Fans: suck or blow?
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.
- What components generate heat, and how distributed my heat sources are.
- Where I can expect the user to be in reference to the device.
- Dust buildup.
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.
- Fan with filter blowing air in. Without a filter the high speed air will pull in dust with the air, but as air slows down it will act like a river slowing down and drop silt.
- Exhaust in a few key locations you can pull in over a large area and expect significantly less dust to enter the enclosure due to low air speed entering the device, but you will still have dust issues. I would prefer everyone used filters, but I do not mind blowing out devices once in a while with an air compressor(just kidding).