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help needed evening out my kiln temp differences


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#41 docweathers

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Posted 21 March 2013 - 11:13 AM

This relates to a project that I've been contemplating for my torchbearer.

I thought about building a heat exchanger around the exhaust port , then pumping the hot air back underneath the kiln so that it will be picked up by the burners. this would hopefully recycle some of the heat and feed warmer air into the bottom of the kiln. this might be particularly useful for me since where I fire the kiln it is often 30 to 40° in there. So it's getting very cold air coming in the bottom. Air few hundred degrees warmer might be helpful.

Has anyone tried anything like this?
Any thoughts about this strategy

Larry

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#42 shoshi

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Posted 27 March 2013 - 10:57 AM


Hello,
I have an Olympic torchbearer gas kiln model 2827G. It's a basic updraft with 4 venturi burners and I fire it manually using cones and a pyrometer with 2 leads to monitor the temperature. I've used it for bisque and oxidation glaze firing-both low and high up to cone 10. The problem has always been the temp difference between the bottom and the top shelves. There is usually around 100 degrees celsius difference (sometimes more) with the top being hotter. I've tried packing the top denser than the bottom with some success. I also tried moving my shelves to act as saggers with the hope it would redirect the flame. That made the bottom even cooler and I aborted that firing until I could rearrange the shelves back to usual configuration. The longer I fire it, the less difference there is but the longer the top shelf is at a mature temperature so it can end up over fired and the bottom under fired. Does anyone else have this problem with this kind of kiln? Any ideas on how to solve this?
Thanks


Hi,
I have been firing a 2827G for the past year with the same disappointing problem. I have tried the diverting flame on two burners, the self 1 1/2 inches from the top, packing more densely with staggered shelves - but to no avail.
I deduced that it was colder at the bottom because more cold air was being drawn in via the rather large burner ports in the bottom of the kiln. Which was also the reason that I couldn't reduce the bottom half of the kiln. What I tried was to take 4" standard insulating bricks and drill a 2" hole near one edge and mitre the edges to fit snugly into the angle so that the hole is centred over the burner. The 4 bricks now support my bottom shelf. The difference in temperature between top and bottom is negligible (the bottom being slightly hotter) and I get reduction throughout the kiln.



#43 shoshi

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Posted 27 March 2013 - 11:04 AM

Hi,I've been diverting the flame on two burners, shelf two inches from the top, and packing denser for the past three firings and it is better than ever before but still a variable difference between top and bottom. So I just want to double check-- your bottom shelf is resting on the insulating bricks and is 2 inches from the floor with the flames traveling up the sides of the kiln with no diversion?
Thanks

#44 R Fraser

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Posted 27 March 2013 - 03:15 PM

This relates to a project that I've been contemplating for my torchbearer.

I thought about building a heat exchanger around the exhaust port , then pumping the hot air back underneath the kiln so that it will be picked up by the burners. this would hopefully recycle some of the heat and feed warmer air into the bottom of the kiln. this might be particularly useful for me since where I fire the kiln it is often 30 to 40° in there. So it's getting very cold air coming in the bottom. Air few hundred degrees warmer might be helpful.

Has anyone tried anything like this?
Any thoughts about this strategy


This sounds like a recuperating (spelling?) type furnace I first read about when making my first blacksmith forge, I found the plans on the Sandia website (many years ago). It called for oval stainless steel pipe(s) running through the exhaust vent that was then collected and used to supply combustion air to the burner(s). These were atmospheric burners, and it was a small furnace < 2 cubic feet. In Nils Lou's book "The Art of Firing" he talks about a similar design for a ceramic kiln, but it is using a power burner with the blower first running through a pipe matrix in the exit flue to pick up heat before feeding the burner itself. I would say that everything from the flue to the burner would have to be stainless steel. I do not think this would be easily to make safe or simple using atmospheric burners.
Richard
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#45 docweathers

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Posted 27 March 2013 - 03:48 PM

Since I'm a welder I don't think it would be that difficult for me to build. What I'm not sure about is whether I would get enough recycling of heat to be worth the trouble.

Larry

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#46 R Fraser

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Posted 28 March 2013 - 02:14 PM

Since I'm a welder I don't think it would be that difficult for me to build. What I'm not sure about is whether I would get enough recycling of heat to be worth the trouble.


The Sandia Forge reported up to 30% improved efficiency if I remember corectly. Nils Lou suggested significantly increased efficiency with his recuperating power burner set up but I do not remember if he made any specific claims. I would be concerned that depending on the location of the recuperators and the temp of the exhaust gases a closed loop system relying on natural draft flow to feed the burners primary air the CFM flow of primary air may be low, possibly too low to offer any meaningfull cooling of the recuperators and thus the burner. This would also limit your ability to control the kiln atmosphere to regulating the secondary air flow. The more I think on it the more it seems that to use a recuperating design safely and effectively you are almost obligated to use a blower fed burner system.

Nils Lou's book "The Art of Firing" has a nice schematic outlining how to set up a recuperating kiln burner using a power burner system along with a ton of other great kiln construction info, probably the greatest information density per dollar of any kiln building books I bought second only to Olson's Kiln book. The plans and breif outline on design are at the end in an appendix if memory serves..

This is a link to the ABANA web site with the forge plans: ABANA Forge plans
Note that this is a very small volume chamber with very short primary air supply tubes to the atmospheric burners in more or less a closed loop design.

Richard
So I built a light saber, am I a Jedi now?


#47 terryr

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Posted 01 January 2014 - 12:18 PM

I am firing the same kiln and having the same problems.  Thanks for all the ideas.  I have only fired this kiln twice with disappointing results top to bottom.

I am also thinking of another layer of insulation brick at the bottom of the kiln.  any updates would be appreciated. 

Thanks 

terry






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