Tuyere / funace / vent sizing

Discussion in 'Burners and their construction' started by Hopefuldave, Jun 9, 2021.

  1. I know there's probably no hard and fast rule, but I hope someone can point me in the right direction...

    I have a 55-gallon drum for the outer shell, plan lining it with 2" of lightweight insulating bricks, internally either 1 or 2" of Posh hard firebricks (1700c Zirconia), definitely 2+2" on the floor, with a 2"? hole for spillages etc. similar construction for the lid. Overall internal diameter 12 - 14", height probably 20"?

    Burner planned is dual-fuel, propane/LPG or waste motor oil pressurised and fed to an atomising nozzle, probably a Nobox7-style annular, with a line to my big compressor for atomising air, a bouncy castle blower with restrictor for bulk.

    Now the difficult bit - tuyere and vent sizes? If I unforget diserroneously, the lid vent should be about 1/4 the furnace bore, not a clue about the tuyere - I have 100mm stainless tube available, or 40mm bore heavy-duty steel pipe (hydraulic pipe, rated 5k psi... stout stuff) and a nearby scrapyard...

    Before you say it, yes, that's a decent sized furnace... A50 crucible for 15kg of aluminium or more bronze (eek!)

    Any suggestions, help, shooting-down-in-flames very welcome!

    Dave H. (the other one)
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    A lift out crucible furnace I presume. Are you planning two-man lifting tongs and shank? Or mechanical lifting equipment? Sounds familiar to me.

    I'll add, if you only plan to do large melts or only have room for one furnace, it's probably the right move, but if you are just doing it because it can accommodate the largest melt you can envision needing, you'll regret it because you'll expend a lot of time and fuel with a furnace that size. I have two furnaces and the larger is built to your dimensions, but I use the smaller furnace 95% of the time.

  3. Mach

    Mach Silver Banner Member


    Rules of Thumb

    Since gaseous fuels are among the most expensive fuel sources, furnaces using these fuels should be as efficient as possible. One of the most important ways to increase efficiency is to size the furnace correctly.

    - For every cubic foot of enclosed space inside the furnace, the burner should output approximately 100,000 to 200,000 BTU/hr.
    - The vent hole diameter should be from 1/3 to 1/4 the inside diameter of your furnace, or 2 to 3 times the inside diameter of the tuyere.
    - The walls of the furnace should be as close as possible to the crucible (with adequate room to operate the crucible lifting mechanism, of course), but the gap between the crucible and furnace walls should never be narrower than 3/4 the burner tube diameter.
    - The gap between the top of the crucible and the furnace lid should have approximately the same area as the vent hole for maximum efficiency. If the lid and crucible lip are flat parallel surfaces, a gap of the correct area will be 1/4 the vent hole diameter.
    - The plinth should be no taller than twice the tuyere diameter.
    - The tuyere diameter should be about 1.4 times the burner tube inside diameter. If the burner has a working flare, use the largest inside diameter of the flare.
    - The furnace wall thickness should be about a quarter the furnace's inside diameter for adequate insulation.
    3Dcasting and Petee716 like this.
  4. Petee716

    Petee716 Gold Banner Member

    I’ve never seen furnace dimensions expressed this way but my personal experience (for what it’s worth) bears them out.

    I haven’t spent any real time studying Nobox 7’s burner design but a typical Hago 1gph siphon burner is a simple plug n play method using the same equipment and works reliably.

    Assuming a cylindrical bore, I would think coating your insulating firebricks with satanite (I think someone here may have discovered a European alternative) would be a lot easier than fashioning hard bricks to fit, although I’m not exactly sure of your plan.

  5. metallab

    metallab Silver

    Nobox7 has indeed very good working very hot burners, but not simple. Rather exotic design.
    I have a good European alternative for Satanite, it is called Fermit, a cement which withstands 1600 C and can be applied easily on a Kaowool lining by just painting it over the lining. Just add some excess water. It is available on Amazon. I have coated my furnaces with it.
    Tobho Mott and Petee716 like this.

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