Insulation

[MRA]

Hi again folks

My 13kg butane-bottle furnace is coming along. I'm thinking about insulation, now I think I know where I'm going with the home-brew refractory lining (see 'Grog').

I don't have much room - 11.5"OD, and I want an 8-ish" ID - so for 1-1.5" thick refractory lining, there's not much space for insulation.

BUT - it struck me that for most insulation (hot tank jacket, rockwool in your loft, clothes) we're told that the main insulating agent is _air_ - since it is a poor conductor of heat. The 'stuff' - fibreglass, rockwool or cotton / wool etc in the above examples - is apparently there to prevent movement of air from carrying heat away by convection, and is normally a better conductor of heat than the air it is trying to hold still around the insulated object. This principle is clearly (ahem) illustrated in double glazing, where the necessary absence of 'stuff' between the panes is tolerable, so long as the air gap is narrow enough to (mostly) inhibit convection currents...I'm told...

So - in a furnace with a small insulating air gap between lining and outer skin - why put any stuff in there at all? Am I missing something (very possible - polite suggestions please ).

cheers

Mark, Manchester, UK

 

[chipenter]

I made mine form a propane bottle but with six inch centre hole , gives a bought four inch refectory when its working at red heat the outside is just warm , cool enough to pick up without gloves .