zappymax wrote:1) is it "better" to use a metal covered hat on top of furnaces (with refractory material inside, under the metal cover ) or just a steel circle with refractory (the two with exhaust holes on top) as heating and cooling cycles will always produce some breaking of the refractory stuff used. Is the metal covered top more resistant ?
A steel band, one that is capable of clamping the refractory material to keep it intact. I'd personally avoid a metal cover, if for no other reason, its ability to conduct heat (exposed to the exhaust stream) and to corrode, possibly dropping oxides in to the heat, contaminating the charge.
2) if i use compressed air with a LP gas venturi , should the air injector be directed in a specific way or just parallel to the LP gas sprayer ? In such a case, should the end of the air injector be in a specific form/design, and be installed just before, just next the gasifier or "more inside" of the gas sprayer into the furnace ? Any advice welcome.
Compressed air? Like from a piston type compressor?
I used old upright vacuum cleaner motors for two furnaces I built, both of which were fired on natural gas. I introduced the gas at a right angle to the air stream on both of them, which appeared to work quite well. I regulated the gas flow as well as the air flow with gate valves. Remember, you really want the gas to mix with the combustion air, not just be introduced to the furnace randomly. The air/gas mixture should be introduced to the furnace on a tangent, so the fire swirls around the chamber, from bottom to top. The flame should not impinge directly on the crucible.
3) The furnace is practically soldered, now need inside lining. Picts will follow.
Soldered? That may not be a good idea, depending on what you expect from the furnace. You're generally better served to weld, or use fasteners that can't fail when heated.
You have options with lining---it can be poured, or you can use a ramable lining. I highly recommend you invest in a commercial refractory, assuming you expect the furnace to perform well and be used reliably. No Portland cement should be used in furnace lining, for example, as it breaks down with heat. Refractories are formulated to perform at elevated temperatures, and generally contain no Portland cement.
4) Is it a good idea to use fiberglass outside of the steel furnace to help keeping heating process and spare fuel ?
My opinion? It's not a good idea to use fiber glass in the melting room, not for any reason. If you've ever seen a fiber glass fire, you'd understand. If you are concerned with heat loss, there are refractory linings that can be used to limit heat loss. You might consider pouring your lining in two layers, the outer one being a light weight refractory, intended to be used for insulation. The inner lining would be one that would tolerate the heat you intend to create. You can also wrap the exterior with an insulating blanket, once again, made from refractory products. They're readily available from foundry supply houses, refractory distributors. Check the yellow pages in your area for a supplier.
Be aware, refractory materials have a shelf life. Outdated materials may fail to cure properly, and will fail in use. Been there, done that.
If you intend to pour clean metals, avoid using a metallic melting vessel. If you insist on using metal, line the item with a refractory wash, to limit oxidation and contamination of the heat. The best scenario is to buy commercially made crucibles. If you limit, or avoid fluxing, they have a respectable life expectancy, and won't contaminate your heats.
Crucible furnaces are not known for their ability to pour cast iron, although some folks have enjoyed mixed success. If your goal is gray iron, I suggest you consider it carefully. Crucible furnaces are very good for non-ferrous materials, however.
Please do post a picture or two!
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