I always recommend to place burner at a tangent. It can be a bit tricky to image what the interior radius will be once the lining is in place, but if you can place burners in such a manner that the flames will follow then interior contour, you'll realize more even heat, and far fewer cold spots.....
Below is my "general purpose" horizontal forge and how the single burner is directing the flame to following the interior contour....
The next image is the welding for I built last year..... it's a vertical design, but it also has the single burner positioned so the flame will following the interior contour... (you can see the burner holder at the bottom of the image)
And finally, a word on burners. I am a firm believer a SINGLE, properly sized/tuned burner is a much better option then multiple burners in a forge. The biggest issue with multiple burners in a forge is getting them "tuned" to burn equally.....almost never happens. This is mainly due to the fact that most design/build multiple burner forges with the fuel coming from a single source, then they "T" it to each burner...... this ALWAYS feeds more fuel to one burner than the other(s). The only real way to make multiple burner forges work well (read that to mean make the burners run evenly), is to treat each as a single burner setup..... having an independent fuel source for each burner, a regulator at each source, and then adjust each burner independently..... which is a real hassle and time waster. It's far simpler to design/build/use a SINGLE burner, sized properly for the size forge you intend to use it in.
