Burnt Earth Posted February 7 Report Share Posted February 7 Hello all, I am just in the process of taking an old electric kiln that was given to my wife and converting it to a propane fired down draft kiln. I have installed 4 burners in the bottom corners and made an exhaust from a 6x6 square tubing I had laying around. I have a few questions regarding firing. The kiln measures inside 30” deep, 24” wide and 36” tall with a curved roof. 1. Where is the best placement for a thermocouple and is it necessary for multiple thermocouples? 2. when doing a reduction fire where do you measure the excess O2? Are you using an 02 sensor in the chimney? 3. How fast do you usually fire one of these kilns? Are there temperature steps you try to achieve before increasing firing rate? Thank you for any advice. I am very familiar with combustion as I install and service large industrial heaters and boilers and am an instrument technician by trade but am very inexperienced in gas kilns. Currently my wife is using electric kilns but we want this one for our off grid property in central BC. Hulk 1 Quote Link to comment Share on other sites More sharing options...
neilestrick Posted February 7 Report Share Posted February 7 Welcome to the forum! Unfortunately, the chimney isn't going to work. It may be the correct size, but the heat is going to destroy it in no time, and it'll likely be glowing orange outside the kiln. You also need a damper in it, and because you're using venturi burners it's probably far too short to provide the secondary air needed for combustion. I highly recommend looking at a kiln design book like Frederick Olsen's 'The Kiln Book'. It's a somewhat outdated but it's got some solid design work in it and it's really about the best we have. It'll give you the calculations needed for flue opening dimensions and chimney height. Your chimney needs to be built from firebrick to at least the height of the kiln, and then you could transition to metal after that if needed, but it's often easier to just keep with brick. Where will the kiln be located while firing? What is the BTU rating of your burners? How thick are the walls, and what type of insulation is behind the bricks? Rae Reich, Hulk and Magnolia Mud Research 3 Quote Link to comment Share on other sites More sharing options...
Burnt Earth Posted February 7 Author Report Share Posted February 7 10 minutes ago, neilestrick said: Welcome to the forum! Unfortunately, the chimney isn't going to work. It may be the correct size, but the heat is going to destroy it in no time, and it'll likely be glowing orange outside the kiln. You also need a damper in it, and because you're using venturi burners it's probably far too short to provide the secondary air needed for combustion. I highly recommend looking at a kiln design book like Frederick Olsen's 'The Kiln Book'. It's a somewhat outdated but it's got some solid design work in it and it's really about the best we have. It'll give you the calculations needed for flue opening dimensions and chimney height. Your chimney needs to be built from firebrick to at least the height of the kiln, and then you could transition to metal after that if needed, but it's often easier to just keep with brick. Where will the kiln be located while firing? What is the BTU rating of your burners? How thick are the walls, and what type of insulation is behind the bricks? Ya I will deal with the chimney when I get it to its permanent home. I wasn’t sure how that steel would be but I’m sure it will survive long enough for testing. It is 3/8” thick so lots of metal to degrade. The burners are currently running #60 orifices and I will have up to 10psi available. I haven’t done the calculations yet but I can easily push these burners to 70,000 btu each. I have them installed in 3” stainless steel pipes through the bricks. the kiln will be living its life outside. I will be welding angle iron to the top and building a steel roof over it for snow and rain protection. the walls are about 6” thick. Double layered with soft fire brick. On the outside of the brick is a refractory blanket about 1/2” thick. That is covered with a galvanized tin. I had the kiln to around 1500f today and shot it with my thermal camera and it is running very cool outside temps. Under 30c after an hour running. I feel insulation is sufficient. I will be installing a baffle in the chimney but for now I will just cover the outlet. I had to cut the chimney short as my shop door is only 7’ tall but I have 4 more feet to put on for increased draft. Currently though the draft seemed sufficient. I haven’t analyzed the stack yet but the burners seem to be running very clean as I had no spot after an hour run time. How fast can you fire these gas kilns to temperature. Her electric kilns take a long time to get to temp? I can really hammer the fire at this thing and get it hot fast but I’m thinking that would crack cups and such. Quote Link to comment Share on other sites More sharing options...
Mark C. Posted February 7 Report Share Posted February 7 wow Neil said it best ( it'll likely be glowing orange outside the kiln) I will add in no time fast. You need a damper in the chimney they work best about waist height. As he said natural draft burners need the stack (chimney) to be tall and even with your 4 feet more of steel its still way short. I have a stainless steel stack after 8 feet of brick on two kilns. look for that stuff at a salvage yards near beer or pulp mills or any large plant that uses stainless piping. 3/8 is good but still not good enough-Yes you can get some life from it but you still need a damper and way more height. You can get a fast fire in maybe 8 hours -remember the furniture need to take the heat and cooling as well. I'm not talking lawn chairs I mean the kiln shelves and posts now to answer the other questions-I would put a digital thermcouple in the door so its swings out of the way and is not in the burner direct path. If you want to be fancy (i use oxy probes) a oxygen sensor will last longer in the chimney except in your case it may not as the flue flow is so tightly contained. The new to be geil I palces my probe in the flue as it will last many hundreds of fires before burning out. My other two gas kilns have the sensors in the middle of the load in the doors. They get all of cone 11 so they do burn out in a few hundred fires. I fire a lot every year over the past 50 years and have a few platinum elements replaced If you are a beginner really just a good eye is all you need for reductiuon firing-a potter who knows thier stuff would be an asset on the 1st firing or two. Most newbies put to much gas in thinking more is better-this is where the cheap digital pyrometer come in as it show the exact temps going up or stalling. You need one that goes at a bit more than the cone you are firing to.. You need to keep that kiln dry-no rain or snow on it-ever as soft brick and fiber are like sponges and once you dry them out (slow start the 1st fire to dry it out) you want it to stay dry. What is the flue dimension 8x8? Pres, Hulk and Rae Reich 3 Quote Link to comment Share on other sites More sharing options...
Bill Kielb Posted February 7 Report Share Posted February 7 (edited) 4 hours ago, Burnt Earth said: will be installing a baffle in the chimney but for now I will just cover the outlet. I had to cut the chimney short as my shop door is only 7’ tall but I have 4 more feet to put on for increased draft. Currently though the draft seemed sufficient. I haven’t analyzed the stack yet but the burners seem to be running very clean as I had no spot after an hour run time. How fast can you fire these gas kilns to temperature. Her electric kilns take a long time to get to temp? I can really hammer the fire at this thing and get it hot fast but I’m thinking that would crack cups and such. Really good to have the combustion background, definitely half the battle. Now some stuff that may help with your background and hopefully allows you to work through the design and tuning. In the end It’s pretty simple basic combustion stuff The split between primary air and secondary air is about 50% Knowing that will allow you to figure reasonable over cuts around the burners where secondary air will enter, if not 1– 2 inch clearance is fine. Too small is generally the mistake. An ideal ratio air / fuel will be approximately 10:1 (oxidation) Reduction will be a dirty flame, producing lots of carbon monoxide and ……… pressurizing the kiln so very little air is drawn into the kiln through the over cuts. Reduction is achieved by closing the damper and and pressurizing the kiln from top to bottom. Very slight adjustments will establish this. Slightly pressurizing the kiln (very slight) will be enough to keep secondary air from coming in the over cuts which drives the kiln into reduction. (Very dirty flame - no soot needed, too rich and decreases reduction) Never go into early reduction below the PEL of the gas 1500 f or better. #60 is pretty small as most propane kilns are regulated down to inches, but at .#60 (for now) you likely will operate this in the 1-2 psi range, here is one of my favorite orifice tables https://www.gordonpiatt.com/wp-content/uploads/2021/07/Propane-Orifice-Chart-7.E.80.6-Rev-3-05.01.pdf easy to figure just what pressure you will need after you figure how much Btuh you need as well. Total guess, start at prox. 300,000 btuh, work your way down as practical.The kiln will be easier to fire and adjust in firing using low pressure. Inches of wc pressure Design your flue by table p41 here can help or 4” - 5” is likely reasonable and allow you to tune the height in the end. https://www.selkirkcorp.com/literature/Chimney_Venting_Sizing_Handbook.pdf Higher increases draw but also makes damper adjustments more sensitive. For an updraft kiln placing the thermocouple as near the kiln as practical in the flue likely give a decent reading of the average temps. Gas kilns are fired supervised by cone so watching when cones bend will be more important than peak temperature. Firing rates 500-600 f degrees per hour are generally considered fast for clay, but once in reduction your flame will have much less energy so 100 -200 f per hour will still be fast. Reduction has its nuances, but simplified, very dirty flame, slightly pressurized kiln not allowing any oxidation. Reduced pieces can and often are reoxidize by plain old air going past them if the kiln is not kept slightly pressurized while in reduction. Onlya handful of metals are affected by reduction. without an O2 meter, you fire by eye, reduction flame poking out spyholes Really nice example reduction flame below from an updraft. Firing with someone who is used to reduction firing is generally a patience saver. Edited February 7 by Bill Kielb Rae Reich and Hulk 2 Quote Link to comment Share on other sites More sharing options...
Burnt Earth Posted February 7 Author Report Share Posted February 7 9 hours ago, Bill Kielb said: Really good to have the combustion background, definitely half the battle. Now some stuff that may help with your background and hopefully allows you to work through the design and tuning. In the end It’s pretty simple basic combustion stuff The split between primary air and secondary air is about 50% Knowing that will allow you to figure reasonable over cuts around the burners where secondary air will enter, if not 1– 2 inch clearance is fine. Too small is generally the mistake. An ideal ratio air / fuel will be approximately 10:1 (oxidation) Reduction will be a dirty flame, producing lots of carbon monoxide and ……… pressurizing the kiln so very little air is drawn into the kiln through the over cuts. Reduction is achieved by closing the damper and and pressurizing the kiln from top to bottom. Very slight adjustments will establish this. Slightly pressurizing the kiln (very slight) will be enough to keep secondary air from coming in the over cuts which drives the kiln into reduction. (Very dirty flame - no soot needed, too rich and decreases reduction) Never go into early reduction below the PEL of the gas 1500 f or better. #60 is pretty small as most propane kilns are regulated down to inches, but at .#60 (for now) you likely will operate this in the 1-2 psi range, here is one of my favorite orifice tables https://www.gordonpiatt.com/wp-content/uploads/2021/07/Propane-Orifice-Chart-7.E.80.6-Rev-3-05.01.pdf easy to figure just what pressure you will need after you figure how much Btuh you need as well. Total guess, start at prox. 300,000 btuh, work your way down as practical.The kiln will be easier to fire and adjust in firing using low pressure. Inches of wc pressure Design your flue by table p41 here can help or 4” - 5” is likely reasonable and allow you to tune the height in the end. https://www.selkirkcorp.com/literature/Chimney_Venting_Sizing_Handbook.pdf Higher increases draw but also makes damper adjustments more sensitive. For an updraft kiln placing the thermocouple as near the kiln as practical in the flue likely give a decent reading of the average temps. Gas kilns are fired supervised by cone so watching when cones bend will be more important than peak temperature. Firing rates 500-600 f degrees per hour are generally considered fast for clay, but once in reduction your flame will have much less energy so 100 -200 f per hour will still be fast. Reduction has its nuances, but simplified, very dirty flame, slightly pressurized kiln not allowing any oxidation. Reduced pieces can and often are reoxidize by plain old air going past them if the kiln is not kept slightly pressurized while in reduction. Onlya handful of metals are affected by reduction. without an O2 meter, you fire by eye, reduction flame poking out spyholes Really nice example reduction flame below from an updraft. Firing with someone who is used to reduction firing is generally a patience saver. Thank you so much. This is exactly the information I was looking for in terms of firing this kiln. the 60 orifices were just what comes in the burners I used. I was planning on drilling these as need be once I actually had some run time on this kiln. I do have a Testo stack analyzer that I can see exactly what is happening with my combustion so I can tune this unit very precisely. Once my valve train is built I will be able to dial the burners any where between 4” WC up to 10 Psi. I am trying to make it as automated as possible with at least 3 different firing rates done through some different controllers I have. I will be installing a ProFire 2100 BMS with flame ionization detection as my safety shutoff and with this I get some different control options and communications so we can monitor remotely. The exhaust is currently 6x6” 3/8 thick square tubing. The kiln walls are approximately 6” thick soft finer fire brick and the exterior is lined with about 1/2” thick refractory blanket covered in a galvanized tin. So we will run oxidizing flame till around 1500f and then go to a reduction firing after that. That’s really good info. Do you know what PPM of CO you would expect to see at reduction. Probably would need to be under 250PPM to prevent sooting. What material are you guys using for dampers in a brick chimney and how are you installing them. Any pictures of that would be handy. For testing I was just going to slide a plate over the top of my exhaust. I am not the potter (that’s my wife) so any pottery terms kind of blow right over my head. I am just taking care of the fire end of things so any help is greatly appreciated. My background is natural draft tube fired heaters up to 20mm btu/h and forced draft boilers up to around 50mm btu/h in oilfield and industrial applications. I have worked on hearth and fore hearth burners in fibreglass production but never anything to do with pottery kilns. I do have a large assortment of tools at my disposal so I’m sure I can get this kiln dialled right in for her. once again thank you for all the help. It’s very much appreciated. Bill Kielb 1 Quote Link to comment Share on other sites More sharing options...
neilestrick Posted February 7 Report Share Posted February 7 Start reduction somewhere around cone 012-08. Your wife can fill you in on cones if you're not familiar with them. I wouldn't waste much time on automating it, because adjustments to the burners are almost always paired with an adjustment to the damper. And every firing is a little different, so adjusting according to what you see happening is very important. You'd need a lot of different sensors to effectively automate a gas kiln. If any of the fiber behind the bricks is exposed where the bricks are broken, I would cover it with new bricks since it may not be rated for high temps. Most people use a kiln shelf as a damper. Google search should turn up some examples. Rae Reich 1 Quote Link to comment Share on other sites More sharing options...
Mark C. Posted February 8 Report Share Posted February 8 Mullet kiln shelf in my 4 gas kilns for a damper Quote Link to comment Share on other sites More sharing options...
Burnt Earth Posted February 8 Author Report Share Posted February 8 13 minutes ago, Mark C. said: Mullet kiln shelf in my 4 gas kilns for a damper Thanks. Do you guys just leave a gap in the chimney that you slide them in? Quote Link to comment Share on other sites More sharing options...
Mark C. Posted February 8 Report Share Posted February 8 6 minutes ago, Burnt Earth said: Thanks. Do you guys just leave a gap in the chimney that you slide them in? I meant mullite and the slot is usually 1 inch with a 3/4 inch thick kiln shelve. My Girl kiln is less and used a 5/8 shelve in a 7/8 slot. Not sure on your steel stack how you are are going to do this. The damper rides on 3 sides the two sides and back to close . In the front slot it sticks out enough to run cool Never used one that was less than 18 inches long . You need expansion space and if the shelve warps that also need space so not to tight Quote Link to comment Share on other sites More sharing options...
Burnt Earth Posted February 8 Author Report Share Posted February 8 With the steel I was just goi g to put in something similar to a wood stove but now my plans have changed and I will build a brick chimney once this kiln is placed in its permanent home. For now I will just use. 3/4” chunk of plate I have to cover the flue opening for testing. The kiln shelf will be an easy fit into the brick one when I build it. im really enjoying building this. It puts a big smile on my wife’s face and I get a kick out of making fire! thanks again everyone. I will keep you posted on my progress. Any more tips you can think of let me know. Cheers Kelly in AK 1 Quote Link to comment Share on other sites More sharing options...
Bill Kielb Posted February 8 Report Share Posted February 8 (edited) 26 minutes ago, Burnt Earth said: Any more tips you can think of let me know. Cheers Nice to see you are on the way! I found this old firing data and its profile from an updraft kiln It’s a simple Reduction profile but also fun to see the data of one of these in action along with rates it can achieve in reduction. It might give you a better idea of how these are often fired by hand. This one is in the 9 hour range and they are trying (more or less) to follow the profile pictured by manually adjusting gas and damper as needed every 15-30 minutes. There kiln hasa bit more capability than the profile below which was specifically created for a Geil downdraft car kiln. Edited February 8 by Bill Kielb Quote Link to comment Share on other sites More sharing options...
Burnt Earth Posted February 8 Author Report Share Posted February 8 58 minutes ago, Bill Kielb said: Nice to see you are on the way! I found this old firing data and its profile from an updraft kiln It’s a simple Reduction profile but also fun to see the data of one of these in action along with rates it can achieve in reduction. It might give you a better idea of how these are often fired by hand. This one is in the 9 hour range and they are trying (more or less) to follow the profile pictured by manually adjusting gas and damper as needed every 15-30 minutes. There kiln hasa bit more capability than the profile below which was specifically created for a Geil downdraft car kiln. Thanks for this. Easy to visualize in chart form. How much propane do you usually use on a 9 hour firing. I have a 250 gallon tank that I will be tying into. For now I’m just using 100# bottles. Should I be buying stocks in a propane company or is the usage not to bad. Quote Link to comment Share on other sites More sharing options...
Bill Kielb Posted February 8 Report Share Posted February 8 22 minutes ago, Burnt Earth said: How much propane do you usually use on a 9 hour firing Good question! Someone here likely firing regularly on their own tanks has that figured out.The recent propane kilns I have tweaked or instructed on have all been 1000 gallon tanks. Propane does provide about 91,000 btu per gallon though so we could back into an estimate from the data. All the more reason to reduce shell losses as much as practical. Less BTU, less gallons. Quote Link to comment Share on other sites More sharing options...
Kelly in AK Posted February 9 Report Share Posted February 9 I’m late to the party, but glad you’re doing a brick chimney. It’s not that the metal will degrade fast, it’s that the metal won’t keep the column of air hot enough to provide draft. There’s a point where it’s throwing off heat fast enough to stall the kiln. Double brick the chimney at least to the height of the kiln, higher is better still. Quote Link to comment Share on other sites More sharing options...
Burnt Earth Posted April 7 Author Report Share Posted April 7 Hi bc there I’m back. Weather has finally improved on the prairies and I’m back testing. I’m just running a test firing for a bisque. How fast is to fast for temperature rise? Quote Link to comment Share on other sites More sharing options...
neilestrick Posted April 8 Report Share Posted April 8 4 hours ago, Burnt Earth said: Hi bc there I’m back. Weather has finally improved on the prairies and I’m back testing. I’m just running a test firing for a bisque. How fast is to fast for temperature rise? A bisque should take a minimum of 6-7 hours, ideally 9-10 hours. If you rush a bisque you may not get good burnout. Go as slow as possible at first so you don't blow things up. Quote Link to comment Share on other sites More sharing options...
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