Expert: Paul Soderman - 9/13/2006

Question
Hello,
I'm working on a project that involves moving air through a 25mm diameter pipe with a 75mm length.
The fan would be sited at one end inside the tube and would be rotated with a piece of string (in the same way a giroscope would be charged) so RPM would be low.
The more air pulled through the tube the better. Can you tell me please what kind of fan or screw design might be best suited to pull the maximum amount of air under these conditions.

Many thanks for your time,

B. Stone


Answer
Bear
I would buy a commercial fan used for electronic cooling of that diameter and modify it to work in your system.  Course you are operating in uncharted waters because all fans at that size operate at high rpm.  The performance at low speed is marginal but positive.  The performance curves are usually available from the manufacturer, but probably don't go down to your speeds.

Fans of that type have simple rectangular blade shapes.  You might do better with paddle shaped blades found in most room fans, but they are large so you would have to fabricate your own blades.  And the performance would be unknown until calibrated.  I think the electronic cooling fan is your best bet.  They are cheap.
Paul