Two fans in a PSU ? Why ?

[Kevin Lawton]

Hi,
I'm putting together another PC which is going to have quite a lot of hard
drives in it.
Normal IDE + IDE RAID + SCSI.
I'm thinking that a bigger than usual PSU would be a good idea, as there
will be quite a current drain when the system boots and all the drives spin
up.
My query is that most of the 450 - 550 w PSUs all seem to have two fans, and
I was wondering if someone could explain the rationale behind this.
Do both fans blow the same way (exhaust, I guess) or in opposite directions
?
If they both exhaust, then how does the air passing through the second fan
get out of the case ?
Does an extra hole need to be cut in the case ? (no problem, if necessary).
If they blow in opposite directions - then what's the point ? If, say, the
usual single exhaust fan on a PSU flows 10 cfm, then that 10 CFM must be
coming through the slots in the PSU case. A second fan blowing the same 10
cfm into the PSU wouldn't actually add anything, would it ?
Think about it:
Single fan: 10 cfm through the exhaust fan, so 10 cfm in dragged in through
the slots - the same air.
Dual fan: 10 cfm exhausted through one fan matched by 10 cfm dragged in by
the other fan - the same air again.
Either way, its the same 10 cfm of air flowing through the PSU - isn't it ?
Also, won't two fans make twice the noise of one fan ?
Okay, I'm not too worried about the noise. After all, if I was building a
'quiet PC' I wouldn't be using so many drives !
I'd just like to understand this latest dual fan trend before buying into
it.
Kevin.

 

[Wooducoodu]

i think it's mostly just marketing bs, two fans must be better than one. it
might be slightly better for the cpu to have that fan right above it pulling
air across it but worse for the psu, especially since a single fan with
vents only at the opposite end would ensure that air was pulled across the
full length of the psus heatsinks.

 

[Kevin Lawton]

| i think it's mostly just marketing bs, two fans must be better than
| one. it might be slightly better for the cpu to have that fan right
| above it pulling air across it but worse for the psu, especially
| since a single fan with vents only at the opposite end would ensure
| that air was pulled across the full length of the psus heatsinks.

Yep, that what I was thinking.
I'm also concerned that the 'extra' fan will be pulling air out of an area
inside the case where the CPU fan is trying to get it from - to push it
through the heatsink.
I would have thought that two fans at the rear and lots of vents at the
other end would benefit both the PSU and the PC in general.
Maybe I should make my own PSU ?
Kevin.

 

[tinklemagoo]

I'm also concerned that the 'extra' fan will be pulling air out of an area
inside the case where the CPU fan is trying to get it from - to push it
through the heatsink.
I would have thought that two fans at the rear and lots of vents at the
other end would benefit both the PSU and the PC in general.
Maybe I should make my own PSU ?<

If 2 fans is ott what about the new Quadruple Super Flower Real 520w
Quadruple Fan Psu Black, AMD and Intel Compliant. Must be noisy as hell?

http://www.super-flower.com.tw/

 

[Ralph Mowery]

My query is that most of the 450 - 550 w PSUs all seem to have two fans,
and

I was wondering if someone could explain the rationale behind this.
Do both fans blow the same way (exhaust, I guess) or in opposite directions
?

One thing that I have not seen mentioned is that the dimensions of the power
supply box will not allow a single big fan. If the supply is twice as wide
as it is high then you can not install one big fan. It will take 2 smaller
ones side by side. Also while probably not the reason , if one fails the
other may save the system from a total meltdown.

 

[w_tom]

Case (power supply) fans serve two purposes. First,
internal air temperature is a function of fan's CFM. For most
every ATX computer, a single 80 mm fan moves sufficient air.
Two fans put in series simply creates redundancy.

Second purpose is to create airflow in 'dead' spots. Air
flow must be, at minimum, so gentle that your hand cannot even
feel it. But to a heatsink, that airflow is the difference
between an extreme calm verse windy day. For example, if a
ribbon cable makes a dead air space over video controller
chip, then that chip may become too hot when room temperature
approaches 100 degree F (within normal operating limits of any
properly constructed machine).

Air flow direction is irrelevant because CFM - air into and
out of chassis - is the important parameter here. Obviously
if two fans try to blow air out and no vent lets air in, the
the overall CFM is minimized. But it does not matter whether
fan sucks air or blows air over CPU. What matters for
internal case temperatures is the overall CFM.

Now that 80 mm fan requirement assumed average CFM. Some 80
mm fans are low CFM - therefore low noise. Two low CFM 80 mm
fans can move same air as one regular CFM fan - at much
reduced noise. At this point, one important point must be
obvious. No one can accurately answer the question without
numbers. For case temperature, CFM is first spec necessary to
understand cooling and airflow. CFM determines case
temperatures. Many case temperatures would not overheat with
no fan when operated in a 70 degree room. But all systems
must also function in a 100 degree F room. Therein lies one
reason for at least one 80 mm fan. This and other reasons
first require specifications - especially numbers - to avoid
useless speculation.

 

[jeffc]

Hi,
I'm putting together another PC which is going to have quite a lot of hard
drives in it.
Normal IDE + IDE RAID + SCSI.
I'm thinking that a bigger than usual PSU would be a good idea, as there
will be quite a current drain when the system boots and all the drives spin
up.
My query is that most of the 450 - 550 w PSUs all seem to have two fans, and
I was wondering if someone could explain the rationale behind this.
Do both fans blow the same way (exhaust, I guess) or in opposite directions
?
If they both exhaust, then how does the air passing through the second fan
get out of the case ?
Does an extra hole need to be cut in the case ? (no problem, if necessary).
If they blow in opposite directions - then what's the point ?

Sure, there'd be no point to that. They blow the same way. Frankly, I'm
not sure why they use 2. With a fan blowing into the power supply, you can
direct the input more, rather than having it "leak" in through all the air
holes. I'm not sure that gains you much. But if you add some lights to
those fans, then windowed computers look more cooler

 

[jeffc]

If 2 fans is ott what about the new Quadruple Super Flower Real 520w
Quadruple Fan Psu Black, AMD and Intel Compliant. Must be noisy as hell?

http://www.super-flower.com.tw/

Holy crap, that's ridiculous.

 

[jeffc]

Case (power supply) fans serve two purposes. First,
internal air temperature is a function of fan's CFM.

And temperature of incoming air. And fan size. It's quite possible to have
lower rpm and also cooler temperatures.

Air flow direction is irrelevant because CFM - air into and
out of chassis - is the important parameter here. Obviously
if two fans try to blow air out and no vent lets air in, the
the overall CFM is minimized. But it does not matter whether
fan sucks air or blows air over CPU. What matters for
internal case temperatures is the overall CFM.

What also matters is the temperature of the air over the CPU. It depends
where the incoming air is coming from, and then it depends if that air has
already passed over other warm components before it got to the CPU.

 

[jeffc]

One thing that I have not seen mentioned is that the dimensions of the power
supply box will not allow a single big fan. If the supply is twice as wide
as it is high then you can not install one big fan. It will take 2 smaller
ones side by side. Also while probably not the reason , if one fails the
other may save the system from a total meltdown.

Actually, that's the only good reason I can think of, although I don't see a
"meltdown" occurring.

 

[SteveH]

One thing that I have not seen mentioned is that the dimensions of the power
supply box will not allow a single big fan. If the supply is twice as wide
as it is high then you can not install one big fan. It will take 2 smaller
ones side by side. Also while probably not the reason , if one fails the
other may save the system from a total meltdown.

I think you will find there is at least one PSU out there with a 12cm fan in
it.

SteveH