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Kiln Design Question

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#1 jrgpots

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Posted 21 April 2014 - 08:13 PM

I am converting an old electric to a downdraft gas kiln.  The burner ports are at the base and the burners enter the kiln horizontally.  The flame is directed upward using an angled bagwall at the opposite wall (18" away from the burners.)  The flue opening is between the burners.  How high do I need to make the base shelf above the floor to give room for the burners?  I have marked the area in question in pic #1.  The exit flue is 4"x 4". So I was thinking of making the height of the base shelf 4" above the floor.  In The Kiln Book by Olson This design is not discussed. He recommends at the bagwall be at least 8" from the burners. Mine will be 18" from the burners.  Most of the electric to gas conversions are updraft, so I am finding it a little difficult to find the answer on my own.

 

Jed

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#2 bciskepottery

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Posted 21 April 2014 - 08:19 PM

This might be helpful; he allowed 9".

http://codyopottery....irth-of-my.html

#3 Mark C.

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Posted 21 April 2014 - 08:24 PM

I think 4 is ok starting point-you can always post it up for better floe.
It's all an experiment at this point
On my I pad mini I am unclear as to where and what level the exit flue is ?
It looks like maybe the flue is in between burners?
Is the flue outside kiln or inside like that leach video
I,m not a fan of an inner flue in electrics.
Mark
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#4 jrgpots

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Posted 21 April 2014 - 08:48 PM

I think 4 is ok starting point-you can always post it up for better floe.
It's all an experiment at this point
On my I pad mini I am unclear as to where and what level the exit flue is ?
It looks like maybe the flue is in between burners?
Is the flue outside kiln or inside like that leach video
I,m not a fan of an inner flue in electrics.
Mark


Yes, the flue is between the burner at the same level as the buners. I will have a chimney outside of the kiln.

Jed

#5 Mark C.

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Posted 21 April 2014 - 10:17 PM

The trick will be to force our flame path up and thru the load before it finds the exit flue.
Pay extra attention to this idea as in this small space the path will want to get out as easy as you let it.
Mark
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#6 jrgpots

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Posted 21 April 2014 - 11:38 PM

Stacking is the trick, I guess....so much to learn. I'll need to design that directs the air through the stack.

I will set the bricks for the chimney next week and may try to fire it in 2-3 weeks once I get a pyrometer.

Jed

#7 Rakuken

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Posted 22 April 2014 - 12:22 AM

Check out this kiln.
https://www.facebook...39501636&type=3

#8 Mark C.

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Posted 22 April 2014 - 01:21 AM

Rakuken
Nice link
The burners are natural draft
But there is no stack on top?
The photos are good but do not show the baffle under 1st shelve
Wish I could see that
Results look good though.
Thanks


Jrgpots
Are your burners forced air or natural draft?
Mark on Molokai
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#9 jrgpots

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Posted 22 April 2014 - 12:34 PM

natural draft right now.  However, I can change them to forced air if needed.



#10 neilestrick

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Posted 22 April 2014 - 02:41 PM

The height of the platform will require some testing, but 4" may work just fine since the burner is coming in from the side. I prefer power burners since you can use a short stack. The height of the stack is a non-issue and makes tweaking the kiln a lot simpler. 


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#11 Marcia Selsor

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Posted 22 April 2014 - 03:05 PM

I saw a nice little converted kiln positioned vertically at the Appalachia center when I was there in 1999. It might be easier to add a stack.
I would take the steel sleeve off and add an inch of fiber if you are planning to high fire. For natural draught you will need a higher stack as several have mentioned.

Marcia

#12 Mark C.

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Posted 22 April 2014 - 07:15 PM

I agree on the addition of fiber for cone 10 use .A stack will make it all work better.
Mark
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#13 jrgpots

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Posted 22 April 2014 - 09:07 PM

I saw a nice little converted kiln positioned vertically at the Appalachia center when I was there in 1999. It might be easier to add a stack.
I would take the steel sleeve off and add an inch of fiber if you are planning to high fire. For natural draught you will need a higher stack as several have mentioned.

Marcia

1.  Could I just add the the inch of fiber around the stainless then surround the fiber with common brick? 

 

2.  How tall would the stack need to be for natural draft burners?

 

Jed

 

 



#14 Mark C.

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Posted 22 April 2014 - 09:43 PM

I saw a nice little converted kiln positioned vertically at the Appalachia center when I was there in 1999. It might be easier to add a stack.
I would take the steel sleeve off and add an inch of fiber if you are planning to high fire. For natural draught you will need a higher stack as several have mentioned.
Marcia

1.  Could I just add the the inch of fiber around the stainless then surround the fiber with common brick? 
 
2.  How tall would the stack need to be for natural draft burners?
 
Jed
You could but it will shorten its life a bit sooner
If it's a beater kiln it will not matter other than the clamps (screw type that keep jacket tight ) may come loose sooner than later
If it's an old one I would just wrap it with fiber and add another aluminum jacket over the fiber to contain it
More stack is always better than not enough
How about 6-7 feet to start
Maybe the last Could be stainless double wall wood stove salvaged pipe?
Is this going to cone 10?
Mark
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#15 jrgpots

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Posted 22 April 2014 - 11:00 PM

Cone 10 is the goal. I can add blowers to the burners, but I have no idea what type of air flow is needed in the blowers.

Jed

#16 Rakuken

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Posted 23 April 2014 - 12:36 AM

Picture of my converted kiln. Up draft, two forced burners for even heat, cone 10.
Aloha, Ken
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#17 JBaymore

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Posted 23 April 2014 - 06:18 AM

Cone 10 is the goal. I can add blowers to the burners, but I have no idea what type of air flow is needed in the blowers.

Jed

 

You need about 10 cubic feet of air at STP for every 1,000 BTUs of fuel being burned for the ability to have complete (Stoichiometric) combustion. If you want to have oxidation conditions in the kiln....... then you add that percentagfe of extra air capacity (ie 10% oxidation = extra 1 cubic foot of air).

 

The best quality high pressure venturi burners (not the ones you pictured ;) ) will only, at max setting, entrain about 65-70% primary air (mechanical linkage).  So they will only pull 7 of those 10 needed cubic feet due to the gas exiting the orifice "bumping into" the air molecules in the mixing tube, and causing the flow of air into the burner thru the primary air ports.  Which means that you are at a level of 30% reduction conditions unless there is sufficient secondary air present.  That also means that you are potentially only getting 700 BTUs of energy out of every 1000 that you put out of the burner.

 

That secondary air to finish off the combustion reactions is coming from draft induced by the kiln system.  Hence the need for the "stack" to help with this.  And the correct size burner ports.  And getting good mixing oof the secondary into the already burning gases coming out of the burners.  This is also why "power burners",  which are not dependent on draft to pull in secondary air, tend to make kilns far less touchy to fire and easier to design. 

 

Note that blowers are rated in various static pressure readings as well as "free air".  The free air reading means that is what the blower puts out in CFM with no resistance to the flow.  When you hook a blower onto a pipe... you induce static pressure .... resistance to the flow of material.  This decreases the output of the blower.  Too complicated to go into here.... but just size the blower a bit larger than what you calculate you NEED.

 

best,

 

.......................john


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#18 Marcia Selsor

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Posted 23 April 2014 - 08:24 AM

Follow John's advice on the burners. I like to insulate. I think if you can do it, remove the metal jacket and stretch it back over the fiber insulation. It helps hold in the radiant heat and will protect the fiber. Don't squish the fiber. That reduces the insulating quality. You will have to see how much draft you need in the stack. I don't know where you are. Hold a newspaper torch at the flu going into the stack. If the flame is drawn in, you are good to go.

Marcia

#19 jrgpots

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Posted 23 April 2014 - 11:08 AM

John, thank you. There is just one thing. Are you making fun of my burners?... Lol. I want you to know that I spent $40.00 making those burners out of the highest quality black pipe I could find.


Marcia, thanks for the advise. I will take off the stainless and wrap with fiber, replacing it afterwards.

Jed

#20 neilestrick

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Posted 23 April 2014 - 11:29 AM

To give you an idea of what size blowers to use, the old Alpine pipe burners used  50cfm blowers on all kilns smaller than 10 cubic feet of stacking space (2 burners). Total interior space of a kiln that size is more like 16-18 cubic feet. So for a 10 cubic foot converted electric you really don't need a big blower. You probably only need one burner.


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