Orion/Baker on mon 6 apr 98
In a very positive spirit (not a contrary one!) I must say that I totally
agree that novices should get the most accurate information possible. I
hope no one was confused by my cross reference to the term ("tip") when I
wrote my short piece the other day about sizing orifices for high altitudes
(thin air). I also see that it's easy for folks to feel a bit confused
about how chimneys and draft really work when so many commonly held beliefs
in our area spring from well intentioned but often incomplete assumptions
about the forces of nature.
It's commonly held that for a kiln to fire well, a large volume of air
needs to flow very rapidly through the chamber and be forcefully "drawn" up
the chimney. Yet the most efficient and even-burning kilns (and furnaces)
should have no more "throughput" than necessary to maintain positive inflow
to transfer heat and provide the atmosphere of choice (oxidation or
reduction). Excessive flow-through wastes heat, and too-rapid, poorly
planned currents can seriously cause or aggravate "hot spots," "cold
spots," and "mysterious" reduction or oxidation dead zones. In such cases,
slowing the transit of hot gases through the kiln can be a wonderful "fix."
As popular as forced air burners may be, they may present serious control
problems.
Few potters understand how chimneys really (really) work -- that "stack
effect" is driven by differential pressure, not by "convection." This
isn't "hair splitting" -- it's a very different principle, and a useful
concept to grasp. For instance, it's fascinating to know that no
temperature difference is needed between the bottom of a chimney and the
top of a chimney for air to flow "up and away" (although, of course, less
dense (lighter) hot air will rise above more dense, cold air anywhere --
with or without a chimney). Certainly the expansion of cool, dense
secondary air inside the kiln chamber adds to overall pressure dynamics and
certainly airflow benefits by it to some degree (all offset by some
internal airflow "drag" within the chimney, depending on its size), but I
think it's important for folks to know that vacuum essentially never occurs
in an operating kiln or chimney.
Properly sized burners operating at the correct flow rate and pressure can
deliver heat very efficiently without great masses of cold secondary air
being drawn in. Of course there is a need for some secondary air but
mainly to serve as a heat transfer (convection) medium. How much secondary
air is needed is probably quite a bit less than many folks would guess is
required.
If a kiln suffers from "combustion inefficiency" (heating problems?) it
would be far better to make sure each burner has a clean, correctly sized
orifice, that it's feed air "throat" is clear of debris, and that the
regulators and gas lines are passing the proper amount of fuel. Anyone who
has operated single, stand-alone burners (like standard raku burners)
should recognize what a well fed flame looks like -- a hearty, blue to
blue-white blast. Attempting to correct poor combustion by providing extra
"secondary" air can be a dangerous evasion -- essentially all of the
combustion should be occurring entirely in/at the burner, not up inside the
kiln chamber.
If a kiln has been properly designed in the first place, tuning up burners,
keeping ports and flues/chimneys clear of debris, and loading thoughtfully
to (a) allow good circulation, and (b) distribute mass as evenly as
possible -- it should operate very well indeed with as little chimney as
possible.
I don't think folks need a ton of technical data to understand these basic
concepts. I'd certainly be willing to correspond with anyone about these
things, but I don't think there's much need for a massive debate about such
simple things.
Best to all, as always,
Ellen Baker
orion@telcomplus.net
WardBurner on tue 7 apr 98
Oh Lord, keep me from being dragged too deep into this....
It was posted that;
<<<mainly to serve as a heat transfer (convection) medium. How much secondary
air is needed is probably quite a bit less than many folks would guess is
required.>>>>
The average Venturi burner that is used by potters entrains 30-50% of it's
primary air through the mixing bell. The remaining air must be delivered as
secondary air. In industry, Venturi burners that are used in closed port
situations have no secondary air entrained around the port. Because of this,
they have air inlets for the injection of compressed air that feeds 100% of
the air required. They wouldn't fire with just the air that enters the back of
the burner. Forced air burners provide 100% of combustion air, but still
benefit from secondary air in most pottery kilns. I feel it is better to err
on the side of too much secondary air than not enough.
Convection provides considerable heat at low temperatures, but at higher
temperatures (once you have color) flame radiation provides most of the heat
transfer.
Marc Ward
Ward Burner Systems
PO Box 333
Dandridge, TN 37725
USA
423.397.2914 voice
423.397.1253 fax
wardburner@aol.com
Vince Pitelka on tue 7 apr 98
Ellen -
Thanks so much for the lecture, but you have introduced a bunch more points
which need to be clarified or corrected.
> As popular as forced air burners may be, they may present serious control
>problems.
Not at all, when they are set up properly and operated properly. Just like
atmospheric burners when set up and operated properly.
>Few potters understand how chimneys really (really) work
Any potter with a natural draft kiln understands very well how chimneys
really work. You can split hairs about convection and differential pressure
all you want.
>internal airflow "drag" within the chimney, depending on its size), but I
>think it's important for folks to know that vacuum essentially never occurs
>in an operating kiln or chimney.
. . . . . . Huh?? Maybe vacuum is the wrong term. How about negative
pressure? Naw, I like vacuum batter. The stack convection or differential
pressure or whatever you want to call it causes negative pressure which
sucks the spent gases out of the kiln and draws the flames and secondary air
into the kiln. Anyone who uses a kiln with a chimney certainly understands
this very clearly. And of course, a vacuum cleaner operates on differential
pressure, right?
>Properly sized burners operating at the correct flow rate and pressure can
>deliver heat very efficiently without great masses of cold secondary air
>being drawn in. Of course there is a need for some secondary air but
>mainly to serve as a heat transfer (convection) medium. How much secondary
>air is needed is probably quite a bit less than many folks would guess is
>required.
>Attempting to correct poor combustion by providing extra
>"secondary" air can be a dangerous evasion -- essentially all of the
>combustion should be occurring entirely in/at the burner, not up inside the
>kiln chamber.
You are right about the necessary amount of secondary air being quite a bit
less than most folks would guess. That is why I like to use refractory
collars in the burner ports, and very careful regulation of damper to
control flame-shape. But otherwise this is VERY misleading. Except in
sealed port burners, and depending on the burner design, secondary air is
usually necessary for complete combustion of the fuel. And no, all the
combustion does not occur in/at the burner, unless you are in hard oxidizing
mode with a very short blue cone flame. On low-velocity burners such a
flame is often quite cool (relatively). A flame is of course just gas
combusting, and in the most efficient flame for temperature climb the gas
continues to combust well into the kiln, and adequate secondary air is
required.
One of the things I really like about a restricted refractory burner port
collar is that the secondary air enters directly around the burner tip, and
drastically affects the shape of the flame. This occurs in almost all gas
burners, but more so with a restricted port. Once you get to know your
burners, you can judge a great deal by the shape of the flame.
>If a kiln suffers from "combustion inefficiency" (heating problems?) it
>would be far better to make sure each burner has a clean, correctly sized
>orifice, that it's feed air "throat" is clear of debris, and that the
>regulators and gas lines are passing the proper amount of fuel.
And make absolutely sure that your flue opening is large enough, your
chimney is large enough and tall enough, and your damper is working
properly. These things are common sense.
>should recognize what a well fed flame looks like -- a hearty, blue to
>blue-white blast.
This of course depends on the type of burner you are using. Personally, I
dislike burners which create a hearty blue to blue-white blast. They are
noisy and tempermental. Give me a lower pressure venturi or tube burner
with a sleepier flame any day.
>to (a) allow good circulation, and (b) distribute mass as evenly as
>possible -- it should operate very well indeed with as little chimney as
>possible.
This of course goes without saying, but adequate stack draft is still
essential for all natural-draft kilns.
>I don't think folks need a ton of technical data to understand these basic
>concepts.
Please take your own advice here.
I'd certainly be willing to correspond with anyone about these
>things, but I don't think there's much need for a massive debate about such
>simple things.
Maybe not a massive one. Maybe just a little one.
- Vince
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
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