CPU cooling - one fan vs. two?

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I have a fairly large aftermarket cooler (Xigmatek Gaia) for my i5-2500k processor. It shipped with one fan (pushing air into the cooler) but has the ability to put a 2nd fan on it (push-pull configuration). I believe the fan is starting to fail, so I'm going to replace it.

My question is, should I go ahead and get two fans for it? I know in cooling the thought is "more is better" but I'm having a hard time believing that two fans blowing an equal volume of air would somehow cool better than just the single fan. My attempts to Google the question were stymied by large amounts of marketing with little info about actual effectiveness, especially in the 1 vs. 2 area. Anyone out there have some know-how about it, especially with regards to the physics of cooling/airflow?

For the record, I am not overclocking now because I have concerns about the fan, but I have in the past and probably will in the future.

techturtle

Posted 2012-09-11T00:51:07.733

Reputation: 8 059

why would you believe the two fans would move the same amount of air ? Surely the only way that'd be true is if the second fan did no work at all ? – Sirex – 2012-09-11T01:07:26.710

2A single low-torque, low-speed fan (e.g. the super-quiet variety) might benefit from the addition of a "puller" fan on the other side of the heatsink. "More is better" - False, more fans are not better, especially if all you are doing is churning the air within the case. There are three key issues: providing adequate intakes for cool air, providing adequate outlets for hot air, and preventing dead zones where hot air might collect. Whether the inlets and/or outlets use forced convection (i.e. fans) is up to you. – sawdust – 2012-09-11T01:13:48.113

Whatever it takes. – Daniel R Hicks – 2012-09-11T01:31:08.840

@Sirex Here's my view: The fan I'm looking at moves ~70cfm. If I put one of these on, it will push 70cfm into and across the cooling fins. If I now put a duplicate fan pulling from the other side, it's going to pull that same 70cfm that the first fan pushed into the fins. It would have more power (i.e., if the air were suddenly thicker and harder to move) but would not pull a greater amount of air or move it any faster. No? – techturtle – 2012-09-11T03:38:17.083

the way i see it, that 70cfm of air is now already moving, so the second fan can use it's own power to spin at a higher rpm and accelerate that air faster. Assuming these two fans aren't on a linked rpm. If they are then yeah, you'll gain nothing. sawdust makes a good point though, if the air isn't then directed out of the case, there's little purpose in this. – Sirex – 2012-09-11T03:49:24.353

Not on a linked RPM, that I know. I can see the pressure applied by the push fan allowing the pull fan to spin a little faster, and vice versa, but I've got to think that the RPM listed by the manufacturer is pretty close, which means at max RPM the fans would both go is the same either way. My case has pretty good airflow, so I'm not too concerned about that at the moment.

– techturtle – 2012-09-11T12:59:20.463

Not directly applicable, but adding an in-blowing fan to a case where there's an out-blowing fan (or vice-versa) can make cooling worse. With only one fan the airflow from the fan will spread out, but with two fans it will tend to follow the shortest path from fan A to fan B. – Daniel R Hicks – 2012-09-11T18:56:51.920

Answers

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Since no one was able to provide a concrete answer, I took this to physicsforums.com and asked them. There was some debate there as well, but the end result I got was that two fans definitely can help. The reason is that the second fan increases the pressure along the length of the radiator. One of the regulars there stated this:

The general rule of thumb is that fans in series double the pressure at the same flow and fans in parallel double the flow at the same pressure.

Basically, if the radiator were a straight, frictionless tube with no gaps, then this rule would apply exactly and there would be no benefit to a second fan. However, in the real world, the radiator is far from perfect. There are blockages on the edge of the fins, friction as it flows along the fins, and lots of places for the air to leak out along the way. What the second fan actually accomplishes is not increasing the overall airflow, but making sure that more of the air flows smoothly along the entire thickness of the radiator.

The increased pressure would allow it to keep the speed up by overcoming some of the resistance of friction. That small volume of air that would have hit the first fin and blown out sideways will now be more likely to stay within the radiator, continuing to gather heat along the entire fin. This explains why hardware/overclocking sites and manufacturers report modest, but not phenomenal, gains by adding a second fan in push-pull configuration.

Thanks to everyone here and at Physics Forums who weighed in. It was an enlightening discussion.

(I did go with two fans because I found a great sale on them.)

techturtle

Posted 2012-09-11T00:51:07.733

Reputation: 8 059

2I don't believe pressure will be doubled. though it will be slightly increased. – Fennekin – 2017-05-13T06:28:31.253

Asked: 2012-09-11T00:51:07.733

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Active: 2016-10-20T18:25:07.023