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Counter-rotation fans

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laslo9

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Jun 20, 2012
I was wondering if anyone has had any experience with COMAIR fans. I just ordered two 120mm x 38mm fans, one that rotates clockwise and the other counter-clockwise to stack. They were relatively inexpensive, $10.95 ea and the specs are impressive:

Maximum Airflow 128.27CFM
Maximum Pressure 0.32inH2O approx 8 mmh2O
Noise Level 43.45dB
Nominal Speed 3000RPM
Operating Input Voltage Range 6V to 13.8V
Power Rating 7.2W

Any thoughts?
 
I guess I'm missing something but what are you trying to accomplish with this? Can't you just flip any pc fan over and have it turning (and blowing) in the other direction? Is this for a CPU cooler?
 
If you stack fans with them both rotating in the same direction it will not increase the airflow as much as one might think, it will, however increase static pressure. Counter-rotating fans blowing in the same direction increase airflow and static pressure. That's what makes the Delta GFB series so desirable, twin motors, counter-rotating blades. A Delta GFB 120mm x 76mm will set you back around $50 plus shipping and are a bit difficult to find. I am hoping to accomplish the same thing with these two COMAIRs. I want to mount these stacked on my Corsair H80.
 
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laslo9 said:
If you stack fans with them both rotating in the same direction it will not increase the airflow as much as one might think, it will, however increase static pressure. Counter-rotating fans blowing in the same direction increase airflow and static pressure. That's what makes the Delta GFB series so desirable, twin motors, counter-rotating blades. A Delta GFB 120mm x 76mm will set you back around $50 plus shipping and are a bit difficult to find. I am hoping to accomplish the same thing with these two COMAIRs. I want to mount these stacked on my Corsair H80.
Click to expand...


YUep, that's the ticket. Hope it works!
 
I guess, if anyone is interested the fans are both COMAIR Rotron's. The clockwise spinner is the GDA1238-12BB, and the counter-clockwise spinner is the MS12K3. The specs are incredibly close with the one exception being the counter spinner uses 11w as opposed the the 7.2w of the clockwise unit. They both are ball bearing with bare power leads (3) so some terminating will have to be done. I will keep you all posted.
 
Here is a quote from an earlier thread:

Originally Posted by omaticrail
All axial fans produce a conical, spiraling airflow in open air. Some fans have static outlet fins to tame this phenomina(sp?); making the flow more cylindrical and straight. A counter-rotating fan takes advantage of that spiraling inertia (wasted energy from the first fan). A properly designed counter rotating blade will look quite different (not just reversed) from the primary fan's blade. The end result is a much higher static pressure, and increased volume.
 
I ordered them from http://www.jameco.com. I got the specs for the GDA1238-12BB from the Jameco site. The GDA1238-12BB is about 10db quieter (considerable). I have a six channel fan controller that I will use to control them. They have a wide operating voltage which is a plus when using a controller. Since the optimum setup (from what I've read) is to have the second fan in the stream running a little slower, I will probably put the MS12K3 in the second spot to try to mitigate the db. If I can get them tuned in it it should be a killer setup...hope... Anyway, the fun is in building it, testing it, and bragging about the results (or not).
 
I just noticed, the GDA1238-12BB is branded a Gryphon, manufactured by Comair.
 
Laslo, have fun playing with the fans. And yes, you should be able to make operable and effective compound fans with what you are playing with. The only reservations I have with that combo is the fact that both fans use rotors with 7 blades, which might make for some resonance problems. The compound fans I have messed with in the past or made such as you are doing have used fans with different blade counts to help combat resonance issues. BTW, wind tunnels also do this too, to counter resonance issues. At least the fans you are using have different rpm ranges. The home made compounds I have made in the past all used 3 blade Delta EFB series fans for the odd rotation fan and 7 blade fans of various makes (Panaflo, Delta FFB and Sanyo Denki) for the regular rotation fans. Wait until you have them bolted together and spun up and see how far away from the fan you will feel coherent airflow. :D
 
laslo9 said:
If you stack fans with them both rotating in the same direction it will not increase the airflow as much as one might think, it will, however increase static pressure. Counter-rotating fans blowing in the same direction increase airflow and static pressure. That's what makes the Delta GFB series so desirable, twin motors, counter-rotating blades. A Delta GFB 120mm x 76mm will set you back around $50 plus shipping and are a bit difficult to find. I am hoping to accomplish the same thing with these two COMAIRs. I want to mount these stacked on my Corsair H80.
Click to expand...


hmmmmmmmmmmmmmmmmmmmmmmm


physics doesn't agree with this.
Stacking fans in series will probably formost, give you a 10% increase in CFM, static pressure increase is true however.
as we are talking about computer cooling, we will be talking a system with high resistance as basical assumption, remember that.
(as a system with low resistance, the aerodynamics can be quite different, but let me not confuse anyone here.)

When you stack fans in Series.
IF rotation is the same, you can actually hurt CFM a little if you are not careful with the spin speed. (Imagine the first fan actually blowing air into the second's blades, and the second is just working with the remainder.) This is not to be confused with fans in 'push/pull config', where they work together to overcome a radiator's air resistance. Here we just talking about stacked fans with nothing in between.

that being say, if you stack fans across radiators with counter rotation, in theory the result will be better than fans spinning same direction.

back to topic and concluding:
according to physics, stacking fans in series will give VERY little CFM boost, and will likely reduce CFM if you don't do your math right. Counter rotation will reduce this 'worsening effect' only. Static pressure increase is evident in both rotation's stacking, with counter spinning giving better static increase.

What is observed in this 'apparent' 10% CFM increase, is just a result of Static pressure's pressence improving the PQ curve. (as in theory, you will never have '0' pressure for max CFM, even in an open tunnel, there will be 'a small amount of pressure' to overcome.
 
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bluezero, thanks for the science! I will be attaching both fans to a controller, each with it's own control so hopefully I will be able to tune it for maximum effect. Unfortunately, my controller doesn't provide any information as to rpm, so I'll be tuning it by "ear". Thanks again for the good info!
 
i did some additional research too.

to improve any CFM boost, it will be to your benefit to add a shroud between the two fans, as it helps the system to have a lower impedence, good luck and post your results. :)
 
Do you think gutting an old 120mm x 25mm and bolting between the two fans would be enough of a shroud?
 
that will be perfect in fact. :)

that's what I do what I need an outlet shroud too.
 
I expect that most of the CFM boost is CFM@pressure, where the CFM is measured through an orifice that causes some pressure buildup. Compound fans do far better in those conditions.
It's also worth noting that a heatsink is an orifice that causes pressure buildup.

I am very curious to see how this goes!
 
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