Furnace design

Discussion in 'Furnaces and their construction' started by Andrew Marquardt, Mar 16, 2023.

  1. I am currently in the planning phase to build a replacement furnace. The outer shell is going to be part of a 120lb gear oil drum. The diameter of the drum is approximately 14" and it will have 2" thick walls. Taking the advice I received on here earlier, my current plan is for a 1" hot face of Kast-o-Lite 30 backed by 1" of fiber insulation. While my current plan is to cast the base as a solid 2" thick disc, would it be better to add 1" of ceramic fiber to back the cast base as with the walls or would it not make much of a difference?
     
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I think for most hobbyists, a lower mass furnace is better because it heats faster, so my vote would be 1". Insulating Fire Brick (IFB) would be a good choice to support and insulate underneath the base. It could even be 1/2" with IFB backing.

    Best,
    Kelly
     
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  3. Thank you for your advice. I have revised my plans to use IFB underneath a 3/4" layer of refractory for the base of the furnace. Now to wait for my shipment of refractory to arrive. I have attached a picture of my planned design for the base and lid lifting mechanism.
     

    Attached Files:

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  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Is that a parallelogram type lifting mechanism? You might want to take a look at Mark's furnace. IIRC, he has a pretty nifty yet simple and effective version of same. Mark, have a picture for Andrew?

    Best,
    Kelly
     
    Mark's castings likes this.
  5. After doing a lot of research and looking at side-swing lid and traditional lid designs, I felt that a parallelogram lift offered a more stable design compared to a side-swing lid design with my base design while not aiming a scorching hot lid at me while the lid is open. Mark's post on another thread was actually the one that had me considering a parallelogram-type lifting mechanism.
     
  6. Tops

    Tops Silver Banner Member

  7. Hi, I just read this thread for the first time and wanted to add some experiences with using the parallelogram lid for a few years: I built mine by welding the lid mounting points and pivot points in place and then making the arms to suit the measured distances. This results in four arms that are all slightly different in length which is not necessarily bad but does require you to number the parts and always reassemble in the original position. The next thing I noticed between the original 1958 vintage furnace lid and my recent copy is that a bit of sloppiness in the holes is an advantage to getting a good seal between the lid and the furnace: you can sort of jog the lid a bit if flames are coming out a gap and get a better seal. That way all the lid weight is borne on the furnace walls and the four arms take no weight when in the closed position.

    I cast the furnace walls first and cast the lid in place to get matching surfaces, the top of the furnace lining could have benefited from some smoothing with a masonry wheel in an angle grinder to get out the irregularities. Also the disc used to cover the bore when casting the lid in situ needs to be rigidly in place so it can't move from all the vibration or you'll get places where the lid refractory goes into a bore a slight amount and binds when opening the lid. In fact the disc over the bore could be slightly higher say 6mm 1/4" or so than the furnace wall lining.

    furnace bore.jpg

    furnace lining 1.jpg

    The current form is 1.5 beer kegs tall with 2.5 inches/65mm of dense castable inside the keg shell and 3"/75mm of insulating fibre outside the shell. This is a kludge to compensate for the outside keg shell reaching 300 deg C in operation but actually works great as the dense castable is contained by a steel shell so the inevitable cracks in the dense castable can't leak oil vapours into the insulating fibre as the stainless shell contains the vapours. This would not be an issue for a propane furnace.

    keg furnace shell 1.jpg

    keg furnace shell 2.jpg
     
    Last edited: Mar 21, 2023
    Tops likes this.
  8. Mark, thank you for the photos and information on your furnace and the lifting mechanism.

    I am wondering if it would be worth it to cast a port into the side of the furnace for a thermocouple probe to slide into in order to measure the temperature inside the furnace during operation. I am trying to finalize the design and it occurred to me the other night that knowing the temp inside may be useful. Would knowing furnace temp be worth it or is it more important to find a way to measure the actual temperature of the metal?
     
  9. I later fitted a thermocouple, only a K type good to 1350 degrees C though a drilled hole in the furnace wall. It gives good data on tuning fuel and airflow for maximum temperatures. I've only melted iron twice and have been unable to duplicate those temperatures again. Here's a discussion of my experiences so far: https://forums.thehomefoundry.org/i...-to-help-fuel-air-mix-tuning.2313/#post-46864
     
    Last edited: Mar 23, 2023
  10. I cast the base and lid of my new furnace this weekend. All told, it took about 65lbs of Kast-o-Lite 30. The lid is solid refractory while the walls and base have a 1" hot face backed by 1" of ceramic fiber insulation and fire brick. The cast tuyere came out much better than I thought it would. While it is curing, my next task is to mock up the frame and lifting mechanism with scrap wood. The final drying-out process will have to wait until it warms up a little here.
     

    Attached Files:

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  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Looks good Andrew. Is that just a separate plug sitting in the lid vent hole?

    Best,
    Kelly
     
  12. Yes, that is just a cap I made with the leftover refractory from casting the lid. I didn't have enough mixed up to cast another correct-size plinth.
     
  13. I fired the furnace today, multiple sessions gradually raising the heat each time. I used my old propane burner for all but the final session, which I did with my new dual-fuel burner running on diesel. The difference in heat output between propane and diesel is amazing. I do need to get a sleeve and weld it the the barrel in line with the tuyere to support the burner in operation.
     

    Attached Files:

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  14. Tops

    Tops Silver Banner Member

    Andrew, congrats on getting the furnace made and the new burner assembled, it all look great! What sort of mechanism are you using to atomize the liquid fuel?
     
  15. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Looks like your in business now. Congrats.Do you have flasks, molding sand, crucible and handling tools? -Time to melt some metal.

    Best,
    Kelly
     
  16. I am using a Delavan siphon nozzle with a .5 GPH nozzle for atomization of the diesel.

    I do have tools and flasks that will work for now, though I think I will remake them to fit better as the flasks are quite crude and have warped, so they don't fit together very well anymore. Likewise, my pouring shank and tongs were built to fit within my old furnace, so they aren't the easiest to use and I have seen much better designs on here, though they will do the job for now. I will have to get a new crucible soon, as I noticed that there is a small crack forming on the lip of mine.
     
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  17. Melterskelter

    Melterskelter Gold Banner Member

    Andrew, You have probably seen Delavan’s recommendations for atomizing air pressure. Initially, I followed those recommendations but eventually figured out that 40 to 45 PSI provided much better atomization and more complete burning in my furnace. At lower pressures I found coke building up quite thickly in a patch where the fuel/air mix impacted the furnace wall. Turning up the pressure eliminated this and reduced overall fuel consumption. I have really liked using the Delavan nozzle.

    Denis
     
  18. I found that out when I tested it when building the burner. I had started at their recommended pressures and didn't get a good burn so I gradually adjusted my 0-30 regulator up to 25 psi, with a needle valve downstream to fine-tune the pressure. I may need get a higher psi regulator given your experience with the pressures. I put in a separate regulator as I like having a regulator for items other than air tools so that I don't always have to adjust my primary FRO on my compressor.
     
  19. Melterskelter

    Melterskelter Gold Banner Member

    I find it very convenient to have a regulator within a foot of the burn tube. That is because, when starting the burner, I turn the pressure to maybe 5 or so pounds, the combustion air to be just cracked open, drop into the furnace a burning diesel-soaked blue paper towel and then crack open the diesel. Once it lights I gradually, at turns, increase fuel, compressed air, and combustion air. Most of the time I do not snuff the furnace and cause a big plume of white vapor. Happens once in a while. But this works better for me by far than trying to have the both air sources fully on and the fuel fully on which rarely if ever works.

    Denis
     
  20. Tops

    Tops Silver Banner Member

    Andrew, are you still thinking of doing a parallelogram lid lifting mechanism?
     

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