140mm less air vs 120mm?

[NeonShadow]

i have a case that can support 2x 120mm fan in front OR 2x 140mm fan i want front fans to be intakes but.. recently i got some 140mm fans... i have 1 120mm at the rear as an exhaust which runs 2200 rpm and says 71 CFM while the 140mm fans i got are 1000 rpm but says 83 CFM so i expect they blow more air which.. it didn't(?) my 120mm fans are like 5x better when feeling the air by hands, can feel it even if my hands are like 7inch away, and 140mm fans can only be felt like 3inch away.. they are directly connected to my psu (molex) tried connecting to 3/4 pin mobo.. still i feel 140mm fans are weak can someone explain how it works?

 

[snurp85]

its a matter of math... the smaller 120mm fan has a high CFM because it pushes air through a small area fast. Since the area is small and focused, you feel it more. The larger 140mm fan pushes overall more air through, but it is spread out over a larger area and thus "feels" like there is less air.

 

[clarkjd]

The 140 mm fans, being bigger move more air, but at a lower velocity(speed). so you would not feel the extra airflow.

 

[AdviserKulikov]

i have a case that can support 2x 120mm fan in front OR 2x 140mm fan i want front fans to be intakes but.. recently i got some 140mm fans... i have 1 120mm at the rear as an exhaust which runs 2200 rpm and says 71 CFM while the 140mm fans i got are 1000 rpm but says 83 CFM so i expect they blow more air which.. it didn't(?) my 120mm fans are like 5x better when feeling the air by hands, can feel it even if my hands are like 7inch away, and 140mm fans can only be felt like 3inch away.. they are directly connected to my psu (molex) tried connecting to 3/4 pin mobo.. still i feel 140mm fans are weak can someone explain how it works?

I'm not sure you understand how speed of fans correlates to the size of the fan.

The larger fans have a larger radius, which allows them to move more air for the same number of blade rotations. You should be able to identify if intake or exhaust is greater by opening a small hole somewhere on the case and holding paper in front of the hole. If the paper is pulled towards the case you have negative pressure (more exhaust than intake) If the paper is blown away from the case you have positive pressure (more intake than exhaust)

 

[synphul]

As others said, what you're feeling is the pressure of the airflow rather than the volume of the airflow. Overall volume may be more while focused pressure is less.

Easy test, take a deep breath and with your mouth open 'huff' the air out on the exhale with your hand in front of your mouth. Then take another deep breath and exhale again, this time as if you're blowing out a candle. Your lung capacity (volume of air) didn't change, only the perceived pressure.

Other things like an intake dust filter may be affecting the fans also. The front intakes are typically filtered while the rear exhaust fan isn't. Filters restrict airflow. Water runs much more freely from the faucet into the sink than it does if it first has to pass through a coffee filter, the original flow of the water didn't change.

 

[NeonShadow]

thanks for all the response! i get it now.. will buy another 140mm then.. soo last question.. my exhaust fan (120mm) which is 2200 rpm 71cfm but.. kinda loud i dont really mind it sometimes cause i cant hear it with my headphones on.. do i really need high cfm for exhaust? im thinking of replacing it +addign 2 more at the top with low cfm but a bit good looking (ring type led 1300 rpm 40 cfm) or should i stick with my current 120 and buy 2 more?

 

[snurp85]

generally speaking, you want negative pressure inside your case (less cfm going in than cfm going out), otherwise you get stagnant air sitting around inside your case and thats not a good thing. Since heat rises, its generally good to have fans at the top blowing hot air out. So get the two fancy ones on top and either keep or take out the back one. I would say just keep it. More air out is good.

 

[madmatt30]

^ well that's totally not true , I'd say 75-80% of pc builders opt for positive pressure systems when possible & the case allows for it.

 

[snurp85]

lol, i figured there would be some debate about this... and a debate it is, there are hundreds of threads relating to just that.

 

[AdviserKulikov]

The positive-negative pressure debate is mostly based off theoretical examples, in practice the pressure difference achievable by a non-airtight case is negligible in cooling effects, but has a minor impact on dust accumulation. If you have a sealed case where you have caulked up all the non fan surfaces, you might see a benefit from negative pressure. If your case hasn't been sealed super tight, then you should go positive pressure just for ease of cleanliness.