Rex Johnson Posted March 2, 2016 Report Share Posted March 2, 2016 ...I'll butt in here for a moment. I've been thinking of converting my Olympic gas kiln into a downdraft, pretty much the same way as High Bridge is doing. Glad to see someone try it first. That said, my kiln is quite a bit bigger, not sure I'd even change the natural draft burners if I did try. At any rate, if I'm seeing right, your flue/chimney size looks way small. A taller chimney will help the draw but if the hole is too small it's going to really struggle to draw heat no matter how much gas or size of burner you have. If someone's mentioned this forgive me. Looking at the one photo, the interior diameter of the chimney needs to be much bigger, usually the size of a brick or looking at the pic, at least twice that size. If I'm mistaken about the photo, I apologize... Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted March 2, 2016 Author Report Share Posted March 2, 2016 It is a 3 by 3 inch internal chimney. My reasons for the size were based on 1 inch square for every 10,000 btu and I think I can get this to cone10 in about 60,000. Not that I have yet. I did double the size of the chimney (not internal, that's still 3by3) after that picture and tried again with hardly any difference which is when I ended up going for the forced air burner. I don't really know if the cross-section is too small and if the chimney or flue should be bigger. I have tried to size everything up from The Kiln Book, Olsen. Link to comment Share on other sites More sharing options...
Rex Johnson Posted March 3, 2016 Report Share Posted March 3, 2016 Good book that. Glad i still have mine. I dunno though, you have a powerful burner but 3" still appears small. I'm thinking 4 bricks (4.5x 9) stacked in an interlock pattern will five you the minimum 4.5x4.5 I would think would be minimum. Just my thoughts. Link to comment Share on other sites More sharing options...
Rex Johnson Posted March 3, 2016 Report Share Posted March 3, 2016 You may have seen this vid before...if you can stand watching Simon bumble about, this is would be a good design, that shows the chimney (frame 6.49). Though it's bigger than yours, the draft and burner placement is everything... Link to comment Share on other sites More sharing options...
Mark C. Posted March 3, 2016 Report Share Posted March 3, 2016 I'm also thinking 3 inch is pretty small-when building flues you can make them larger and brick them down to tune them. My car kiln for example has bricks in flue to make it smaller-this was all done tuning the kiln during the 1st fires. I have yet to read about someone who makes an electric conversion act like a well oiled gas kiln-I was hoping you would be the 1st. I think your exit may be to small even for forced gas burner One thing to keep in mind is the gas kilns that are made from the factory with lid exit flues-like Olympics-they seem to have larger exit flues than your 3 inch. I have a small single burner trash can kiln which I do not use(west coast kiln Co.)-its natural draft and very small but the exit flue on lid is 4 or 5 inch round. Link to comment Share on other sites More sharing options...
neilestrick Posted March 3, 2016 Report Share Posted March 3, 2016 I agree. Make it too big. You can always damper it down during the firing. Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted March 4, 2016 Author Report Share Posted March 4, 2016 Thank you for the ideas on flue size. Before I do make it bigger I think I will do a firing with the burner, don't want to change two things then have no idea which did the job. The first firing I ever did the exit flue from my kiln was a 2 inch diameter circle with area 3.14 inch. This one stopped at 900-950. I then realised that this was way too small even for my estimates and I made it into the 3by3 square so 9 inch area and doubled the height of the stack. This one still stalled out at the same temperature even though my exit flue was 3x as big, maybe it was still too small. These two firings made me think it probably was not too much of a flue size issue but I could be wrong. I wish it didn't take me 6 hour to fail but hey, that's ceramics Link to comment Share on other sites More sharing options...
Rex Johnson Posted March 4, 2016 Report Share Posted March 4, 2016 I didn't see your placement of the burner, but the whole idea of a downdraft is to create a flame flow that targets up and swirls down to the flue. That's the idea in Simon's two burners pointing reward, targeting up and swirling back down towards the flue. If you decide to rebuild your chimney/flue here's an easy flue solution. Take a piece of kiln shelf and cut it to fit/slide horizontally into the flue. This will give you alot of adjustment by sliding the shelf in/out.. That way you can experiment with gas/air flow and draw. I can take a pic if you'd like to see the one on my larger kiln which has two Stiktite burns with squirrel cage blowers. Crude-ish but functional. It has a basic chimney made of a 4 brick stack and the same flue set up. In regards to stalling, I use to fire full airflaps open on the blowers , full gas valves turnedon, and it would get to like cone 8-9 and stall. I'm finding that with less gas I'm actually getting more heat and a faster ramp up! It's all about air/fuel mixture and draw. I'd make that round flue square or rectangular as well. It's too constricted. I wanna see you succeed Link to comment Share on other sites More sharing options...
Magnolia Mud Researdh Posted March 4, 2016 Report Share Posted March 4, 2016 Joel, A few suggestions for your problem solving: 1. Check your premises, especially if your experiments don't meet your expectations. Not having a large enough flue or tall enough stack may not be your problem. Is your burner tuned so that the air / fuel is balanced to produce a flame temperature near the theoretical maximum? Propane burners used by potters often have more than adequate pressure to induce enough air into the burner to have excess oxygen for complete combustion. The excess air just cools the flame. Computer tools are available to help predict the flame temperatures. Checkout Gaseq [ http://www.gaseq.co.uk/ ] If you have too much air then you will NEVER reach your target. From the discussion so far it seems that you are assuming that you don't have enough air. A good reference for trouble shooting kilns is the book by Nils Lou, The Art of Firing. [ http://www.worldcat.org/title/art-of-firing/oclc/60201735 ] 2. Is the flame staying in the kiln long enough to heat the kiln? In an empty downdraft kiln the pathway is likely to be significantly different than when the ware is obstructing the flow. Also, how are you measuring your progress to your target temperature? Cone packs or thermocouple probes, or quality of the fired pots? Remember that heat transfer is dominantly by radiation. Thermocouple probes that can "see" a cold surface will generally under-report the temperature of the gas surrounding it by 10's of degrees unless the gas velocity is very high because the probe loses heat to the cold surface (not because it is a bad probe) (it can also over report if it sees a hot pot) All the measuring tools report the temperature of the tool not the ware. The best estimate of the correct temperature for your pots is the pots themselves. In my own experience with an electric to gas conversion kiln (using natural gas), I was able to cure stalls by partially blocking the secondary air ports with kaowool and lowering the fuel supply pressure. Nils Lou reports similar responses in his book for other kilns. He claims that most studio potter kilns have flues that are oversized, not undersized. He recommends that the kiln chamber be pressurized with flames blowing out the peep holes an inch or more. 3. Since you have converted to a forced air supply, you should do an Internet search for "Abernathy Kiln" [ http://www.bing.com/search?q=abernathy++kiln&qs=n&form=QBLH&pq=abernathy+kiln ] (also discussed in the Mel Jacobson's book KILNS: "DESIGN, CONSTRUCTION AND FIRING IN THE 21ST CENTURY" [ http://www.21stcenturykilns.com/ ])The Abernathy kiln design does not have a stack - just a small hole or two in the kiln to let the combustion gases out. It works beautifully. Good look on your trials. LT Link to comment Share on other sites More sharing options...
Mark C. Posted March 5, 2016 Report Share Posted March 5, 2016 Nils Lou has two versions of that book the old one and his newer one-they vary a little bit with the newer having a tad more in it. Link to comment Share on other sites More sharing options...
neilestrick Posted March 5, 2016 Report Share Posted March 5, 2016 Since you're going to be using a power burner, you can take the stack down to the height of the kiln and just use a shelf on top as a damper. No need to do anything fancier than that. The size of this kiln is both a blessing and a curse, but it can work. Link to comment Share on other sites More sharing options...
jrgpots Posted April 15, 2016 Report Share Posted April 15, 2016 Joel, How is your kiln build going. I haven't seen an update recently. Have you fired up the power burners, yet. Hopefully, you are so busy firing it that you have not had time to update the rest of us. Jed Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted April 15, 2016 Author Report Share Posted April 15, 2016 I have been meaning to post an update but got a bit distracted trying to work out posting a video with it from instagram. I have figured out getting a direct link to the mp4 but still can't get it into the posts. https://scontent-lhr3-1.cdninstagram.com/t50.2886-16/12951943_1173488249391294_1706430324_n.mp4 Had a pretty successful firing with the power burner, all cone9 flat and cone10 down except at the flue. I started without the extra chimney pipe but ended up adding it on top at 1000c because it seemed to be slowing. I think the flue could still be slightly on the small side but there is no easy way to make it bigger (cross section) which is why I have been avoiding it. Still very much getting used to the burner and how much air to let in. On the lowest setting I could get it went to 400c in 30 min and cracked half the pots. Need to work out how to make it climb slower or maybe buy a bunsen burner for the first 400c. Overall I was really happy with it and some more work will get it flowing even better. Link to comment Share on other sites More sharing options...
neilestrick Posted April 16, 2016 Report Share Posted April 16, 2016 YES! I was wondering if that power burner did the trick or not. I saw your Instagram post and have been eagerly awaiting the results. Congrats on the success! You should be able to slow the blower way down and block most of the air in order to slow down the start of the firing. A small venturi burner could also function as a pilot burner that would take care of the first few hundred degrees. Link to comment Share on other sites More sharing options...
What? Posted April 17, 2016 Report Share Posted April 17, 2016 Very Nice (Borat voice). Link to comment Share on other sites More sharing options...
Joseph Fireborn Posted April 17, 2016 Report Share Posted April 17, 2016 Wooo wooo I knew you would get it. Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted April 18, 2016 Author Report Share Posted April 18, 2016 I think I have a bit of an issue with the 120v transformer and using that to control the speed. Also my lowtech cardboard should be replaced with something better. I had been thinking about using the old venturi but it feels too clunky to swap in and out. I am going to look into using the transformer to get the 120v and make a little/buy a triac dimmer thingy. Link to comment Share on other sites More sharing options...
neilestrick Posted April 18, 2016 Report Share Posted April 18, 2016 Magnetic business card instead of cardboard. Link to comment Share on other sites More sharing options...
Magnolia Mud Researdh Posted April 18, 2016 Report Share Posted April 18, 2016 It works! Observation & comment: The position of cardboard throttle of the blower is a clue that your blower is significantly over sized. Throttling it for several hours will lead to overheating of the motor. (I've been there & done that!) Other than getting a smaller blower, adding a large throttled discharge vent to the blower outlet before connecting to the burner will allow you better control. Keep tinkering! LT Link to comment Share on other sites More sharing options...
Biglou13 Posted April 18, 2016 Report Share Posted April 18, 2016 Magnetic business card instead of cardboard. magnetic refrigerator magnet from local pizza joint Link to comment Share on other sites More sharing options...
jrgpots Posted April 19, 2016 Report Share Posted April 19, 2016 I used a cd disc. I drilled a small hole in the edge of the disc and threaded a screw through it. I then clamped the screw to the motor housing. The disc slides to increase or decrease the air flow. It works. Jed Link to comment Share on other sites More sharing options...
neilestrick Posted April 20, 2016 Report Share Posted April 20, 2016 Magnetic business card instead of cardboard. magnetic refrigerator magnet from local pizza joint Do they sell pizza in Newcastle Upon Tyne? Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted April 20, 2016 Author Report Share Posted April 20, 2016 It looks like pizza, but is not necessarily made out of dough or cheese. Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted April 20, 2016 Author Report Share Posted April 20, 2016 It works! Observation & comment: The position of cardboard throttle of the blower is a clue that your blower is significantly over sized. Throttling it for several hours will lead to overheating of the motor. (I've been there & done that!) Other than getting a smaller blower, adding a large throttled discharge vent to the blower outlet before connecting to the burner will allow you better control. Keep tinkering! LT The video is at my very first setting of the burner so it did get opened to about 2/3 of the way. It is about 50cfm and my burner needs around 30cfm if I remember that right @ 2psi. I think it will be better once I can get a dimmer switch on the 120v so it can run open at a slower speed. The motor was a little warm to the touch but not overly hot at the end. Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted November 26, 2016 Author Report Share Posted November 26, 2016 Used a different camera to get some better footage of the burner. Link to comment Share on other sites More sharing options...
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