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Can too much chimney draw cause temp differences in a gas kiln?


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I have a small paragon kiln I converted to a downdraft gas kiln. The temp differences in the kiln are significant. As it is, firing to cone 6, the top cone is all the way over and the bottom cone hasn't moved at all. I probably need to get some 5 and 7 cones to better know how much difference there is, but I'm guessing the top is closer to cone 7 and the bottom 5ish.  Added wrinkle, my pyrometer only show 2200 when the top cone goes down. Can too much draw from the chimney cause something like this? The design is a bit janky I know, but supposedly others have made it work. Would using a damper to put back pressure into the kiln help even out the temperature? I'd be happy with a cone difference.

The attached picture is how the kiln is constructed. Flame enters on the left and exits bottom right. Pyrometer is opposite the chimney towards the top.

Feel free to tell me this design is crap and I need to start over if that's the right answer.

Thanks!

Kevin

Small Gas Kiln.jpg

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6 minutes ago, Mark C. said:

A few things 

cones tell the story better than a pyrometer

Maybe your thermocouple is wearing out?is that a digital guage or analog gauage

The chimney is the exhaust -whats the other exhaust?

It's a new digitial guage. Only been used 3 or 4 times. Here's another picture without the bottom shelf so the exhaust path is clearer.

 

Small Gas Kiln-exhaust.jpg

Edited by kriggins
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You need a damperto control reduction-I very much doubt you have to much suck in chimney. More likely you have not diverted the flames enough to get them into the load.

in many of these conversion kilns the heat/flame is not diverted enough to heat the load well before the flame/ heat exits. The key is to divered it into and thru the load before it gets out.

 

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The ones that seem to work the best use a wall to force the flames to stay in chamber before going down the verical flue slot to bottom and out to chimney . I would work on the flame path design.

On the pyrometer note-most never show exact temps that cone shows even when next to them-use that pyro as a guide to tell you how the kiln is climbing or stalling not to measure end point measurements . Its a rough guide at best at top end temps.

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So maybe a bag wall on the left to force the flame up? The tricky bit is getting the flame to interact with the load on the bottom shelf before going out of the chimney. Maybe move the walls under the shelf to force the flame back towards the front of the kiln. Or maybe the opposite. Hmm. Some things to play with.

Will firing the kiln without load and just cones help me understand what's going on with the flame path or does the lack of load make that a bit worthless?

This design is based on off something I think I found here. I'll have to search some more. Thanks for the tip on sorcery. Will look that up too.

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HERE is a link to a very successful conversion method.

Are you using the same size kiln shelves that were used in the kiln as an electric? If so, you likely don't have enough air flow. You'll need to either use smaller shelves or make some cuts on the current shelves to allow movement of air and heat.

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1 hour ago, kriggins said:

So maybe a bag wall on the left to force the flame up? The tricky bit is getting the flame to interact with the load on the bottom

So just some food for thought, even updraft kilns can fire hotter on the bottom. Almost all kilns have a damper so they can be managed while firing and ......... heat in a kiln  (red heat) after about 1500 degrees is primarily by radiation, then conduction, then convection. Convection is great in an oven but in a kiln at high temperatures it is the lessor contributor.

Not to say kilns should not breath appropriately but having this in mind often helps when things end up appearing non intuitive. Lastly, no matter the kiln but especially gas fired kilns,  firing at high rates (degrees per hour) lends to uneven firings. Slowing things to 100 degrees per hour or less  within 200-300 degrees of the final firing temp often can help even out a kiln a bunch.

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3 hours ago, Bill Kielb said:

So just some food for thought, even updraft kilns can fire hotter on the bottom. Almost all kilns have a damper so they can be managed while firing and ......... heat in a kiln  (red heat) after about 1500 degrees is primarily by radiation, then conduction, then convection. Convection is great in an oven but in a kiln at high temperatures it is the lessor contributor.

Not to say kilns should not breath appropriately but having this in mind often helps when things end up appearing non intuitive. Lastly, no matter the kiln but especially gas fired kilns,  firing at high rates (degrees per hour) lends to uneven firings. Slowing things to 100 degrees per hour or less  within 200-300 degrees of the final firing temp often can help even out a kiln a bunch.

Speed could certainly be part of my issue. Trying to fire this thing slowly is a challenge.

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13 minutes ago, tomhumf said:

I made a similar kiln, not sure the thread is updated to what I do now.  I use a brick on the top of chimney as my damper . I fire in oxidation, reduction then neutral. I get probably within half a cone top to bottom. 

 

Your thread was one that I read when researching for my kiln. Ours do look very similar.

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I would classify this as a cross draft kiln. Using Fred Olsons principals I built a crossdraft. After firing this wood eating  behemoth for 20 years ive come to learn one immutable thing. ..this type of kiln requires the WARES be stacked loose at the bottom (allowing more flame to flow thru) and tightly at the top (restricting flame path).  this method works better than bagwalls or baffles

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5 hours ago, kriggins said:

Speed could certainly be part of my issue. Trying to fire this thing slowly is a challenge.

Can you turn down the gas pressure? The relationship for gas and energy (btuh) is exponential so turning up the gas say 10% often yields an increase in thermal output of more than 20%. Of course as the kiln heats up, it’s shell  losses and internal absorption continue to escalate as well. Combine all that with a fairly touchy damper often makes for a cyclic ride.

We like to demonstrate to set a low gas pressure, optimize damper closure for best rate of rise and then open a few percent more so the operator can focus mainly on slowly ramping up with minor changes in  gas pressure and their firing speed. At some point the damper will become optimum on its own and will need to be opened a bit more as more gas and energy is added to the system. This step approach seems to free up the operator. Early on its really easy to overshoot because the kiln takes minimal energy to climb so small adjustments are in order.

Edited by Bill Kielb
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I built mine after the design Neil Linked.

Slow is the most reliable method to keep things even IMO. 

I have been thinking about adding a moveable "damper like" object I can shove into the kiln, to deflect the flame path from the top top towards the end, so I can manually even things out. A vent to release a little heat from the top in the beginning has less efficient merit, but if it means the difference between good wares and not. To hell with efficiency!

 A combination of clays and glazes matching most repeatable temps is easier IMO, depending on what you produce.

 Is it the Coriolis effect? I watched a YouTube video where a kid noticed a difference by working WITH your hemisphere's Coriolis effect.

I have been firing another small kiln a bit differently. Observations from that says you will likely have quite the temp difference from that back left corner to the front right corner, as that flame loses energy. So you may want to cone up the back and fronts of shelves too. 

Sorce

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