Initially I had incorrectly remembered this system as a very-negative air-pressure system, however there are 2x 140mm fans blowing air into the case. The over-all balance of pressure is negative flow though.
This blog asks the question “does a negative air-pressure system contribute to more dust”? There are some who say it does, but my experience seems to indicate the opposite.
For those who don’t know, I live in Big Bear mountains, 7000 feet. We tend to have very low humidity (one doctor said it’s lower than the Sahara Desert). We get a fair amount of dust, and a few of my clients have had thick dust in their systems. (positive flow)
I built the following NAS system about a year ago, and it has been in operation for at least 9 months out of 12, with the fans blowing 24/7.
Here you see the top of my NAS system where it sits in a corner. Notice all the dust on the top of the system. Also note how it lands everywhere around the fan.
Here I have swung open the door and we’re looking down at the “bottom floor” of the system. I forget the term for that, it’s not the literal floor of the system, the power supply is right under that, with a fan pointed up.
Here you see some dust on the platform. The motherboard stands vertically in the case, of course. There’s very minimal dust on it. You can see the CPU fan is spinning. Also notice the inward pointed fan on the front of the case (120mm).
The fan in the top of the fan is a large fan (180mm, not visible in photo), rear fan (120mm, on the left edge of photo), are pointing out. The power supply fan (presumed 120mm, can’t reach it atm) is blowing upward into the case. For the top and rear I used a ruler measuring the fan blade diameter and rounded up to a common number. For example the 120mm is technically “115 mm” on the rear.
Rear shot — No fan on the rear of the Power Supply here, it’s inside blowing upwards into the case. Interesting to note, there is less dust on the rear intake of the ps than on the roof of the case. Could this be because dust settles down from gravity more than it floats around? Notice less dust from the outward pointed fan next to the ethernet cable? Only around the edges away from where the fan is blowing. Same effect on the top of the system from the 180mm.
Notes —
Fans tend to have a reverse blowing effect at the edges, especially noticable on the 180mm on top. I ran my hand back and forth inside the case and noted that I could feel a good blast of air going down at the edges of the fan, but felt like nothing near the center. When I lifted the magnetic micro cover on the top of the case, the inside blast decreased noticably and also I felt more air exiting the case on top from the fan. I wonder if I should cut a hole out of the micro cover at about 160-170mm diameter, above where the 180mm fan is located, to help with outward pressure. Theoretically dust isn’t going to fall “through” a blast of air, so cutting a hole shouldn’t be letting in any more dust and may be helping the negative pressure.
For those who are curious about my choice of cases — this one is a terrible choice for a NAS — I can’t hardly place drives in there. I plan to switch cases as my case for the i7 7700 is a nice large tower case with 2 racks for 4x 3.5″ drives each, and extra slots on the back-side for a few 2.5″ drives. Contrasted with the large fan but lack of hard drive bays in the mid-tower, the mid-tower is probably better for cooling a warm system. I don’t currently use any hard drives on larger case, so the potential is wasted. I’ll probably retire the 7700 to the NAS job and upgrade the 10k at some point, just swap some gear around.
Why the decision on the large fan? It’s quiet and moves a higher volume of air.