For general forging things to shape the vertical design or any wool lined forge is pretty good, I have been considering building one just for those times that I need to quickly fire up a forge and do some quick shaping.
However, for welding I never understood what anybody saw in that design, yes it is cheap to build but if you have to replace wool everytime you use it, it seems pretty darned expensive to me. When I was forced to weld in such forges when away from home they eventually got tolerable after so much cement, satanite and castible refractory replaced the damaged wool that they were behavbing like a solid forge. Also I have found that for me the most even distribution of heat I can have is best for welding, and the vertical design either has all the heat coming up from below or, if it attempts to use the vortex, has a hot spot just inside each door. Once again all of these issues can be a benefit in forging, but tend to hinder me in welding.
It is no secret that I have a great fondness for the horizontal cylindrical design. The billet passes right down the center of a flame vortex that heats very evenly. Cast refractory takes longer to heat up but then holds that thermal mass and throws it back at the work from all directions. If I had the time to cast my own shell I would get some high alumina refractory cement and cast the floor from it and some regular refractory for the top. This is how I made my two previous shells and I am only on the third shell in all the years I have been forging. The second shell was not dead,and still sits in my back room, I just thought I would give another shell a try. I would get a thick build up of slag and flux in the bottom and when it got to around 1" thick I would use my time after one heating session to hammer and chissel the crud out and discard it. This always left me with a clean floor of the orignal refractory. Yes wool is cheaper to buy, but I spent around $200 on materials and went around 10 years on it, so one completely beats the other in overall savings.
Here is another tip for you- avoid the tendency to go massive with the solid refractory. It just takes longer to heat up, cracks up worse and wastes material. A cylinder with 1-1/4" to 1-1/2" walls is more than enough. Wrap it up with wool on the outside to protect and insulate it and band it down so pressure keeps the arc principle going and they are virtually indestructable.
