Quote Originally Posted by Boogerlad View Post
how could you make a counter rotating fan? can you buy one that's relatively quiet? they seem interesting.
Can't make them....the post below is a very good description of what it is.
Quote Originally Posted by delewin View Post
Thank you Vapor for your reply.

Can you give me some examples of "counter rotating fans, as I have not seen this in a PC application?

It has been used in propellor aircraft, such as the old "fairey Gannet"



Are you describing a situation like:

Fan 1 rotates clockwise and pushes air in direction A.

Fan 2 rotates anticlockwise and pushes air in the same direction as fan 1.

When you put Fan 2 in front of Fan 1, the blades spin in opposite directions, but the air flow is in the same direction.

This leads on the the next questions:

As all these fans are DC voltage, can you simply reverse the (wires) voltage and turn them around to achieve the counter rotating effect that you describe?

As the "pushing" surface would now be the back of the blade, the shape of the blade would have a big impact on the performance / suitability. That is, a strait, flat blade would be the most suitable for this type of modification.

In the group of fans that you tested, which ones use a strait blade or in your opinion, would be suitable for the reversing modification?

The other question follows on. These fans are originally designed to rotate in one direction. What adverse effect on lubrication / life cycle would this modification create?

If Fan A is on one side of the radiator and Fan B is on the other, can you explain why it would be advantageous to have Fan B counter rotating to Fan A?

Thank you for your informed response.

Regards,

David
Yes, that's exactly what counter-rotating is One fan spins one way, the other in the other direction, but all moving air in the same direction. Delta's 120x120x76mm beast is a counter-rotating pair. Delta's tri-blade EFB's spin the 'wrong way' and can be paired with a regular fan for a 'super fan.'

You can't reverse the wires...the fan just won't spin (unless it's really, really old or cheap). They have control PCBs that prevent that kind of thing. Even if you could, no blade design is particularly good for reverse direction, IMO.

Why it's advantageous has to do with the fact that fans spin the air as much as they push the air. When you pair it with a fan that spins in the same direction, it can't do too much with the air other than add a little velocity in both the forward and spinning direction.

However, when you pair fans that spin in the opposite direction, one fan is forcing air into the other at a greater airspeed at the blades, and the counter-rotating fan will deflect the 'spin' into forward velocity...

Basically, think of it as a playground swing. Your daughter is swinging on it but she's not very good at leg pumping because she's 4....so you push her. You normally stand behind her and beginning pushing her just as she gets to the top--you get more leverage. That's what a counter-rotating pair essentially is. It's the ideal situation...

A like-rotating pair is like standing between the swings and as she reaches the bottom (on the way forward) you give her a push, but because she's already moving quickly in the same direction, it's basically a little extra nudge--there's not a whole lot you can add.

Putting a radiator between the fans mostly eliminates the 'spin' of the air so either way you're only dealing with the forward velocity, regardless of which fan you use, you'll get the same result. But if you want to put fans only on one side, counter-rotating is very effective if you have the right fans.

I tried a Sharkoon 2000 + Tri-Blade LE pair and had very impressive results....they both push like 72ish CFM IIRC in the open air, so they were a good pair. I was pushing nearly 100CFM in open air with the pair, and with not much more noise (3dBA since I doubled fans of the same amount of noise). On the radiator, I was pushing a whopping 90CFM or so...it was a tiny amount of loss. Most 100CFM fans drop down to around 75CFM...