Changing fans - wanting same airflow and less noise

George Hudson · 20 Mar 2020 at 07:38

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.

George Hudson · 20 Mar 2020 at 09:48

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

George Hudson · 22 Mar 2020 at 19:12

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 · 23 Mar 2020 at 12:45

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 · 23 Mar 2020 at 13:13

Thanks Doyll. That is an interesting read.