Janet Harper on thu 17 feb 00
Hi Clayarters,
I'm firing a 20 cu ft downdraft propane-fired kiln with a 14 ft stack. The
firing is nice and even temperature-wise ( cone 10 flat all around). My
problem is getting enough reduction at the floor level. The char test
indicates that the reduction is fine up high but I'm just not getting it down
low. I'm closing the damper to about 50% of the chimney opening and closing
some of my primary air at the burners (venturi) to reduce. I am getting a
nice 6-8" flame out of the upper ports but the bottom port is sucking air in
at that point. Would closing the damper more be the answer? When I tried
that I got smoke with the reduction and I always thought that meant I was
overdoing the reduction. I would appreciate your wisdom on this problem.
Thanks!
Janet Harper
Knoxville, TN
Richard Selfridge on fri 18 feb 00
Hi Janet,
I use a burning stick test. I think the thing to think about this problem
is to see a difference between reduction and pressure in the kiln. A good
way to test the pressure of the kiln is to observe a spy hole at the floor
and at the entrance to the stack. A soap size hole in the kiln at this spot
will give you important info and should be incorporated in any kiln design.
The visable flame on an inserted splinter of wood there should waver
between coming out the hole and being sucked toward the flue, that is it
should be verticle. This tells you that the whole of the envelope of the
kiln is under pressure, not just part way down. I would close the damper
till this balance is attained.
Next you need to check the "color" or visibility of this flame. If the
kiln is in oxidation there it will show a visible yellow flame. If it is in
reduction the flame on the burning stick will "go out" even though the
stick continues to smoke and burn. What you want is for the flame to waiver
in a lazy way from being just visable to going out.
Finally there is the question about how much air you let into the kiln
with the propane. If it smokes too much when you close the damper it may be
that even though it is not pressurized to the bottom, the fuel mixture is
too rich and that letting more air into the kiln will increase the pressure
and decrease the smoke. The air as well as the fuel creates the pressure. I
think you will find that the burning stick will tell you all you need to
know.
Regards, Richard Selfridge
http://www.compusmart.ab.ca/selfridg
ps. Are you going to Denver?
At 02:17 PM 2/17/00 -0700, you wrote:
>----------------------------Original message----------------------------
>Hi Clayarters,
>
>I'm firing a 20 cu ft downdraft propane-fired kiln with a 14 ft stack. The
>firing is nice and even temperature-wise ( cone 10 flat all around). My
>problem is getting enough reduction at the floor level. The char test
>indicates that the reduction is fine up high but I'm just not getting it down
>low. I'm closing the damper to about 50% of the chimney opening and closing
>some of my primary air at the burners (venturi) to reduce. I am getting a
>nice 6-8" flame out of the upper ports but the bottom port is sucking air in
>at that point. Would closing the damper more be the answer? When I tried
>that I got smoke with the reduction and I always thought that meant I was
>overdoing the reduction. I would appreciate your wisdom on this problem.
>Thanks!
>
>Janet Harper
>Knoxville, TN
>
>
Hank Murrow on fri 18 feb 00
>----------------------------Original message----------------------------
>Hi Clayarters,
>
>I'm firing a 20 cu ft downdraft propane-fired kiln with a 14 ft stack. The
>firing is nice and even temperature-wise ( cone 10 flat all around). My
>problem is getting enough reduction at the floor level. The char test
>indicates that the reduction is fine up high but I'm just not getting it down
>low. I'm closing the damper to about 50% of the chimney opening and closing
>some of my primary air at the burners (venturi) to reduce. I am getting a
>nice 6-8" flame out of the upper ports but the bottom port is sucking air in
>at that point. Would closing the damper more be the answer? When I tried
>that I got smoke with the reduction and I always thought that meant I was
>overdoing the reduction. I would appreciate your wisdom on this problem.
>Thanks! Janet Harper, Knoxville, TN
Dear Janet; Sounds like you are doing most of this in a way that would
produce the results you want. However, try NOT closing down the primary air
at all. Try achieving reduction only with the damper. If your kiln 'leaks'
a lot around the door and through cracks in the brickwork, you may not be
able to get enough backpressure to get the reduction down on the floor.
Generally, a good rule of thumb is, "The tighter the kiln, the more
uniform the atmosphere". Hoping my suggestions help, Hank in Eugene
Jan Cannon on sat 19 feb 00
Janet,
I always get smoke with reduction! I fire a lot of celadon which needs a
good heavy reduction and I get perfect reduction everywhere in my kiln (20
cu ft. stacking space) except right next to the target bricks. It's been
said many times on this list that the presence of smoke in a reduction
firing indicates over reduction, but later someone admitted, "yeah, some of
the pieces on the bottom don't reduce too well, so I put things there that
don't need it." Well, I wouldn't call that a good reduction firing. Don't
worry about the smoke. The extra two or three dollars you spend on gas, in
a kiln the size you are firing, will be well worth it for the pots that
don't end up as shards. Good luck.
Jan Cannon Pottery
19 Garen Road
Charlotte, VT 05445
802-425-6320
jan@jancannonpottery.com
www.jancannonpottery.com
----- Original Message -----
From: Hank Murrow
To:
Sent: Friday, February 18, 2000 10:59 AM
Subject: Re: More reduction to floor
> ----------------------------Original message----------------------------
> >----------------------------Original message----------------------------
> >Hi Clayarters,
> >
> >I'm firing a 20 cu ft downdraft propane-fired kiln with a 14 ft stack.
The
> >firing is nice and even temperature-wise ( cone 10 flat all around). My
> >problem is getting enough reduction at the floor level. The char test
> >indicates that the reduction is fine up high but I'm just not getting it
down
> >low. I'm closing the damper to about 50% of the chimney opening and
closing
> >some of my primary air at the burners (venturi) to reduce. I am getting
a
> >nice 6-8" flame out of the upper ports but the bottom port is sucking air
in
> >at that point. Would closing the damper more be the answer? When I
tried
> >that I got smoke with the reduction and I always thought that meant I was
> >overdoing the reduction. I would appreciate your wisdom on this problem.
> >Thanks! Janet Harper, Knoxville, TN
>
> Dear Janet; Sounds like you are doing most of this in a way that would
> produce the results you want. However, try NOT closing down the primary
air
> at all. Try achieving reduction only with the damper. If your kiln 'leaks'
> a lot around the door and through cracks in the brickwork, you may not be
> able to get enough backpressure to get the reduction down on the floor.
> Generally, a good rule of thumb is, "The tighter the kiln, the more
> uniform the atmosphere". Hoping my suggestions help, Hank in Eugene
>
Janet Harper on sat 19 feb 00
Hi Hank
Thanks for your reply. My door is not as tight as I would like, especially
when it gets hot and expands. I'm not sure how to fix that other than trying
to stuff the cracks with fiber. The cracks are pretty thin though so it is
hard to stuff them with anything. I'll try to close the damper more next
time...I think that will be my answer. Again many thanks...........Janet
Paul Taylor on sat 19 feb 00
----------
>From: Janet Harper
>To: CLAYART@LSV.UKY.EDU
>Subject: More reduction to floor
>Date: Thu, Feb 17, 2000, 9:16 pm
>
>----------------------------Original message----------------------------
>Hi Clayarters,
>
>I'm firing a 20 cu ft downdraft propane-fired kiln with a 14 ft stack. The
>firing is nice and even temperature-wise ( cone 10 flat all around). My
>problem is getting enough reduction at the floor level. The char test
>indicates that the reduction is fine up high but I'm just not getting it down
>low. I'm closing the damper to about 50% of the chimney opening and closing
>some of my primary air at the burners (venturi) to reduce. I am getting a
>nice 6-8" flame out of the upper ports but the bottom port is sucking air in
>at that point. Would closing the damper more be the answer? When I tried
>that I got smoke with the reduction and I always thought that meant I was
>overdoing the reduction. I would appreciate your wisdom on this problem.
>Thanks!
>
>Janet Harper
>Knoxville, TN
Dear Janet
I have a 40 c foot propane down draft with a high chimney to take it to
above the roof.
I have preheated secondary air
Any way The delicate balancing act between gas pressure, primery, and
secondary air ,damper was a real head ache All these work in harmony change
one change them all. until I bought an oxygen analyzer.
Then I discovered that the finesse of adjustment was beyond belief and for
all those years I had been making gross adjustments. The meter saves 20% >on
gas and gives me control whatever the weather.But maybe You can not afford
one.
So you could have been only 2.5% away from a good firing.
Here is what I used to do. Use the temperature rise rate on the pyrometer to
help and keep notes
When you get to reduction temperature first adjust the venturies I use
secondary air so I can not give you an accurate setting. I do not Know your
Jet sizes or venturi type. There is a slight difference in the flame color
some reckon you want a slight green color as opposed to blue.I turn the air
down a bit and the flue up.
Then set the gass pressure and the venturi setting. Your guess is as good
as mine.
Next set the damper. I have a hollow kin prop set one fifth of the packing
height above the ground ( bottom shelf) - my flue is underneath the bottom
shelf. So that's a hole in the kiln just above the bottom shelf.
I adjust the damper until a flame held up against the hole is neither drawn
into the kiln nor blown away from it -neutral . these adjustments take some
patience waiting for the pressure to build.
Then if there is smoke every where turn up the air .Adjust the flue
If the pyrometer refuses to rise turn up the gas and air then re adjust the
flue.
If there is no flame at spy hole turn up the gas. adjust the flue.
If you ever get it right you are doing better than I
Regards Paul T
Lana Reeves on sun 20 feb 00
Janet-- You could try clamming up the door with clay. I've done it with a
kiln that has a leaky door. Lana
kilnkat@rcn.com =^..^=
-----Original Message-----
From: Janet Harper
To: CLAYART@LSV.UKY.EDU
Date: Saturday, February 19, 2000 8:35 PM
Subject: Re: More reduction to floor
>----------------------------Original message----------------------------
>Hi Hank
>
>Thanks for your reply. My door is not as tight as I would like, especially
>when it gets hot and expands. I'm not sure how to fix that other than
trying
>to stuff the cracks with fiber. The cracks are pretty thin though so it is
>hard to stuff them with anything. I'll try to close the damper more next
>time...I think that will be my answer. Again many thanks...........Janet
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