Not sure why everyone uses axial fans for cooling the extruded filament.
A radial blower and a flex pipe seem much more appropriate for this task.
With the flex pipe the stream of cool air can be precisely directed at the tip of the hotend and a small ( 12V, 125mA ) radial blower creates enough pressure to push the air through the flex pipe.
First tests show very promising results. Now I can maintain 245°C on the hotend with full cooling ( without the head wrapped in kapton tape ). Also the extruded filament seems to be cooled much better than with the standard 40mm fan+nozzle which I used before.
This thing is just the adapter that connects the blower, the printer frame and the first segment of the flex pipe.
For the flex pipe and nozzle please refer to http://www.thingiverse.com/thing:43272 and http://www.thingiverse.com/thing:63366
The blower is fixed in place with glue ( 2K epoxy ). For the flex pipe I tried to integrate the ball joint into this design but I came to the conclusion that it is easier to also use glue to attach the first segment of the flex pipe.