The blower is something from a furnace -- not a gun burner blower but I don't know just what. It came with an intentional constriction just where the the flange joins it to the obviously cobbled-up outlet pipe. That's been removed.
The air gate has two butterfly valves linked so that when one is open to any degree, the other is closed to the same degree. The motor on the blower is 1/50 HP (!) and it starts slow. I won't want to start and stop it and I don't think I want to try to restrict its airflow. This dual-port gate allows the blower to run unrestricted while adjusting the flow to the forge.
The forge air will go straight through. The diverted air will go out the tee.
I did this on one of Sam Allen's forges where the forge, rheostat and blower were institutional property, maintained (at least in theory) by staff electricians etc. So I didn't want to mess with the blower itself there, either. The students were altenately burning their work of waiting forever for a heat because the steps on the industrial rheostat were just perfectly wrong for small work. The air gate fixed that right up: use the high setting and then adjust with the gate.