Really dumb PSU fan question???

PSUs with two fans (like the Antec Trueblue 480W PSU, one at the back
and one on the top/bottom).....the fan that is on the underside of the
PSU does it blow air out or in?

I'm assuming it sucks air in from the inside of a PC case and then
blows it out the back of the case. Otherwise hot air would get blown
directly over the CPU socket.

It blows air out.

--
DaveW



"Darth Joules" wrote in message
...
PSUs with two fans (like the Antec Trueblue 480W PSU, one at the back
and one on the top/bottom).....the fan that is on the underside of the
PSU does it blow air out or in?

I'm assuming it sucks air in from the inside of a PC case and then
blows it out the back of the case. Otherwise hot air would get blown
directly over the CPU socket.

"Darth Joules" wrote
in :
PSUs with two fans (like the Antec Trueblue 480W PSU, one at the back
and one on the top/bottom).....the fan that is on the underside of the
PSU does it blow air out or in?

I'm assuming it sucks air in from the inside of a PC case and then
blows it out the back of the case. Otherwise hot air would get blown
directly over the CPU socket.

Consider the power supply is a box. You put 2 fans, one in any side of
the box. If both blow into the box or both blow out then there is
almost no airflow. Only if the fans different in airflow rate would
there be any differential velocity in airflow. So if one of the fans is
different by 180 degrees then get airflow *through* the box. Think of
two people at each end of hose. Both blow and there is no airflow
(other than to pressurize the hose over ambient air pressure). Both
suck and all that gets accomplished is a slight vacuum and still no
airflow. If one blows and one sucks then you have airflow (with the
possible danger of the one doing the sucking getting their lungs blown).

Even with one fan blowing in and one blowing out, the slower fan can
hinder the higher airflow from the faster spinning fan. The slower fan
presents an obstruction. You'll probably get a bit more airflow than
with just the one slower fan by having the faster fan increase air
pressure inside the box, but it's probably of not much more than the
slower fan by itself. The exhaust fan in a 2-fan power supply is
usually the variable speed unit. The underside fan (blowing into the
power supply's case) is often a constant-speed unit. So although the
power supply has a temperature-controlled fan, the other isn't. The
point of adding the 2nd underside fan is not to increase airflow as the
variable exhaust fan will mostly limit how much air goes through. The
underside constant-speed fan is a backup: if the exhaust fan goes bad
(doesn't spin, gets stuck and won't start to spin, or doesn't get enough
voltage to get it spinning) then the underside fan spinning constantly
at the high fixed speed ensures the power supply and case interior still
gets cooled.

--
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On Mon, 08 Dec 2003 21:05:45 GMT, le (Darth
Joules) wrote:

PSUs with two fans (like the Antec Trueblue 480W PSU, one at the back
and one on the top/bottom).....the fan that is on the underside of the
PSU does it blow air out or in?

I'm assuming it sucks air in from the inside of a PC case and then
blows it out the back of the case. Otherwise hot air would get blown
directly over the CPU socket.

Yes, it *always* blows in.

le (Darth Joules) wrote in message ...
PSUs with two fans (like the Antec Trueblue 480W PSU, one at the back
and one on the top/bottom).....the fan that is on the underside of the
PSU does it blow air out or in?

I'm assuming it sucks air in from the inside of a PC case and then
blows it out the back of the case. Otherwise hot air would get blown
directly over the CPU socket.


Man, that's a DUMB question. Just kidding -
You got it right. It sucks air in.

Nick

"Darth Joules" wrote in message
...
I'm assuming it sucks air in from the inside of a PC case and then
blows it out the back of the case. Otherwise hot air would get blown
directly over the CPU socket.

Some of your answers I found a bit confusing but the fact is, your
assumption is correct.

Hank

On Tue, 09 Dec 2003 01:24:03 GMT, kony wrote:

On Mon, 08 Dec 2003 21:05:45 GMT, le (Darth
Joules) wrote:

PSUs with two fans (like the Antec Trueblue 480W PSU, one at the back
and one on the top/bottom).....the fan that is on the underside of the
PSU does it blow air out or in?

I'm assuming it sucks air in from the inside of a PC case and then
blows it out the back of the case. Otherwise hot air would get blown
directly over the CPU socket.

Yes, it *always* blows in.

That is, the underside power supply fan, blows into the power supply.

"Darth Joules" wrote in message
...
PSUs with two fans (like the Antec Trueblue 480W PSU, one at the back
and one on the top/bottom).....the fan that is on the underside of the
PSU does it blow air out or in?

I'm assuming it sucks air in from the inside of a PC case and then
blows it out the back of the case. Otherwise hot air would get blown
directly over the CPU socket.

Originally, ATX specs called for the PSU to suck outside air from behind the
PC, through the PSU and then down over the CPU. This was to maximize the use
of the airflow while keeping the number of fans low.

....of course everyone quickly found out that the air coming out of a PSU is
not very good at cooling a CPU.

ATX supplies now suck air out of the case and out the rear of the PC. Any
other fans on the power supply should also be sucking air from the case.