Fan Ducting

[Michael Carter]

I have several fans here that I tried out for airflow. None are marked with a CFM rating. A couple are 115VAC and some are 12VDC.

One of the 12VDC fans put out quite a bit of air so I thought I'de use that one for my cockpit vent. This is a four inch (102mm) fan and I reduced the effective area to 2" using a oil funnel over the fan and cutting a two inch hole in the small end to attach it to the hose.

I think what I did was effectively create half of a venturi because after attaching the funnel and turning on the fan I could feel barely any airflow out of the funnel. Is this the reason? Creating a low pressure area right in front of the opening? If so, where is all of that air going that the fan is putting out?

How can I effectively get the same or faster airflow at a reduced opening size to fit my duct tube? I thought that if I reduced the opening size that the airflow would increase, not really more airflow, but faster airflow.

Any HVAC engineers here? I'm stumped.

[Westozy]

Hi Mike,

PC 5v power supply fans make good vent blowers...just a thought! I have saved a couple for mine.

gwyn

[AndyT]

The problem is that you have actually restricted the amount of airflow that can go thru your ductwork. When you think of moving air, think about water. In order to increase that amount of water thru a pipe, you have to increase the pressure by the ratio of how much you decreased the opening times 2 (Or was it 3?)

So, if you have a 2" fan and reduce it to a 1" duct, you have to increase the power of the fan by 2. Reduction is 2 to 1 (1 x 2 = 2) You need a fan that is twice as powerfull to get the same airflow.

This is also the same reason a Cessna's vents only blow so strong no matter the speed of the plane. The ductwork is designed to restrict the flow to a specific rate. (Cubic Feet per Minute)

I don't remember the exact formula but this should get you in the ballpark.

[Michael Carter]

Thanks guys. I think I found the 'why' of the problem here:

http://ateam.lbl.gov/Design-Guide/DGHtm/basicducthydraulics.htm

Pretty interesting actually.