Flames coming out.

Discussion in 'Furnaces and their construction' started by orson thompson, Nov 11, 2020.

  1. Howdy all,

    I built a foundry and fired it up, at first I struggled to keep the propane burning, after a few minutes I grabbed my air hose with an air nozzle and started blowing air into the burner and then it stayed burning. I slowly added more propane and air and it started to roar and the flame was coming out the hole in the top.
    Is this normal or am I doing something wrong?

    Attached Files:

  2. DavidF

    DavidF Administrator Staff Member Banner Member

    Perfectly normal, although your vent hole looks small. What is its diameter??
  3. master53yoda

    master53yoda Silver

    When you have the fuel /air correct you will a small amount of flame exiting the furnace/firebox, your picture looks good, you may have a little to much air but it is really close
    Art b
  4. I had an oops when I was pouring the refractory for the lid and had to improvise so I grabbed a can of brake parts cleaner and used that for the vent hole.
    What size should it be?
  5. What do I look for to know if the air/fuel is correct?
  6. Zapins

    Zapins Gold

    Agree with vent hole being too small. Needs to be 3 inches across for propane.

    The louder the sound (as long as it is stable and not sputtering) the hotter it is. You want it loud and vibrating the ground. You shouldn't be able to hear people talking comfortably at normal volume right next to it when its working right.

    You know the fuel mix is right based on the sound but also you should have some flames coming out the vent hole about 5 to 6 inches tall. No flames exiting the top means you have too much air or the propane pressure is too low. Too tall flames out the top or flames that look like wobbly candle flames rather than a sharp constant jet means too much gas or not enough air.
  7. Melterskelter

    Melterskelter Gold Banner Member

    You could cut a larger vent with some abrasive, water, a piece of 3” pipe with a stem chucked in a slow turning drill press and some patience.

  8. Petee716

    Petee716 Gold Banner Member

    What is your burner design? Pictures?

  9. Zapins

    Zapins Gold

    Looks like the burner is entering the furnace directly in the center? It should be offset so flames spiral along the walls and don't point directly onto crucible. This way the flames will heat the metal evenly and won't destroy your crucible.
  10. FishbonzWV

    FishbonzWV Silver

    It depends on what metal you are melting.

    With aluminum you want a lean flame that does not come out the vent.
    If you have flame exiting the furnace, that means you are putting in more gas than can be burned internally which is a rich atmosphere. Molten aluminum will absorb the excess hydrogen and your castings will have a bad case of porosity.
    I tune my burner by sound, low roar = rich. Tweak it to the highest pitch, it's a fine line between rich, neutral, lean.

    For brass/bronze you want a rich flame like Zap described.
    Last edited: Nov 12, 2020
    Melterskelter and Tobho Mott like this.
  11. I think its the camera angle, here is a short video of the first time I fired it up. It might need more of an angle but what's the fun in doing it right the first time? If I do right the first time then I only get to do it once.
  12. FishbonzWV

    FishbonzWV Silver

    Pull your burner back into the tuyere about 3/4 to one inch.
    The tuyere will act as a flare for flame propagation.
    Petee716 likes this.
  13. Zapins

    Zapins Gold

    Yes more of an angle towards the closest wall is better. Flames should skirt along the walls. I agree with pulling it back. You might need to play with the distance you pull it back. Maybe 0.75 to 2 inches. No metal pipe inside the furnace chamber where flames can touch it.

    That burner needs a blower attached to it. It isn't getting enough oxygen to hit proper melting temperatures. I've got a 135 cfm radial blower on mine that I got off ebay. Hair driers work ok too. Shop vacs can work.
    Last edited: Nov 12, 2020
  14. Zapins

    Zapins Gold

    Here are 2 videos from different similarly built furnaces.

    This is when I first start it up and the furnace is cold with very low amount of gas and no blower on. Watch how the flames spiral around the walls and encase the crucible on all sides.

    Your regulator also seems like it might not be capable of putting out the pressure you need to melt properly. What is it rated at?

    What metal are you planning to melt? Aluminum you won't have trouble with probably even as it is now. Bronze you'll need to tune it and iron needs a lot of tuning to hit and maintain the Temps you need.

    Here are 2 videos of propane furnaces at melting temp.

    Having a high pressure regulator let's you dump a huge amount of gas into the furnace fast and let's you properly fill the space with flames. If your pressure drops in the tank as it freezes up or your regulator can't supply it then your melting temp drops dramatically.

    I'd also recommend learning this stuff at night when it is dark and you can better see what the flames are doing. It's very difficult during the day to learn these things.
    Last edited: Nov 12, 2020
  15. metallab

    metallab Silver

    This is not completely true. A loud roar can also mean that too much air is entering (i.e. too lean flame).
    I once did a test by putting two thermocouples, one on the level of the top of the crucible and another in the exhaust hole.
    Lowering the air input increased the temperature at both points until a certain optimal point. The roar also slightly decreased.
  16. Zapins

    Zapins Gold

    Interesting. I'm sure the sound it makes is influenced by a lot of things. Shape of furnace, crucible location, area of gas combustion inside furnace or above furnace, air flow, and fuel ratios. There is a certain sound a properly adjusted furnace makes and it is pretty loud.
    Last edited: Nov 13, 2020
  17. Does the vent hole have to be 3 inches or can it be bigger? And what kind of abrasive and where might I find it?
  18. Zapins

    Zapins Gold

    I'm unsure how big you can go. But your furnace is approx same size as the ones I've built and thats what has worked well for mine.

    Too big and the heat escapes out the top. Too small and it prevents enough gas/air from entering the furnace and makes it hard to get up to temperature.

    I'd imagine any sort of sand would be a good start or blasting media like aluminum oxide.

    Stone chisels work too or just repouring the lid.
    Last edited: Nov 14, 2020
  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    How thick is the lid? You're in for a heck of a chore. Cured refractory is very hard. If you have extra refractory, I'd suggest recasting the lid. You could make your own hole saw and use abrasive slurry, but refractories are mostly aluminum oxide so will be just as hard as your cutting media. If you go buy a diamond coated hole saw at the big box stores, you'll need a stout drill and a lot of determination but at least you'd have a chance of cutting the refractory, but the cost of that and time would be similar to just recasting the lid.

    My 2 cents

  20. Melterskelter

    Melterskelter Gold Banner Member

    I think most people use a 3 or 4 inch hole.

    To cut the hole, I’d use coarse aluminum oxide or better yet si carbide grit like 40, 60, or 80. Then make a stem on a piece of steel pipe of the desired diameter to make what is effectively a toothless hole saw.

    Now mount your lid on the slowest-turning drill press or milling machine you have. Use clay, caulking whatever to build a little dam around the area to be drilled. Plug the existing hole with wood/wax/POP, modeling clay etc.

    put a couple tablespoons or so of abrasive in the area to be cut and add a few tablespoons of water.

    Now just bring the holesaw down onto the lid with modest/light pressure and let the abrasive roll/grind under the saw. You will get a feeling as to how much abrasive and water to use and how often to recharge the mix with fresher grit. I’d be thinking it would take maybe 15 mins to cut the hole. How fast the holesaw will wear remains to be determined.

    This is the technique used to cut glass and looking at YouTube may offer some refinements. Using silicon-carbide grit might be better as the cement uses al oxide as an ingredient.

    Alox and sicarbide grits are available on eBay.


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