don hunt on thu 11 apr 02
I finished building a 36cf propane fiber downdraft recently and could
use a little advice in the tweaking stage. It fires very nicely to
about 2000f then stalls. The rest of the way temperature increase is
acheived by reducing gas pressure. Is this typical of a downdraft? The
updrafts I had fired before took increases of gas all the way up. It
has 8 75k btu venturi style burners. I have been experimenting with
burner orifice size as I don't get more than 3inches of pressure with
the stock orifices. A reduction of about 40% gives me around 2-3 inches
with no visible difference in the flame, am I on the right track? I
have a valve on each burner, so I could change the orifices in the
middle of a fire if it didn't seem right. I haven't fired with them
yet.
The exhaust flue is built on Nils Lou's theory. The initial opening is
5.75 x 5.75. The flue box itself is 8.5 x 9.75 and 19.5 deep. The
opening into the stack is 5.75 x 5.75 and the chimney itself is 8.5 x 8
and about 35 inches tall. Should the chimney be taller?
As much as I love pottery itself, building this kiln was just as much
fun. I have fired it 4 times now and it seems to do cone 10 on about 30
-35 gallons of propane. Does this seem about right? It has 10 inch
thick modules for the roof, which I really like.
Thanks for any help
Don Hunt
Jeremy McLeod on fri 12 apr 02
don hunt wrote:
> The exhaust flue is built on Nils Lou's theory. The initial opening is
5.75 x
> 5.75.
If I remember a recent reading of Nils' book correctly he was suggesting
a initial flue opening of 4.5" x 7" = 31.5 sq. in.. Your 5.75" x 5.75"
equals
33.06 sq. in. I'm a novice in the world of kiln design and don't know if
2 square inches are a critical difference or not.
Hopefully the gurus can comment on this matter.
Jeremy McLeod
Kurt Wild on fri 12 apr 02
At 08:34 PM 4/11/02 -0700, you wrote:
>I finished building a 36cf propane fiber downdraft recently and could
>use a little advice in the tweaking stage. It fires very nicely to
>about 2000f then stalls. The rest of the way temperature increase is
>acheived by reducing gas pressure. Is this typical of a downdraft? The
>updrafts I had fired before took increases of gas all the way up. It
>has 8 75k btu venturi style burners. I have been experimenting with
>burner orifice size as I don't get more than 3inches of pressure with
>the stock orifices. A reduction of about 40% gives me around 2-3 inches
>with no visible difference in the flame, am I on the right track? I
>have a valve on each burner, so I could change the orifices in the
>middle of a fire if it didn't seem right. I haven't fired with them
>yet.
>The exhaust flue is built on Nils Lou's theory. The initial opening is
>5.75 x 5.75. The flue box itself is 8.5 x 9.75 and 19.5 deep. The
>opening into the stack is 5.75 x 5.75 and the chimney itself is 8.5 x 8
>and about 35 inches tall. Should the chimney be taller?
>As much as I love pottery itself, building this kiln was just as much
>fun. I have fired it 4 times now and it seems to do cone 10 on about 30
>-35 gallons of propane. Does this seem about right? It has 10 inch
>thick modules for the roof, which I really like.
Don:
My kiln 20 cu. ft., flat top, Nils design, slight stalling at 2000. light
reduction from 1650 F. to end, flame at peep hole about 3" with damper set
so if it is pulled out just a hair the flame will disappear. Your stack
sounds fine to me. I use 25 gallons of propane. If your kiln was
compared (proportionally cu. ft. to cu. ft.) exactly to mine I figure you'd
use 31 gallons of propane so you are probably just fine.
Kurt
----------------------------------------------------------------------------
----------------------------------------------------------------------------
-
Kurt Wild
1000 E. Cascade
River Falls, WI 54022
phone: 715-425-5715
email: kurtwild@attbi.com
web site: www.uwrf.edu/~kw77/
most current work: www.uwrf.edu/~kw77/Available.html
Tony Ferguson on fri 12 apr 02
Hi Don,
Where are you located? I am very interested learning about fiber
downdrafts. Do you have any pictures you could share of the building
process? When you say your chimney, do you mean 35 inches above the kiln or
in total? Like you, my experience with gas is with up drafts. How long
does it take you to reach cone 10, cooling, time when you can unload the
kiln? Are you hoping to fire faster because there is significant stall?
Thank you!
Tony Ferguson
Stoneware, Porcelain, Raku
www.aquariusartgallery.com
Web Site, Marketing & Photographic Services for Artists
Workshops available
218-727-6339
315 N. Lake Ave
Apt 312
Duluth, MN 55806
----- Original Message -----
From: "don hunt"
To:
Sent: Thursday, April 11, 2002 8:34 PM
Subject: Fiber Downdraft
> I finished building a 36cf propane fiber downdraft recently and could
> use a little advice in the tweaking stage. It fires very nicely to
> about 2000f then stalls. The rest of the way temperature increase is
> acheived by reducing gas pressure. Is this typical of a downdraft? The
> updrafts I had fired before took increases of gas all the way up. It
> has 8 75k btu venturi style burners. I have been experimenting with
> burner orifice size as I don't get more than 3inches of pressure with
> the stock orifices. A reduction of about 40% gives me around 2-3 inches
> with no visible difference in the flame, am I on the right track? I
> have a valve on each burner, so I could change the orifices in the
> middle of a fire if it didn't seem right. I haven't fired with them
> yet.
> The exhaust flue is built on Nils Lou's theory. The initial opening is
> 5.75 x 5.75. The flue box itself is 8.5 x 9.75 and 19.5 deep. The
> opening into the stack is 5.75 x 5.75 and the chimney itself is 8.5 x 8
> and about 35 inches tall. Should the chimney be taller?
> As much as I love pottery itself, building this kiln was just as much
> fun. I have fired it 4 times now and it seems to do cone 10 on about 30
> -35 gallons of propane. Does this seem about right? It has 10 inch
> thick modules for the roof, which I really like.
> Thanks for any help
>
> Don Hunt
>
>
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Hank Murrow on fri 12 apr 02
>I finished building a 36cf propane fiber downdraft recently and could
>use a little advice in the tweaking stage. It fires very nicely to
>about 2000f then stalls. The rest of the way temperature increase is
>acheived by reducing gas pressure. Is this typical of a downdraft? The
>updrafts I had fired before took increases of gas all the way up. It
>has 8 75k btu venturi style burners.
*****Dear Don; I would expect that with 8 75K burners for a
total of 600K BTU that a flue opening of nearer 50-60 sq.inches might
be required. Let's hope Nils can elucidate on this question. My
lifting fiber kiln uses about 150K BTU and has a flue opening of 21.5
sq.inches. See http://www.murrow.biz/hank/kiln_and_tools.htm for a
look at this kiln.
>I have been experimenting with
>burner orifice size as I don't get more than 3inches of pressure with
>the stock orifices. A reduction of about 40% gives me around 2-3 inches
>with no visible difference in the flame, am I on the right track? I
>have a valve on each burner, so I could change the orifices in the
>middle of a fire if it didn't seem right. I haven't fired with them
>yet.
*****Can you resize the orifices (smaller) to increase the
pressure to nearer 6-8" w.c? Most burners are made to run at 6" to
10" w.c. (unless the are made for high pressure propane)
>The exhaust flue is built on Nils Lou's theory. The initial opening is
>5.75 x 5.75. The flue box itself is 8.5 x 9.75 and 19.5 deep. The
>opening into the stack is 5.75 x 5.75 and the chimney itself is 8.5 x 8
>and about 35 inches tall. Should the chimney be taller?
****Probably a good idea here. It is hard to develop much
draft with less than 6-8 feet of chimney.
>As much as I love pottery itself, building this kiln was just as much
>fun. I have fired it 4 times now and it seems to do cone 10 on about 30
>-35 gallons of propane. Does this seem about right? It has 10 inch
>thick modules for the roof, which I really like.
>Thanks for any help
****Hoping this helps, Hank in Eugene
Craig Martell on fri 12 apr 02
Hello Don:
If you have to reduce gas flow thru the burners to get the temperature to
climb in the kiln you're not getting enough air to combust the fuel at
higher pressure. You could also be turning the burners up too much at
2000F and you may be able to increase pressure at higher temp. where
combustion rate is more rapid.
A couple of things to consider are: Do your burners put out enough BTUs to
drive the kiln? You say the kiln is about 33 ft3 so the minumum input at
full pressure should be about 330,000 BTUs per hour. I'm talking about
total interior dimension at 33 ft3 and not just stacking space. For the
max input, just multiply 33 by 16,000. If your burners aren't rated for
this type of output you may be overdriving them. If this is the case, you
can put more fuel thru the orifice but the burners aren't capable of
entraining sufficient air to give you the BTUs you need. The inefficient
combustion puts the kiln into heavy reduction and you stall. The other
thing is that your short stack isn't creating the draft and pull you need
to move enough secondary air thru the kiln to handle the higher BTUs. I
think this is the first thing to look at. Your stack is pretty short for
that size kiln. I'd recommend about 8 ft. You should look at some kiln
building literature that tells you how to figure stack height. Fred
Olson's book, "The Kiln Book" has this info. Downdrafts need a much
stronger draught than updrafts and you achieve this with proper flue
sizing, inlet and outlet, plus proper stack dia and height. I think your
stack diameter is fine but you could use some elevation. I fire a 66 ft3
downdraft and my stack is 15' high. I have the Nils double venturi flue
too.
Also, the BTU input info I gave you is for soft bricks fired to cone
10. The input factors for fibre are a bit lower.
good luck, Craig Martell in Oregon
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