New oil burning furnace build

Discussion in 'Furnaces and their construction' started by Mark's castings, Jun 6, 2024.

  1. I'd been editing my earlier post about the B301 air gap of 2.9" when you made that B301 post about the airgap. The B702 scales up but still maintains around 2.75" air gap for an A70 crucible. So that seems about minimum for a given crucible size.

    Looks like you've been busy too, so yours will be optimized for radiant heat coupling obviously.
    Last edited: Jun 9, 2024
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Yes indeed and using a new (to me) refractory material. I'll start another thread in not-too-distant future.

    My comment on the air gap was just contrasting or perhaps reinforcing it was about half the gap for NG as least at the A20 size furnace.

  3. My existing 11" bore was based on gas furnace dimensions and 1-1.25" between the crucible and bore. The original gas furnace gets hot with high gas consumption and melts hotter materials including monel which is up there with iron's melting point. My similar bore furnace struggles with iron and the experiment with an A6 crucible seemed give enough extra combustion volume to melt a smaller mass of iron. In a way I'm scaling up the existing furnace with an A6 to accommodate an A25. In the photo below you can see an upside down A25 for size comparison to the right.

    So the new furnace goal is to reduce the refractory mass, boost the volume and have the same or less surface area.......hmm might have to calculate all that now.

    A6 iron run 2.jpg
  4. Finally started the form work for the "top hat" profile furnace refractory. I have some 12"/30cm PVC tube lying around that I've used for the previous furnace build. This time round I have to soften it with a hot air gun and wrap it round some plywood discs of the planned 16"/40cm diameter. I worked out I needed an extra 12.5"/31.5cm of circumference and cut that out with the reciprocating saw and split the PVC tube. Cardboard of the right size would be nice but a little hard to obtain locally. I may need some longitudinal wood strips to maintain disc spacing and prevent unwanted curling too. I marked out four 39cm discs on the plywood and will cut them out tomorrow morning. I'll reuse them for the refractory pour to reinforce the PVC during vibration but make them individually removable.

    PVC tube 1.jpg

    plywood disc 1.jpg
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  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    That PVC should make for a sturdy mold. Will it be used for ID and OD? Thought through the demolding strategy for the ID? When you say top hat, does that mean you'll cast a flange onto one end of the hot face cylinder? I ask because even though I do so myself, I worry it's a source of stress and breakage due to large temp gradients and non-uniform expansion across the diameter. It can also be a source of heat leak.

    On my previous build I cast the flanges separate using insulating refractory and then used dense castable for the hot face. On the most recent resistive electric build I referenced earlier in this thread, it's built up as a monolithic block with integral flanges and coil shelves but uses a very low-density, highly insulating, moldable refractory. Since my furnaces are lift off, and also get jostled around because they're portable, I must have a flange but if they weren't, I wouldn't have one.

    Looking forward to the build.


  6. Hi Kelly, The PVC would have been moulding the bore of the furnace, late last night I found a coil of some colorbond steel: this is about a 0.5mm thick sheet steel with a plastic coating used for roofing iron and rolling into corrugated iron, gutters etc. It's a pretty common material here in Australia. Anyway this sheet is just the right height and at 4.2 metres long will wrap on itself over four times at the desired 40 cm O.D.. It also has a clear plastic film coating one side that can be peeled off which I'll face outwards to the concrete side. The advantage is there's no hot working to try and bring to the correct diameter and if I'm careful I can reuse the material for other things. with four-five discs of plywood there should be plenty of strength when vibrated.

    Regarding the flange, I'll be casting a radius fillet to reduce the stresses of a sharp 90 degree corner. The flange will form the top of the furnace lip and should be ok even if it cracks from the heat. The bottom of the cylinder will fit into a shallow groove in the base refractory.

    colorbond coil.jpg
    Last edited: Jun 18, 2024
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    What I was getting at is the ID of the flange is at hot face temp and the OD at a much lower temp. In my case the gradient will be from 2000F - 200F because the material is a very good insulator.......that must mean huge OD hoop stress since the ID wants to expand a lot and the OD hardly at all.

    Your (higher) refractory material will likely be a better conductor but you'll also have a much higher hot face temp and high temp gradient so same issue......and then we induce the condition over and over with each heat in our furnaces....

  8. I'm expecting cracks but that it'll be usable even when cracked, ideally the metal shell will keep it in position. I did get a pretty loud bang from the current furnace on it's first firing which I assume was caused by a crack.
  9. I decided to see if the PVC would take on the new diameter of 40cm/16": I cut five plywood discs and with five ratchet straps on the outside of each plywood disc it took shape pretty well while cold. I worked over the PVC with a hot air gun in vertical stripes trying to get it to the 80 degree C softening point and with enough penetration of the 6mm wall thickness tube. One area I did screw up was with the diameter, I allowed for the tube thickness when cutting the plywood discs but obviously overcut and ended up with a 95mm/3.75" airgap instead of 100 or so I was aiming for for the A25 crucible. Using the PVC instead of the sheet steel lets me make the outer formwork with the sheet steel and some plywood rings.

    Furnace bore form work PVC.jpg

    After a few hours of cooling I undid the ratchet straps and the PVC held it's shape pretty well. There's a few ripples in the surface from straight at the cut sides but it shouldn't affect things. All in all it worked as planned. I had four scrap ply corners per disc and cut them all to make three legs for each of four discs and attached them with screws, they keep the five discs evenly spaced. Because the plywood discss weren't full bore width there's two gaps on each disc where you can grab the disc and remove it if needed.

    Furnace bore form work 2.jpg

    Furnace bore form work 3.jpg
    Last edited: Jun 19, 2024
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  10. The furnace bore has been cast in 1600 degree C refractory concrete. I opted for a simple tube with no top ring, so I'll cast a socketed base, a socketed top ring for the rim and a lid with the remaining refractory. The PVC tube had several lengths of tape stretched across the joins to give a decent closing force and tightly wrapped with sandwich wrap as a release agent. I wound the 4.25 metre long by 50 cm wide strip of colorbond steel so the removable plastic film was facing the refractory concrete and used some ratchet straps to get the correct diameter. Between the inner PVC tube and the outer colorbond sheet steel is a plywood ring to set the gap that the refractory goes in: 4cm across. I had cut plywood spacers for the top to set the 4cm gap but they were not needed at all. The much vaunted grout mixer appears to have a dead gearbox, I thought it was stalling from the concrete but the motor fan was spinning at full speed so no stalled motor. I defaulted to a tarpaulin where I could spread out a bag of refractory at a time and sprinkle a pre measured amount of water on it and mix it in, turning over several times with a shovel and pulling the tarp corners over. It worked like a charm and was much faster than mixing in a bucket by hand. I made a crude vibrator from a reciprocating saw with a steel rod hose clamped to the blunt blade, it worked really well but the rod came loose twice as the hose clamps loosened from the vibration. I can recommend such a tool: it readily got the refractory flowing like a thick liquid. I made several passes to ensure good mixing and packing and being able to plunge full depth helped too.

    Final dimensions are: 40cm/16" bore, 4cm/1.6" wall thickness, 45 cm/17.7" height.

    furnace bore 1.jpg

    concrete vibrator 1.jpg
    Last edited: Jun 21, 2024
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  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Good scramble! Glad it went well.

    How much refractory did that take? ......circa 150lbs...3 bags?

  12. About 2.4 bags or 60 kilograms or 130 lbs, I'll need a base, a top ring and a lid. I'm considering some kind of standoff for the base so I can put fibre insulation under it without crushing. I've been thinking I'll leave the inner PVC in place until fully assembled and then remove the ply discs and the PVC should come out. That should give some extra support to the concrete tube.
    Last edited: Jun 21, 2024
  13. Tobho Mott

    Tobho Mott Administrator Staff Member Banner Member

    I used a recip saw vibration tool for my oil furnace lining too, filed down one end of a long thin bar to match the shape of the back end of a saw blade so it could just chuck up in the saw. Never thought of trying a hose clamp, what a time saver... I guess you could weld something on as well. In any case it worked well for me too.

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  14. It was your use of a saw in the first place that I got the idea from, I did think about welding it but not fabricating the same blade end profile. It was a little bit floppy but worked well enough even with the hose clamps coming loose twice during use before settling down.....I could split the rod so the old blade goes down the middle which would allow for a decent long weld to give a permanent setup.
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  15. Here's some photos of the concrete liner and the colorbond sheetmetal after removal. I have to say the reciprocating saw vibrator really worked beautifully: this is the most consistent refractory casting I've made to date. Removing the colorbond steel outer did pull some fine bits of concrete with it, waxing would have helped with that but it still came off freely with a few tears likely caused by the vibrator.

    furnace bore 2.jpg

    colorbond sheet 1.jpg

    I'm really pleased with the result to date but I'll leave the internal PVC in place for some mechanical support, I don't want this to happen:

    Last edited: Jun 22, 2024
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  16. So 48 hours after the bore refractory was cast, I flipped it over onto a XPS sheet to cushion the edges and rolled it out of the way and stood it upside down. There's a few bubbles but otherwise the casting is very uniform from the vibration doing it's job. I need the ply base and the PVC tube and colorbond sheet as the formwork for the top ring : it should need minimal modification to reuse all that again. I'll add some clearance so the bore will fit without binding. Of particular note, the sandwich wrap worked as a release agent but is very hard to remove from the bore and also the refractory got between the layers with vibration, the vibrator likely tore the sandwich wrap too. Next time for the ring, I'll just wax the PVC and save the sandwich wrap for casting the lid onto the ring. The bore came apart readily, the plywood discs came out with no troubles once you cocked them sideways and the small 6mm strip of PVC formed a keyway to release the tension on the PVC tube unlike last time where I had to cut it vertically with an angle grinder in a few places to get it to release.

    Pulling the key strip made removal easy.
    PVC tube 2.jpg

    Tufts of sandwich wrap that can be burned off later
    furnace bore plastic.jpg

    The refractory got under the sandwich wrap and uniformly coated the PVC tube.
    furnace bore plastic 2.jpg

    Layers of sandwich wrap sheet.
    furnace bore plastic 3.jpg
    Last edited: Jun 23, 2024
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  17. Tops

    Tops Silver Banner Member

    Mark, thanks for the cylinder casting pics, I need to do this job on my build too.
    PS I will be using the concrete forming tubes, probably covered with some sort of vinyl sticky back shelf paper and/or packing tape on the contact surfaces.
    Last edited: Jun 23, 2024
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  18. HT1

    HT1 Gold Banner Member

    dont bother just burn them out much easier then trying to remove
    V/r HT1
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  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The IDs of those tubes are waxed, water resistant, and self-releasing. The ODs are not and if not sealed can become weakened and compromised by the water in the wet refractory. I just use packing tape but if you have it, I'm sure PSA sheet barrier will due. I also wipe mine down with paste wax. HT1 is right, they will burn out easily. You don't even have to clean it up. What little ash there is will blow right out the vent.

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  20. HT1

    HT1 Gold Banner Member

    Ive never had that issue ( failure) but im sure its a possibility especially if your refractory is too wet

    V/r HT1

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