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Interesting choice: 2 x 140mm vs 3 x 120mm intake fans

post #1 of 22
Thread Starter 
Hi all, I originally posted this within the Corsair Air 540 owner's thread, but thought I'd put it in the Air Cooling as a standalone thread to get everyone's opinion.

Here's the content of my question, as was posted over there. I think I'll edit it in the Corsair thread to point here:
Quote:
I'm thinking of using 3 x 120mm fans for the front intake, instead of the two 140mm fans, to increase airflow and have positive pressure in my case.

I wonder, 3 x 120mm fans running at the same speed as 2 140mm fans, does it give more airflow? From my faint physics recollection, and my 30 second google search (literally, just quickly browsed the first physics forum I found), I gather airflow is additive with fans in parallel. So I would have to sum the airflow of the 3 x 120mm fans, and compare to the sum of the airflow of the 2 x 140mm fans. Obviously I would have more airflow per fan if I use faster rpm, and consequently louder fans. But if I want to compare at an approximately equal sound level, I'd be dealing with 3 x smaller airflow vs 2 x higher airflow.

Thoughts anyone?

*edit to add*

So found the following informative article, which explains that when fans are added in parallel, airflow is theoretically multiplied by the number of fans, while in practice the airflow is reduced by outflow turbulence, pressure characteristics, etc:
http://www.ebmpapst-ad.com/media/content/technical_articles/TA_Using-fans-in-series-and-parallel.pdf

Comparing the sound, cooling, and airflow chart for the Corsair AF120 Performance from silentpcreview to that the of AF140 Quiet
(http://www.silentpcreview.com/article1342-page4.html)
(http://www.silentpcreview.com/article1346-page4.html)







Testing methodology:
http://www.silentpcreview.com/Fan_Roundup_6_Scythe_Noiseblocker_Antec_Nexus_Thermalright

If I look at 1,100rpm, silentpcreview's airflow measurements look about the same between the AF140 and the AF120 Performance. The larger fan diameter translates into the smaller thermal rise, I believe (thermal rise is measured on a Thermalright heatsink designed for 170mm fans, so the larger surface area of the fan, the better the thermal performance). So if I take 3 x AF120 Performance, and run them at 1,100rpm, I'd have more airflow than the 2 x 140mm. In addition, total fan area of approx 360mm exceeds the total fan area of 280mm, notwithstanding the fact that the 140mm fan area is partially obstructed by the required shape to support the 120mm fan mountings and sides, while it doesn't look like the 120mm fans would have that problem:



Finally, depending on the fan I use, like the AF120 performance, for example, I would have the advantage of pushing them to 1600rpm, albeit at the cost of louder dBA levels.

Have I completely misunderstood how it works? Hopefully more knowledgeable people can chime in to correct my logic. graduated.gif
Maybe I should start this in a separate thread.

Edited by Atokade - 7/7/13 at 9:30pm
post #2 of 22
Quote:
Originally Posted by Atokade View Post

Hi all, I originally posted this within the Corsair Air 540 owner's thread, but thought I'd put it in the Air Cooling as a standalone thread to get everyone's opinion.

Here's the content of my question, as was posted over there. I think I'll edit it in the Corsair thread to point here:
Do you have fan filters in the front of case where you are trying to put these fans?
 
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post #3 of 22
FYI the testing was done on Thermalright Archon (Dimension: Length 155mm x Width 53mm x Height 170mm; fin area 155m wide x 127.7mm tall) which came with a 140mm fan (TY-140).

I know this isn't directly what you are talking about but as it's all about cooling I'll throw it in. wink.gif

Cooling isn't simply getting lots of cfm into case. We want airflow not airblow to cool out systems. This means we want cool air flowing to everything that needs cooled and the hot air exhausting case without mixing with the cool intake air.

Components like CPU & GPU have variable speed fans so why not do the same with case fans? As components get hotter and need more cooling the CPU & GPU fans increase in speed / airflow but case fans run the same all the time.. why? Doesn't it make more sense to have case fans do the same and increase their airflow as CPU & GPU do? I think it does so all my builds use PWM case fans controlled by CPU (& sometimes GPU) PWM signal. In this way the case fan speed increases and decreases same as CPU (& sometimes GPU) fans do.

Cooler / radiator fans have higher static pressure ratings than case fans.. But the resistance in most cases is similar to most coolers.. First there's a grill, than filter, often a HDD cage, cables, etc. restricting flow.. So why are case fans lower pressure rated? It doesn't hurt anything to have higher pressure rating on case fans.. it only means they will flow air better if there is more resistance.
post #4 of 22
Quote:
Originally Posted by doyll View Post

why are case fans lower pressure rated? It doesn't hurt anything to have higher pressure rating on case fans.. .

I know what you are saying, but to answer this (rhetoric) question: a airflow oriented fan usually has higher CFM at the same noise than a pressure oriented fan when you have no or low resistance.
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post #5 of 22
Quote:
Originally Posted by Elohim View Post

Quote:
Originally Posted by doyll View Post

why are case fans lower pressure rated? It doesn't hurt anything to have higher pressure rating on case fans.. .

I know what you are saying, but to answer this (rhetoric) question: a airflow oriented fan usually has higher CFM at the same noise than a pressure oriented fan when you have no or low resistance.
Key words there.
True but no resistance is almost a myth in real world. We always have at least resistance.. unless we are setting the fan in open space.. like on a table.

Just a grill has significant restriction.. add a filter (most of us use them) and restiction doubles.. or more.
post #6 of 22
Sure, but we dont have actual numbers what kinda restriticon a so called "average case" has and at wich point what airflow/pressure ratio will be benefical at wich noise level. So it's also all a bit speculation. I for one have no fan grill for my exhaust fan in my Lian Li case.
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post #7 of 22
The only thing I've seen of grill restiction is from Silverstone

DEMCiflex rate their filters at 20%

Any idea what CPU cooler airflow restriction actually is?
Edited by doyll - 7/8/13 at 2:53am
post #8 of 22
Thread Starter 
Quote:
Originally Posted by OneGun View Post

Do you have fan filters in the front of case where you are trying to put these fans?

Hey, thanks for asking. Yes, it comes with a magnetic filter. I removed it for a clearer shot of the 140mm fans. I think I may have a pic.. Ah yes:

Quote:
Originally Posted by doyll View Post

Key words there.
True but no resistance is almost a myth in real world. We always have at least resistance.. unless we are setting the fan in open space.. like on a table.

Just a grill has significant restriction.. add a filter (most of us use them) and restiction doubles.. or more.

Hmm, that's a good point to add. Testing that in my case would probably require I try static pressure fans and compare vs airflow fans and see what effect it has on on my components, although I'm not sure if I want to go out and buy 6 fans. Hmmm...
To clarify, my CPU is being cooled by an H110, and will have two EVGA GTX 770s in SLI using the ACX cooler. If you take a look at Gleniu's build log of the Corsair 540 (page 2), you'll see the amount of grills and openings in the back, and the general layout of the interior. It's very reminiscent of the direct airflow idea of the Silverstone Fortress and Raven series. My idea was to pack more airflow into the case to force exhaust out the back while keeping the same sound levels. My Corsair AF140s run at their max, 1150rpm, which gives a nice, low hum, and if I replace with three 120mms, I'd want to have equivalent sound levels.

Ato
post #9 of 22
 
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post #10 of 22
Thread Starter 
Quote:
Originally Posted by OneGun View Post

Check out this guide it might help..
http://www.overclock.net/t/1394467/daves-el-cheapo-heatsinks-air-cooling-guide

Thanks!
Based on the link you gave, I did some more research, and I think I'll go with 3 x 120mm SP fans instead of 3 x 120mm AF fans or 2 x 140mm fans.
Linus from LinusTechTips used some industrial test equipment in his Corsair fan review video (Youtube video here) that shows that the CFM ratings for AF fans are pretty much ratings if there is 0 pressure. As soon as some pressure is present, the airflow does not hold up as well compared to SP fans. The consequence is that SP fans, to provide an equivalent CFM, must have higher RPMs, and as such generate more noise.



In the above graph you can see the SP120 outperform the AF120 at all points on the graph where pressure > 0. However, at 12V it also runs at 2,350rpm (!) vs 1,650 rpm for AF120, and is 4 dBA louder, which is pretty significant. The same can be said for the Noctua F12 (SP) vs the the P12 (more AF than SP).

Seeing as I have a pretty fine fan filter, I expect a decent amount of pressure, and believe that SP fans would provide better airflow compared to AF fans with the filter. If that's true, I could make a gross generalization and extend that reasoning over 140mm AF fans based on Corsair's given static pressure ratings for the AF140 (0.84mm/H20) and the AF120 (1.1mm/H20) in conjunction with the silentpcreview graphs from the first page, and finally conclude that 3 x 120mm SP fans should provide better airflow than 2 x 140mm fans (at 12V), but will be much louder... sigh.
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