Jon Singer on sat 30 oct 99
Micheal E Rector says, in a recent posting,
"As the temperature in a kiln increases the pressure inside
the kiln also increases."
?? Can anyone substantiate this? I would think that the change in
pressure in a kiln could never be large enough to make a difference.
(A kiln has a big hole in it [the flue, or in the case of electrics, the
spyholes], so what is supposed to be holding in the increased pressure?)
It sounds really fishy to me, but then I haven't built kilns, so I could
easily be missing some key item here.
Cheers --
jon
Cameron Harman on sun 31 oct 99
As the temperature inside a kiln increases the heated gasses
inside expand, however, as they expand they have a reduced
density. In a properly designed kiln with the correct flue system
there will be no change in pressure inside a kiln as the
temperature rises.
It is possible that with improper flue systems the pressure can,
indeed, rise as the temperature rises. Remember that natural draft
kilns have been around since the beginning and they operate quite
well with a constant pressure throughout the burn. however, the
short stack and hood design is relatively modern and does not
allow the stack to increase in temperature as the kiln increases
in temperature.
As the gasses inside the kiln expand, even with a decreased
density there could be an increase in kiln pressure, but the stack
acts as a pump to remove the hot gasses. As the stack heats its
pumping action also increases which actually is enough to offset
the pressure rise in the kiln. If you do not have a stack that can
heat sufficiently the there is not enough pumping action and the
kiln pressure will rise somewhat.
In either event the rise in kiln pressure is not a great deal. It
is enough, however, in some cases to cause problems. For example,
in a car type kiln you can get enough pressure increase to force
hot gases through the sand seals and burn the steel on the sides
of the car.
A bit wordy, but I hope it helps.
Cameron
--
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Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
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Jon Singer on mon 1 nov 99
I think Cameron Harman's comment of October
31st is very nicely put. I have a related comment/
question:
Isn't it true that a gas-fired kiln that is oxidizing
has _less_ pressure inside than outside?
That is, if you pull a spyhole plug and you don't
see flames coming out, it's because _air_ is going
_in_, right? (Unless it's precisely balanced, that is.)
In other words, there is _no_ direct relationship
between temp & pressure in an open system that
is far from equilibrium, which is exactly what
you'd expect anyway.
Best --
jon
Nils Lou on tue 2 nov 99
Jon, Flames don't come out. What you perceive as flame coming out is
unburned fuel igniting in the ambient air. If the kiln is oxidizing it can
have just as much pressure--hot air expands. Just pass your hand (rapidly)
across the peep opening and you will feel the pressure. Your first premise
is not correct.NL
On Mon, 1 Nov 1999, Jon Singer wrote:
> ----------------------------Original message----------------------------
> I think Cameron Harman's comment of October
> 31st is very nicely put. I have a related comment/
> question:
>
> Isn't it true that a gas-fired kiln that is oxidizing
> has _less_ pressure inside than outside?
>
> That is, if you pull a spyhole plug and you don't
> see flames coming out, it's because _air_ is going
> _in_, right? (Unless it's precisely balanced, that is.)
>
> In other words, there is _no_ direct relationship
> between temp & pressure in an open system that
> is far from equilibrium, which is exactly what
> you'd expect anyway.
>
> Best --
> jon
>
Gavin Stairs on tue 2 nov 99
At 03:48 PM 11/1/99 -0500, you wrote:
>----------------------------Original message----------------------------
>I think Cameron Harman's comment of October
>31st is very nicely put. I have a related comment/
>question:
>
>Isn't it true that a gas-fired kiln that is oxidizing
>has _less_ pressure inside than outside?
>
>That is, if you pull a spyhole plug and you don't
>see flames coming out, it's because _air_ is going
>_in_, right? (Unless it's precisely balanced, that is.)
Yes, that's true, but it doesn't mean you don't still have an excess of CO on
the inside. It just means you'll have the devil of a time making it even
reduction.
The question of whether you have excess CO and whether your kiln is in balanced
reduction are two separate questions. If you have a little O2 chasing a lot of
CO, you may still get flashed and oxidized pots. This is exciting, but
unpredictable. Like wood firing, maybe.
However, if your kiln is adjusted so that the secondary air dampers can be
almost all the way open, and still be burning nicely, and not too lean, then it
is a simple matter to induce reduction by moving the stack damper in by a bit.
Then the UPPER part of your kiln will definitely be at positive pressure
relative to the atmosphere, and you MAY see red flame coming out of the peep
holes. The lower part of the kiln will still be sucking, but gently, gently.
It is a corollary that you need to be sure the lower part of the kiln is well
sealed, and probably need to close down the secondary dampers a bit: burn
mostly in primary air, for well mixed reduction, with no free oxygen.
This is not mysterious. To fill the kiln with CO excess, and no free O2, you
have to burn up the O2 with fuel, and have a bit of fuel in excess. If the O2
(air) is coming into the kiln in an uncontrolled and unmixed manner, as through
peeps, cracks and secondary air vents, then it will have a hard time mixing
with the rest of the atmosphere to get used up, so there will be free O2 in the
kiln: poor reduction, or oxidation flashing. But if the air mostly comes in
through the burner as primary air, it will have a chance to mix intimately with
the fuel, and all of the free O2 can be used up: instantaneous reduction
atmosphere, everywhere.
If you have a primary air reduction going, then you may not see much of a flame
at the peep at all, since you needn't have very much excess CO. This is
efficient reduction. It is also subtle, so you may need an O2 meter to
recognize it. Or have some experience firing this way.
If you want to reliably recognize reduction by puffing and blowing, you have to
set up like I suggest, almost balanced with atmospheric pressure, then up the
back pressure slightly with the stack damper, and shut down the secondary air.
You may have to open up the primary a bit, to compensate. You will get a rich
flame, not too long, and CO coming from the burner. The back pressure will put
the upper part of the kiln into gentle positive pressure, and so may see a hint
of flame. If you test with a splint at a peep, it will char, but not burn. If
you see bright flame coming from a peep, then you have heavy reduction, and you
can open up the primary air a bit.
The key is to know where your air is coming from, and to balance it with fuel.
Otherwise, you're blind. You may have reduction on one side and not on the
other, or a little patch of oxidation right beside the burner, etc.
This is not the most energy efficient way to fire, and not the right way to get
a long flame to heat the kiln evenly. For that you want lots of secondary air,
and a rich burner setting. Going to a reduction setting is a matter of cutting
down the secondary air. There are two dampers to help you do this: the stack
damper and the secondary air damper.
It took me a while to understand the role of secondary air. In a phrase, flame
length. Secondary air takes a while to mix with the fuel rich primary stream,
so burning can be delayed until the gas stream is well within the kiln. The
sooty primary flame is then an efficient radiator of heat, and the long flame
can heat the whole kiln efficiently. However, in this mode, there is O2 all
over the place. So you can't use secondary air to establish reliable
reduction. It has to be primary air, so the flame will be shorter, and the
heating will not be as efficient. If your burners are blowing like a jet
plane, they are probably in oxidation. The reduction flame is quieter, because
there is no excess of oxygen to burn up the fuel close to the burner.
Whether you have to adjust the primary air to get a good reduction depends on
how you set it up for oxidation and max efficiency. If you have a large kiln,
and need a very long flame, then you may have just a bit of primary air, and a
lot of secondary air. In that case, you need to shut down the secondary and
open up the primary somewhat. However, if you have a very small kiln, like the
originator of this thread, then your most efficient heat rise may be with a lot
of primary air, and only a bit of secondary. Then you may have to cut back on
both the primary and the secondary air to establish reduction. She would also
have to damp the stack (or exit hole) until the secondary flow was slight, for
most efficient burning. Then, reduction is established by only a bit more
damper. Her problem may be that if you have no secondary damper, the back
pressure needed to close out the secondary air may also make the burner
unstable. There are limits to how much back pressure you can stand in a
normally aspirated kiln.
I think I'll have to build a demo kiln to illustrate all this. Stay tuned.
Gavin
Gavin Stairs
Stairs Small Systems (S3)
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
(416) 530-0419 stairs@stairs.on.ca
Cameron Harman on tue 2 nov 99
With respect to Jon Singer's comments:
If a kiln is well designed it will have a positive pressure
inside. The pressure inside a kiln will vary from bottom to top.
The pressure will increase with height inside the chamber. It will
always do this and can not be stopped.
The hobby gas kilns use a simple burner system that cannot
generate enough external pressure to allow it to burn into a
chamber that has much pressure even at the base. However, a well
designed kiln will be designed such that the pressure at the floor
of the kiln is nearly zero or slightly negative. This lower
pressure allows the burners to burn properly by letting them suck
in surrounding air for complete combustion.
These burners do require pressure inside the chamber in order to
burn reducing, however.
If the pressure inside the chamber gets lower it causes the entire
kiln to suck in cold air which upsets the temperature uniformity.
You should feel hot air escaping from peep sights anywhere around
the kiln. I must add, however, that I have less experience with
this kind of kiln and I am sure there are folks who have operated
their kilns satisfactorily with negative pressure inside the
chamber. Just because something is correct or incorrect does not
necessarily mean that operating in that way is bad.
The pressure inside the kiln is regulated by the exhaust system
and should remain constant over the entire range of temperatures.
So, if your kiln is pulling air into the chamber, don't get upset,
its just that it could be made to work a little better. That
doesn't mean that it has to be changed, especially if you are
happy with the results you get.
Good firing!
--
**********************************************************
Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
get your free ezine: http://www.kilnman.com/ezine/ezine.html
THE place for total kiln and drier support
**********************************************************
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