This is the fan used: Delta AFB0412HB-R00. It has a high flow rate, and high pressure.
The fan attaches to the empty space after removing the second extruder on a DUO.
Cardboard paper line the fan shroud to prevent plastic from melting. Cardboard paper is also thin, so the air can get close to the nozzle.
A torture test on a small part. This is the Bridge to Nowhere. It managed to do it, but it does have a droop underneath on the first layer.