Enclosure cooling : blow fresh air or suck hot air ? - Page 1
Forum section: electronics / projects

author: jujun
author_rank: Regular Contributor
timestamp: July 21, 2019, 12:02:27 pm
content: Hello,I often see fan that suck heat from the inside of the enclosure to put it outide, but when air is heated, it's have a lower density, so It must be more efficient to blow fresh air inside an enclosure, and let the heated air flow outside by itself. Is it right ?Do you know any good practice about enclosure cooling ?Thank youJ

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 21, 2019, 12:35:28 pm
content: Not sure about entire enclosures, but I do know it's vastly more effective to blow on a hot component to cool it, than to suck air by it. Internally, find out what's hot and arrange the ducting so air it forced towards it. It probably doesn't matter how that's achieved, fan pressurizing the case, or exhausting it.

author: jaromir
author_rank: Unknown
timestamp: July 21, 2019, 12:39:13 pm
content: Fan blowing cold air has higher life expectancy than fan sucking hot air.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 21, 2019, 01:02:53 pm
content: I would say the key point whatever you do is airflow. It would very much depend on the enclosure itself, where the openings are located and what's inside, which will all influence airflow.Blowing outside ("cold") air in tends to accumulate more dust around the fan. A filter would of course be needed, but it will usually tend to clog faster in this setup, one of the reasons why it's rarely done IMO. Likewise it will tend to suck the most cold air in from the fan's opening, whereas blowing hot air out will tend to in turn have cold air sucked in from pretty much all openings in the enclosure. Those are general considerations, of course again it would require a proper airflow analysis to conclude.Really it's all in the airflow. In a "perfect" airflow situation, I don't think it would make any difference as to efficiency. Whatever volume of air you're blowing out will be replaced by air sucked in, or conversely. Now airflow is never perfect in a real enclosure, there are many obstacles and turbulences, openings are often not balanced/symmetrical, etc. If you're forcing cold air inside the enclosure, I think the cold air will mix with the hot air before eventually getting out, whereas forcing hot air out will tend to make the hot air get out faster, so there should be less inertia in a typical "crowded" enclosure. (And again in an ideal setup where the enclosure is very large, contains only negligible obstacles and has symmetrical in and out openings, it would probably not make a difference.)

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 21, 2019, 02:15:57 pm
content: 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.For best results, you'll wantgoodairflow. That is, you'll want the air to flow freely from the input vents to the output vents inside the enclosure, passing over heatsinks and components that need cooling. You don't want laminar flow, where the airstream stays in a tight bunch, though; you'll want a little bit of turbulence to ensure there are no pockets of nonmoving air.I've built silent and low-noise but high-performance PC enclosures for myself and some family members for fun, both normal-sized and mini-ITX ones. I use both ingress and egress fans, in push-pull configuration, with plastic and foam baffles to direct the airflow. Foam baffles also absorb some noise from e.g. fan motors, generating more easily tolerated wider spectrum noise instead. (Whoosh instead of whine.) When using input air filters, the fans were about equally powered; without input air filters, the intake fan a bit more powerful (not just in airflow, but also capable of generating a small pressure differential without dropping the airflow too much).Use the largest fan you can, even if it is in a suboptimal position. Small (Ø50mm or smaller) fans have to rotate too fast to move any air, and will whine. Larger fans can rotate slower, and in the 80×80×25mm class (smaller 12V PC case fans) you have a large selection of fan types to choose from. The 120×120×25mm 12V fan class has some excellent fans that can push quite a bit of air at 800 RPM, being basically silent; I like both Noctua and Scythe fans, but like I said, there is quite a lot of variety available today.Using a much larger fan also means that laminar airflow is usually not a problem, as the flow is generated in a relatively large area. This includes situations where there is very little room before/after the fan -- although in that case, I'd pick a fan with lower airflow but higher pressure differential capability.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2019, 03:21:11 pm
content: I suspect pulling is better than pushing, but it's probably a small difference -- assuming air flow over required surfaces is adequate in either case (ducting applied to heatsinks, etc.).The difference to me is this: dust deposition is greatest where the velocity and shear are greatest. This occurs at the fan itself, immediately downwind of it, and to a diminishing extent at a distance.If you can have laminar flow, expect very little if any dust deposition at all.A wide intake, that's not being stirred up by fan blades, may not provide laminar flow, but it can be low turbulence at least. It won't be high velocity, so don't expect stupendous power density.That's only about dust, of course, which drives long term reliability, again as long as temperatures remain adequate.It may well be a higher priority, over the specified lifetime of a product, that smaller size or lower cost is the bigger target, and for that, the direct, downstream airflow from a higher speed fan is required. Probably this dominates in most consumer equipment: laptops, PSUs, graphics cards and such.So I think it divides into two overlapping regimes, where for short lifetime, dust really doesn't matter and the intense flow is desired, versus longer and longer lifetimes where gentler flow is required, and also better quality fans (probably brushless ball bearing?); and screens, filters and sound dampening are also desirable.Two handy examples: my TDS460 oscilloscope has a huge fan in its side, 5 or 6 inch dia. Not very fast, so on the side of lower flow. It is "sucking" mode. It draws air over stacks of boards, not really ducted to heatsinks or anything (that I recall at least). It accumulates very little dust, surprisingly little I would even say.Other example: my 21" Trinitron monitor. This is an extreme example because, obvious gimme, it doesn't have a fan at all, it's convection only. It is tall enough to develop a modest flow, though, and there are perforations at the bottom and top to draw in, and let out, the air. It collects essentiallynodust -- even more remarkably, this includes the high voltage connections! And this despite being almost old enough to drink (if it were human), and yes having been in service much of that time, not storage!Tim

author: fcb
author_rank: Super Contributor
timestamp: July 21, 2019, 06:09:02 pm
content: Interesting question.I tend to have the fan suck in ambient air. My thinking is (1) it's easier to position the fan to blow across the cooling structure (heatsink/pcb/shunt) and then exhaust from one or more vents (2) it keeps the insides at a +ve pressure, if it's going to suck in dust I like it to be in one place so I can filter it if I choose (3) it keeps the fan bearing cooler.I have made an exception before for some equipment that was mounted at head height, it was louder and more annoying (and forced a small flow of air into the users face) to have a fan sucking at the back and exhausting at the front. In that case I reversed the fan direction - never actually noticed any change in performance or fan bearing life

author: David Hess
author_rank: Super Contributor
timestamp: July 22, 2019, 02:52:45 am
content: Quote from: jujun on July 21, 2019, 12:02:27 pmSince air density is roughly proportional the absolute temperature, the change in temperature between inlet and outlet temperature in typical applications has an insignificant effect.QuoteI prefer an intake fan so air can be filtered for dust and positive pressurization keeps contamination out. But in low velocity applications, this is less important so exhaust fans are almost as good and probably more convenient.Quote from: fcb on July 21, 2019, 06:09:02 pmStrategically placed vents and internal shrouds to direct the air flow make a huge difference.

author: bdunham7
author_rank: Super Contributor
timestamp: July 22, 2019, 03:51:55 am
content: Quote from: jujun on July 21, 2019, 12:02:27 pmI built a HTPC (Home Theater PC) where I learned quite a bit about this. My goal was to keep my fanless GPU, fan-cooled CPU and motherboard VRMs as cool as possible while keeping as quiet as possible. I also wanted to keep my hard drives at a reasonable temperature--35-40C is ideal. I ended up using a total of 7 fans--3 intake, 2 exhaust, 1 CPU and 1 PSU (that never turns on). It turns out that removing the hot air to keep the overall enclosure temperature down is only one tiny portion of the overall picture. It actually matters more that you have internal airflow circulating to transfer the heat from the component to the air. Even if the air is warm, say 40C, it is still cool enough to keep a CPU or VRM under 60-70C, which is good for them. The GPU has a huge heatsink, so just a little airflow keeps it about 5C above the enclosure temp. So moving the air in and out is only the first part--the key is using that air efficiently to cool whatever components need cooling.I know exhaust fans are the most common way of doing this, but I would prefer to use an intake fan, a filter and if needed, some shrouds or ducts to direct the air to where it's needed most.A good example to think about is the TEFC (Totally Enclosed Fan Cooled) motor design. With those, you have a completely sealed motor case that acts as the heatsink, while a fan inside cools the components solely by circulating air internally. You could do the same with an electronic component if it needed to be sealed or you wanted to keep dust out. Just use a big aluminum finned case and an internal fan.

author: Tom45
author_rank: Frequent Contributor
timestamp: July 22, 2019, 03:39:23 pm
content: I once had a fan get noisy and fail in a tower PC. So I replaced it and another fan failed within a year or so. Time to investigate.Turned out that whoever had built it had installed the multiple fans so that they all tried to exhaust air from the case. Never had another fan failure after I changed the direction of half of the fans.I'm in the camp of having a filter over any fan blowing into the case. And providing a path for hot air to leave, either by passive venting, or one or more exhaust fans.

author: GregDunn
author_rank: Frequent Contributor
timestamp: July 22, 2019, 03:51:49 pm
content: Quote from: Nominal Animal on July 21, 2019, 02:15:57 pmThis. 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.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 22, 2019, 03:57:59 pm
content: 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...

author: Siwastaja
author_rank: Super Contributor
timestamp: July 22, 2019, 04:18:12 pm
content: 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.

author: Berni
author_rank: Super Contributor
timestamp: July 23, 2019, 05:55:42 am
content: 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... etcLike 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".

author: Siwastaja
author_rank: Super Contributor
timestamp: July 23, 2019, 06:06:56 am
content: It's the same amount of total airflow, of course, but the airflowpatternis 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.