Push-pull with dissimilar fans?

[LoneWolf121188]

Can you do push-pull (specifically for a rad in my case, but I'm curious in general) with two different fans running at different speeds? I have a 1200 RPM 180x32mm fan on my rad, and I can't fit another 32mm thick fan but I might be able to fit a 25mm. But the only fan I can find that fits that runs at 700 RPM. Would that cause problems? Would the slower, lower pressure fan somehow bottleneck the airflow?

 

[givmedew]

No it wont bottleneck it. I would recommend that the more powerful fan be the push and the lesser fan be the pull. Also if the lesser fan is almost silent full out you may want to set it to always run full out and have the other fan hooked up to your motherboard.

also

Try to bench the 2. Only thing is that push pull give so little benefit that the difference will fall into margin of error and room temp changes.

 

[Bobnova]

Fans (the vast majority of pumps, really) are quite bad at pulling, you'll almost certainly have better results with the more powerful fan in pull.
Depending on the blade profiles you may or may not get any benefit from the new fan. It probably won't hurt, but it is possible.

 

[macforth]

Bob. interestingly, Martin got a surprise when he viewed some results of testing the push v pull scenario. He was testing shrouds at the time:

First testing: http://martinsliquidlab.i4memory.com/Radiator-Fan-Orientation-And-Shroud-Testing-Review.html

Second testing : http://martinsliquidlab.org/2012/01/15/radiator-shroud-testing-v2/7/

 

[ivanlabrie]

Those are interesting...I liked the whole push fan with shroud thing.

 

[givmedew]

Bob doesn't understand pump, fans, heat etc OR maybe he does but just wants to say exactly the opposite of what I say. I had basically given up trying to help this person the second BOB showed up.

I was just like great here we go again. Bob, you would think that sucking air off the radiator would work better I bet you would even have thought it made a huge difference and was not even worth comparing!

Thanks for posting the martin info it is sad that his site is going down soon. I have read that entire site and have used it multiple times when people made claims like bob. I was once told that you absolutely cant put 2 pumps inline with each other. Should probably check and see if that was bob as well lol. I used martins info to face slap the person who did that to me last time.

-----

OP did you get a chance to try this out? I hope the misinformation did make you have to re set up the fans multiple times.

I don't think you will be able to see much of a change but like someone else mentioned the fans are basically a pump so the other fan will not slow the first fan down and it doesn't matter what order they are. I just recommended the better as push since push is the way to go so obviously you want the best fans in the spot that makes the biggest difference.

 

[LoneWolf121188]

Bob. interestingly, Martin got a surprise when he viewed some results of testing the push v pull scenario. He was testing shrouds at the time:

First testing: http://martinsliquidlab.i4memory.com/Radiator-Fan-Orientation-And-Shroud-Testing-Review.html

Second testing : http://martinsliquidlab.org/2012/01/15/radiator-shroud-testing-v2/7/

Great links, thanks!

 

[Bobnova]

Yup, very good links. They don't address the issue at hand, but they do show interesting push/pull results. I hadn't seen the second one. The first I read back when it came out.

 

[RnRollie]

You can use dissimilar fans.. within reason of course mixing a 5000 Rpm 38 Delta with a slim 600 RPM scythe might not be advisable

If you want to get most out of you push/pull you should try to find a fan which turns in the opposite direction (turbine principle) as your "main" 180x32 ... and those might be very hard to find

 

[LoneWolf121188]

If you want to get most out of you push/pull you should try to find a fan which turns in the opposite direction (turbine principle) as your "main" 180x32 ... and those might be very hard to find

Care to elaborate on that?

 

[wagex]

im no physist or anything but i think it would help being that the slower of the fans can move more air in free air than the higher fan can move through the radiator otherwise it seems it could hinder it

 

[bmwbaxter]

Bob doesn't understand pump, fans, heat etc OR maybe he does but just wants to say exactly the opposite of what I say. I had basically given up trying to help this person the second BOB showed up.

I was just like great here we go again. Bob, you would think that sucking air off the radiator would work better I bet you would even have thought it made a huge difference and was not even worth comparing!
.

Maybe a bit less of the personal attacks?

 

[ivanlabrie]

I second the turbine effect someone mentioned above...It's been tested to increase pressure, but not so much CFM. It helps if the radiator is quite restrictive.

 

[RnRollie]

Care to elaborate on that?

if you put two fans "on top" of each other , the 2nd fan does not add a lot. Its not as if you double the CFM or pressure or performance.

However, if you have one fan turning clockwise and the other anti-clockwise, the performance/pressure goes up by a lot (although still not double of course )

NOTE: "on top" is not to be taken literally... image them to be seperated by a 2" tube or two shrouds.

some interesting references
http://en.wikipedia.org/wiki/Coaxial_rotors
http://en.wikipedia.org/wiki/Contra-rotation

 

[ivanlabrie]

QFT!

There were some Papst fans that spun in the opposite direction most 120mm fans spin...those are handy for this endeavor.

 

[Bobnova]

With push/pull on a radiator or heatsink the fins in the heatsink/radiator will straighten the air out, so counter-rotating fans aren't needed.
If you don't have a straightening device between the fans, counter-rotating fans are needed.

 

[Tech Tweaker]

I've never encountered any real problems from running fans rated at different RPM's or airflow.

I have two different model and brand fans on my socket 939 computer's CPU heatsink, a 2800RPM one for push and a 2000RPM one for pull. I see no decrease in performance because of the different fans. Actually, it performs a little better than it did back when I was using a lower-rpm fan for intake, that's mostly due to having more airflow though and not necessarily due to having the pull/exhaust fan spinning at a slower speed.

I originally had two matching fans, but when I saw this other fan that had more airflow, and higher rpm, but about the same noise-level I jumped on that and upgraded the push/intake fan while leaving the pull/exhaust fan the same.

 

[ivanlabrie]

Good point...you don't see heatsink arrays inside them turbines for a reason eh?
Sadly you do see birds in there at times

 

[Bobnova]

Turbines actually do have similar structures inside 'em, they're a big stack. Moving blades, stationary blades (pointed the other direction), moving blades, stationary blades, etc.
All the moving blades turn the same direction, but the stationary blades kill the rotation of the air between each rotor.

EDIT:
Here's a lovely picture from wikipedia: http://upload.wikimedia.org/wikipedia/commons/5/56/J85_ge_17a_turbojet_engine.jpg

 

[ivanlabrie]

Interesting and way off topic

Hey, wait a minute! You could always use push pull plane turbines in your heatsink!