Crucible size vs furnace diameter?

[cds333]

I am trying to decide between two possible shells for a furnace build. One would be a standard grill-size propane tank and the other would be a slightly smaller green freon tank.

I would be using a #3 or #4 crucible which would fit better in the smaller tank, however I think I would prefer the heavier gauge metal of the propane tank as it is easier to weld handles and hinges to.

 

If I were to go with the larger propane tank, how much efficiency would be lost by having a larger amount of air space between the kaowool and the crucible?

What are the implications of having such a design? i.e. a small crucible relative to the diameter of the furnace?

Thank you!

[ThomasPowers]

Is this a propane, natural gas, charcoal or coke fueled furnace?

[cds333]

Propane with a single 3/4" venturi burner 

[ThomasPowers]

Have you thought of adding another layer of insulation to cut down the interior size?  You do want enough of a gap to get a good swirl around the crucible to transfer heat. 

[cds333]

I had considered that but that would double the cost of the most expensive consumable.

Would doubling the kaowool actually give a significant increase in heat retention efficiency such that the costs would balance out?

Also is there a known preferred gap between insulation and crucible?

Thank you!

[ThomasPowers]

Consumable yes; but as you add it in layers you just replace the inmost layer as needed.   There is probably a documented good gap; but I don't know it.  Anything in the backyardmetalcasting.com  archives? (I heat my crucibles with coke where you want the fuel in contact with the crucible.)