Some furnace rules of thumb

 

dennis — Today at 9:34 AM

I recall seeing those “rules of the fingers” on “the home foundry”. What I recall is that they all relate to the bore of the furnace. One is that your exhaust needs to be between .25 and .33 of the bore. Another is your tuyere - where the burner goes - is a bit smaller than your exhaust. How much smaller - good question.
The rules are most crucial when one wants or needs maximal performance, e.g. high temperatures. (This might well explain why aluminum is easier to melt than say, bronze.)

[9:37 AM]

Another I recall is that oil wants more room than gas. Propane, say, can burn readily with an inch per side, though more is wise if you need it for your tongs. Oil wishes at least twice that.