How much static pressure for exhaust fan?

[bit_user]

Apologies for this not involving overclocking, but I didn't see a forum devoted strictly to cooling.

I bought a Lian Li PC-Q08 Mini-ITX case, and I'm replacing its fans. Both because I hate LED fans and to reduce noise. I'm just putting 3 SSDs and a i3-4370 in this box, so cooling should not be an issue. Eventually, it may also have a 10 GigE NIC, once prices come down.

For the front intake fan, I've selected a fixed-speed NF-A14 ULN 140 mm. At 800 RPM, it moves about 80 m3/h. For the top exhaust fan, I'd like a PWM fan that can outstrip the intake during high-load periods. This will maintain positive pressure for the 99% of the time the box is under low load, but will run the case into negative pressure territory as needed. Positive pressure is good when your intake fan has a dust filter, as this keeps the case & various internal heatsinks immaculate, in my experience.

Now, my question is which exhaust fan to choose? Given that the fan grating is a rather fine mesh (being on top of the case, I can only assume they were trying to keep out dust, while it's turned off), do I get the higher CFM fan with lower static pressure, or is it worth giving up some CFM to get more static pressure to push the air through that top grating?

The two exhaust fans for consideration are (both 120 mm PWM fans by Noctua): NF-S12A and NF-F12. Their performance is 107.5 m3/h and 1.19 mmH20 @ 1200 RPM vs. 93.4 m3/h and 2.61 mmH20 @ 1500 RPM, respectively. Not surprisingly, the slower fan is quieter @ max speed.

If the fan grating *does* create significant resistance, then I could conceivably get more flow with the higher-pressure fan.

Thoughts?

 

[jaimelmiel]

IT is not that important on exhaust fans. I like the first one. Static pressure is more importanot when it comes to intake and heat sink or radiator fans.

 

[bit_user]

Thanks for the reply. I wouldn't normally be concerned about this, except for that fine mesh. You can still see the fan through it, but it looks like it blocks a lot of air.

Anyway, since I wrote my first post, I thought of another reason to go with the higher-pressure / lower CFM exhaust fan. The reason is that there's less risk of me inadvertently running @ negative pressure, due to the smaller CFM range. In other words, the lower CFM fan would have to get running pretty fast to outstrip the intake. Admittedly, this is a pretty poor reason, but I don't know of an easy way to check the direction of airflow of the other vents. Perhaps I could make some smoke near them and see if it gets sucked in or pushed away. An incense stick might be good for that.

In my opinion, people don't give enough thought to dust. It's all well and good if your new build has awesome cooling performance, but how easy is it going to be to maintain? I once had a Pentium 4 that I needed to clean out once or twice a year to keep the CPU temperature alarms from sounding. Since then, I build positive pressure cases with intake filters that are easy to clean, and I've never had to clean out the inside of those cases.

 

[jaimelmiel]

Then go for the one with the more static pressure.