Fan direction (In vs Out) for an external CPU side tunnel

[BeakerChem]

I have one of those new fangled air ducts to allow fresh air to get to the CPU through the side of the case. A fan already blows up and out on the CPU, but if I modded in a fan inside the port, would it work better to have it blowing in or out of the case?

Opinions?

 

[furballi]

There is no need to add another fan. Simply position the intake duct to within 1/3" of the CPU fan. If you must add a fan, then have it suck air into the case.

 

[Heckler 5th]

what heatsink are you using? i have an xp-120. i just recently finished installing a duct like this myself. my 120mm 108cfm sunon fan is on the outside of the side window with a tapered metal pipe on the inside to within 1/8" of the heatsink. with the fan sitting on the heatsink, sometimes reversing direction will gain a couple of degrees max. but i think that blowing cooler air from outside the case onto the heatsink is the best method in almost every case. furthermore, separating the fan from the heatsink works even better. my load cpu temp is now 43C with the fan at about 55-60% (down from 48-49C).

 

[BeakerChem]

Right now I have the stock heatsink that comes with the AMD X2 4800+ mounted with Artic Silver 5. I may get an aftermarket heatsink later after I test out temps.

So, so far two votes for blowing inward eg. Hmmm...

 

[furballi]

The big Zalman 7700Cu is fantastic! It cools the CPU, RAM, MOSFETs, and chipset with one big 120 mm fan.

 

[Navid]

I have no experience with this duct. But, my understanding is that the air around the fan moves away due to centrifuge. So, there is a small negative pressure in the center of the fan. If you have a pipe that comes close enough to the fan, that negative pressure is enough to pull air in through the pipe.

It makes no sense to put a fan in such a pipe to send air out as it would be working against what the fan on the heat sink is doing!

 

[BeakerChem]

Originally posted by: Navid
I have no experience with this duct. But, my understanding is that the air around the fan moves away due to centrifuge. So, there is a small negative pressure in the center of the fan. If you have a pipe that comes close enough to the fan, that negative pressure is enough to pull air in through the pipe.

It makes no sense to put a fan in such a pipe to send air out as it would be working against what the fan on the heat sink is doing!

Hmm, my understanding was the reverse. That hot air from the heatsink beneath the fan is push up and away from the CPU. Hence why I thought the fan should point outward to help expell hot air through the vent.

Hmm. I can't seem to find a goo theoretical treatment of this online, still looking though. What I found so far are peoples complete ducting to the outside where they expell air.

I may have to just see what the pressure inside is compared to the pressure outside and reverse accordingly.

 

[CycloWizard]

Originally posted by: BeakerChem
Hmm, my understanding was the reverse. That hot air from the heatsink beneath the fan is push up and away from the CPU. Hence why I thought the fan should point outward to help expell hot air through the vent.

Hmm. I can't seem to find a goo theoretical treatment of this online, still looking though. What I found so far are peoples complete ducting to the outside where they expell air.

I may have to just see what the pressure inside is compared to the pressure outside and reverse accordingly.

Do you have a picture of the setup? If so, I can probably tell you the best course of action.

 

[BeakerChem]

Originally posted by: CycloWizard

Originally posted by: BeakerChem
Hmm, my understanding was the reverse. That hot air from the heatsink beneath the fan is push up and away from the CPU. Hence why I thought the fan should point outward to help expell hot air through the vent.

Hmm. I can't seem to find a goo theoretical treatment of this online, still looking though. What I found so far are peoples complete ducting to the outside where they expell air.

I may have to just see what the pressure inside is compared to the pressure outside and reverse accordingly.

Do you have a picture of the setup? If so, I can probably tell you the best course of action.

This is what I have setup so far:

ImageShack picture of the MB in the case with the new PSU

This is the case duct from the Aspire website:

ImageShack image of Duct on side panel

Something that strikes me is that I am going to have a 120mm fan right next to the CPU and CPU heatsink, and yet nothing directly over my 7800GTX GPU. Maybe I should dump the duct and just put in an intake or outake fan alone to help cooling.

SO MANY CHOICES! Arg! Fun tho...

 

[Navid]

This is what I thought you were talking about.
http://cache-www.intel.com/cd/00/00/06/20/62085_62085.pdf

 

[CycloWizard]

Ah, now I think I see what you're saying. Well, I'd say sticking a fan in there isn't going to do much, regardless of which direction it's blowing. One thing that might be beneficial is to stick an additional fan somewhere far away on the case that is blowing in the opposite direction as the CPU fan (i.e. if the CPU fan is blowing away from the CPU/up the port, use the new fan as an intake. if the CPU fan is blowing air onto the CPU, then use the new fan as an exhaust). This should generally provide you with good airflow to maximize cooling. The optimum placement of this new fan depends on too many factors to say here, but generally if it's far enough away from the CPU, its exact placement won't be too important.

 

[BeakerChem]

Originally posted by: Navid
This is what I thought you were talking about.
http://cache-www.intel.com/cd/00/00/06/20/62085_62085.pdf

Wow, what a reference! Thanks! Let me dig through this a while an see what I can figure out from it. It looks like what my system is set up to do though!