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Minimum distance between fan and obstruction

post #1 of 5
11/11/12 at 1:12pm
Thread Starter 
I have two exhaust fans parallel to my video card to cool the VRM heatsinks I installed. Space is limited, so I would like to have the video card as close to the fan as possible. The video card is water cooled, so there won't be an issue of conflicting airflow patterns. The fans are positioned on either side of the waterblock. Obviously any obstruction will reduce flow, but is there a distance at which an obstruction will begin to drastically decrease the flow?

The fans are http://www.cougar-world.com/products/fans/vortex_pwm.html
Edited by tulanthoar - 11/11/12 at 1:14pm
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post #2 of 5
11/11/12 at 7:46pm
This depends on the strength of the fan motor. If the fan motor was infinitely strong a obstruction would change the pattern of airflow, but not reduce the volume of air flow.

In real life, if you have a obstruction, it pushes the air back towards the fan making the motor work harder....and if it is too close the motor slows down.

Best practice is to keep the fan a few inches away. You can physically hold the fan and see how close you can get it.
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post #3 of 5
11/12/12 at 12:06am
Obstructions effect both intake and exhaust. Fan needs approximately the same flow area as area of fan. 140mm fan is 154sq.cm in area. 140mm fan 40mm from flat surface has 176 sq.cm to draw air from. Over simplified as 140mm fan actually has less area than area of circle.. center hub is not part of airflow through fan.. but the intake / exhaust area doesn't take loss of flow due to things like turning air direction, grille restriction, HDD cage restrictions, etc. so all balances out.
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post #4 of 5
11/12/12 at 12:20pm
Thread Starter 
The fans are drawing from the obstructed side and exhausted to ambient pressure, so I'm hoping that will help flow.

doyll, I'm confused how you made your second calculation. Are you calculating the area of the flow channel? So the fan diameter * distance from obstruction * # of open sides (3 in this situation- the fan is resting on the case)?
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post #5 of 5
11/12/12 at 2:05pm
Kinda.. I call it flow area. The available space for fan to draw air from.

Fan circumference X fan distance from surface = area.of intake


If 1 side of fan is setting on a flat surface - - open on 3 sides: I would use

(1/2 of circumference + diameter) X face of fan to surface measurement (clearance)


Another way of looking at it is you have 1/2 of the circumference open and 1/2 of diameter on each side.

so 1/2 Cir + (2 X 1/2 Dia) X face of fan to surface (clearance)

Hope that makes sense.
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