One interesting property of fans is that "they blow better than they suck".

Although the mass of air going into the fan is, obviously, the same as that
coming out, the shape of the surface at which the air velocity is half the
maximum velocity is vastly different.

This surface is roughly hemispherical on the inlet side with a radius about
the same as the diameter of the fan, if I recall rightly. On the outlet
side, it is shaped more like an exclamation mark or a baseball bat, with
the small end at the fan.  I forget the exact figure, but it extends
something like 15 diameters.

Since the rate of cooling of a hot component is strongly affected by the
rate of airflow across it, the best way of cooling a SPECIFIC component
is to blow on it.  I vaguely remember reading that the ATX fan was
reversed so that the flow from the power supply blew at the (really hot)
CPU.

Overall, the airflow through the box should be essentially the same either
way, and so should the heat removed (equals watts actually used).

If the box is crowded with big, hot, cards and drives, sucking air past
them (all more or less equally) may be best.  If you have one long card
on the far side from power supply, and a bunch of small cards with
considerable open space, I suggest you "stir things up" by aiming the
new fan so that it swirls the air inside the case.

Boyd Ramsay

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