Insulation to forge volume ratio?

[Hefty]

Hi all,

I'm making plans for a second forge. My first one is made from a 9kg gas bottle (I think that's about the same size as a US 20lb tank) and I wouldn't say it is too big for my needs, but it's too big for some of the smaller work I do.

I'm looking to make this one intentionally very small. I'm aiming for less than 1 litre or less than 60 cubic inches of volume running off 1 (or 2 turned down) 1/4" burner(/s).

My question is: does the amount of insulation for a smaller forge scale in comparison to volume, the same way burner to volume ratio does? Or is it more about insulation to max temperature? I.E. Will I still need two inches of ceramic fibre outside the castable for this small of a forge, or would 1 inch be sufficient/efficient?

Cheers,

Jono.

[Latticino]

Heat transfer rate through a forge liner does not vary with forge interior volume.  For a given surface area it is proportional to the thickness of the insulation.  However the quantity of heat transfer is proportional to the overall surface area of the liner.  So generally speaking for the same internal temperature, the skin of your forge should be at a temperature that is related to the thickness and type of insulation (not the internal volume), but the overall amount of heat lost will also be related to the exposed surface area.  It is a little more complicated than that, but essentially this is a good theoretical model.

However, the key in this particular case is that you can likely afford to lose more heat in a smaller forge (as your burner will most likely be oversized), so the skin temperature can be relatively higher.  You can't go wrong with 2" of insulation, but if you have enough burner and don't mind the skin temperature being higher, you may not need the full 2".  These are all just rules of thumb in any case

[Hefty]

Thanks Latticino, that's exactly the information I was after! I'll spend the extra for the 2 inches