A simple, easy to print (20mins) fan duct for Tevo Tarantula.
(An interchangeable version https://www.thingiverse.com/thing:2346783)
If you print small parts / bridge / overhang, extreme cooling is needed. According to http://www.desiquintans.com/coolingtests2 and http://www.desiquintans.com/coolingtests, blower fan seems like the way to go as it supplies higher air pressure and can be directed better. I chose dual blowers for max cooling and evenly-distributed airflow (surround-style rarely provides even, symmetric air in practice) and medium nozzle open to match the fan's CFM and power. The duct has no right angles or compression spots that slow down the airflow and is directed underneath the hotend to avoid cooling the hotend and heat block.
Printing orientation : Print the duct upside down.
Installation : Just connect the fans in parallel. Combine the +'s and -'s of each fan together to form a single pair of +, -, then connect them to the board as you would with a single fan.
The design snug-fits this blower fan:
Fusion360 file is attached for educational / modification purposes.
Using this duct with 0.2mm layer:
5cm Bridge Torture Test Photo #3 http://www.thingiverse.com/thing:12925/#files.
Cooling Tower Test Photo #4 http://www.thingiverse.com/thing:1015641
Printing tips: For most / big prints where each layer has enough time to cool, fan speed can be set to 50-70% without any noticeable degrade in quality or use the wide-nozzle design. But for small parts / overhang, more cooling just generally gives better results and it's good to have a setup that provides such a power if ever needed. This duct at 100% is very powerful and at lower layers, air will bounce off the bed and partially reach the hotend which could lower the temperature of the hotend. In cura, fan speed can be gradually increased with "regular fan speed at layers" options. Set this to some layers or certain height once there's a good clearance between the bed and hotend. You can also increase the hot end temperature to compensate or run PID tuning with fans on at your desired speed if you really need to run at high speed.
Update 5/25/17: A new platform with a little wider clearance.
Update 3/16/17: Added fillet. Added a wide-nozzle design.
Update 11/5/2016: Increased the distance between two fans to improve the heat sink airflow + A new snug-fit platform.
Update 11/11/2016: Added fusion 360 source file.