In a square or rectangle body forge, multiple burners make the heat distribution more even. I run a NC whisper lowboy for most general forging work. It does nicely with its three small burners.
If building a new forge from scratch, a round chamber and a properly sized blown burner are the best choices. A venturi burner will certainly work, but the later addition of PID control is more of a problem. For most normal size forges, a single burner does fine. ( see below for more on that)
My advice to most all forge builds is to use:
2" of Hi-Z wool,
1/4-3/8" of satanite,
a good coat of ITC-100,
Bubble alumina floor,
blown burner,
PID control
Other really nice add-ons are:
A slide out tong/bar rest - This is simply two pieces of pipe welded/attached on each side of the shell so a "U" shaped slide can be pulled out. The slide end should be at the port level.
A flame minder to reduce the gas flow when there isn't a bar/billet in the forge - This is simply a valve with a by-pass setup that is flipped when you are not heating a bar/billet, so the forge runs at a lower flame setting. Flip it back when you put in the work for the next heat. Over the course of a day, it can save a lot of propane. You can build one or use the commercial types used by welders. For the best setup, use a solenoid, and turn it on and off by a foot switch.
Full PID control with a DC blower. This allows perfectly balanced atmosphere for all settings.
As to one burner vs two or three, I have a different opinion. It isn't just a matter of BTU output. If the burners are individually controlled, properly sized and places, and are well made, multiple burners can be better. There is a reason car engines have six or eight cylinders instead of one big cylinder. It helps even out the work load. On the new cars, it also allows one or more cylinders to be shut down when not needed. A three burner forge could roar at welding heat or purr at HT temps. That would be a lot more difficult with only one big burner.
