push/pull configuration, how about pull/pull?

strangesnow · Post by **strangesnow** » Tue Oct 09, 2007 7:17 am

I know there is lots of debate about a push fan-rad-pull fan configuration and whether or not that is any good versus just a straight rad-pull fan. I was wondering if anyone has done a build where you have rad-pull-pull and whether or not that makes a difference? I don't have the tools to figure that out, aside from just a general temps at idle/load to see the difference.

I am using a coolit freezone and have turned off the fan and instead put a 120mm fan on the outside of the case to pull air. Since I've got one, I'm wondering if 2 fans in series pulling makes a difference (of course, keeping with acceptable noise)?

andyb · Post by **andyb** » Tue Oct 09, 2007 8:20 am

I used to have a pull-pull setup using 80mm case fans some time ago (not silent), it created a massive throughput of air compared to a single identical fan, the pressure is also much higher as well, as the fans are aiding each other, I woyuld guess 50%+ more throughput than a single fan.

This is not a reccomended setup for silence, as you will get lots of turbulance noise, and its not good for the fans as they will accelerate each other (like overvolting), but if you use 2 fans in series in this way that are undervolted this is a non-issue. As far as turbulance noise is concerned moving them apart will make a difference, you would need a tunnel with space between each fan (and ideally the rad).

Rads however usually need lots of pressure, I think only experimentation will help you with your answer, but a single high pressure (38mm deep fan) should be looked into.

Andy

Felger Carbon · Post by **Felger Carbon** » Tue Oct 09, 2007 9:29 am

Andy, just one week ago, I tried the 2-stage fan experiment. Just as you say, you do double the pressure but you get more turbulence from the two fans interacting (rather, the blades interacting) as the RPMs are slightly different. So I can confirm everything you say.

But a new thought occurred to me... what if the two fan's RPMs were identical, so the two sets of blades rotated in lockstep? This should eliminate (or minimize) the additional turbulence.

Impossible? Uh-uh. The magic phrase is "phase locked loop". During my working engineering career that was one of my specialties. The tach signal provided by each fan can be input into a PLL, and the output would modulate the +V of one of the fans to keep the rotation in sync.

I don't see a commercial use for this, or even a significant DIY demand, but it's definitely possible. You would need to use fans with high-quality bearings, such as the Enermax Enlobal Marathon fans, or possibly the GW NCB fans.

strangesnow · Post by **strangesnow** » Tue Oct 09, 2007 9:49 am

all quite interesting... i have two noctua NF-S12-800 that i would love to try this experment on. I understand in concept your idea to sync the fan but wouldn't it be extremely difficult? is it just that the rpm needs to stay constant or that the fan blades need to be perfectly in sync?

Felger Carbon · Post by **Felger Carbon** » Tue Oct 09, 2007 12:09 pm

strangesnow wrote:...or that the fan blades need to be perfectly in sync?

Just the blades in sync. Not sure exactly how perfectly is needed. Possibly something like +/- 3 degrees or less?

strangesnow · Post by **strangesnow** » Tue Oct 09, 2007 2:07 pm

as i think about the electronics and calibration required, probably the same effect could be had with a duct between the fans...

Felger Carbon · Post by **Felger Carbon** » Tue Oct 09, 2007 2:55 pm

strangesnow wrote:...probably the same effect could be had with a duct between the fans...

Long duct. I wound up using 1" double-sided tape to tape the fans with a separation of .6", case-to-case. Still lotsa extra turbulence, though not as much as with zero separation. Got room for a long duct?

strangesnow · Post by **strangesnow** » Tue Oct 09, 2007 4:15 pm

i've put this fan on the outside of the case already... is that 0.6inches you said? i could probably duct even more than that. The back of the case already looks funny with a fan sticking out so a little bit more can't hurt! Will try that out and let you all know.

Alternatively, i am thinking of putting a fan on the other side of the rad and see if a push/pull helps with temps and noise. anyone have experience about whether or not both fans should be the same cfm?

andyb · Post by **andyb** » Tue Oct 09, 2007 6:42 pm

But a new thought occurred to me... what if the two fan's RPMs were identical, so the two sets of blades rotated in lockstep? This should eliminate (or minimize) the additional turbulence.

Wouldnt you have a high chance of "beating" noises.?

A few questions / nutty ideas.

Would using 2 different fan designs, one 7 blade and one 9 blade for example, reduce or increase the turbulance.?

Would using a fan in the airflow exit position that has much steper blades (noctua) overcome this as the airflow path is more linear.?

Another solution would be to create a "V" or an "A"shaped duct to use 2 fans "side-by-side" at an angle from each other both blowing out or in through the same hole (radiator) do the same job without the turbulance problems, or would the lack of individual fan pressure still be the problem with overall airflow through the radiator even though the quantity of fans has doubled. I would guess that 2 fans side by side "sucking" through the radiator would have better overall airflow than 2 fans "blowing" using the same setup, would you concur.

Andy

ddrueding1 · Post by **ddrueding1** » Tue Oct 09, 2007 8:46 pm

Am I the only one that instantly thought of taking the blades/hub off a second fan and attaching them to the hub of the first? Using the motor from just one to drive 2 sets of blades?

strangesnow · Post by **strangesnow** » Tue Oct 09, 2007 9:24 pm

ddrueding1 wrote:Am I the only one that instantly thought of taking the blades/hub off a second fan and attaching them to the hub of the first? Using the motor from just one to drive 2 sets of blades?

I wonder how the RPM would get affected... maybe just like an undervolt?

strangesnow · Post by **strangesnow** » Tue Oct 09, 2007 9:26 pm

andyb wrote:
But a new thought occurred to me... what if the two fan's RPMs were identical, so the two sets of blades rotated in lockstep? This should eliminate (or minimize) the additional turbulence.
Wouldnt you have a high chance of "beating" noises.?

A few questions / nutty ideas.

Would using 2 different fan designs, one 7 blade and one 9 blade for example, reduce or increase the turbulance.?

Would using a fan in the airflow exit position that has much steper blades (noctua) overcome this as the airflow path is more linear.?

Another solution would be to create a "V" or an "A"shaped duct to use 2 fans "side-by-side" at an angle from each other both blowing out or in through the same hole (radiator) do the same job without the turbulance problems, or would the lack of individual fan pressure still be the problem with overall airflow through the radiator even though the quantity of fans has doubled. I would guess that 2 fans side by side "sucking" through the radiator would have better overall airflow than 2 fans "blowing" using the same setup, would you concur.

Andy

I like the idea of a Y duct altho that starts to look quite like a large protrusion out of the back of the case! I am curious about the performance... only one way to find out i guess which is to try!

jaganath · Post by **jaganath** » Wed Oct 10, 2007 12:44 am

ddrueding1 wrote:Am I the only one that instantly thought of taking the blades/hub off a second fan and attaching them to the hub of the first? Using the motor from just one to drive 2 sets of blades?

the starting torque would be much higher,might burn the motor out....but it has the advantage that the blades are always perfectly in phase.

Felger Carbon · Post by **Felger Carbon** » Wed Oct 10, 2007 1:06 am

ddrueding1 wrote:Am I the only one that instantly thought of taking the blades/hub off a second fan and attaching them to the hub of the first? Using the motor from just one to drive 2 sets of blades?

There is the small matter of the (usually 4) supporting spokes that center the hubs. This is what is known in engineering circles as a "physical conflict".

andyb wrote:Another solution would be to create a "V" or an "A"shaped duct to use 2 fans "side-by-side" at an angle from each other both blowing out or in through the same hole (radiator)...

Each fan of area A is trying to force air thru a space whose area is A/2. This makes for a serious "lack of pressure" problem. So now you need to turn each of your side-by-side fans into dual series fans to get more pressure, placing us back where we started except with 4 fans (if I'm keeping count correctly).

Felger Carbon · Post by **Felger Carbon** » Wed Oct 10, 2007 1:32 am

I propose a thought experiment: take a closed box. On one side, mount a 120mm fan, cutting the usual hole for said fan. On the other side, cut a 1/8" diameter exhaust hole. Would the usual ~40CFM produced by the 120mm fan be forced thru the small hole? Obviously not.

Now mount a second 120mm fan beside the first. We now potentially have 80CFM! But the leave the 1/8" outlet hole unchanged. How much more air would now be forced out of the small hole? None at all; the problem is that there isn't enough pressure and adding a second parallel fan doesn't increase the pressure at all.

Parallel fans make a great deal of sense when not much pressure is needed to maintain airflow. There's a sticky on this subject near the top of this forum...

andyb · Post by **andyb** » Wed Oct 10, 2007 3:43 am

Now mount a second 120mm fan beside the first. We now potentially have 80CFM! But the leave the 1/8" outlet hole unchanged. How much more air would now be forced out of the small hole? None at all; the problem is that there isn't enough pressure and adding a second parallel fan doesn't increase the pressure at all.

This is pretty much as I expected, there is no balance between in and out in terms of physical space for the air to travel through, this is (in my opinion) one of the biggest problems with the P180 if your design is to use front to back airflow and you seal off the vents on the back, there are 2 fans sucking air through one hole.

Andy

ddrueding1 · Post by **ddrueding1** » Wed Oct 10, 2007 7:23 am

Felger Carbon wrote:
ddrueding1 wrote:Am I the only one that instantly thought of taking the blades/hub off a second fan and attaching them to the hub of the first? Using the motor from just one to drive 2 sets of blades?
There is the small matter of the (usually 4) supporting spokes that center the hubs. This is what is known in engineering circles as a "physical conflict".

The 4 supporting posts are only on one side. All you would have to do is attach the blades on the other side.

ddrueding1 · Post by **ddrueding1** » Wed Oct 10, 2007 7:59 am

I should stop talking and just build one...

(magic of television)

...done!

Used Vantec Stealth fans because I don't mind destroying them. All I did was pull the hub (including magnet) off the fan and krazyglue it to the hub of the other. Some very rough alignment of the axis by hand led to a not-entirely stable, but operational fan.

Image 1: Side View

Image 2: Top View

The image where it is running was aborted due to a blood sacrifice; those spinning blades (with newly increased mass) made 5 lovely gashes through my finger before I could get clear.

*Playing with my camera skills: Each image is 5 pictures spaced 1EV apart and merged using Photoshop's "Merge to HDR" feature, then compressed to 8-bit color using local adaptation.

Building fan: 2 minutes
Fooling with pictures: 15 minutes.

strangesnow · Post by **strangesnow** » Wed Oct 10, 2007 11:50 am

ddrueding1 wrote:I should stop talking and just build one...

(magic of television)

...done!

nice work!

to prevent your gashes, you need to cut off the supporting struts and reattach that frame (makes for better flow anyway).

so is the fan noisy? hows the flow?

strangesnow · Post by **strangesnow** » Wed Oct 10, 2007 11:52 am

also, a question for someone who knows... do you align the fan blades on top of each other (looking straight down and you won't see the other blade) or do you mount it so that it's almost like one long blade (the edge of one blade matches the beginning edge of the other blade?)

ddrueding1 · Post by **ddrueding1** » Wed Oct 10, 2007 3:18 pm

It sounds a lot like a single fan, except for the misalignment and rumbling due to it. Air movement is about the same as a single fan, but this is in free air.