Dave said:
Many computers were cooled with only one cooling fan. That is the one
that was mounted on the back of the power supply. Air would be pulled
through the case, then through the power supply, then exhausted out the
back of the power supply. Therefore it made sense for the bottom/back
of the PSU to be mounted as high as possible in the case. That's the
direction all the hot air was going anyway, so it made cooling more
efficient to have the PSU way up top.
The PSU still assists somewhat in cooling the case...and more
importantly the components in the case. The only advantage to having
the PSU bottom mounted is that the PSU fan will be drawing more cool
air into the PSU itself. In turn, this means you need a shitload of
airflow to cool the case (and components like the northbridge, CPU and
GPU in particular), because much of your cool intake air is now going
down low where it's not cooling a damned thing. -Dave
There is an equation, to work out what a shitload is.
CFM = 3.16 x Watts / Delta_T_degrees_F [ Formula for case cooling ]
If the entire contents of the computer case dissipated 200W
(components and PSU waste heat), and you wanted to meet the
"well cooled" criterion of 7C difference between case and
ambient (7C = 10F), then the equation gives you
CFM = 3.16 * 200 / 10 = 63.2 CFM
That amount of air can be moved by a single fan.
If you allow a larger delta_T, it means the components in the case
will have a hotter environment to work in, your hard drive gets
warmer and so on. You don't have to shoot for 7C if you don't want to.
Paul