Of Push/Pull and CFM Mathematics

Strafe · 06-10-2002, 09:12 AM

I could have sworn a while back the general consensus was that the push/pull method of putting fans on a radiator was not recommended as it would just create turbulence. Lately I have seen more people using this method and recommending it. So what's the current recommendation?

On to the math... I've got a BI-X, and HWLabs recommends using a 100CFM fan on it. If i was to push/pull would I be able to use 2 less powerful fans? I'm not sure about the math but it would seem to me that the CFM would remain the same but the static pressure would increase. Am I right?

BTW, I request we sticky a FAQ to the to of this forum with answers to questions like this...

bigben2k · 06-10-2002, 09:56 AM

There is no general consensus, just a lot of talk... this is a forum after all! (so no FAQ!)

The push-pull setup is good/bad:

it's good if you're trying to achieve a higher flow rate (of air) through a rad that has a lot of resistance. Both fans should be similar, and the idea is that the second fan helps the first one achieve its rated cfm at no load.

It's bad if there is little or no air flow resistance, because the second fan can't help the first one achieve its maximum air flow rate, as it's almost already there. The second fan only draws power, with little or no benefit.

Overall, the push-pull is good, if it's setup properly, and will be quieter than using a single more powerful fan, but not as energy efficient.

The other debated aspect is running fans side by side, and we've had that discussion already: see the thread about dual 120mms.

ambient · 06-11-2002, 09:12 PM

So then the stronger fan should be pulling and the weaker pushing? If the fans weren't rated the same. Say like on fan is rated for 75cfm and the other 100cfm.

Brad · 06-12-2002, 02:32 AM

if you've got enough room to have dual fans, then why not just go for it, I doubt you'll ever find a situation where they decrease performance.

I'm not so sure about running two differant fans, I can't see why you would want to though

bigben2k · 06-12-2002, 08:02 AM

Quote:

Originally posted by ambient
So then the stronger fan should be pulling and the weaker pushing? If the fans weren't rated the same. Say like on fan is rated for 75cfm and the other 100cfm.

There's no perfect setup... The more powerful fan will be restricted by the less powerfull fan, so your actual flow rate will be closer to the weaker fans maximum spec, unless the stronger fan is WAY more powerful, in which case it'd blow right through the weaker one.

You've got to keep in mind an objective here: are you trying to run silent, or to maximize air flow? I gotta ask because if you don't care either way, then let's talk about a blower!

Sirpent · 06-13-2002, 04:08 PM

I think what we are discussing here is how to achieve the best cooling effect with a fixed cap on the noise level, or how to achieve the required cooling effect producing as little noise as possible (no real difference between the two formulations). The noise is mostly caused by turbulence in the air passing through the radiator, not by friction in the bearings of the fans (assuming the fans are decent). Now, the level of this turbulence noise is a function of the air flow through the radiator (we ignore shrouds and other things that can decrease/increase turbulence - their effects for one and two fan configurations are roughly the same). The cooling effect is also a function of the air flow through the radiator. The bottom line: there is no difference between using a single fan and a push-pull arrangement, same noise level = same cooling effect.

chazz469 · 06-13-2002, 05:50 PM

Quote:

Originally posted by Sirpent
The bottom line: there is no difference between using a single fan and a push-pull arrangement, same noise level = same cooling effect.

I beg to differ. I have the push/pull setup on my rad right now. When removing the "push fan", my temp raises about 3°C, put the fan back on and it drops back down again. Some may see this as negligble, but every little bit helps. I'm using the DD Cooler Cube, which has 30-some-odd fins within a 4 sq. in. area, so it's pretty dense. And I think the success or failure of the use of a 2-fan method, would totally depend on your RAD.

Sirpent · 06-14-2002, 10:48 AM

Quote:

When removing the "push fan", my temp raises about 3°C

No surprise. But the noise level decreases.

06-10-2002, 09:12 AM	#1
Strafe Cooling Neophyte Join Date: Sep 2001 Location: Chiba City Posts: 62	Of Push/Pull and CFM Mathematics I could have sworn a while back the general consensus was that the push/pull method of putting fans on a radiator was not recommended as it would just create turbulence. Lately I have seen more people using this method and recommending it. So what's the current recommendation? On to the math... I've got a BI-X, and HWLabs recommends using a 100CFM fan on it. If i was to push/pull would I be able to use 2 less powerful fans? I'm not sure about the math but it would seem to me that the CFM would remain the same but the static pressure would increase. Am I right? BTW, I request we sticky a FAQ to the to of this forum with answers to questions like this... Last edited by Strafe; 06-10-2002 at 09:14 AM.

06-10-2002, 09:56 AM	#2
bigben2k Responsible for 2% of all the posts here. Join Date: May 2002 Location: Texas, U.S.A. Posts: 8,302	There is no general consensus, just a lot of talk... this is a forum after all! (so no FAQ!) The push-pull setup is good/bad: it's good if you're trying to achieve a higher flow rate (of air) through a rad that has a lot of resistance. Both fans should be similar, and the idea is that the second fan helps the first one achieve its rated cfm at no load. It's bad if there is little or no air flow resistance, because the second fan can't help the first one achieve its maximum air flow rate, as it's almost already there. The second fan only draws power, with little or no benefit. Overall, the push-pull is good, if it's setup properly, and will be quieter than using a single more powerful fan, but not as energy efficient. The other debated aspect is running fans side by side, and we've had that discussion already: see the thread about dual 120mms. __________________ WBTA (Water Block Testing Alliance) Nordic Hardware WC 101

06-11-2002, 09:12 PM	#3
ambient Cooling Savant Join Date: Jan 2002 Location: Planet Earth Posts: 312	So then the stronger fan should be pulling and the weaker pushing? If the fans weren't rated the same. Say like on fan is rated for 75cfm and the other 100cfm. __________________ water cooled 1.4 tbird @ 1580 143x11 vcore 1.88 vio 3.55

06-12-2002, 02:32 AM	#4
Brad Thermophile Join Date: Oct 2001 Location: Nuu Zeeelin Posts: 3,175	if you've got enough room to have dual fans, then why not just go for it, I doubt you'll ever find a situation where they decrease performance. I'm not so sure about running two differant fans, I can't see why you would want to though __________________ 2x P3 1100's at 1400, Abit VP6, 2x Corsair 256mb PC150 sticks, 20gb 'cuda ATA-III, 2x 40gb 'cuda ATA-IV in raid 0. 20" Trinitron. No fans 2x 2400+ at 2288mhz (16.0 x 143), Iwill MPX2, 2x Kingmax PC-3200 256mb sticks, 4x 20gb 60gxp in Raid 5 on a Promise SX6000. Asus Ti4200 320/630. Cooled by Water

06-13-2002, 04:08 PM	#6
Sirpent Cooling Neophyte Join Date: May 2002 Location: here, there .... Posts: 20	I think what we are discussing here is how to achieve the best cooling effect with a fixed cap on the noise level, or how to achieve the required cooling effect producing as little noise as possible (no real difference between the two formulations). The noise is mostly caused by turbulence in the air passing through the radiator, not by friction in the bearings of the fans (assuming the fans are decent). Now, the level of this turbulence noise is a function of the air flow through the radiator (we ignore shrouds and other things that can decrease/increase turbulence - their effects for one and two fan configurations are roughly the same). The cooling effect is also a function of the air flow through the radiator. The bottom line: there is no difference between using a single fan and a push-pull arrangement, same noise level = same cooling effect.

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