So it's 1:35am... what else to do but wade into a debate!
This is relevant to the very first post of this threead:
I assembled system as a lab utlity PC and decided it should be encased rather than on an open platform like most of the stuff in the lab. Just to keep it a little safer from accidents. It needed to be as quiet as possible so acoustic measurements and recordings of super quiet stuff could be made in the room.
The system is a heavily modded study steel mid tower. There are 2 fans: A Nexus 120 on the back panel as an exhaust, and the Yate Loon 120 in a Seasonic Super Tornado 300.
There are two large holes in the front:
1) A 120mm hole at the tradition front bottom with a very open hole in the bottom edge of the bezel. A Samsung HDD sits on foam on the floor of the case directly behind this hole.
2) The two top CD bays are completely open. The 3rd bay holds a CD drive.
The HS for the Athlon XP 2500+ (?) is a Thermalright SI-97. Its fins run up and down. It is fanless. The Nexus 120 spins at ~5.5V -- the min it will run. The PSU was left stock -- it never
ramps up, I think its rpm is no higher than the Nexus. I think the system measures ~18 dBA/1m. It's under a desk so you have to put your head under it to really hear the PC at all.
The airflow in this system is not huge; each fan is probably not moving more than 15 cfm tops. The system is constantly loaded with folding@home running 24/7.
When the case side is off, the CPU temp hits around 60C. Put it back on and it gradually drops over about a 10-15 minutes down to 52-53C. Put your hand in the CD bay hole at the top and it feels cool because of the air tunneling in. Put your hand at the bottom front edge of the bezel, and again, your fingers feel cool because of the airflow.
The air flows from the front holes to the 2 fans perpendicular and so close to each other. From behind the case, the PSU exhaust feels a slightly warmer than the case fan and has less pressure; the airflow is obviously a lot more blocked by the PSU innards.
Turning the back case fan up to full does improve CPU temps a bit, but much less than you would expect; the two fans spinning slowly together are very efficient at pulling the air in from the front of the case without any conflict which each other.
My thought about this dual-fan-at-right-angles set up is that it works fine as long as neither fan is spinning real fast, and if the intake vent/s are unrestricted enough to allow the air to come in and get to the fans. If the intake vents are too small, then yes, the two fans could fight each other. With the CPU/HS right at the "crossroad" in tront of the 2 fans, the concentration of directed airflow is high despite the low fan speeds. The air simply has to go across the HS fins to get to either fan.
Dust has not been a problem but it's only been running 2-3 months.