Orion/Baker on wed 8 apr 98
After careful consideration of today's replies about burners and chimneys,
I can only say, "shucks" -- y'all are welcome to believe anything you'd
like, but...
(a) A kiln can be designed to require little or no chimney.
(b) Bright (blue to blue-white) flame indicates hot, efficient combustion
(even little gas flames, like on a gas kitchen stove, should be blue to
blue-white).
(c) A good, well tuned natural draft (venturi-type) burner will get the
majority of its heat generating feed-air from "primary" airflow (this can
be verified, if anyone wants to write directly).
(d) Secondary air is called "secondary air" because its role really is
"secondary"! A given quantity of gas at a given pressure needs a given
quantity of oxygen-rich air to combust -- and beyond that, "bonus" feed air
serves as a convection medium at any temperature.
(e) Too much draft can "rob" a kiln of its heat (and too much secondary
air can rob a burner of flame temperature, by the way)
(f) Too much draft can cause or aggravate hot or cold spots within the
kiln chamber.
(g) Although radiation becomes the principal mechanism for heat transfer
in the kiln chamber at high temperatures ("glowing and above"), throughout
the entire firing burners deliver their heat in the form of hot gas
(primary and secondary, both). We couldn't see any point whatsoever in
bringing radiation into the discussion of gas/heat delivery.
(h) "Forced air" burners are not ideal for every natural draft kiln by a
long shot. Industrial "forced air" burners are essentially always used in
conjunction with high pressure gas delivery, where "blowtorch" type heat is
the order of the day (like in steel furnaces, calciners, glass furnaces,
foundries, boilers, etc.). Even there, the amount of air "forced in" is
delivered to balance with the fuel inflow.
(i) The willful and wanton misuse of the term "vacuum" should disallow its
user from ever again scolding anyone else for the "misuse of terms." :)
This information stands on its own merit. Many sound sourcebooks about
combustion and heat transfer are available -- the overwhelming majority of
which should be found in the "science" departments of bookstores and
libraries.
For some REAL fun, I suggest anyone with a real passion for fire should
take a moment to visit the "hobby section" of Purdue engineering professor
George H. Goble's website, http://ghg.ecn.purdue.edu. The second half of
the site is devoted to his famous experiments burning barbecue charcoal
aided by liquid oxygen (LOX)! Not all science/engineer types are staid,
restrained, or dull --- that's for sure! In my experience, as a whole they
love to test the limits!
Best regards, as always
Ellen Baker - Glacier, WA
orion@telcomplus.net
Vince Pitelka on thu 9 apr 98
Ellen -
I am really not trying to get in the last word, but some things below beg
for a response.
>(a) A kiln can be designed to require little or no chimney.
Of course. Forced draft kilns need little or no chimney.
>(b) Bright (blue to blue-white) flame indicates hot, efficient combustion
>(even little gas flames, like on a gas kitchen stove, should be blue to
>blue-white).
It is a question of flames in different circumstances. And I never said
that the flame should not be blue, I simply questioned your reference to a
"a hearty, blue to
blue-white blast." This may be the healthiest flame under many
circumstances, but in kilns I prefer a slower, sleepier blue flame.
>(d) Secondary air is called "secondary air" because its role really is
>"secondary"! A given quantity of gas at a given pressure needs a given
>quantity of oxygen-rich air to combust -- and beyond that, "bonus" feed air
>serves as a convection medium at any temperature.
Secondary air is not ''bonus" feed air. It is not the "primary" combustion
air. It is the "secondary" combustion air. Does this really need to be
clarified?
>(e) Too much draft can "rob" a kiln of its heat (and too much secondary
>air can rob a burner of flame temperature, by the way)
>(f) Too much draft can cause or aggravate hot or cold spots within the
>kiln chamber.
You are restating the obvious.
>(h) "Forced air" burners are not ideal for every natural draft kiln by a
>long shot.
Huh? Who said they were? And besides, the above statement is a
contradiction, because with forced-air burners it would not longer be a
natural draft kiln.
>Industrial "forced air" burners are essentially always used in
>conjunction with high pressure gas delivery, where "blowtorch" type heat is
>the order of the day (like in steel furnaces, calciners, glass furnaces,
>foundries, boilers, etc.).
I'm not sure what you mean by "industrial" forced air burners. I have seen
lots of high-pressure forced-air burners on crucible furnaces, etc, and they
are entirely unappropriate for kilns. Low-gas-pressure, low-air-pressure
forced air burners, such as the ones Marc Ward sells or the ones found on
Unique and Alpine kilns work extremely well on both updraft and downdraft
kilns. They are very quiet and efficient.
>(i) The willful and wanton misuse of the term "vacuum" should disallow its
>user from ever again scolding anyone else for the "misuse of terms." :)
I try to use terminology correctly. I have a collection of old steam
guages. Some of the very low pressure ones read pressure on one side of
zero, and vacuum on the other. In other words, pressure is the opposite of
vacuum. A convection tower creates very slight pressure at the top, and
very slight vacuum at the bottom. OK?
>This information stands on its own merit.
No, it doesn't. If you are going to make absolute statements of fact, you
must be willing to verify them. All of the statements I have made are based
on many years of watching exactly what happens in all kinds of kilns.
- 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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