[GordonS]

Aren't Noctua fans quite large, like 12cm in diameter? In a 1U chassis, your fan can only be around 4cm in diameter, because they have to be mounted at the back. You can't mount them horizontally, because of course rack units are meant to be stacked close together.

[jowsie]

They start small enough for cooling VRM's and go up to iirc 20cm

[Macha]

Note that small noctuas are not magic, like other small fans they're noticeably louder than larger fans.

[numpad0]

Servers people are talking here are like 5cm thick and like 125W per CPU