Ivor and Olive Lewis on sun 18 mar 07
Dear Vince,
Perhaps we have similar experiences but explain them in differing ways
As a child I was fortunate to have, as a toy, a Bunsen Burner. It was a =
sad day when we moved home and our new residence did not have a gas =
supply and a happy one when I started attending Grammar School where =
Chemistry was on the timetable..
Do you know that it is possible to ignite this device with a cold =
platinum wire? But, I believe, only if there is Hydrogen present.
Adjusting the gas flow of a BB to a minimum with the maximum possible =
potential air imput will cause the flame to travel into the burner tube =
and ignite at the jet.
I currently fire with two venturi burners that have flame retention =
rings. If, when the burners have been ignited, I move the shutters to =
fully open them and turn the gas flow to a minimum, the flame travels =
back to the jet. It would be impossible to candle this kiln without =
closing the shutters to their minimum setting. Flashback is inevitable
By the way, I know what you mean, but is "...Diminishing Vacuum..." a =
good way to describe restricting the flow of combusted gases through the =
chamber into the stack ? Just think about it ! ! !
Sincere regards,
Ivor
Vince Pitelka on sun 18 mar 07
Ivor Lewis wrote:
"I currently fire with two venturi burners that have flame retention rings.
If, when the burners have been ignited, I move the shutters to fully open
them and turn the gas flow to a minimum, the flame travels back to the jet.
It would be impossible to candle this kiln without closing the shutters to
their minimum setting. Flashback is inevitable"
Ivor -
That's right, of course. We have an old Norman updraft with ceramic venturi
burner tubes, and we do the same thing. The small target pilot burners do
not provide adequate heat for an overnight preheat, so we turn the burners
on very low, and the flame jumps back to the orifice.
It doesn't seem that "flashback" is an appropriate term in the case you
describe. Flashback is the same phenomenon, but it happens at higher burner
settings and is usually accompanied by a loud popping noise. Tube burners
without flame retention tips are especially susceptible to flashback, and
the flame often jumps back and forth between tip and orifice, producing a
popping noise each time. "Back-burning" seems a more appropriate term in
this case.
Ivor wrote:
"By the way, I know what you mean, but is "...Diminishing Vacuum..." a good
way to describe restricting the flow of combusted gases through the chamber
into the stack ? Just think about it ! ! !"
Dear Ivor,
I don't need to "Just think about it ! ! !," because that is exactly what is
happening. When you close the damper in the exhaust flue, you are not
"restricting the flow of combusted gases through the chamber" unless you
close it enough to create positive pressure (back-pressure) inside the
chamber and significant flames at the spyholes. Until that point, the flow
of hot gases up the stack creates a very slight vacuum in the firing
chamber, drawing secondary air in through the burner ports. Thus, closing
the damper diminishes secondary air.
You have only to watch the flame shape at the burner ports to witness this.
When you open the damper wide, it really sucks in the secondary air, and the
flame shape narrows considerably. When you close the damper enough to
create positive pressure (back-pressure) in the firing chamber, the flame
entering the burner port flares out to fill the port, indicating that little
or no secondary air is being drawn in.
Are you implying that you actually have a different explanation for these
phenomena?
- 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/
Linda Blossom on sun 18 mar 07
I am thinking about this and wondering what the
outcome is on this little debate because I have one
burner that I always light in the morning after the
other burner has heated things up. This is because
this burner always backflames whether I turn it low
or high when the kiln is cold.
Linda
Adjusting the gas flow of a BB to a minimum with the
maximum possible potential air imput will cause the
flame to travel into the burner tube and ignite at
the jet.
I currently fire with two venturi burners that have
flame retention rings. If, when the burners have
been ignited, I move the shutters to fully open them
and turn the gas flow to a minimum, the flame
travels back to the jet. It would be impossible to
candle this kiln without closing the shutters to
their minimum setting. Flashback is inevitable
By the way, I know what you mean, but is
"...Diminishing Vacuum..." a good way to describe
restricting the flow of combusted gases through the
chamber into the stack ? Just think about it ! ! !
Chris Trabka on mon 19 mar 07
>I currently fire with two venturi burners that have flame retention rings.
If, when the burners have been ignited, I move the shutters to fully open
them and turn the gas flow to a minimum, the flame travels back to the jet.
It would be impossible to candle this kiln without closing the shutters to
their minimum setting. Flashback is inevitable
Ivor,
I have only one kiln, a gas kiln. For a bisque firing, I candle one bank of
burners (I have 2 banks) for at least 4 hours. The flame looks like a
candle and starts from the burner orface (not the tip of the burner). This
allows my kiln to warm up and dry out slowly and remove latent moisture
from the ware (and kiln). At the end of 4 hours the temp is about 200F to
300F.
Chris
Ivor and Olive Lewis on tue 20 mar 07
Dear Chris Trabka,=20
Thank you for your notes about your firing schedule.
I fully understand the need to apply heat slowly and the reasons why =
that must be done. But for some reason I have great misgivings about =
working with a gas burner with the flame burning from the as jet nipple. =
Unfortunately my classroom notes taken from John Coulter when he was =
teaching Studio Practice are no longer with me. But as long as I can =
remember, and that is back to my childhood, it was considered dangerous =
to work any gas fired appliance with the flame issuing from the jet =
nipple.
Just from memory, I think it is about the incomplete combustion of the =
gas. At that time Coal Gas, which contains Carbon Monoxide was used. But =
if Propane, Butane or LPG are incompletely burned then there is the =
potential for that gas to be released into the vicinity of the kiln. The =
appearance of a blue flame at the head of the burner nozzle is an =
indicator that fuel is being used efficiently. This can be achieved at =
low gas pressures by restricting primary air induction. There is also =
the thought that the jet nipples may be made from an alloy that would =
deteriorate due to heat conducted from the flame.
My own burners have pilot lights which supply sufficient heat to take =
the kiln to 200 deg C in a couple of hours. I would never consider =
working with burners with the flame issuing from the Jet Nipples. I am =
certain this is inappropriate practice. I still maintain that Primary =
Air Shutters are there to be exploited.
Perhaps Mark Ward might like to comment.
Best regards,
Ivor
Ivor and Olive Lewis on tue 20 mar 07
Dear Vince Pitelka,=20
Please think long and hard about the nature of the mechanics of a kiln. =
You are dealing with buoyancy, the ability of an expanded hot rarefied =
(low density) gas to float on a dense unheated gas. It is the motion of =
the various gases that shape the phenomena you describe. Gravitational =
force and heat power the motion of gas through a kiln, unless it is =
fired with a fan assisted burners.
Best regards,
Ivor
Vince Pitelka on tue 20 mar 07
Ivor Lewis wrote:
"Please think long and hard about the nature of the mechanics of a kiln. You
are dealing with buoyancy, the ability of an expanded hot rarefied (low
density) gas to float on a dense unheated gas. It is the motion of the
various gases that shape the phenomena you describe. Gravitational force
and heat power the motion of gas through a kiln, unless it is fired with a
fan assisted burners."
Dear Ivor -
It sounds like we agree. On a downdraft kiln with a good chimney, when the
damper is open quite a bit, the force of hot gases rising up the chimney
creates a suction at the flue, and thus creates a very slight negative
pressure (could also be called a slight vacuum), in the ware chamber, and
atmospheric pressure causes secondary air to rush into the ports around the
burner tips in order to compensate. Closing the damper diminishes the
negative pressure, and less secondary air enters. Close the damper further,
and at some point you get positive pressure rather than negative pressure in
the firing chamber, and no secondary air enters. Now, don't dissapoint me
by finding something to quibble about with this explanation.
- 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/
mel jacobson on wed 21 mar 07
ok
At 02:22 AM 3/18/2007, you wrote:
>Dear Vince,
>Perhaps we have similar experiences but explain them in differing ways
>As a child I was fortunate to have, as a toy, a Bunsen Burner. It
>was a sad day when we moved home and our new residence did not have
>a gas supply and a happy one when I started attending Grammar School
>where Chemistry was on the timetable..
>Do you know that it is possible to ignite this device with a cold
>platinum wire? But, I believe, only if there is Hydrogen present.
>Adjusting the gas flow of a BB to a minimum with the maximum
>possible potential air imput will cause the flame to travel into the
>burner tube and ignite at the jet.
>I currently fire with two venturi burners that have flame retention
>rings. If, when the burners have been ignited, I move the shutters
>to fully open them and turn the gas flow to a minimum, the flame
>travels back to the jet. It would be impossible to candle this kiln
>without closing the shutters to their minimum setting. Flashback is inevitable
>By the way, I know what you mean, but is "...Diminishing Vacuum..."
>a good way to describe restricting the flow of combusted gases
>through the chamber into the stack ? Just think about it ! ! !
>Sincere regards,
>Ivor
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
>melpots@pclink.com.
John Baymore on wed 21 mar 07
If you look at the equations for inducing natural draft, you'll see that
stack effect is dependent on a numbetr of factors, but very much on the
temperatuire of the effluent at the exit point of the chimney. As a broad
generalization, the hotter the exit point, the greater the induced draft
effect will be.
The hot, less dense combustion effluent wieghs less per cubic foot (or
meter if you prefer ) than the surrounding cold air, and therfore it is
boyant (floats) when suspended in some colder surrounding ambient air.
Think of it as the "hot air baloon effect". In fact you can visulaise the
top of the chimney having a constant stream of little 1 cubic foot (or
meter) hot air baloons popping out the top. The hotter they are, the more
of them will pop out in a given unit of time. The more that pop out in say
1 minute, the higher the pressure differential, and therefore the effluent
flow also will be for a given chimney structure.
As this effluent exits the confines of the stack, it leaves space in the
chimney for an equal volume of materials to replace it. This creates a
pressure differential between the bottom of the chimney and the
surrounding air. "Nature abhors a vacume", so if the base of the chimney
is open to the general surrounding atmosphere, the "laws of nature" will
attempt to equalize this pressure differential back to zero.
Since the stack of a kiln (chimney) is attached to the kiln chamber, and
the chamber of the typical studio potter-type kiln has open port burners
mounted on it, there is then space (X number of the above mentioned hot
air baloons per unit of time) for the combustion products from the burners
to be injected into the kiln as well as POSSIBLY some air surrounding the
kiln to be PUSHED into the kiln by the induced pressure differnential that
is created across the "orifice" that is formed by the burner port opening.
Flow through an open port burner port is very similar to the flow of a
given gas thru any orifice (opening). So the normal orifice equations
apply there. But note that burner ports and exit flues and such have
awful friction losses, unlike the typical gas burner orfice.
If the amount of materials that the kiln's burners are injecting into the
space left by the exitring gases, then the kiln will be running at
equilibrium, and even if there are open port burners, effectively no
secondary air will flow into the burner ports, because there is no
pressure differeential to drive the flow. If the burners are putting in
MORE combustion products than the induced exit flow, then the kiln chamber
will move into positive pressure conditions, and hot gases / flames /
effluent will leak OUT of the chamber thru any available opening.
Secondary WILL flow if there is still a pressure differential between the
exterior and the interior.
The pressure differential existing between the exterior of the kiln and
the interior of the chamber is affected by the stack effect generated
(mainly) by the chimney's hot effluents. The other factors in the
generation of the pressure differntial (and hence stack flow) are such
things as the diameter of the cross section of the chimney, the materials
the chimney is made of (friction losses), comstrictioon quality (more
friction), the geometry configuration of the chimney exit (discharge
friction), the length of the chimney structure and any bends (duct
losses), and so on.
In troubleshooting kilns I often utilize an inclined tube manometer to
measure the pressure differential at the burner port(s). I also often
utilise the same kind of unit (see the Dwyer instrument company catalog)
to install onto the unit permanantly in order to measure draft similar to
using gauges on gas burners. It is another instrument to give you
feedback when you are firing.
Hope this helps in the discussion.
best,
...................john
John Baymore
River bend Pottery
Wilton, NH
JBaymore@compuserve.com
http://www.JohnBaymore.com
John Baymore on wed 21 mar 07
OOPS... that last post should have said..........
"If the amount of materials that the kiln's burners are injecting into the
space left by the exitring gases is equal to the volume of materials
exiting the kiln, then the kiln will be running at equilibrium, and even
if there are open port burners, effectively no secondary air will flow
into the burner ports, because there is no pressure differential to drive
the flow."
Sorry about the brain lapse. Still tired from NCECA.
best,
................john
John Baymore
River Bend Pottery
Wilton, NH
JBaymore@compuserve.com
http://www.JohnBaymore.com
Ivor and Olive Lewis on thu 22 mar 07
Dear John Baymore,=20
Gravity rules the process of expelling the cooling gases from a kiln =
chamber. Without minimisation of gas density the process would stall. =
You should be able to calculate the velocity at which secondary air will =
flow into a kiln if you know the distance between the fire ports and the =
top of the stack.
By the way, what sort of pressures do you get in the kiln chamber and =
the stack? What are the reading below and above the Damper ?
Interesting technology.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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