For a fan, should air be sucked in, or should it blow air out?
I'm talking about an enclosure mounted fan.
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Airflow is the key. Any direction will do. Just keep in mind where the hot components inside your enclosure is.
However, if you blow into the enclosure, you have the option of putting a dust filter on your fan. Whereas if you have your fan blowing out, air will enter your enclosure through all sorts of holes, and lots of dust may eventually accumulate.
In my experience, the choice of direction determines where the inevitable dust build-up will occur. Dust seems to accumulate wherever the air enters the case.
With fans blowing in, you tend to get the most dust build up right in front of the fan. You wouldn't want this happening on the heat sink of your CPU, as the dust acts like an insulating blanket.
On the other hand, with fans blowing out, you get dust in the case near every possible way for air to get in. One place this can be a problem in a PC is when the air comes in through the openings for removable drives.
Probably the best design I've seen involves having two fans, one blowing in and the other out. The exhaust fan sits near the heat sink of the CPU to ensure good air flow there, w/o the dust problem, and the other fan pulls air into the case to keep the pressure in the case high enough to prevent sucking air (and dust) in through all the other openings.
When I design something like airflow, there are a few important points.
In order. If you have single units that generate a bulk of your heat, they need something near them. You have options, a fan on them in extreme cases similar to a high end processor or a fan on the enclosure next to them. If you have generally generated heat, you just need to focus on getting general air flow.
After you know where fans are needed, you need to think about the user. Why would your o-scope have a fan on the front that blows in? It is generating a lot of heat there, but if it blew forward and blew warm air in the user's eyes all the time I doubt they would thank you. Design air to exhaust in a direction the user can be expected not to be.
Now, on dust buildup, you want to limit this. I have two directions for this.
If you are talking about a fan on an enclosure, then the whole point of this fan is to cycle hot air out of the enclosure and pull cool air in. You have to have the appropriate vent on the other side of the enclosure to allow this to happen. Some enclosures have fans blowing both directions to help the process along. If you only have one, make it blow out, and near the biggest source of heat.
All the air that gets in will get out and the air resistance is the same in both direction so it is more or less the same thing. There are however some scenarios where it does make a difference. For example, if you have:
| Low power sensible components | High power components | Fan ---> |
Then you definitely want the fan to take the air out because otherwise, the high power component would heat the sensible component.
Another aspect to keep in consideration is that air speed is faster and more directional at the output of fan and slower and more distributed at the input. If you have a single very high power component that is difficult to cool down, you may want to point airflow directly to it, even if it will cause the rest of the unit to be hotter.
| Heat resistant components | Single very hot component | Fan <--- |
Generally, airflow isn't reversible in electronics without ramifications.
Complex software like Flowtherm is used to pattern airflow and heat patterns for a specific air direction. Usually parts are placed to maximize cooling with specific direction and speed of air.
Going against this could potentially lead to hot spots and bad things like thermal runaway.
I do believe in a simple thing about cooling: it's better to cool the components instead of draining the heat. Why? Let me show you a simple reason, that was done on my lab. power supply on pictures below:
This is my lab bench supply with 80x80mm FAN mounted on back. It is blowing air from OUTSIDE to the INSIDE. Why? Look another photo below. It's not the best, but that's all I got on me right now.
If I would be sucking air from inside to the outside, the fan would suck the air from closest gaps to the fan. But instead of this, the air is being blown into the power supply across the components, which will cool them all and go through any hole in the power supply cover as it wants.
I think you should have a negative airflow within the case itself. Dust to me is not a problem. I have 2 exhaust fans (fans sucking out) and air entering the front of the case via the vent holes (with filters) I have never, ever had a heat problem with this configuration. This way not only is the CPU and Ram and North/South bridges kept cool but the GPU will also blow out, therefore keeping itself cool. If you live in a stupidly warm room, this might not work but I like to keep my room at about 18c.