Click a step. The 3D scene builds up to that step and flies the camera there.
Every choice I made, why, and what I turned down. Confidence is mine, not tested.
Switch the mode with the bottom-bar buttons. The particles show the airflow path for each one.
Keep the four modules, change how you use them. Each module is already two rotors in series (push-pull): that doubles pressure, which is good for pushing through a bend, a pipe and a charcoal bed, but it does not add flow. Four modules in parallel give you the flow.
Depending on module size they put out roughly 1-2x the air a 10 in charcoal bore burns well at full speed. So: one manifold, a plate over unused inlets, and PWM speed control. Do not throttle by pinching the duct: it stalls the fans and wastes pressure.
The change I would make is where the air enters, not the fans: from the bottom via a plenum and buried tunnel, so the fans stay 8 ft away and the pit's depth finally does something useful.
I don't know your exact part numbers. These are per module (two rotors). Read the label and fix them.
Method: fan curve is a straight line from your max flow to your max pressure, scaled for PWM (flow ~ speed, pressure ~ speed squared). System curve = duct loss + tuyere loss + Ergun pressure drop through a lump-charcoal bed. Burn rate comes from stoichiometric air for charcoal (about 8 m3 per kg) with 50% excess. Melt time uses 14% down to ~10% useful efficiency for the charge, plus the heat the dense castable wall soaks up on a cold start (the biggest single cost; a lighter insulating lining would cut it). Treat everything here as +/- 50%, and the "sweet spot" band (37 to 110 CFM per ft2 of bore) as my reasoning from stoichiometry and forge practice, not a published standard.
Read this before you dig, light or pour.