gas forge construction advice sought

[Salem Straub]

Let me preface this by saying that I have built several forges, most of which have had a 2" kao-wool liner and one or more venturi burners. I like this combination for fast heat and economy of construction cost, as well as good welding heat and gas economy. However, the first propane forge I ever built had a castable refractory shell with venturi. I stopped using it altogether once I got another, better one built, and it now gathers dust. I did not like it because it took forever to achieve welding heat (only 1" of castable inside an 8" well casing) and the burner was poorly designed.

Recently, I got that old first forge back out with the idea of building a vertical welding forge out of it. I rebuilt the burner, but now I'm faced with this dilemma: I'm going to add 1" of Kao-wool, should I add it inside or outside the castable? With it inside, I would have the quick heating of kaowool and the fuel economy I associate with that, but the heat chamber would be fairly small, and the liner would suffer eventually from flux. With the blanket added outside, I would have a larger heat chamber, and a more durable interior, but I am only afraid that it would still cost me a fortune in gas to weld much in. Is this unfounded? (I have rebuilt and streamlined the venturi burner, smaller orifice now, better mix.) Oh, and the inside of the castable is already coated with ITC-100 from when I used to use it.

I appreciate any thoughts you all might have on this.

 

[okzj]

Won't make any difference. conduction losses are linear. Put it on the outside so you can utilize the high temp ITC-100 and castable where the high temps are going to be.

If you put it on the inside, you would get it hot quicker simply because you're burning the same amount of gas in a smaller space, but you need space inside.