"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." Area? I have trouble visualising this. Does it mean the gap distance should equal the diameter of the vent? Cheers Charlie
Probably means that as a minimum the cylindrical area formed by the diameter of the crucible and the air gap from the top of the crucible to the underside of the lid. I've had that gap vary a fair bit but when the crucible is close to the lid you can get swirling vortexes that stir the aluminium (bad thing) and with iron it even formed a lump of molten slag twister on top of the metal that periodically broke off and flew out the lid to randomly rain lava on the surrounding grass and equipment, melting holes in plastic buckets etc.
To me that suggests that if you have a 6 inch vent for instance, it would have an area of about 28.25 sq in. You'd want an equivalent area between the top of the crucible and the furnace lid, which given it's greater diameter should be easy to accomplish. Why it's talking about area and not volume, I don't know; it's easy enough to imagine a 6" vent as a 2D circle, but the space between the top of the crucible and the furnace is less intuitive to me. I'm no physicist, but I think this is due to the properties of fluid motion (gas being a fluid) in that you want even flow through the furnace to allow for more heat to get into the melt.
Thank you dtsh; I am in the same boat as you, trying to relate area to a place which is largely about volume, Cheers Charlie
