vince pitelka on wed 27 mar 02
> I am convinced the principal benefit of dampering the flue is reducing
> the amount of secondary air flooding into the kiln through the burner
ports.
> Secondary air is, by nature, poorly mixed with the firing gases,
likely
> to stay low in the kiln because it's denser, and likely to short-circuit
to
> the flue opening and create a cold spot at the floor of the kiln, or in my
> case with forced-air burners, create a cold spot at the back of the kiln.
Dave -
That is a primary concern, but it must be seen in balance with the concept
of excessive heat-loss through the stack. Both work together. Closing down
the damper a bit decreases outflow form the kiln and also reduces secondary
air, and both together increase heat retention within the kiln. Some
secondary air is of course necessary, because flames are just hydrocarbon
gas continuing to burn as it leaves the burner tip, and secondary air is
necessary to allow complete combustion.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Dave Finkelnburg on wed 27 mar 02
Snail,
I am convinced the principal benefit of dampering the flue is reducing
the amount of secondary air flooding into the kiln through the burner ports.
Secondary air is, by nature, poorly mixed with the firing gases, likely
to stay low in the kiln because it's denser, and likely to short-circuit to
the flue opening and create a cold spot at the floor of the kiln, or in my
case with forced-air burners, create a cold spot at the back of the kiln.
Every kiln is different, of course, so your mileage may vary.....
Regards,
Dave Finkelnburg in windy Idaho
Earl Brunner on thu 28 mar 02
Right, you said better than I. I was just trying to point out that as you move the
damper in, you can go to far for the desired result. My kiln is a small Geil, and
the stack is very short (just to the top of the back of the kiln). So even to get
any measurable back pressure out of the bottom port, I have flame out the top of
the stack. (and I have stalled the kiln and am actually losing temperature.
Interestingly, today I tried the firing down idea. It's interesting to me that it
takes much less gas to maintain 1900-2000 degrees than it takes to get there in the
first place. I held it between 2000 and 1900 (with a gradual drop over two
hours). Can't wait to open it up tomorrow.
vince pitelka wrote:
> > This makes sense, but it should be pointed out that by dampering the flue
> down too
> > much, you actually increase the velocity (not decrease) the velocity of
> the air
> > moving through the kiln (at least that is what it "looks like") , yes
> you get
> > back pressure, but if you haven't increased velocity, you at least begin
> to move
> > the combustion at least partly out of the primary chamber of the kiln and
> into the
> > flue, wasting gas and gaining no temperature climb within the kiln.
>
> Whoa Earl, are you trying to confuse? I offer the following for the sake of
> those learning the process. Closing the damper increases the velocity of
> gases right at the damper, and induces a sort of "vacuum cleaner" effect in
> the immediate vicinity of the damper, but it decreases velocity at the
> burner ports, and thus does not increase the velocity of gases through the
> kiln. And your statement about the gas burning in the chimney is a bit
> misleading. When you close the damper a little, creating slight
> backpressure, you eliminate the excess secondary air, and you get a neutral
> flame, which is still efficient combustion but with no unnecessary cold air
> entering the kiln. When you close the damper a little more you get a
> partial reduction, with some unburned hydrocarbons throughout the kiln
> (that's what you hope for anyway, if you are seeking a partial or "climbing"
> reduction), but with little or no gases burning in the exhaust flue beyond
> the damper. When you close the damper a little more you get a heavier
> reduction, with a heavier hydrocarbon atmosphere throughout the kiln, and
> with significant hydrocarbon gases burning in the exhaust flue beyond the
> damper.
>
> It is just a question of the degree of "dampering down" and the degree of
> air movement in the kiln. Closing the damper to any degree decreases air
> velocity through the kiln, but excessive "dampering down" stalls out
> combustion, filling the kiln with excess hydrocarbon gases, which can only
> combust in the exhaust flue beyond the damper where oxygen is available.
> Best wishes -
> - Vince
>
> Vince Pitelka
> Appalachian Center for Crafts
> Tennessee Technological University
> 1560 Craft Center Drive, Smithville TN 37166
> Home - vpitelka@dtccom.net
> 615/597-5376
> Work - wpitelka@tntech.edu
> 615/597-6801 ext. 111, fax 615/597-6803
> http://www.craftcenter.tntech.edu/
>
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--
Earl Brunner
http://coyote.accessnv.com/bruec
mailto:bruec@anv.net
Earl Brunner on thu 28 mar 02
This makes sense, but it should be pointed out that by dampering the flue down too
much, you actually increase the velocity (not decrease) the velocity of the air
moving through the kiln (at least that is what it "looks like") , yes you get
back pressure, but if you haven't increased velocity, you at least begin to move
the combustion at least partly out of the primary chamber of the kiln and into the
flue, wasting gas and gaining no temperature climb within the kiln.
I depend so much on my digital pyrometer anymore. Moving my damper as much as a
1/4 of an inch can make the difference between heat climb and a stall.
The dynamics here are multifaceted.
vine patella wrote:
> > Dave:
> > I am convinced the principal benefit of dampering the flue is reducing
> > the amount of secondary air flooding into the kiln through the burner
> ports.
> > Secondary air is, by nature, poorly mixed with the firing gases,
> likely
> > to stay low in the kiln because it's denser, and likely to short-circuit
> to
> > the flue opening and create a cold spot at the floor of the kiln, or in my
> > case with forced-air burners, create a cold spot at the back of the kiln.
>
> Dave -
> That is a primary concern, but it must be seen in balance with the concept
> of excessive heat-loss through the stack. Both work together. Closing down
> the damper a bit decreases outflow form the kiln and also reduces secondary
> air, and both together increase heat retention within the kiln. Some
> secondary air is of course necessary, because flames are just hydrocarbon
> gas continuing to burn as it leaves the burner tip, and secondary air is
> necessary to allow complete combustion.
> Best wishes -
> - Vince
>
--
Earl Brunner
http://coyote.accessnv.com/bruec
mailto:bruec@anv.net
Snail Scott on thu 28 mar 02
At 07:03 PM 3/27/02 -0700, Dave wrote:
> I am convinced the principal benefit of dampering the flue is reducing
>the amount of secondary air flooding into the kiln through the burner ports.
> Secondary air is, by nature, poorly mixed with the firing gases, likely
>to stay low in the kiln because it's denser, and likely to short-circuit to
>the flue opening and create a cold spot at the floor of the kiln, or in my
>case with forced-air burners, create a cold spot at the back of the kiln.
Dave-
Makes good sense r.e. downdraft designs. Less relevant
for unpdraft, I suspect, but probably still a factor.
I figure a bit of backpressure helps the hot and cold
gases mix better, too, and causes the heat to linger
long enough to create a more even heat environment.
-Snail
vince pitelka on thu 28 mar 02
> This makes sense, but it should be pointed out that by dampering the flue
down too
> much, you actually increase the velocity (not decrease) the velocity of
the air
> moving through the kiln (at least that is what it "looks like") , yes
you get
> back pressure, but if you haven't increased velocity, you at least begin
to move
> the combustion at least partly out of the primary chamber of the kiln and
into the
> flue, wasting gas and gaining no temperature climb within the kiln.
Whoa Earl, are you trying to confuse? I offer the following for the sake of
those learning the process. Closing the damper increases the velocity of
gases right at the damper, and induces a sort of "vacuum cleaner" effect in
the immediate vicinity of the damper, but it decreases velocity at the
burner ports, and thus does not increase the velocity of gases through the
kiln. And your statement about the gas burning in the chimney is a bit
misleading. When you close the damper a little, creating slight
backpressure, you eliminate the excess secondary air, and you get a neutral
flame, which is still efficient combustion but with no unnecessary cold air
entering the kiln. When you close the damper a little more you get a
partial reduction, with some unburned hydrocarbons throughout the kiln
(that's what you hope for anyway, if you are seeking a partial or "climbing"
reduction), but with little or no gases burning in the exhaust flue beyond
the damper. When you close the damper a little more you get a heavier
reduction, with a heavier hydrocarbon atmosphere throughout the kiln, and
with significant hydrocarbon gases burning in the exhaust flue beyond the
damper.
It is just a question of the degree of "dampering down" and the degree of
air movement in the kiln. Closing the damper to any degree decreases air
velocity through the kiln, but excessive "dampering down" stalls out
combustion, filling the kiln with excess hydrocarbon gases, which can only
combust in the exhaust flue beyond the damper where oxygen is available.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Dave Finkelnburg on thu 28 mar 02
Snail,
My apologies! I was, indeed, thinking, speaking only of a downdraft
kiln when I wrote, ">> Secondary air is, by nature, poorly mixed with
the firing gases, likely
>>to stay low in the kiln because it's denser, and likely to short-circuit
to
>>the flue opening and create a cold spot at the floor of the kiln, or in my
>>case with forced-air burners, create a cold spot at the back of the kiln."
You wrote, ">I figure a bit of backpressure helps the hot and cold
>gases mix better,...."
Actually, because dampering reduces the secondary air coming into the
kiln, there's less cold air to mix with the hot combustion gases. Thus the
mix is automatically more uniform.
I know you have this figured out--you've been firing successfully a lot
longer than I have and I don't mean to disagree with you. The dynamics of
the inside of a gas kiln are too complex to try to reduce to two paragraphs.
Good potting!
Dave Finkelnburg
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