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Enclosure cooling : blow fresh air or suck hot air ?

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GregDunn:

--- Quote from: Nominal Animal on July 21, 2019, 02:15:57 pm ---For dust ingress, it is useful to have the pressure inside the enclosure very slightly higher than ambient.  The pressure differential creates a kind of a barrier for dust.

--- End quote ---

This.  Pressurizing the case with filtered air tends to keep the accumulation of dust down and also tends to purge the case of hot spots; we learned this when building military electronics for extreme environments and it was a sort of rule of thumb we followed.

SiliconWizard:
Just a couple additional thoughts:

Although exhaust fans are often better IME (but again still consider airflow), it should be taken into account that they will obviously tend to run hotter than fans taking cold air in, because the outside air will obviously cool them down more (just because they are in the front line), which again doesn't necessarily imply that the internal temperature of the case will be lower. That may reduce the fan's life especially if it's low-quality and additionally not brushless.

Another obvious one: again airflow! If you're putting several fans on opposite sides of the enclosure, it's in most cases preferable to keep one airflow direction inside, thus making fans on one side exhaust fans, and intake fans on the other side... Running fans on opposite sides all as exhaust fans would create turbulences and actually hinder airflow in a severe way especially if there aren't any other openings in the case...


Siwastaja:
When the fan moves air through existing physical channels (think about an enclosed heatsink, air entering from one end, exiting the opposite), it does not matter much. The result is very close either way. Increased fan life when the fan runs cold, and the ability to filter dust better when all the air only enters from one single place, are arguments for blowing air in, but they are not always significant.

When you have localized hotspots in free space, the difference can be massive. Demonstrate it for yourself easily: run the fan, put your finger 10cm away from it, first on the sucking side, then on the blowing side.

The same effect can be seen if you make a fume extractor for your soldering iron: if it sucks, it... well, tends to suck. They require well-designed hoods and need to come very close to your workpiece. OTOH, if you blow, even a very small fan can blow the smoke away, even sitting far away. The only reason they use the ones that suck is so they can actually remove the smoke from the room, not just spread it everywhere.

Sometimes this applies to equipment, especially if the fan is added as an afterthought to cool parts such as capacitors and inductors. Options then are either simply blowing air towards them, or constructing complex plastic air channels to direct cooling properly.

Berni:
Its all just a mater of whats more practical for the particular equipment you are cooling.

The total amount of air moved trough the fan does not depend on direction. Its all just a function of the fans pressure vs flow curve at a given RPM and how restrictive the case is to flow. The restrictions determines the static pressure the fan has to overcome and this lets the fan move a certain amount of air. Yes there are tiny differences in density due to hot/cold air or positive pressure inside the case compressing the air, but the effect is too tiny to matter.

However the enclosure can present a different restriction depending on the direction of air flow because one direction might have more turbulent flow than the other (This is the working principle of a tesla valve, a one way valve with 0 moving parts) or one direction might be helped by convection if you have a tall case and are blowing air vertically.

There are still reasons why you might chose one over the other. Like positive pressure giving you better dust mitigation because only a filter on the fan is needed. Blowing air can also be an alternative to ducting the air because the momentum of the air carries it in a straight line out of the fan. Then again sucking the air off a heatsink can be attractive because you can blow the hot air directly out and keep the hot air from heating up everything else in the case (A lot of larger lab PSUs do this), but does require ducting to the heatsink to make sure all the air is forced to pass over the fins rather than sneaking in from the area around it. If both sides of the heatsink can be ducted then the fan could be placed on the other side of the heatsink to blow air trough it and then duct the air out... etc

Like most engineering its matter of weighting out the benefits and downsides. Some applications find certain benefits more important than others. Hence why you can find all sorts of fan configurations in products rather than one single "best way".

Siwastaja:
It's the same amount of total airflow, of course, but the airflow pattern is completely different. On the sucking side, it's similar to how a bare LED die emits light, in all directions 180 degrees. Hence, if you measure the cooling effect at different points near the fan, it decays quickly the farther you move.

On the blowing side, it's like a directional antenna, or like an LED beam with a narrowish (say 30 degree) lens. The cooling effect can be 1000x more at a distance of, say, 20cm from the fan.

Try it. Easy to demonstrate.

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