Rule of thumb for fans

[javafiend]

Is there some kind of rule of thumb for determining intake and exhaust fan ratios?

Thanks!

[jreffy]

There's several different ways you can setup your machine.

1) neutral pressure (intake = output)
2) positive pressure (intake > output)
3) negative pressure (intake < output)

As for which one is best, I have no clue, however I'm sure you can dig up some very long debates about it.

[Zephik]

For me, I just make sure that I have air blowing over all the "hot spots" in a computer. But you want to vent that air as well. Once the air hits those "hot spots", like the CPU, then it becomes warm and adds to the total temperature of the case which adds to the temperature of all your components. So you want to vent that warm used air and constantly bring in fresh cool air.

For most computer cases, I think it goes like this,

Front Intake
Side Intake
Back Exhaust
Top Exhaust

I think, if you have a higher intake than exhaust in that setup, you would have effective cooling. imo.

-SF

[b4i7]

you want your air to flow...not hit points where it just circulates

[ajmilton]

http://www.amd.com/us-en/Processors/ComputingSolutions/0,,30_288_13265_13295%5E13333,00.html

decent discussion on cooling. amd recommends negative air pressure inside the case, by the way.

[javafiend]

Do you generally factor in the power supply fan when considering overall cooling?

[ajmilton]

yeah. power supply fans exhaust.

[jreffy]

Do you generally factor in the power supply fan when considering overall cooling?

It depends. In my mod (link in my sig) I modded how my PSU is mounted in my case so that the PSU exhaust is pressed up against the top panel of my case and I made a blow hole. So in that situation, I would not consider my PSU fan into my air-flow scheme.

However, if that same PSU was mounted on the bottom of my case with the fan blowing up, I would definitely factor that into my cooling in that it's another air-flow/heat source to vent out of the case.

[nil8]

According to ATX spec, the power supply fan was the original fan to remove heat from the processor way back in the day. It must be considered.

Negative pressure has given me better cooling results, but it also accumulates more dust and debris.

[ajmilton]

seal it up good and filter :p

honestly though, i think you do get better cooling with neg pressure because (i think) it ensures more proper airflow - if you have positive pressure, the hot air isn't necessarily leaving the case quickly, which could bork your components. or at least make them less efficient

[jreffy]

seal it up good and filter :p

honestly though, i think you do get better cooling with neg pressure because (i think) it ensures more proper airflow - if you have positive pressure, the hot air isn't necessarily leaving the case quickly, which could bork your components. or at least make them less efficient

Common sense forces me to agree with this. So I think your answer is negative cooling.

Just as it was already mentioned, this creates more suction on the intake fans, which will pull in more dust and particles. Make sure you have good filters for your intakes (as well as making them easy to clean/replace) and you should be golden.

[nil8]

The Coolermaster Centurion 5 is a good sleek case with onboard filters and sports 120mm fans. I would check it out of you need a medium quality midtower.

[simon275]

Rule of thumb for fans dont stick your thumb in one.

Pretty much 120mm fans are good. It really depends on how much heat there is which is prortional to how powerful your components are. This will determine fan speed and number of fans. So if you are running a core 2 duo and a 7900gt say just a standard modern system you will need an intake and a exhaust fan running at 12v for full speed.

I have found filters no matter what is says on the packet cut down airflow a lot. So you may need to go for a top blow hole fan sucking out hot rising air or maybe if you have a big tower case then you can fit another fan on the front or in normal cases a fan in the side panel blowing air over the VGA is good as well.

[nil8]

A good power supply with 2+ fans (90mm or above) can remove enough heat that a blowhole isn't necessary. Still not a bad idea to have one, just up to you really.

Filters will always cut down air flow, it's what they do. The more clogged they are the worse their airflow is as well, so maintain your filters.

[a.Bird]

Negative pressure has given me better cooling results, but it also accumulates more dust and debris.Nothing a pressurized can of air can't take care of every few months. :p

[fragged]

This doesnt really make sence, if you made an airtight box, had one fan blowing inwards at 2000cfm, and another outwards at 1000cfm, wouldnt it just slow down/speed up one of the fans by the other ones push/pull pressure

I did year 10 physics, so i still have nfi tho...

[Illusion]

no case is air tight they all have vents and what have ya. se neg presure does make more sence. but i have to admit that i do use pos pressure in my box.

[javafiend]

So, would it stand to reason that since negative pressure pulls in more dust, would positive pressure help to keep dust out or am I just way off base?

[nil8]

No, positive pressure still has dust, it's just in different places.
With negative pressure you have a lot of dust on your cables, around your air entry points and on the bottom of the case.

With positive it's mostly around where the excess air leaves the case, so any opening in the case. It also collects around the entry points, as it will with all setups.

[b4i7]

haha...i had a friend that stuck his finger in a processor fan once....thinkin its"the same speed as a case fan" cut a big gash in his finger....

[fragged]

Yeah, some pretty valid points there

@b4i7

my CPU fan is 80mm.... I'd happily stick my finger in there than the small ass 6000RPM fan on my northbridge O.o

[slytherock]

positive or negative, It's not the biggest thing. Most important, your air must go everywhere without getting stock in one place. Make the test (With a side window, it's better), push smoke into your intake and see where it goes.

[RSzetec]

Id say negative is best, been mucking about with the fans in my pc and with positive i was at 33c and with negative im at 27c(case temps)

[Gardens]

in my unproffesional opinion i would say that negative pressure would be far better. The pressure drop between fans would create an increase in velocity causing better flow over your components and quicker hot air removal.

[nil8]

Sly did touch on an important factor, which is hotspots. All cases have places where air doesn't move as much or as well as the rest of the case. These areas are hotspots. The problem is that without a wind tunnel, it can be rather difficult to find them and eradicate them.
In most cases, their existance doesn't hurt much.

I think the general consensus here is negative pressure systems. That's what most people have anyway. The best way to get experience is to play around with it. See what brings your temps down the best, use it, then figure out if it follows our recommendations.