Changing fans - wanting same airflow and less noise

George Hudson

George Hudson

George Hudson

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I have a 360mm front mounted radiator with 3x120mm fans on the back in a pull configuration, therefore pulling air from outside the case through the radiator and into the case. Infront of the radiator I have 2x200mm fans pushing external air through the radiator, so a push-pull configuration albeit with different sized fans.

The 2x200mm argb fans are not speed controllable, therefore run flat out and they are audible at 28dBA according to the specs. So, I am wanting to replace the 2x200mm fans with 3x120mm that can hopefully be run slow and still achieve the airflow keeping in mind that the 200mm fans are wider than the radiator so not all of the airflow they produce is going through the radiator.

I also have 3x120mm argb fans in the top of the case fitted to another radiator, that are very quiet when running at 40% power and I was thinking of getting another 3 of these 120mm fans to replace the 2x200mm fans on the front radiator.

Now for the airflow maths and noise calculations.

A single Cooler Master MF200R (200mm) fan running at 800 RPM produces 90 CFM of air flow at 0.88 mmH20 pressure with 28dBA noise output. Therefore two of those in parallel produces 180 CFM at 0.88 mmH20 and a noise output of 28(?) dBA.

A single Cooler Master SF120R fan running at 2000 RPM produces 60 CFM of air flow at 2.1 mmH20 pressure with 30dBA noise output. Therefore three of those in parallel produces 180 CFM at 2.1 mmH20 and a noise output of 30(?) dBA.

The 3x120mm fans would have to run at 2000rpm to achieve the same 180 CFM airflow as the 2x200mm and that the noise output of the 3 fans in total would still be 30dBA (the level of one fan or would it be higher).
But, if only 60% of the area of the 2x200mm fans actually covers the radiator then does that mean the 2x200mm fans are only pushing 108 CFM (60% of 180 CFM) through the radiator and therefore the 3x120mm fans (covering all of the radiator) only need to run at 60% speed to produce the same airflow? Does pressure effect these calculations? And how is the noise level effected? When running a push-pull configuration should each individual fan have the same CFM and pressure specs? What happens if if you are trying to push through more air than the pull fans are trying to draw through?

I hope someone can explain. Thanks.
 
With a large portion of the fan not being over the rad, the fans centre deadspot, I wouldn’t even be surprised if when you removed those 200mm fans that it didn’t make much of a difference at all.

3x120 fans actually mounted onto the rad will be far superior, The static pressure you will actually be getting with a 200mm fan sat next to a 360 rad I imagine is minimal.

I’d remove the 200s, put the 3x120s you have on the back on the front pushing in cold air. Push pull doesn’t offer much in terms of performance. Potentially more importantly, you need to be looking at static pressure for radiators, not cfm.
 
Only things which are fully usefull in advertised fan specs are size, weight and RPM inside tolerances.
Rest are as reliable as anything coming out of mouth of politicians, or not telling anything about real world situation.

Like 2000RPM and 30dB means BS. (or they measured it from other room behind wall)
Even 1500RPM would make that questionable.
And advertised airflow is equal to measuring top speed of car in free fall when dropped from airplane.
Also static pressure is about as accurate real world performance measure as measuring how much noise car's engine makes with hood open, gear at neutral and car sitting in parking lot.

https://martinsliquidlab.wordpress....w-specs-are-poor-measures-of-fan-performance/
Hence those aren't good for comparing fans in real world.
And when decreasing speed from max, those not real world performance values also drop.


Also airflow goes always through the path of least resistance. (/impedance in fan jargon)
Hence fan which spreads outside edges of airflow obstacle wastes its work mostly there.
Motor hub also causes dead spot with minimal if any airflow, until airflow from around it combines some distance away from fan.
So pretty sure that most of the work of moving air through radiator is done by those 120mm fans.
Already big slow fan's weak pressure means it would never have much chances to actually push airflow through radiator.
 
Thanks for prompt reply and I appreciate your input. I have been doing a lot of data logging since building my system in December in order to optimize ramping curves. Running the pump and all fans at the same settings over a 2 hour period of Heaven benchmark, one test with the 2x200mm fans in place and the other with them removed, on removal of the 2x200mm fans I see an increase in water Delta-T from 8 to 11 degC, so 3 degC increase, CPU average temp increased by 3 degC and the GPU increased by 4 degC. I thought the differences seen were on the high side for those fans but that was just one test. I found this topic on EK website. https://www.ekwb.com/blog/push-pull-or-push-pull-on-radiators/

I have to admit that without the 200mm fans the system is noticeably quieter even though those fans are rated at only 28dBA, but maybe that is me listening for them. I think I need to do more testing before I purchase another 3x120mm fans and fit them to the front radiator maintaining a push-pull configuration.

Regarding noise calculations, I just found this https://www.silentpcreview.com/forums/viewtopic.php?t=19297
 
This is looking/sounding really good. I bought the 3x120mm fans and fitted them yesterday. I have slowed some of the fans and have all 10 fans at 40% and it is very quiet. Not silent, more of a quiet background hum, but big difference to the 2x200mm fans that were previously in the front. My average Delta-T has dropped 1 to 2 degC also after testing using Heaven benchmark for 400 minutes. I cant help thinking that the H500M case should have the option of being sold with no fans because as soon as a radiator is fitted in the front position, the 200mm fans just seam to create noise and turbulence with little help to moving air though the radiator. The 200mm fans are probably great if only a top mounted radiator is in place.
 
George Hudson said:
I have a 360mm front mounted radiator with 3x120mm fans on the back in a pull configuration, therefore pulling air from outside the case through the radiator and into the case. Infront of the radiator I have 2x200mm fans pushing external air through the radiator, so a push-pull configuration albeit with different sized fans.

The 2x200mm argb fans are not speed controllable, therefore run flat out and they are audible at 28dBA according to the specs. So, I am wanting to replace the 2x200mm fans with 3x120mm that can hopefully be run slow and still achieve the airflow keeping in mind that the 200mm fans are wider than the radiator so not all of the airflow they produce is going through the radiator.

I also have 3x120mm argb fans in the top of the case fitted to another radiator, that are very quiet when running at 40% power and I was thinking of getting another 3 of these 120mm fans to replace the 2x200mm fans on the front radiator.

Now for the airflow maths and noise calculations.

A single Cooler Master MF200R (200mm) fan running at 800 RPM produces 90 CFM of air flow at 0.88 mmH20 pressure with 28dBA noise output. Therefore two of those in parallel produces 180 CFM at 0.88 mmH20 and a noise output of 28(?) dBA.

A single Cooler Master SF120R fan running at 2000 RPM produces 60 CFM of air flow at 2.1 mmH20 pressure with 30dBA noise output. Therefore three of those in parallel produces 180 CFM at 2.1 mmH20 and a noise output of 30(?) dBA.

The 3x120mm fans would have to run at 2000rpm to achieve the same 180 CFM airflow as the 2x200mm and that the noise output of the 3 fans in total would still be 30dBA (the level of one fan or would it be higher).
But, if only 60% of the area of the 2x200mm fans actually covers the radiator then does that mean the 2x200mm fans are only pushing 108 CFM (60% of 180 CFM) through the radiator and therefore the 3x120mm fans (covering all of the radiator) only need to run at 60% speed to produce the same airflow? Does pressure effect these calculations? And how is the noise level effected? When running a push-pull configuration should each individual fan have the same CFM and pressure specs? What happens if if you are trying to push through more air than the pull fans are trying to draw through?

I hope someone can explain. Thanks.
Click to expand...

I assume you know this but just in case you don't .. YOu say MF200R at 800rpm produces 90CFM of airflwo at 0.88mm H2O. If that is what you were saying you are incorrect. fan produces 90cfm with no resistacne / 0.00mm H2O or 0.88mm H2O with no airflow / 0.00cfm at 800rpm. Fan airflow specs are maximum possible airflow with no resistance or maximum pressure fan makes when airflow stops moving .. maximum amount of pressure measured in water column by air pusing into sealed container .. pressure reached when airflow stops .. definition of "static" (like in "static pressure") is lack of movement, as in no air moving. :D

You are correct about MF200R being almost wothless and a good 120mm / 140mm fan being able to move more air with less noise, although 3x 30dB fans makes 34.8dB.
1x 30dB fan = 30dB
2x 30dB fans = 33dB
3x 30dB fans = 34.8dB
4x 30dB fans = 36dB
5x 30dB fans = 37dB
​
Air flows out of most fans in a cone shape with most of the air being at edges of cone. This means a 200mm fan on a 120x360 radiator has most of the airflow ourside of sides with best airflow in stips at each end.

Also keep in mind the best way to create good airflow from fans into a case is to seal all openings in fan mounting panels not covered by intake fans so the air they are pushing into case cannot leak back to intake side of fans and end up going in circles instead of on through case.

Running fans push/pull does not increase airflow spec, but does increase static pressure spec. But to do this there has to be an airflow straightener between then like a radiator.

Hope that all makes sense.
 
Excellent technical info Doyll. Thanks. The CFM I quoted was just what I found on manufacturers website and I obviously did not understand its true representation. I now need to start sealing up vents around the fans. Thanks again.
 
George Hudson said:
Excellent technical info Doyll. Thanks. The CFM I quoted was justwhat I found on manufacturers website and I obviously did not understand its true representation. I now need to start sealing up vents around the fans. Thanks again.
Click to expand...
No problem. Any other questions just ask.

I rarely use exhasut fansin my buillds, in fact I haven't used any in .. can't remember last time and I can remember last 10-15 builds. Using both intake and exhaust fans is similar to push / pull fans on cooler or radiator .. assuming case has more exhaust vent area than intake and exhaust vents are where we want the air intakes are pushing into case to flow to and out.

You might find below link is to basic guide about how airflow works and how to optimize case airflow, including what I use as a low-cost way to monitor airflow into cooler / radiator of interest too:
https://forums.overclockers.co.uk/t...-i-put-my-temp-sensor.18564223/#post-26159770
 
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