Orion/Baker on sat 4 apr 98
At high altitudes where air is "thin" and has less oxygen per cubic foot,
gas burner orifices ("tips") would need to be smaller to maintain the
correct gas-to-air mix. A gas burner orifice sized for standard air
conditions at sea level will burn "rich" in thin air -- sort of like a
"flooded" combustion engine. Don't try to guess at how much adjustment is
needed -- check with the local gas company and/or burner manufacturer for
the proper burner gas flow requirements (orifice and pressure).
It follows then that at high altitudes, a kiln designed for sea level use
will then need one or more additional burner(s) to offset the lower
fuel-flow resulting from the orifice downsizing to maintain the same "heat
power" rating. (Adding burners just means adding more flame & heat, with
the original volume of fuel).
To size natural draft ports and flue size at any altitude, the "rule of
thumb" is: the total area of the input ports should be about equal to the
area of the exit flue (chimney inner diameter). Simple example: if you
have four 2" x 2" input ports (4 square inches each), 4 ports X 4 square
inches each = 16 square inch exit flue/chimney opening (4" x 4") required.
Chimneys are basically needed only to carry exhaust fumes away -- they
don't serve any specific role in their own right. Without getting into
what "stack effect" is -- how it works and how to figure for it -- the
general rule of thumb is don't add any more chimney than you absolutely
need to carry the gases away.
Ellen Baker - Glacier, WA
Vince Pitelka on sun 5 apr 98
Please don't interpret this as contrariness on my part, but several things
need clarification in your message.
>At high altitudes where air is "thin" and has less oxygen per cubic foot,
>gas burner orifices ("tips") would need to be smaller to maintain the
>correct gas-to-air mix. A gas burner orifice sized for standard air
Your advice is sound, but we have many novices on this list and they need to
learn the right terminology. The orifice is of course the small gas jet
within the burner which injects gas into the airstream in a burner (ands in
a natural draft burner creates the velocity which draws air into the burner
tube). The "tip" is the end of the burner tube. Most burners utilize a
flame-retention tip, which keeps the flame from jumping off the end of the
>Chimneys are basically needed only to carry exhaust fumes away -- they
>don't serve any specific role in their own right. Without getting into
>what "stack effect" is -- how it works and how to figure for it -- the
>general rule of thumb is don't add any more chimney than you absolutely
>need to carry the gases away.
This is confusing. For any natural draft kiln, the chimney is CRITICALLY
important, because it creates the convection vacuum which draws the flames
and secondary air into the kiln and circulates the heat and atmosphere
throughout the kiln. The chimney does of course carry exhaust gases away,
and must do so efficiently or the combustion efficiency will decline. But
it is very important to emphasize the necessity of adequate stack draft
(chimney convection) for proper operation of any natural draft kiln.
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Marcia Selsor on mon 6 apr 98
I have to add some comments regarding chimney size and needs. For a natural
draft as oppsed to forced air, the chimney draught is critical.
When I was in Banff (6,000 ft.) we were firing an Olsen fast fire wood
kiln built exactly to specs. There was insufficient draught in the chimney and
the kiln would not climb above 1900 degrees. -twice. In order to get it to
climb, we opened the damper all the way. This caused Les Manning to frak when
he saw the flames shooting out the top of the chimney. I told him there was no
draught on the flu. Finally, Les
called Fred Olsen. Fred agreed that the chimney must be higher at high
altitudes for his wood kiln design.
I agree it is necessary for venturi burners, too. I have been firing with
venturi burners in Montana for the past 18 years, since we moved into new
facilities. They need a good draught in the chimney to be controllable.
Marcia in Montana
> ----------------------------Original message----------------------------
> At high altitudes where air is "thin" and has less oxygen per cubic foot,
> gas burner orifices ("tips") would need to be smaller to maintain the
> correct gas-to-air mix. A gas burner orifice sized for standard air
> conditions at sea level will burn "rich" in thin air -- sort of like a
> "flooded" combustion engine. Don't try to guess at how much adjustment is
> needed -- check with the local gas company and/or burner manufacturer for
> the proper burner gas flow requirements (orifice and pressure).
> It follows then that at high altitudes, a kiln designed for sea level use
> will then need one or more additional burner(s) to offset the lower
> fuel-flow resulting from the orifice downsizing to maintain the same "heat
> power" rating. (Adding burners just means adding more flame & heat, with
> the original volume of fuel).
> To size natural draft ports and flue size at any altitude, the "rule of
> thumb" is: the total area of the input ports should be about equal to the
> area of the exit flue (chimney inner diameter). Simple example: if you
> have four 2" x 2" input ports (4 square inches each), 4 ports X 4 square
> inches each = 16 square inch exit flue/chimney opening (4" x 4") required.
> Chimneys are basically needed only to carry exhaust fumes away -- they
> don't serve any specific role in their own right. Without getting into
> what "stack effect" is -- how it works and how to figure for it -- the
> general rule of thumb is don't add any more chimney than you absolutely
> need to carry the gases away.
> Happy firing!
> Ellen Baker - Glacier, WA
KarateHiro on thu 9 apr 98
On this matter, I cannot but resist repeating what I learned many years ago as
a neophyte potter. Maybe another common sense approach is in order at this
First, I agree and side with Orion/Baker on technical matters, although I am
not that technical. Rather, I call myself a practical or pragmatic person. I
lived in a high altitude location many years (I don't remember if it was a
mile high or not, I am not good at conversion or exact measurements), but now
I live in a low, more hospitable locale at about the sea level.
1) The carburetor of a passenger automobile adjusted optimally to a low
altitude will be very inefficient at high altitude (and vice versa.), since
the amount of oxygen in air changes to be far less than at lower altitude.
What happens is too much gas for the amount of air (oxygen) available. And gas
vapour escapes into thin air, just like that, unburned. The tauted fuel
injection really has nothing to do with this outcome. To remedy this
situation, all the car engine needs is a carburetor or nozzle adjustment. That
should fix it. Typically, less gas is fed into the carburetor or something
like that. That is it, if one wishes to drive around in thin air for any
length of time, and to be efficient and not waste the energy, and avoid loss
of power, slower acceleration, difficulty to sustain high speeds, and so on.
This is akin to the size of oriface adjustment to a smaller tip or nozzle.
Exactly as pointed out by Orion/Baker.
Well, a car engine converts maybe 20% or less of the gasoline energy input
into the locomotive energy output, in the olden days. The rest is converted
into heat and goes up into the air through exhausts or hot engine block, aided
by that radiator fan with forced air.
The natural airflow alone to cool the engine down is not enough. I don't know
the exact numbers then or today. Not relevant to this discussion. I was told
that a power generating station, with the best equipment, the conversion rate
is something like way less than 20%. The rest heats up whatever -- water,
machines, or air. Same as auto engines. Hence the electricity is more
expensive than gas in terms of energy efficiency, if gas is used to generate
the power. It does not matter if it is 14% or 25% here. Because of the
enormous quantity of energy used to generate the power, a 1% increase in this
ratio means a fortune to those who can invent a better conbustion or
conversion system. But such progress is slow in coming. People have tried all
sorts of things already. Not much room to change things, actually. Maybe the
Japanese would try this desperately. Their economy seems at the brink of
2) Forced air burner system, as I learned long time ago, does not need any
stack. Zero. That is what industrial standards were about. I can cite book
authorities here. What industries do now are different matters, since they
have other ideas, and physical limitations (like floor space, ceiling
clearance and the building itself), in addition to somewhat "stupid"
engineering decisions at times, I guess, made things look complicated and
If the combustion is complete in a forced air burner system, all it takes is a
contraption to exhaust the hot spent air, without any harm. That's it. In a
natural draft burner system, in contrast, for example, reduction woodfire kiln
or naturally aspiring burner system kiln with or without ultra-violet safety
shutt-off devices, the stack must draw or pull the exhaust air up and away. No
fans here. Of course it is ok to install the exhaust fan or fans to force the
air up the chimney, but such in-line fans would be prohibitively expensive.
But would serve the same purpose as the forced air into the kiln chamber by
way of the burner ports.
In case of the natural air aspiration type burners, the oriface or nozzle tip
size becomes critical, since it regulates the amount of gas or energy. And the
stack size and height must be adjusted or match the pulling power of exhaust
air out of the kiln. The stack acts like forced air. The taller and larger,
the more air pull. It accelerates the airflow throughout the kiln. Without the
natural draw from the stack, things stall and the kiln will not reach the
temperature...at high fires, this factor becomes pivotal, literally. And as
you have guessed, the proper balance must be struck to have a good burn.
You see, the forced air means more oxygen (air) for whatever the given
equipment you may have can handle. It accelerates things. Like a turbo fan on
an auto engine which forces more air into it. Once properly adjusted, the
combustions is near perfect and forced air will move things along. No natural
draft is needed. It will come right off the kiln at the exhaust point.
The altitude compensation could be done for the thinner air and less oxygen,
like a car carbulator. But in this case with a forced air burner, it is like
the auto engine with a strong turbo fan but no major adjustment to the
carburator. (I don't know if such a thing exists. I simply give you some idea
on this matter. Not sicentific, you see.) It follows that, if the oriface size
or the amount of gas has been already properly adjusted, no special work for
the high altitude for the forced air system. It's not the altitude but the
amount of oxygen the forced air supplies that makes the combustion ok, under
normal circumstances. No extreme cases, mind you.
Remember, a car engine is adjusted for a low altitude. So, the high one
becomes a special case. But it we start from the minority picture of the high
altitude, as in Mexico City, no big deal here. We will be adjusting to the
lower altitude situation.
3) In my common sense, it is far better to have many nozzles, rather than a
few large ones, if the fuel efficiency and firing flexibility are your goal,
regardless of the altitude, forced air or natural draft. They are simply more
versatile in actual use. It may mean more work, but we love to fiddle around
and derive pleasure from such things. We do not do things like in the
industry. We have something beyond the economy and efficiency in mind. So if
we must start with the same kiln at low altitude which has been moved to the
higher altitude, it makes eminent sense to just add more burners. That should
do the trick as Baker contends.
What we fire are not a repeated similar-shot deals. We must adjust what we
fire. Hence, in my way of thinking, many smaller burners are betrer than one
or two larger ones. The installation and repair costs would come about just
the same, I guess. But this is not related to the high altitude, and I am not
really qualified to answer technical questions on this. Ask Fred Olsen or a
genuine ceramic engineer on this list (who seems to know more about the famed
ITC or its competition) with a long history of kiln design, construction and
firing experience if you wish to pursue this subject matter further.
4) In industrial applications, it is customary to build in an extra margin of
safety or risk avoidance, upon designing a system, be it a kiln or any other
contraption. If the engineer cuts it too thin and wants to build the very best
and the most efficient, with least costs, no on would praise it even if things
work just fine. That is expected. But, for a myriad of reasons beyond the
engineer's control, should that stuff does not work properly, it could lead to
the loss of one's job and reputation. Overbuild it, by spending a lot of extra
money, and no one would be wiser, and the job is secure. Other non-engineers
would not notice the difference. That's how it works. Trust me on this
generalization. I have never taken advantage of others. The opposite of my
having been duped into dubious schemes is rife, since I am not that
technically competent in the past. I learned from bad mistakes. That is all.
Well, if I make a living out of big bucks, the scenario would run like this.
In real life situations, this means larger burners than actually necessary,
with more insulation or bricks, bigger and higher stacks, and exotic
paraphrenalia, including ITC or whatever strikes your fancy, even if we should
not be tempted to use them. But we do all sorts of extravagant things as
reasonable human beings. We do get sucked into "miracle" schemes. That's human
nature. Don't we love those gadgets? And the "industries" must love such a
person in the position to know and influence others.
I have heard so many stories when kilns (not necessarily the potters or
ceramic kilns) did not function properly, and when the culprit engineer got
the job to fix it up, or at least volunteer an explanation on it (not the
design factor, you know). You see, you can always intimidate the technically
uninitiated big-money spender. Just like the "industries" do to us, saying
that we exist on the fringe or the margin of them, like an appendage.
5) How come is it so? I dare volunteer that the truth is the other way
around. The ceramic industries, which are reputedly the technical mainstream,
"derive" their existence from the same sources which give us the honor to be
at the "fringes". I do not wish to collect suitable stats on this assertion,
but I do not think I am too far off the mark. Making many toilet bowls give a
large sales volume. And huge consumption or buying power of clay. And they may
even mine the clay themselves. And if anyone pursuade me that clay mining is a
high-tech business, the more power to that person who knows how to extract
things out of ground.. Using the clay properly maybe high-tech. But that's
what potters are all about. And that is where I draw the line. Cheap dinner
sets in bundles? The same thing. But we do not make toilet bowls. Or, compete
against cheap dinner sets from low labor foreign lands. Not many of us,
At any rate, the industrial designs are not so flexible, since they tend to be
on the large to huge scale. Like the power generating station that I quoted
earlier. Industrial production is, for various reasons, geared toward volume.
They can absorb the higher risk associated with volume. This is not the case
with average potters, who can ill afford to set both the temperature and
atmosphere rigid, single-sided, and monotonous. That's the kind of life from
which we have managed to extricate ourselves. There is no special honor or
glory in being business-minded. Everyone does it. Some very well. Others not
so well, or quite poorly.
6) Granted that we must strike a proper balance somewhere. The industries have
huge investment and the sales volume to match. The potters are numerous in
number, on the other hand, but the aggregate volume or individual investments
may not match those of the "industries". But there are so many of us, to give
a huge counterbalancing power. Problem is that potters are not a unified
force. The politics, you know. But the degree of technical sophistication on
the part of the potters advances day by day, and by leaps and bounds, thanks
to the clayart technical gurus. Their expertize, I quarantee, is what
"industries" would look at us with envy.
The upshot? We are less likely to be intimidated by "pseudo-science" and
"know-it-all" tactics. More power to us all. The downside is that there are
still those who must profess to be the experts, since their livelihood and
personal pride depends on such utterances and their followers (the turf thing,
I imagine). And new entrants have no obvious defense against such onslaughts.
How can they tell what's what? The experienced can tell the difference, but
not the newcomers. And we cannot police such tries. The politics get in the
way. The situation is far more complex than it looks. (Did you know that some
websites which are not moderated do not tolerate any "advertisements"? It does
not matter if they are genuine or false. That one was simple.)
7) The "industries" as we have heard are much fewer in number. Their politics
are different from ours. They cannot afford to be politically slick and
sleazy. They know each other. Too much. Things like that do not work. Either
they cooperate with each other tacitly, or go to the other extreme of openly
cutting each other's throat. They have other problems, different from ours.
For us, the studio potters, politics and pottery do not mix. I have recently
noted an excellent post by Gavin on this, really impressed. So there it is in
black and white.
When there is just one potter or pots from one hands, there is no politics.
When there are more than one potters and the pots produced by two sets of
hands, the politics begins. Personally, I have tried to be my own boss, and
avoid direct competition (confrontation) with any potters. My days may be
numbered, however. I cannot prevent others from imitating me. That is why. Of
course I say this with tongue in cheek. Thank goodness I do not live in
- - - - - - - - --- --- ----
Well, if I offended anyone, it is not by design. I have tried to be as
truthful as I can. The court of law may still side with me. ... ... By the
way, my name is not Clinton.
April 8, 1998.
PS. The situation with the tax works are getting desperate for me. The
fatigue factor has finally descended on me. They haunt me from dusk till dawn
(I still remember the movie title). I am just human, I guess. I cannot tell
what or who will suddenly change into who or what. Hope I can resume my
normal life soon. But it has been a real drag. Cheers to you all who may or
may not have similar problems. HM
Jeff Lawrence on sun 12 apr 98
My first higher firing experience at altitude involved the following:
(1) 7200 feet of elevation above sea level
(2) "Lazarus" -- a California Kiln (4 venturi burners, downdraft design, 16
cuft stacking volume) that fired to ^10 in 8 hours at sea level
(3) kiln site between two buildings, just to make things worse
(4) a pyrophobic landlord on the periphery, wringing his hands at every turn.
Firing #1, with 7" gas pressure, stock 2" circular ports and no stack beyond
the built-in ~3-4' flue ... stalled at some contemptibly low temperature
I raised the stall point over several firings by the following:
- "butched out" the burner ports from 2" circles to 4" squares
- deregulated the gas to get 2 lbs of pressure
- added 12 feet of stack
- bribed my landlord with a six-pack; not a kiln alteration per se, but a
condition for the possibility of future firings
Last results: reached ^6 in 5-6 hours, oozed up to ^10 in 6-7 more,
depending on the weather. Monstrous huge reduction after ^3-4
(guesstimated), spitting flame from every crack. When I cut back on gas to
ease up on the reduction, stall ensued. More gas, and Lazarus continued to rise.
I stopped firing this kiln because the results were rock-solid reliable
squamous disasters, at a cost of $35/firing.
If I rebuild larger the 3-4' of currently fixed stack (the opening, 6"X9",
is limited by a frame welded to the kiln body), I expect much better firing
behavior. I base this on my observation that every adjustment toward more
air entrainment and chimney pull raised the temperature attainable. If I
could add 40' of chimney, it might do the same thing, but there are limits
to my tenacity.
Hope this saves somebody some time!
Sun Dagger Design
Rt 3 Box 220
Espanola, NM 87532