Blow or suck - Page 1
Forum section: electronics / beginners

author: AlfBaz
author_rank: Super Contributor
timestamp: October 03, 2013, 02:17:50 am
content: I'm mounting a fan onto what is essentially a square heatsink tube, with fins on the inside.What would be the most effective with regards to cooling and the fan, pushing or pulling air?

author: David_AVD
author_rank: Super Contributor
timestamp: October 03, 2013, 02:32:29 am
content: I'm pretty sure the ones I've seen in audio power amplifiers have always been sucking the air from the heatsink "tube".

author: IanB
author_rank: Super Contributor
timestamp: October 03, 2013, 03:34:27 am
content: Blowing will add the waste heat from the fan to the exit air, so the air leaving the fan will be slightly warmer than the air entering.On the other hand, blowing will most likely give greater airflow through the heat sink than sucking.I would expect blowing to be best, but you could test either way by fixing a temperature probe to the heat sink and see which direction runs coolest overall. If you do this, please report back and let us know what you found...

author: David_AVD
author_rank: Super Contributor
timestamp: October 03, 2013, 03:37:39 am
content: It also depends on what's on the other side of the heatsink tube. The fan is almost always very close to the front or rear panel, so is exhausted quite well.If you blow air into the tube, you have to make sure is can exit properly instead of circulating inside the enclosure. Make sense?

author: AlfBaz
author_rank: Super Contributor
timestamp: October 03, 2013, 04:19:21 am
content: Hmm interesting.I'll run some tests shortly and post results.The only thing I could think of with regards to direction is less load on the fan when sucking as any air restriction is a reduction in load. What perplexes me is that if the fan is working less then either you have a more efficient setup or you are no maximising available work

author: pickle9000
author_rank: Super Contributor
timestamp: October 03, 2013, 05:17:43 am
content: When blowing air into a case you will almost always see a dust catcher or filter. My understanding is that you can get better flow inside a case by blowing into it (common on high end industrial stuff). It will typically be a more expensive setup where the flow is charted through the case to a vent. Exhausting air has no need for filtering because the goal is to blow out as much as possible.

author: Skimask
author_rank: Super Contributor
timestamp: October 03, 2013, 06:02:04 am
content: Mongo always blows into an enclosure with a filter (even if it is just a dryer sheet) on the outside.If you suck the air out, might not catch ALL of the little bitty holes that'll let the dust in.

author: AlfBaz
author_rank: Super Contributor
timestamp: October 03, 2013, 06:24:08 am
content: Just did a couple of quick tests in the hope that something obvious would be seen but there doesn't seem to be much difference. At first glance it appears that blowing was a little better but that may be due to variations in the timing/testing.So it would appear that the method you choose would more dependant on overall layout and possible environmental factors such as dust etc...

author: AlfBaz
author_rank: Super Contributor
timestamp: October 03, 2013, 06:35:35 am
content: Oh, I forgot to mention.Thermistor mounted on top of the heatsink channel, plenty of heatsink compound. Heatgun clamped in position and pointing to the side.Turned heatgun on when temperature hit 40degC I turned the fan on for about 2 minutes at which point the heat gun was turned off and the fan still on.

author: AlfBaz
author_rank: Super Contributor
timestamp: October 03, 2013, 12:02:11 pm
content: It's no device at the moment. I have a stack of IRFP450's laying around and plan to use them to see how well this system can pull heat. If all goes well I might end up turning into a heavy duty variable DC load for lead-acid battery banks and large PSU's

author: dfmischler
author_rank: Frequent Contributor
timestamp: October 03, 2013, 12:24:58 pm
content: Blowing is generally preferred because maintaining the box at a positive air pressure increases the air density so that a bit more heat can be removed at any given temperature. This probably goes double for devices that are operated well above sea level (avionics, anyone?). But I'm sure there are plenty of situations where sucking simply works better.

author: rsjsouza
author_rank: Super Contributor
timestamp: October 03, 2013, 03:13:54 pm
content: Also, if I recall correctly from fluid mechanics classes, a turbulent flow has a better temperature exchange when compared to a laminar flow (don't take my word for it, I am pulling this from classes 20 years ago). Sucking air from the inside will mostly create a laminar flow. Blowing to the inside will tend to create turbulence on the edges of the fins.

author: IanB
author_rank: Super Contributor
timestamp: October 03, 2013, 04:33:36 pm
content: A couple of notes on fluid mechanics topics:Quote from: David_AVD on October 03, 2013, 03:37:39 amActually, airflow needs a circuit much like electricity. If you blow air into the enclosure it must have an unobstructed exit, but the converse is equally true. If you suck air out of the enclosure there must be an unobstructed entrance so that fresh air can replace the extracted air. The direction of airflow does not materially alter this equation--it's similar to placing a resistor before or after a load in an electric circuit. The resistance impedes the flow either way.Quote from: rsjsouza on October 03, 2013, 03:13:54 pmTurbulent or laminar flow depends mainly on the air velocity and the size of the flow channel. It is likely that air flow in moderately large channels will be turbulent whether the fan is pushing or pulling, but very small channels and/or very low air flows could result in laminar flow.(The laminar/turbulent characteristic of fluid flowing in an enclosed channel is determined by a dimensionless group called the Reynolds number, which is proportional to the flow velocity. If the Reynolds number is greater than about 4000 then the flow will be turbulent.)The strongest impact on heat transfer comes from changing the air velocity since this is easiest to achieve (with a more powerful fan). Other factors like air density, air temperature and so on do affect the heat transfer but those parameters are mostly invariant in a standard room.

author: SeanB
author_rank: Super Contributor
timestamp: October 03, 2013, 05:08:09 pm
content: As well most fans are more efficient with a short smooth tube at air inlet and outlet, so to get the best airflow you need to have a tube taking the air into the fan, a short tube connecting the fan to the tube so that the air flow can stabilise and not get chopped by the blades as they turn. I have found that having a filter with an area about at least 2x the area of the fan with a smooth taper to the fan is best to get a smooth air flow into the fan, and then a short tube to the heatsink.

author: Smokey
author_rank: Super Contributor
timestamp: October 03, 2013, 05:37:51 pm
content: Blowing or sucking... The age old question... Depends on who you talk to...Seriously though, it depends on a lot of things.1) Type of fan. Check the specs for static pressure. That will give you a good idea of what it's capable of and how it moves air most efficiently and what happens when you obstruct it.2) Fan Surroundings. What kind of obstructions are there around the intake/outlet of the fan. Is there a big enough cavity for the air to move easily? If you only have space on either the intake or the outlet of the system, that can push you in the suck or blow direction.3) The system surroundings. Are you going to be putting that fan/heatsink assembly in something like a rack? Will that rack have enough clearance on the input/outlet for the air to move easily? That could be another factor.Forced airflow is really a systems problem that you have to both solve on the component level and on the system level at the same time.

author: TimNJ
author_rank: Super Contributor
timestamp: October 03, 2013, 05:51:23 pm
content: When I was into the PC scene, the general consensus was that blowing air over fins provides significantly better cooling than sucking.

author: rsjsouza
author_rank: Super Contributor
timestamp: October 03, 2013, 06:47:30 pm
content: Thanks for jolting my memory, IanB! Your explanation brought long forgotten knowledge - I was remembering the details as I was reading your post, including the Reynolds number.

author: Skimask
author_rank: Super Contributor
timestamp: October 04, 2013, 03:55:50 am
content: Soooo very many wrong turns this thread could have taken...How did it stay on topic for more than 2 posts?

author: c4757p
author_rank: Super Contributor
timestamp: October 04, 2013, 03:57:57 am
content: By being genuinely interesting to an engineer.

author: Skimask
author_rank: Super Contributor
timestamp: October 04, 2013, 04:03:37 am
content: Aye...excellent point.Some things are just plain worth keeping on track...

author: digsys
author_rank: Unknown
timestamp: October 04, 2013, 10:54:29 am
content: This is a beast I made a few years ago, still getting around to doing code for it. It's a high flow push-pull with turbulencevanes fitted. Currently it can do 2KW with 3KW peaks. It should be able to do 5KW peak once I fit copper bus bars onthe TO264s. The exhaust fan will definitely need to be an "oven" type to take the heat.www.eevblog.com/forum/testgear/dmm-that-has-dc-watts/msg268399/#msg268399

author: AlfBaz
author_rank: Super Contributor
timestamp: October 04, 2013, 02:53:11 pm
content: Quote from: digsys on October 04, 2013, 10:54:29 amVery nice!QuoteI have another fan sitting around and was considering push/pull also. Just feeling by hand there seemed to be greater air flow with both fans bolted together. I'll have to set up the test again and see if there's a much more pronounced difference than the first test.In your setup, it looks like a larger 110mm fan. If you don't mind me asking where did you source the heatsink and what did they call it. The one I got was form jaycar and buggered if I could find the right search term to find something similar on google

author: digsys
author_rank: Unknown
timestamp: October 05, 2013, 12:46:21 am
content: Quote from: AlfBazIt's from one of the dies we had made from Alcan/Capral/New name for Industrial P/Supplies we make.Should be a few pallets in stock, I'll check on Tue and let you know. There are a couple threads in these forums on the urls ofon-line suppliers. I did make a list somewhere, I'll try to find them :-) There are quite a few profiles available.

author: AlfBaz
author_rank: Super Contributor
timestamp: October 05, 2013, 04:14:37 am
content: Thanks, much appreciated

author: victor
author_rank: Regular Contributor
timestamp: October 06, 2013, 08:42:34 pm
content: Fans usually doesn't create much negative pressure, so they are best blowing than sucking.If you system have a constant power dissipation is common to set up the fan as a exhausting, like the power supply of your computer.But if you have something that goes from 2W to 20W power dissipation in seconds, you will want the fan blowing on the heat sink, like CPU cooler fans. (my computers goes from 40 degree C to 70C in seconds when you open something cpu intensive)

author: MakerMike
author_rank: Newbie
timestamp: October 07, 2013, 12:46:42 am
content: IMHO it depends on what is in the other side of the fins. Blowing air from the outside directly onto fins will give it the greatest cooling, if the outside intake is on the other side sucking would coll it the best.

author: Hamster
author_rank: Regular Contributor
timestamp: October 07, 2013, 12:51:12 am
content: In this situation the way the heat sinks are orientated, i don't think the direction will matter.However, if the other end will be in a compartment with other electronics, i would "SUCK" the air out to prevent the hot air heating the other components.or blowing the hot air AWAY from the other components.if the heat-sink is in free air, and the exhaust will not effect anything, and there would be no heat source from the fan inlet, is see no benefit of it sucking or blowing.i follow this up with i have ZERO fluid dynamics education, except that, if a open bottle of bear falls over, its going to get the ground wet.

author: Orpheus
author_rank: Regular Contributor
timestamp: October 09, 2013, 10:14:13 am
content: It's probably worth noting that the biggest fan in the world couldn't generate 15 psi of negative pressure inside a case (and if it did lower pressure = lower airmass = less thermal transfer, so at 10-14.7 psi your heatsink would increasingly be in a thermos bottle) but a huge fan could generate 100s/1000s psi positive pressure -- higher thermal transfer but you'd probably want to precool that air before introducing it to the heatsink (compression concentrates intrinsic heat to a higher temp)IOW, the asymptote works strongly in your favor in the regime of typical fans

author: smoothtalker
author_rank: Regular Contributor
timestamp: October 09, 2013, 11:11:53 am
content: I would say blowing is better. Ive no way to verify. When your food is hot you blow it instead of sucking it. Right??

author: AlfBaz
author_rank: Super Contributor
timestamp: October 09, 2013, 12:04:50 pm
content: Just a quick note on air flow quantity.Whilst looking at data sheets it seems that increasing air flow at some point starts to have insignificant effect on increasing heat transfer away from the heat sink. Curves have a reverse exponential characteristic.Knowing squat about the physics, it would appear to me that the radiation of heat by the metal is probably a constant and once you have sufficient air flow to immediately remove any heated air, increasing the flow of ambient temperature air will have little effect

author: IanB
author_rank: Super Contributor
timestamp: October 09, 2013, 03:26:12 pm
content: Quote from: AlfBaz on October 09, 2013, 12:04:50 pmThe important factor at play here is actually conduction rather than radiation. Radiation can be a significant mode of heat removal from heat sinks in still air, but it becomes much less important with forced cooling.Heat is transferred from the heat sink surface into the moving air stream by conduction across a "boundary layer" of air touching the metal surface. The coefficient of heat transfer here (the "film coefficient") increases without limit as the air flow increases. (Heat transfer coefficients are like conductances, they are the inverse of resistances.)Now the cooling of a device requires the heat to follow a path of several thermal resistances in series. There is from the die to the case, from the case to the heatsink, through the heatsink from mounting point to cooling surfaces, and from the heatsink surface to the surrounding air.As with electrical resistances, when there are several thermal resistances in series the larger resistances become limiting. So once the heat transfer coefficient between the heatsink surface and the surrounding air becomes good enough then other thermal resistances in the system start to dominate and put an upper bound on the maximum heat flow possible.

author: ciccio
author_rank: Frequent Contributor
timestamp: October 09, 2013, 07:48:40 pm
content: Years ago I designed a range of very high power audio amplifiers, intended to be rack mounted.The air flow was from front panel to rear panel (air was flowing inside the rack) because we did not want to have an hot front panel.We tested, with the same fan, both solutions (blowing-in and sucking-out) , and the result was that blowing-in was better, for the following reasons:1) the cooling channel was cooler (about 5 degrees lower), because cool air is denser than hot air, and what cools is air mass, not air volume.2) the fan temperature was lower, increasing fan's operating life3) a fan spinning in hot air seem to increase her speed, resulting in higher noise.The problem was that blowing-in required mounting the fan just inside the front panel, and panel aesthetic was compromised (there was no space for an internal tunnel, and the fan was almost visible) .Another problem was that the fan's noise was coming from the front panel, and this was objectionable.We opted for the less than optimum solution: an higher speed fan sucking air out from the rear panel (noise was inside the rack, and less audible, and panel design was simpler and more elegant).But the industrial version of these amplifiers, where aesthetics was less important, was designed the correct way.

author: AlfBaz
author_rank: Super Contributor
timestamp: October 09, 2013, 09:27:33 pm
content: Quote from: IanB on October 09, 2013, 03:26:12 pmNow that makes perfect sense, thank you Ian@ciccio Thanks for sharing some good points to consider

author: madires
author_rank: Super Contributor
timestamp: October 09, 2013, 10:14:10 pm
content: I'd say it depends on the design of the air flow though the box. If the box has some venting holes in the front or in the side panels mount the heatsink with the fan blowing out of the box. It doesn't matter much if the fan is located at the end or the beginning of the heatsink tube unless there's a considerable difference of the temperature which might stress a fan at the end of the headsink more. The air stream will also cool stuff whithin the stream from the venting holes towards the heatsink. But the air stream won't be distributed equaly. If you put the box into an industrial environment and prefer a closed box and a dust filter add a second fan behind the dust filter to suck air in. When you need more reliabilty put a fan at each side of the heatsink or stack two.