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Question two 140mm or 120mm pwm intake fans

Conradfwebster

Junior Member
Mar 10, 2021
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Given that positive pressure equals less dust and only one 120mm rear exhaust fan is possible, is it better to have two 140 or two 120 pwm intake fans? 140's can run slower but would they be excessive even though they have to pull air in through side front vented Fractal Design Define 7 Compact or C case?
 

Paperdoc

Platinum Member
Aug 17, 2006
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The pair of 140's probably is excessive. Assuming you set the mobo CHA_FAN or SYS_FAN headers to do their default automatic fan control based on the temperature measured by a sensor on the mobo, all of those three case ventilation fans will receive the same signals (PWM) to set their speeds as percentages of full speed. So if all fans are identical, the two front intakes could pull in EACH as much air as the ONE rear exhaust fan can move out. Even allowing for the small reduction in intake fan capacity due to the intake air flow path and the front dust filters, the two intakes combined still will have more capacity than the single exhaust. This combination means the actual air flow rate at any particular fan speed setting would be somewhat more than one single unconstrained fan can do, and the case internal pressure will be slightly above ambient to reduce dust intake at leak points.

Now, change that scenario to having the front fans in the larger 140 mm size. For the same PWM signal as before, such a fan will run slower RPM's but will likely produce a higher air flow rate. Since the 120's would already have supplied more than the rear exhaust fan can do, this new even higher intake rate is pretty much wasted - it only will change very slightly the overall air flow rate and internal pressure. It might very well mean slightly LESS noise generated by the front fans because they are running more slowly. But in terms of cooling capacity, the difference would be small.

IF you anticipate that you will be operate often at maximum workload and high temperatures, having a slightly higher maximum air flow rate for slightly more cooling under those conditions would make the 140's an advantage. Otherwise the 120's will do the job well.
 

Conradfwebster

Junior Member
Mar 10, 2021
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Thanks that sounds logical. 2 x 120mm intake and 1 x 120mm exhaust it is. The mobo has 65w tdp, gpu100w with dark rock 4 fan so I will go with your recommendation .
 

Soulkeeper

Diamond Member
Nov 23, 2001
6,633
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I have two 120mm front intake fans, one 120mm side intake, and no exhaust fans.
The warm air comes out of the holes where the exhaust fans would be just fine.
 

WilliamM2

Golden Member
Jun 14, 2012
1,786
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You don't have to run all the fans at the same speed. I have two 140mm intakes, and one 140mm exhaust in my define R6.

I run the front fans slower than the exhaust fan. makes it quieter, and still has positive pressure.

That case is similar to the define R6, and in my opinion they have too many fan mounts with open grill area, which makes air pour out everywhere instead of flowing through the case. I modified mine in a way that may seem counter intuitive, and it lowered CPU temps about 12f from better airflow. The space left below the front two fans had air coming out and recirculating back into the case, I fixed that. I also blocked the bottom to force air to go all the way thought to the back. A bonus is it's even quieter like this.
 

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Paperdoc

Platinum Member
Aug 17, 2006
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WilliamM2, your post shows how someone who really understands case cooling can make real improvements with creative ideas. Congrats!
 

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