How much total CFM should I get?

[Alexander Culletto]

Well I plan on making my current case negative pressure, so that it sucks out any hot spots. I have 4 fan slots, all with fans in them already (Like factory 24cfm fans maybe)

Its for this build right here ! http://gyazo.com/65185e2a72cee58b45bb1841e759729d

Now heres the thing, they're 80mm fan slots, no bigger :/. The PSU will also be doing some exhaust, as that big fan on it will be pointing down at the top of the case. Suggestion as to how much Intake CFM/Exhaust CFM I want? Theres 2 in the front, 1 on top, and 1 on the rear! 😀

 

[volcanoscout]

If the case fans are all the same size and specs, just set your intakes to a lower RPM. If you set your intakes at 50%/2000 rpm, they should each be pulling in @26.4 CFM for a total of 52.8...call it 53 CFM. If you then set your exhaust fans at 60% or 2400 rpm, they should be exhausting at a rate of @ 32 CFM each, or 64 CFM total. Voila - negative pressure. Any additional from the PSU is just icing.

 

[Alexander Culletto]

well I know this, I just wanted to know how much CFM should I need to keep this system cool and not worry about any hot pockets

 

[mouse24]

well I know this, I just wanted to know how much CFM should I need to keep this system cool and not worry about any hot pockets

I don't think anyone can give you a specific answer for this. Its not hugely important, you either have good fans or you don't. Theres just to many factors, case layout, obstructions, ambient temps, heat output of various components, etc.

 

[Alexander Culletto]

well i'm gunna go for around 40CFM intake and maybe 80 exhaust + the psu. Hope this will be enough

 

[volcanoscout]

Mouse24 is right - there are too many variables to give you a pat answer. All you can do is estimate and track your temps. Try various differentials and see how well they work.

 

[Goodeggray]

You have the wrong idea about neg. pressure in computer cases. When you have neg. pressure in a case it means basically the air pressure inside the case is lower than the air pressure in the room. It means the fans are pushing out more air than they are pulling in. To equalize the sir pressure inside the case air is suck into the case from any opening and cracks. This is not what most builders want because that air is not filtered so the insides of the pc get dirty very quickly(think of a vacuum cleaner). What you want is positive air pressure(more air coming in that going out) the hot air is push out of all the openings so less dust and the hot air is forced out as wall as being exhausted by the fans.

 

[Alexander Culletto]

https://www.youtube.com/watch?v=4NYHC3DXynM

 

[Goodeggray]

You are misinterpreting the video. Positive air pressure is more air is intake by the fans than exhausted by the fans. Negative more air is exhausted by the fans than pulled in by the fans. You want positive not negative pressure in a pc case.

 

[Alexander Culletto]

No I ment I found a video saying what you were explaining I understand. I just see so many negative ones too so its hard to decide. I'm just gunna go positive then, maybe 200 CFM in on a new case. I can't see how that won't cool anything.

 

[volcanoscout]

You have the wrong idea about neg. pressure in computer cases. When you have neg. pressure in a case it means basically the air pressure inside the case is lower than the air pressure in the room. It means the fans are pushing out more air than they are pulling in. To equalize the sir pressure inside the case air is suck into the case from any opening and cracks. This is not what most builders want because that air is not filtered so the insides of the pc get dirty very quickly(think of a vacuum cleaner). What you want is positive air pressure(more air coming in that going out) the hot air is push out of all the openings so less dust and the hot air is forced out as wall as being exhausted by the fans.

I've been assuming that he was taking the dust ingress factor into account, which is why I didn't mention it, but he's not wrong about the cooling aspect. Removing heat at a higher rate than introducing cool air can be more effective than the reverse. It's just that the degree of effectiveness depends on getting the proportions right, and for most people the result is outweighed by the much higher rate of dust introduction.

I have a negative pressure fan profile that I use when I'm benchmarking my CPU or GPU SLI which usually gets me a 3C-4C advantage over positive. It's only about a 5-10% (estimated) differential between intake CFM and exhaust, but it certainly works. Of course I don't leave it like that for normal use - I don't like cleaning it anymore than the next guy

 

[Alexander Culletto]

But if you have all that air blowing in there and no where for it to go fast enough, I saw another site that showed it eventually started to heat up more components as that hot air kept sitting in there overtime. And yeah positive seems to be the best way to ignore dust, and eliminate it a good ammount

 

[Goodeggray]

Good, pushing out the hot air is more effective than pulling in cool air and dust from the openings. The dust becomes and insulating layer around the parts to keep heat in. The easy way to do positive presurre is to use the same size and brand and model of fans and have more intake fans than exhaust fans. Always filter intake fans.