eflyguy

Barring any other influence, warmer air will rise, that is correct.

In a case with virtually any amount of airflow, it doesn't matter which way your fans move the air, top down or bottom up. As long as the air is moving, it never heats up enough have any influence on that airflow.
..a

__________________
Short Stroking - Why, and How

dkev

Well see here's the thing. Most of us like to run a little positive pressure. So what is positive pressure? Well basically your taking in more air then your exhausting. So you have air moving around in your case heating up while waiting to exhaust. This is one of the main reason so many cases have so much passive cooling now. So really, no matter what you do, you gonna get dust..lol.

__________________

PHENOMII X2 550BE OC CLUB

cathode

The best solution to avoiding dust is to not have skin, and not allow anyone in your house who has skin. Some studies find that up to 95% of household dust is composed of dead skin cells.

eflyguy

The air is not hanging around long enough to make a difference.

Tell you what, I'll reconfigure my fans to make this case "positive pressure", and repeat the test - at least, I'll do upright vs. inverted. I'll have all the fans "inlets" except the 140mm on top.
..a

__________________
Short Stroking - Why, and How

billbartuska

Some numbers:
Density of air...
0.0745 lb/sqft @ 70°F
0.0731 lb/sqft @ 80°F
A difference of 0.0014 lb/sqft

A typical fan (120mm, 67 CFM) puts out a pressrue of 0.12 in H2O
0.12 in H2O [4 °C] = 0.6243 lb/sqft

0.6243/0.0014 = 445.9

So, the pressure a (single) fan puts out is 450 times greater then the "pressure" of air that is 10 °F warmer.

Or......

Weight of air:
0.07492 lb/ft3 @ 70°F
0.07353 lb/ft3 @ 80°F
A difference of 0.00139 lb/ft3.
Since a typical case is about 2 ft3, that's 0.00278 lbs. of air in a case

Again, A typical fan (120mm, 67 CFM) puts out a pressrue of 0.12 in H2O
0.12 in H2O [4 °C] = 0.6243 lb/sqft

So, the weight of air a (single) fan will lift is (0.6243/0.00278) 224.5 x more than the weight of air in a case.

Conclusion: Ain't no convection in a computer case.

__________________
EDGE ...if you're not on it, you're taking up too much space.

eflyguy

Thank you, Bill.

Assuming you have ANY reasonable airflow (i.e. not a Dell or HP desktop!) then you can pull air thru in pretty much any direction..
..a

__________________
Short Stroking - Why, and How

dkev

You spoke about the air not hanging around long enough to heat up. Ok, well if that was the case, then your exhaust would be the same temp as your intake and your system temps would be through the roof because there was no heat transfer.
It's not like I'm not on your side, and 6 weeks ago I would have pretty much agreed with most of your statements. But on my last build, the exhaust temps are high enough to heat up the room I have my pc in. Which also contains the thermostat for the house. In order to maintain 72f in this room (my den) several of my rooms literally get down to 66-68f. I attribute most if this to my video card. The problem gets worse when I am gaming. The problem became apparent because my old case didn't have the exhaust capacity that my new one does.

__________________

PHENOMII X2 550BE OC CLUB

GoHigh

I don't think he meant NOT having an exhaust, but rather changing the normal direction of the airflow, as dictated by 99% of the PC cases. There really hasn't been a "groundbreaking" innovations in terms of traditional airflow design.

I do have to say though that for most instances, typically due to where we place our PCs, inlet from the front DOES make the most sense. It wouldn't make sense to let air draw from the rear, since GPUs and PSUs typically exhaust hot air out the back. That would cause the hot air to draw back into the case, which wouldn't make sense.

One thing I DO NOT agree with, is that the theory of +/- airflow in a case.

In theory, I guess it does make sense, but PC cases do not have enough of a seal to create any kind of vacuum effect to influence positive or negative effect in terms of airflow. I have tried this with about six difference cases (Antec 300, Antec 900, Antec 1200, Thermaltake Armor, and CoolerMaster Stacker 830), and it makes nae difference in terms of temps whether I had more airflow coming in or coming out.

IT DOES however make a difference WHERE the air is drawn from, and typically the more direct path to fresh cool air a component has, the better.

As far as INTAKE from the top and EXHAUST from all the other fans on the Antec 1200, 900, or the 300, the biggest drawback will be cooling of the GPU. (I tried this so trust me) All these cases have an option to place a 120mm fan on the side, which actually is a HUGE difference maker in terms of dropping 2-3c off the GPU temps. The problem with only drawing air from the top is that by the time the air gets to the GPU, it already bypassed the CPU, NB, RAM, and somtimes MOSFETs, which makes a difference of about 3-4c in terms of ambient temps.

Also, another flaw is that AIR does not travel in straight line, but rather gets dispersed at a wide angle. With typical case design, that means that about half of the air would get drawn out of the case via REAR and FRONT fans even before having an opportunity to get to the other components.

Hope all this helps.

eflyguy

Sorry for not being precise. The air is not hanging around to heat up enough for convection to be an issue..

Yes, it is heating up, otherwise it would not be drawing any heat from the cooler and other components - in my test rig, enough to notice the warmth blowing on you hand over the exhaust fans..
..a

__________________
Short Stroking - Why, and How

dkev

I think a lot of people forget just how warm the exhaust air is. If it feels warm to you then it's well over 100f, since your skin temp is 98f. One thing I should point out also, if your bringing in air, your gonna bring in dust. Doesn't matter how much positive pressure you have. So there's the issue. Most system fans don't have the ass to pull air through a denser filter material. That so called filter material they put in these cases is a joke. And the ones that use better filters cut the fan efficiency by half. That's why it is better to use high static pressure fans vs high rpm. But anyway this is a great thread and I am glad we could debate about it.

__________________

PHENOMII X2 550BE OC CLUB