I changed up the original fan adapter from http://repables.com/r/212/ to take a BD5015 radial fan because the original 30x30x10 fan is not available in Europe. I strongly recommend using radial fans when you are using a fan duct, because a radial fan can exert a much higher static pressure than an axial fan. So in return the airflow will be higher, even if an axial fan has a higher nominal air flow. I have added the datasheet for characteristic curves . With the fan at full, the hot end only reaches about 205°C so in fact one of those fans will be more than sufficient. I print ABS at 18% fan speed to avoid shrinkage, while still having good layer adhesion. PLA at about 70%, thats with 3 fans.Print with ABS on Kapton at 85°C bed and 235°C hot end. Put original fan to about 30% after reaching 2mm hight.