot of space. Most kiln builders and kiln
gurus suggest 16,000 to 20,000 btus per cubic foot for a hard brick
interior. Even though you have great backup insulation, you still have to
saturate the hards before they radiate and this takes a lot of energy. The
figures I've seen are: 8,000 btus per brick for 2300F soft brick and 35,000
btus per brick for hards. Big difference. I have a 55 cubic toot (total
space) salt kiln with hards inside. My input for that kiln is 980,000 btus
per hour at peak. It will fire in 12 hours. I use propane though and
that's somewhat different.
I think your problem is not enough line pressure from the main meter. I've
been in on firings of hard brick salt kilns and they always had a 2psi
meter and were fired at about 11WC. The drop from 5.5 WC to 4.5 WC in each
manifold would indicate that the line pressure is too low. At least that's
my take on things. I would assume that the pipe is sized correctly to get
the proper volume of gas to the burner manifold, so it's most likely the
line pressure that's giving you a headache.
If the kiln was fired with insulating brick on the hotface you'd be
fine. But hards need more muscle and btus.
regards, Craig Martell Hopewell, Oregon
Hendrix, Taylor J. on tue 6 apr 04
Ash Wizard Aerni,
Is this a kiln you have restacked at your new digs? Everything else
remains the same? So the only difference between the old pile and this
new pile is the gas lines? You didn't add any new bells or wistles?
Regulator new? Elevation similar to the last kiln's home?
I of course am just asking questions because that's all I got.
Though I do love a mystery!
Taylor, in Waco
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Richard
Aerni
Sent: Tuesday, April 06, 2004 7:43 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Gas pressure, orifice sizes, pressure drop at burners
Hi Jim,
Thanks for the input and financial support!
It was raining, so barometric pressure was down, but somehow I don't
think
that was the issue. ...
Vince Pitelka on tue 6 apr 04
> Questions: Is my logic sound concerning the higher gas pressure? If so,
> what would those of you in the know recommend...smaller orifices/higher
> pressure, or stepdown regulator with the original orifices (say set the
> regulator to 9 WCI or so)?
Richard -
My recommendation would be none of the above. If you really do need more
pressure, then just get them to boost the pressure and then go fire the
kiln. You shouldn't have to change anything if you are only raising the
pressure from 7 to 14 WCI. In fact, for best results with the higher
pressure, you SHOULDN'T change anything else. If you were changing from 7
WCI to 7PSI you would need new orifices, but not with the kind of change you
are talkin about.
What kind of burners did you use? How big is the kiln? Were you
maintaining reasonable back pressure (light partial reduction) during the
latter part of the firing?
You have had plenty of firing experience, so I'm sure that you are aware
that a noisy low-pressure burner is a very inefficient flame, usually far
too much air. The burner should be quiet, and the flame should be blue with
licks of orange or yellow. Also, where the flame enters the burner port, it
should gently fill the port. If the flame tapers in abruptly as it enters
the port, it indicates far too much secondary air, which of course just
stalls the kiln.
You may have read on Clayart about Michael McDowell's experiences with his
new downdraft natural-draft car kiln. It is about 40 cubic feet, and has
six MR-100 burners, and standard 7 WCI gas pressure. He really has to hold
that kiln back or it fires to cone 10 way too fast. On the first firing,
the rear two thermocouples failed (they were defective from the factory),
and he still had to hold it back on only four burners to keep it from firing
too quickly.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Ivor and Olive Lewis on wed 7 apr 04
Dear Richard Aerni,
Even if the temperature stalls you are only considering energy and not
time. Would firing an extra few hours make the ^10 cones sit?
I will make the assumption that you have sufficient gas entering the
kiln to achieve the higher temperature and that the rest of your
pipework for delivery and burner technology are adequate for the task.
This leaves the question hanging "What reduces the efficiency of the
burning to prevent access to the energy you are putting into the kiln
chamber". The suggested answer is always "Deficiency of Oxygen"
Flames licking through the chamber suggest burning is incomplete. What
does this imply? It seems to me as though all of the energy is not
being released. So more oxygen is required to release energy to take
the temperature higher.
You say that the Oxyprobe reading are "spot on" but this is not
factual. Does this mean you are achieving the degree of reduction
anticipated to give the surface colouration you desire? Does this mean
that your damper is in rather than out? Or is in the range of 0.01 or
less? You say there is adequate draft. Do you know the average
velocity for the system?
My suggested solution is to increase both the input of oxygen to
release energy and additional fuel to maintain your reducing
atmosphere according to Oxyprobe readings.
There are three ways, all of which might be needed.
1 Increase flow of secondary air by enlarging the burner ports.
2 Increase the volumetric through put;
either (A) by pulling the damper to get a greater opening if you have
a strong draft,
or (B) by increasing stack velocity by about 10%.
If the first does not work then the second will be needed.
You would have to calculate the new stack height and add pipe to
achieve this.
3 Increase fuel input.
The second kiln I built refused to go beyond about cone 4 until I
changed the stack height, I added about 8 feet. Worked like a dream
and gave real control at the damper.
Hope you find a solution.
Best regards,
Ivor Lewis. Redhill, South Australia
Roger Korn on wed 7 apr 04
Hi Richard,
You are on the right track here. I normally design my systems for "2 pound gas", approximately 50 wci, which means that any domestic appliances served by the same gas line need regulators to drop the 2 psi pressure to 7-8 wci. The gas companies, once presented with the layout of the consumer-side system, have been ok with this. The advantages are huge: - 100 foot runs of 3/4" pipe are adequate for large kilns, saving on cost. And since the pressure regulator for the kiln is located at the kiln distribution manifold, the variations in pressure across the manifold are small. You might ask your gas company about this approach.
Hope this helps,
Roger Korn
Richard Aerni wrote:
...
Now here is where I am looking for input. I've two manifolds of three
burners each. At full gas, the first burner (first off the gas line) has a
pressure of 5 1/4 WCI at the gas cock on the burner. Second is a bit less
the 5 WCI, third is 4 1/2 WCI. This is identical on the other side. I
understand about pressure drop with open orifices and burners (the gas
coming into the manifold was 6 1/2 WCI), but I'm thinking that it just
hasn't got enough ooomph to get it to cone 10.
I've arranged with the utility company to double my gas pressure, to 14
WCI. This way, I can either use a stepdown regulator on the main line so
that the burners when full open will give me 7 WCI (and the BTUs I figured
I needed to get to cone 10), or else I can drill myself new orifices on the
burners which when operating at 14 WCI will still give me the 100,000 BTUs
I figure I need per burner (assuming 1 3/4-2 1/2 WCI drop from the main gas
line).
Questions: Is my logic sound concerning the higher gas pressure? If so,
what would those of you in the know recommend...smaller orifices/higher
pressure, or stepdown regulator with the original orifices (say set the
regulator to 9 WCI or so)?
--
McKay Creek Ceramics
In OR: PO Box 436
North Plains, OR 97133
503-647-5464
In AZ: PO Box 463
Rimrock, AZ 86335
928-567-5699
John Baymore on thu 8 apr 04
Richard,
Hi. As you know these things are often very hard to diagnose from afar
without actually seeing the installation.
That being said...............
What is the total gross cubic footage of the new unit?
What is the height of the chimney on the new unit? ( You gave the XC and=
taper in another posting somewhere.)
At the last hour to two hours of your firings, exactly what percentage op=
en
is your damper? =
Is the new kiln in a closed building? If so, have you planned in supplyi=
ng
adequate make up air? Related.... it there a general dilution ventilatio=
n
system (or any other type) located in the kiln building with the kiln?
Keep in mind that with using those home-made "Alfred" pipe burners, the
kiln operation there is "all about secondary air". Even the good quality=
venturi casts in commercial burners cannot entrain 100% primary (far from=
it) particularly at the orfice mixture pressure ratings you mention. Tho=
se
homemade pipe burners are even less efficient in doing that. So you are
VERY dependant on secondary air for combustion. That makes many other
variables of kiln design and operation even more critical than they are f=
or
other kiln types. So the first place I would go in diagnosing the issue
you have here .............(unless your BTU input figure relative to the
total cubic footage is off) ........ is in looking at issues that affect
the secondary air entrainment and internal chamber mixing (and hence
combustion) issues. =
A kiln that is dependent on high levels of secondary air for complete
combustion is VERY touchy to fire. This dependence also makes the kiln
very dependent on the INTERNAL mixing in the chamber itself of the
entrained secondary air with the partially aerated mixture coming out of=
the burners. If this mixing does not occur before the gases exit the
chamber..... the heat is not realized in the chamber....and goes up the
stack as unburned complex hydrocarbon compounds. Damper setting will be
CRITICAL...... and some precise calibration (for repeatability) will be
important.
This characteristic also will potentially affect the evenenss of
atmospheric effects within the chamber. Depending on the type of work on=
e
does.... this is either an asset....or a liability . (Us woodfirers
strive for those variations )
An important point to consider is that the oxyprobe you mentioned only
measures the atmosphere in the loaction that the probe is located. (True=
of most any instrumantation.) If this location actually IS representativ=
e
of the overall chamber......... then you can go by it in an "exact" sense=
. =
But in most cases I have seen in craft potter type kilns I have seen, it =
is
not necessarily so.......... due to many of these types of kilns having
internal mixing issues. So I'd take the readings you're getting on the
probe with a grain of salt (whole other discussion.) They are certainly
useful to say that you likely aren't
"orders of magnitude" off of an appropriate mixture...... but when it
comes to more subtle details.... only testing multiple points in the kiln=
at multiple points in the firing will give you any indication of the
precise usefullness of the readings. And I think that your issues lie in=
the "subtle" range.
There are three likely "root causes" that I can see from the information
provided so far:
1.) Possible under-rated BTU input for the kiln size.
2.) High dependence on secondary air for combustion.
3.) Excessive heat loss from oversized chimney cross section.
My initial "guess" here is that the issue is really a combination of the
three acting a bit synergistically. =
I think you are likely SLIGHTLY under-rated on heat input for the
kiln.....but not grossly so. I'll know more when I know the exact cubic
footage. If you are much over about 50-52 cubic feet... then you are
light. But keep in mind that I think some of the heat input figures for
hard brick kilns that are "out there" are a bit generous. I typically us=
e
12,000 BTU/ cu ft input for a 4 1/2" hardbrick lining backed with
IFB......... and it has always worked fine (lots of kilns built behind th=
at
number ). If the other factors weren't ALSO present..... I think you'=
d
likely "get away with it" .........but albeit a slow firing at the upper
end. =
I think it is also likely that your chimney cross section is WAY oversize=
d
for most kilns unless that is a honking HUGE unit. (and if it is that
huge... then you ARE light on BTU's.) The relationship of 600,000 BTU /
hr. to a chimney that size is WAY non-typical and very suspect in a gas
kiln. In general, the flues on most gas kilns in this country (USA) are
quite oversized. Even designing for depending on heavy secondary air
entrainment.... that is a BIG cross section. The taper will exacerbate i=
t
a bit too.
With that cross section I'd be expecting an input in the 1,500,000 BTU
range and up.
And I think that =
with those home-made burners you are running, it is a design that will
always be a bit "touchy" as to the way it is stacked, weather conditions,=
wind, the operator using it, control system inaccuracies, surrounding
structure influences, and so on. Small changes to most anything will
likely have large impacts when compared to kilns with burners that entrai=
n
more primary air.
So that and $0.50 will buy you about 1/4 of a cup of coffee somewhere the=
se
days . Hope these thoughts are a help in getting things straightened
out.
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)
"Earth, Water, and Fire Noborigama Woodfiring Workshop: August 2004 Date=
s
TBA"
Professional Kiln Design, Construction, and Workshops since 1972: Visit
http://www.johnbaymore.com/kilndes1.htm
Richard Aerni on sat 10 apr 04
John,
Thank you for your lengthy and informative and well thought out reply.
I'll try to respond to the questions you ask, and tell you more if it seems
relevant. I've developed an action plan at this time, and I'll lay that
out for comment (since I've made my private travails and misery public, the
least we can do is to make it somewhat educational...if not
entertaining...for those in the crowd who wish to follow!) The generosity
and knowledge shown by this group is truly wondrous to behold. Thanks one
and all.
On Thu, 8 Apr 2004 10:08:34 -0400, John Baymore
wrote:
>Hi. As you know these things are often very hard to diagnose from afar
>without actually seeing the installation.
>
>That being said...............
>
>What is the total gross cubic footage of the new unit?
62 Cubic feet total interior
>
>What is the height of the chimney on the new unit? ( You gave the XC and
>taper in another posting somewhere.)
Total height of the chimney is 15 1/2 feet. I'll probably be adding
another four or five feet onto the total in order to clear an abutting
roofline, though the kiln was approved without that addition. The head
plumber of the building (it is an old industrial building...I'm on the
first floor, but the kiln is in a bay that is one story, with the stack ten
feet away from the rest of the building) fears I will kill inhabitants of
the upper floors with all of the carbon monoxide I am pouring out of the
kiln.
>
>At the last hour to two hours of your firings, exactly what percentage open
>is your damper?
Damper was open 3 1/4 inches out of a total possible opening of 12 inches.
During the firing I had the damper open between two inches and 3 1/2 inches
at various times. This is consistent with how the previous generations of
this kiln was fired (at least 1000 times).
>
>Is the new kiln in a closed building? If so, have you planned in supplying
>adequate make up air? Related.... it there a general dilution ventilation
>system (or any other type) located in the kiln building with the kiln?
The kiln is indoors, but the building is not closed. When firing, I keep
the loading dock door open, which is about 20 feet from the kiln. I also
keep some of the studio windows open, which are in a different part of the
studio, about 75 feet from the kiln. I paid an HVAC specialist $1500 to
diagnose ventilation issues and install an exhaust fan and damper system
above the kiln, though in my humble layman's estimation it is woefully
underpowered. It runs during the firing (it's rated at 330 cfm), and
because I feel that it's underpowered, I also ran my spray booth fan during
the firing, which is rated around 1000 cfm. I realize that my kiln at
maximum operating range will use 570 cubic feet per hour of gas, which
under optimum combustion conditions needs 5700 cubic feet of air, so I am
sensitive to the need for fresh air in the studio (from the outside as well
as Terry Gross!).
>
>
>Keep in mind that with using those home-made "Alfred" pipe burners, the
>kiln operation there is "all about secondary air". Even the good quality
>venturi casts in commercial burners cannot entrain 100% primary (far from
>it) particularly at the orfice mixture pressure ratings you mention. Those
>homemade pipe burners are even less efficient in doing that. So you are
>VERY dependant on secondary air for combustion. That makes many other
>variables of kiln design and operation even more critical than they are for
>other kiln types. So the first place I would go in diagnosing the issue
>you have here .............(unless your BTU input figure relative to the
>total cubic footage is off) ........ is in looking at issues that affect
>the secondary air entrainment and internal chamber mixing (and hence
>combustion) issues.
>
>A kiln that is dependent on high levels of secondary air for complete
>combustion is VERY touchy to fire. This dependence also makes the kiln
>very dependent on the INTERNAL mixing in the chamber itself of the
>entrained secondary air with the partially aerated mixture coming out of
>the burners. If this mixing does not occur before the gases exit the
>chamber..... the heat is not realized in the chamber....and goes up the
>stack as unburned complex hydrocarbon compounds. Damper setting will be
>CRITICAL...... and some precise calibration (for repeatability) will be
>important.
I'm aware that these homemade burners do not mix and burn as well as many
commercial burners. I've got secondary air port covers that I move in and
out as the firing progresses. Having fired kilns like this one over 1000
times, I have a sense of where the secondary air should be. Nevertheless,
when the kiln stalled, I made adjustments to secondary, then primary air,
and to the damper, keeping close watch on temperature rise (via pyrometer),
character of flame throughout the kiln (via peeps), and the amount of flame
at the damper. BTW, the amount of flame at the damper was never excessive,
in my opinion. I didn't have the sense that combustion was occuring in the
chimney. And, I adjust my damper in increments that run along the lines of
1/8 ths of inches. Damper settings are very important. I use the oxyprobe
not as a bible, but more as a supporting reference to how things are
going. It will register sudden atmosperic changes, and also the subtle
temperature rises and drops during adjustment periods of supply air and
damper that the eye cannot see until they have progressed past where they
can be easily and quickly rectified. I use my eyes, ears, nose and brain
to fire the kiln, not an easily malfunctioning digital instrument.
> This characteristic also will potentially affect the evenenss of
>atmospheric effects within the chamber. Depending on the type of work one
>does.... this is either an asset....or a liability . (Us woodfirers
>strive for those variations )
>
>An important point to consider is that the oxyprobe you mentioned only
>measures the atmosphere in the loaction that the probe is located. (True
>of most any instrumantation.) If this location actually IS representative
>of the overall chamber......... then you can go by it in an "exact" sense.
>But in most cases I have seen in craft potter type kilns I have seen, it is
>not necessarily so.......... due to many of these types of kilns having
>internal mixing issues. So I'd take the readings you're getting on the
>probe with a grain of salt (whole other discussion.) They are certainly
>useful to say that you likely aren't
> "orders of magnitude" off of an appropriate mixture...... but when it
>comes to more subtle details.... only testing multiple points in the kiln
>at multiple points in the firing will give you any indication of the
>precise usefullness of the readings. And I think that your issues lie in
>the "subtle" range.
>
I agree. The oxyprobe is in the same placement in the kiln where it has
been for the past 600 firings of the previous kiln. I don't take it as a
measure of what's happening in the kiln as a whole.
>There are three likely "root causes" that I can see from the information
>provided so far:
>
>
>1.) Possible under-rated BTU input for the kiln size.
This is my prime suspect.
>2.) High dependence on secondary air for combustion.
Not sure about this one.
>3.) Excessive heat loss from oversized chimney cross section.
I'm curious about this one. Where do you read about chimney cross
section? I am aware of what Olsen writes about concerning chimney height
(the old 3 foot for each foot of downward pull plus one foot per foot of
horizontal pull), and that taper can increase the draft, but what I also
read in Olsen (principle 6, page 61 of second edition of the kiln book) is
that the chimney diameter should be approximately 1/4 to 1/5 the diameter
of the kiln chamber. Mine kiln chamber is 54 x 42, so that would speak to
a chimney diameter somewhere in the 10 inch to 13 inch range. I would love
to have more references to this factor, if you've got them.
>
>
>My initial "guess" here is that the issue is really a combination of the
>three acting a bit synergistically.
>
>I think you are likely SLIGHTLY under-rated on heat input for the
>kiln.....but not grossly so. I'll know more when I know the exact cubic
>footage. If you are much over about 50-52 cubic feet... then you are
>light. But keep in mind that I think some of the heat input figures for
>hard brick kilns that are "out there" are a bit generous. I typically use
>12,000 BTU/ cu ft input for a 4 1/2" hardbrick lining backed with
>IFB......... and it has always worked fine (lots of kilns built behind that
>number ). If the other factors weren't ALSO present..... I think you'd
>likely "get away with it" .........but albeit a slow firing at the upper
>end.
>
>I think it is also likely that your chimney cross section is WAY oversized
>for most kilns unless that is a honking HUGE unit. (and if it is that
>huge... then you ARE light on BTU's.) The relationship of 600,000 BTU /
>hr. to a chimney that size is WAY non-typical and very suspect in a gas
>kiln. In general, the flues on most gas kilns in this country (USA) are
>quite oversized. Even designing for depending on heavy secondary air
>entrainment.... that is a BIG cross section. The taper will exacerbate it
>a bit too.
> With that cross section I'd be expecting an input in the 1,500,000 BTU
>range and up.
>
>
>And I think that
>with those home-made burners you are running, it is a design that will
>always be a bit "touchy" as to the way it is stacked, weather conditions,
>wind, the operator using it, control system inaccuracies, surrounding
>structure influences, and so on. Small changes to most anything will
>likely have large impacts when compared to kilns with burners that entrain
>more primary air.
I would agree that it will not fire itself off automatically. I have
always trusted that I could diagnose and fix problems for a good firing.
But then, I haven't had an experience where I've not reached temperature
since my old partner Mike Frasca and I were designing and building this
first generation of the kiln back in 1980. There is more stacking room
between shelve stacks and front and back wall than in my previous kilns.
The btu/cubic foot ratio is the same as in previous kilns. I do wonder
about the influence of the surrounding buildings on draft, but truly I did
not notice inadequate draft in the firing...quite the reverse, actually
(assuming I can calibrate draft by eye and "feel").
What I plan on doing is to drill out the orifices and make the btu input
even larger (by a total of 150,000 BTU for the whole kiln) which under
optimum conditions should give me 13,000 BTU per cubic foot. If this
doesn't give me a satisfactory result, I will proceed with raising the gas
pressure to 14 WCI. (this aspect of the story has taken so many twists and
turns as to be almost comical to the dispassionate observor. Suffice to
say I am being finely ground down by the beaurocratic meat-grinder of the
utility company. I shall save that story for another day, however.) If I
get the gas pressure raised to 14 WCI, I will shrink the orifice somewhat,
but not enough to put me in the below 15,000 BTU per cubic foot range.
>
>So that and $0.50 will buy you about 1/4 of a cup of coffee somewhere these
>days . Hope these thoughts are a help in getting things straightened
>out.
>
Once again, thanks loads for the time and thought you put into this. And,
belated thanks to all of the others out there who have done likewise, both
on and off clayart. There is no small measure of reassurance in being able
to noodle through this stuff with others who have knowledge and experience
that I may not have. The dialogue is important, as is the contact and
support, in keeping my spirits out of the dumper.
best wishes,
Richard Aerni
Rochester, NY
Earl Krueger on sat 10 apr 04
On Saturday, Apr 10, 2004, at 06:31 US/Pacific, Richard Aerni wrote:
> Once again, thanks loads for the time and thought you put into this.
> And,
> belated thanks to all of the others out there who have done likewise,
> both
> on and off clayart. There is no small measure of reassurance in being
> able to noodle through this stuff with others who have knowledge and
> experience that I may not have. The dialogue is important, as is the
> contact and support, in keeping my spirits out of the dumper.
Hey Richard, thanks for bringing this up to Clayart. I had no idea
there were so many complicating factors involved in firing a gas kiln,
even though I have read Olson's book a couple of times. The
discussions have been very educational and have generated thought
processes on factors I wasn't even aware existed.
And thanks to all those who have been giving their $1000 worth.
Earl K...
Bothell, WA, USA
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