Inlet or Exhaust ?

[rs125]

Hi

I bought a new power supply the other day (after being advised by people on hear ) and installed it but it only has the one fan on the back of the power supply - now the old one had two,one on the outside (back of case)and the other was on the bottom of the power supply ..i went out and got another fan today and have installed it to the back off the case so that it blows cool air in..is this the wright way or should i turn the fan around so that it blows the hot air out ???

thanks


gavin...

[Kubie]

rs125,

Turn it around to blow hot air out.

Carl

[rs125]

Originally posted by Kubie rs125, Turn it around to blow hot air out. Carl

Thanks Kubie ive just swapped it around

[reboot]

If you're wondering why, there's a huge technical explanation for it, which I'll paraphrase:
Negative air pressure cools faster than positive, thus having a fan sucking air out of the case creates negative pressure (or slightly less positive pressure) inside the case, allowing fresh air to be drawn in as needed to circulate, absorb heat, and then be exhausted.
Clear as mud?

[Tuf]

From engineering standpoint you are absolutely correct a fan will pull through a device much more efficiently than it will push it through the same device.

But the the terminology is incorrect, negative air pressure will cool poorer as it is less dense than atmospheric pressure. Positive Air pressure will cool even better.

[reboot]

Heh, I got it backwards...I knew it didn't quite sound right.

[Spyda]

i was going to say, if that was true then howcome all CPU fans blow down onto the heatsink instead of sucking air off the heatsink and pulling cold air from the bottom up

[TwoRails]

a fan on the end of a pipe, drawing air out, creates smooth, laminar air flow and less dead air pockets.

a fan blowing into the pipe, and trying to push the air out will create turbulent air flow and dead air pockets.

The same is true with computer cases, and pressure differential is negligible (unless, of course, someone puts 6 fans all sucking out, or in )

TwoRails

[Tuf]

I don't think any of us are hanging on the edge of cooking our computers or anything. But the ideal situation is to have slightly more fan capacity blowing in than exhausting. Your exhaust becomes more efficient and the denser air cools better. It's better than simply having a fan blowing out the back.

How much better than having just one fan blowing out the back?

Some, but not a great deal. Unless it is causing air to the front part of the case to circulate when it otherwise wouldn't.

In reality if your computer cools pretty well adding a bunch of fans won't make much of a difference. If your computer runs really hot it will work wonders.

[TwoRails]

I've gotten to the point where I will run only the rear fan(s) that comes with the case. I'll run the front only if the case comes with one.

I guess a lot depends on case design, but I've noticed that on my systems, having a front fan does not show a drop in temps when testing.

I used to subscribe to the front fan pretty heavy, as I figured it would stir up the air in the case better. But, they didn't drop temps, so I don't see a need for the (slight) additional cost and noise.

One day, I'd like to get a bunch of thermal probes and plaster them all over the insides of a case, wire each fan to an on / off switch, and have a ball testing! It would be fun

TwoRails

[HAL9000]

Originally posted by Tuf From engineering standpoint you are absolutely correct a fan will pull through a device much more efficiently than it will push it through the same device. But the the terminology is incorrect, negative air pressure will cool poorer as it is less dense than atmospheric pressure. Positive Air pressure will cool even better.

I'll argue that until the day I die. A postitive air pressure does not allow heated air to escape. When I was flying model airplanes and helicopters, to keep the engine cool, you needed 2 to 2 1/2 times exhaust area as intake to keep the engine from overheating. Allowing a larger intake than exhaust did not allow the heated air to escape and would cause overheating of the engine EVERY time. Go to some model air races and check out the planes. The intake will be literally a slit about an inch long and about 3/16ths of an inch high, while the back of the cowling is left almost entirely open with just some fairings around it for aerodynamics. Now lets eliminate the argument of the air blast form a fast mocing plane. Look at a model helicopter, no air blast over the engine except from a cooling fan. The intake is a hole around the crankshaft that is maybe1 1/2 in diameter, but a lot of that is filled by the engine shaft itself. The outlet is a good 2" x 3/4". Doing a rough calculation shows the areas to be roughly equal, but not so, remember to take out the area occupied by the output shaft of the engine which will significantly reduce the intake area. Positive air pressure allows heat to build with nowhere for it to escape, so it sits waiting to leave taking on more heat. What does heated air do? expand, creating more pressure, it's self defeating. 'Boot was right the first time.

[Kubie]

My thoughts, (what I have of them), is that the negative air pressure is negligible at best. With the exhaust fan pulling air out of the case the pressure is constantly equalized by air flowing in from the air vents.
With the hot air being exhausted out near the top of the case, cooler air is drawn in and therefore air movement.
Hot air is constantly being replaced by cool air.
Case in point.
On my system, I wasn't using any fans, save for the p/s fan and the cpu fan. Adding a exhaust fan dropped the cpu and mobo temps by 8C-10C.
Carl

[Force Flow]

This is quite interesting

Just let me get this straight (since it seems to be bouncing back and forth): is it better to have more fans blowing out or more fans blowing in?

[scott_d]

were adding more fuel to the fire, force, but i think it is better to have more out than in, it just works best on my box, i've tryed it both ways. right now im gettin 32c cpu/32c box...cant argue w/ that and it never goes over 35 on my 2000+ xp

[Force Flow]

Doesn't go above 35!?! I have a 1900+ that won't go below 47. That's not fair...

[Floppyman]

Remember there are other factors that come into play here on how well your pc cools than just fan placement, including the case itself, how tidied up it is (i.e. cables etc.), what other components are in there etc. Air must be able to circulate in the case for the pc to cool down efficiently.

[scott_d]

i have all my cables very neatly arranged..can hardly see any of em, cept at the top where the psu cables are all in a jumble. also, it is about 0 degrees outside here in nebraska, and hte register vent in my room is closed.

my box's name is frosty

[Kubie]

scott_d,

And this isn't even Nebraska.

Cpu 28C Mobo 29C

Carl

[scott_d]

is it a pentium?? it must be b/c mine is hard to beat for an amd

[Kubie]

scott_d,
Yeh, its a P4, but I just finished playing a game too.

BTW, I'm a Husker too.

Carl

[Force Flow]

Jeez... what do you guys put in your machines to keep them so cool?

[scott_d]

fairy dust..........and i have an expensive heatsink

you from lincoln kubie?

[Kubie]

force_flow2002,

Stock Intel hs/fan and one ehaust fan. The covers are on the case.

scott_d,
Columbus

Carl

[Force Flow]

Originally posted by scott_d fairy dust..........and i have an expensive heatsink

Can you get me some of that fairy dust?

What heatsink are you using?

[scott_d]

sure...the fairy dust is about 6 bucks for 3 grams, comes in a little syringe and is called arctic silver 3...not really dust, more like paste, lol

my heatsink is a big shiny slk-800

[Force Flow]

How's the noise level on that thing?

[scott_d]

not too bad, till the smart case fan 2 thinks things are gettin too hot, then speeds up
talked (bragged??) about it all night here ~ http://forum.pcmech.com/showthread.p...threadid=55628

[Force Flow]

How much did it cost and what's the full name of it?

[scott_d]

i picked it up for about $30 at www.svc.com and its a thermalright slk-800

[Force Flow]

Hmm... add that to $15 syringe of artic silver...add S&H for HSF...

That'll run about $50! Oh well... perhaps I'll find something cheaper...