Positive, Negative, or Equal Air Pressure?

[Gage8]

Right now the pressure is about equal with 2 80mm pulling and another 2 pushing air.

I am thinking about putting a 120mm (63.05CFM@1800rpm) on the top of my dragon and making the 4 other 80mm as intake and possibly add one on the side for gpu.

What do you guys think?
What kind of pressure does everyone like: positive, negative, equal

Gage

 

[Finnkc]

Well people will say go with a positive pressure to blow dust out instead of in. However I have taken too many physics classes to know that whatever goes out of the box has to go in at equal amounts.

If x is out flowing air and y is in flowing air. X has to equal Y. SO just as much dust can come in as out.

So I would go with positive pressure but try and control the area where the air is take into the box via a filter or something.

 

[Gage8]

i heard on another forum that positive pressure raises heat levels more than negative b/c the heated air stays in the case longer. Any thoughts on that?

Gage

 

[Alphanos]

Positive pressure would increase the density of the air inside the case. I could be wrong, but it seems to me as though this would increase the thermal energy required to raise the air temperature (the specific heat capacity of the air). That should allow the air to absorb more heat from your computer without raising case temperature as much.

However, you do need air flowing out of the case to circulate. I think positive pressure is good to a limited extent, but you wouldn't want like 8 fans blowing in and 1 blowing out. Try to keep in/out fairly balanced, but if you have an odd number of fans or something I'd recommend making the extra blow in.

 

[deadseasquirrel]

Originally posted by: Gage8
i heard on another forum that positive pressure raises heat levels more than negative b/c the heated air stays in the case longer. Any thoughts on that?

Gage

i don't know if thats true in ALL cases, but it was for me. my set-up:

antec sx835 or something like that w/ true 330w
2 vantec stealths
2 antec case fans

when i set it up so that there are
3 intake
2 exhaust (1 being the psu fan)

case temps are 34-35C (ambient room is about 28C and the case is under a desk with little circulation)

when i set it up so that there are
2 intake
3 exhaust (1 being the psu fan)

case temp is 30C.

i have a filter that came with the antec case and it removes easily without having to open the case at all. so its easy to clean.

for me positive pressure isnt necessary (because of the filters and dust not being a problem). case temps dropped a lot by having another exhaust fan.

for a case that sits under a desk, in a room at 81F, in Texas, during the summer, i think 30C is the best i'm gonna do.

deadseasquirrel

 

[Bookie]

81f in your room? Time for an air conditioner. I'm in texas too but we keep out house at a cool 70f at all times. Makes for cool computers too. My vote would have to go for equal pressure to alleviate any strain on your fans, though I have two intakes and one (PS) outake for my case. I do have good ventilation though so there isn't much pressure in my case.

 

[huesmann]

Any positive air pressure will be very slight. Not enough to increase the density appreciably.

 

[deadseasquirrel]

Originally posted by: Bookie
81f in your room? Time for an air conditioner. I'm in texas too but we keep out house at a cool 70f at all times.

i'd love to, but cannot afford to. 5 months of unemployment will do that to ya. :| i keep downstairs at 81 and up at 84.

 

[Finnkc]

lol fans don't compress air all that much ... in a computer case that is. But I wouldn't go as Alphanos
said 8 blowing in and 1 blowing out.

4 in and 4 out

or like 2 in 3 out

 

[Cogman]

Originally posted by: Finnkc
Well people will say go with a positive pressure to blow dust out instead of in. However I have taken too many physics classes to know that whatever goes out of the box has to go in at equal amounts.

If x is out flowing air and y is in flowing air. X has to equal Y. SO just as much dust can come in as out.

So I would go with positive pressure but try and control the area where the air is take into the box via a filter or something.

umm, I belive this is backwards. I belive that you mean a Negitive case pressure to blow out the dust (+ means there is higher pressure inside case then outside, - means the case pressure is lower then the outside pressure...). anyways I've heard that equal pressure is ideal, but if you are going to go one way or another you should go with negitive as (like someone else said) more cool air will come in from holes anyways.

 

[Zepper]

Wow, an air conditioned out house! they must be rolling in it...
.bh.
:moon:
sorry, couldn't resist.

I'm for positive pressure (slight). It does keep air (and dust) from being sucked in through every crack and crevice. Though as another said, the intake air does have to be controlled to come in through a filter and the filter kept clean. Better to blow than suck, as it were. .bh.

 

[huesmann]

Well, you know those Texans like their creature comforts. If you're gonna have an outhouse it might as well be air conditioned!

 

[Gage8]

thanks for the replies guys, helps me out

 

[JulesMaximus]

I think positive air pressure inside a case would create eddies in the airflow which would probably be disruptive to the airflow through the case and probably trap more heat. I would say equal in equal out or slightly negative would be ideal.

 

[sodcha0s]

Positive pressure will most definitely increase case temps, how much depends on your particular setup. Negative pressure will tend to pull dust in every crack that is not sealed, including your CDROM and floppy/zip drives. So, is cooling or cleanliness more important?

Equal pressure would be best of course, but is not easily achieved. Is not as simple as "2 fans in, 2 out..." There are MANY other factors involved. Besides that, it is impossible to actually tell if you have equal pressure without controlled testing, and if you use intake filters your case pressure will drop as the filters get dirty.

My preference is a slight negative pressure with some sort of opening (like a drive bay grille) to allow outside air an easier way to enter rather than through my CD drives.

 

[mechBgon]

Bigger picture: The SX800-series cases will stay pretty cool even with very quiet 80mm fans (front intake, rear exhaust), that's how my 800-series Plus880 is running. I use the super-quiet NMB 18dB fans.

The top deck of the case does stagnate a bit, and I helped my case with that problem by flipping the power supply over (instructions here, very simple and reversible). One low-RPM 80mm exhaust in the top panel wouldn't hurt, but methinks a 120mm is going to be overkill!

 

[maluckey]

After years of thinking the brute force air flow would cool things without thinking about it, I got a shocker. 120CFM exhaust and 84 CFM intake, with 5000 RPM 80mm CPU fan, was three degress C. higher temp than with 2400 RPM CPU fan. It got weirder when I dropped the intake to 41 CFM, and droped Mobo AND CPU temps 1 degree more!

I guess that the pressure stopped flow, and these stagnant pockets raised overall temps.

 

[Zepper]

maluckey,
When you've got that much exhaust CFM (plus PSU fan) and so much less on intake, you're creating somewhat of a reduced pressure in the case.
. The lower density air will cool less effectively. It would be better to remove the intake fan and cover the hole with a piece of air conditioner filter.
.bh.

 

[NesuD]

Originally posted by: JulesMaximus
I think positive air pressure inside a case would create eddies in the airflow which would probably be disruptive to the airflow through the case and probably trap more heat. I would say equal in equal out or slightly negative would be ideal.

My experience completely agrees with you. I have always had slightly better cooling performance with equal or slightly negative air pressure ranther than positive. I know what you mean Maluckey. I have an Antec sx10xx series case with two intake and 2 exhaust fan mounts. I have 4 Y.S. Tech 50 cfm fans in them plus whatever the P.S. fan pulls. I have a fanbus in the system and run the front fan and rear fans on their own switches. If i run the all 4 fans wide open it actually hurts my cpu temps. If I run them at half speed which incidentally is incredibly quieter I am able to maintain a better cpu temp woth my cpu fan running slower make it all even quieter yet. Just a guess here but I think that all the extra turbulence created by those fans running wide open hurts the efficiency of the airflow through the case.

 

[maluckey]

Yeah, I was hoping that the all-aluminu Lian-Li would make up for the turbulence by acting as some sort of giant heat sink. It didn't quite measure up that way.

On a comic note, you all mentioned slight negative pressure in the case !!?? Small animals and children were at risk near this thing! I went as far as flipping the fan and tunneling the CPU fan (80 CFM) to out side air. That gave me 200 CFM out, and THAT worked wonders on Mobo temps, but drove up CPU temps. I flipped the Fan, and 160 CFM in and 120 out, and it raised the Mobo temps a lot, while dropping the CPU to barely above room temp. Now, the fan blows in, the exhaust turned down (RPM wise), the intakes removed, the hearing protection on the shelf, and the CPU fan at 3100 RPM. Life is good.

 

[Finnkc]

Originally posted by: Cogman

Originally posted by: Finnkc
Well people will say go with a positive pressure to blow dust out instead of in. However I have taken too many physics classes to know that whatever goes out of the box has to go in at equal amounts.

If x is out flowing air and y is in flowing air. X has to equal Y. SO just as much dust can come in as out.

So I would go with positive pressure but try and control the area where the air is take into the box via a filter or something.

umm, I belive this is backwards. I belive that you mean a Negitive case pressure to blow out the dust (+ means there is higher pressure inside case then outside, - means the case pressure is lower then the outside pressure...). anyways I've heard that equal pressure is ideal, but if you are going to go one way or another you should go with negitive as (like someone else said) more cool air will come in from holes anyways.

lol yep my mistake ...