confusion over fan laws - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: October 03, 2017, 07:57:37 pm
content: I have read two seperate internet pages that state different things about the relationship between power input, airflow and pressure for fans running at the same speeds but of different diameters. One of these pages is wikipedia but it credits another article that I went through and found the laws stated as reported on wikipedia.So some say that the airflow is proportional to the fan diameter, others that is is square. I thought square sounds about right as the area increases with the square of the diameter.... Some report the pressure relationship as being the square others the cube, This then impacts greatly on the power input required as 2+1=3, the power increase is the ratio of diameters to the power of three or in the other case poser of 5! (2+3)does anyone have any ideas ? is this a centrifugal versus axial thing ? a centrifugal (squirrel cage) if doubled in diameter would in fact have just twice the blade area so maybe yes twice the flow although the speed of the circumference would be twice as well so the sqaure ? for a centrifugal fan if you increase the diameter the blade surface goes up by a square....For reference:http://www.engineeringtoolbox.com/fan-affinity-laws-d_196.htmlhttps://en.wikipedia.org/wiki/Affinity_lawshttps://www.pdhonline.com/courses/m125/m125content.pdf- see at the end, this document is the basis of the wikipedia article.

author: katzohki
author_rank: Frequent Contributor
timestamp: October 04, 2017, 04:23:11 pm
content: Yeah square sounds about right, but there's probably some math out there that makes it wholly more complicated.Toss in blade pitch and depth as well as number blades to complicate things. I believe that one of the Mac desktops had a really deep fan which allowed it to move a lot more air at a slower rotational speed which cut down on noise.Seems like a "non-trivial" problem. I've always just gone with the CFM that I need vs what the fan provides without trying to figure out how they got that number

author: Simon
author_rank: Unknown
timestamp: October 04, 2017, 04:25:39 pm
content: I think the basic laws that admittedly won't hold true all over the range assume that the blade design between two fans being compared is the same.

author: Ian.M
author_rank: Super Contributor
timestamp: October 04, 2017, 07:11:53 pm
content: The vortex induced by the blades slings the air away from the hub lowering the pressure near the root of the blades, + the coarse pitch at the root isn't going to be as efficient at moving air so the airflow of a fan at constant RPM with the same ratio of blade area to swept area wont scale with the square of the diameter, nor will it scale with the diameter. It is likely to be proportional to an intermediate power of the diameter between 1 and 2. The exact power is likely to depend on the blade design.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: October 04, 2017, 08:45:06 pm
content: Look up cooling fan data sheets and they show plots of pressure vs flow. You should be able to learn something from that.

author: tooki
author_rank: Super Contributor
timestamp: October 05, 2017, 06:44:11 pm
content: Quote from: katzohki on October 04, 2017, 04:23:11 pmYep. The Power Mac G5, which followed a G4 model that was so loud it had the nickname “wind tunnel”, had huge cooling needs with those hot PPC970 CPUs, but was very quiet even under load, because it used atonof fans (9, I think) whose total cooling was far more than it needed under full load, so that the fans ran at a fraction of maximum speed in use. It was software controlled, so when Mac OS X wasn’t booted (e.g. when in one of the firmware screens like the startup disk selector or Target Disk Mode), the fans ran at full blast, which was enough to make heavy drapes move. (Similarly, if the fan controller didn’t receive a control command from the OS for a while, it would turn the fans back up as a fail-safe.)Most Macs since then have continued with this basic strategy of oversizing the cooling so it can be run very slowly and quietly. On most desktop Macs, if they’re totally shut down, when powering them up, the fans will turn on full blast for a second until the OS begins to load.

author: Simon
author_rank: Unknown
timestamp: October 05, 2017, 07:47:19 pm
content: Quote from: Ian.M on October 04, 2017, 07:11:53 pmIndeed, and i beleive that the laws hold more true for centrifugal fans than for axial due to the varying linear speed of the blade at any point. on a centrifugal the whole of the blade moves to a new diameter whereas on the axial it just grows so some of the blade is at the new diameter.I have indeed a set of 3 datasheets for 3 different axial fan models, all 3 are in fact identical and simply their top speed is limited, they are 30, 65 & 125W the flow rates sort of follow fan laws.A manufacturer makes near 400W twin centrifugal blowers, they came out with a 740W version, what did you get for your double power ? exactly what fans laws estimate, 20% more flow. I discovered the same on a fan that we had to slow down to reduce power, at 80% speed we used 50% power, exactly as predicted by fan laws.

author: ratio
author_rank: Regular Contributor
timestamp: October 05, 2017, 10:08:29 pm
content: Loren Cook (http://www.lorencook.com/) has some interesting information about various fan designs in their Android app "Engineering Cookbook" They're geared more towards large HVAC fans, and a US company, but the designs are recognizable even scaled down to CPU fans. They have another app, "CookTools", that sports a fan law calculator, although it is intended to calculate the changes that will occur to a fan when it's operating parameters are changed.

author: b_force
author_rank: Super Contributor
timestamp: October 06, 2017, 01:54:42 am
content: Well, obviously it all has to do with the shape of the blades.Also, keep in mind that the noise goes with 50*log(delta RPM).So in many cases you can actually better have two fans running than one.A second fan roughly adds 4-6dB (they are not exactly coherent sources)

author: Circlotron
author_rank: Super Contributor
timestamp: October 06, 2017, 11:29:02 am
content: Quote from: Ian.M on October 04, 2017, 07:11:53 pmSo an axial fan is also partially centrifugal. This, I believe, is why rear drive cars with a viscous coupled belt driven fan at the front has the fan blades hanging 20-30mm out of the shroud, to utilize also the centrifugal component of the airflow. Interesting that computer fans don't use this technique.

author: Simon
author_rank: Unknown
timestamp: October 06, 2017, 11:32:46 am
content: Quote from: Circlotron on October 06, 2017, 11:29:02 amI don't think that fan cowl was made intentionally short for that reason, it's main purpose is to make sure air is sucked through the radiator, they obviously reasoned they could save some plastic and make it shorter. I have never seen a design that intentionally puts halve the blade out of the cowl.

author: Simon
author_rank: Unknown
timestamp: October 06, 2017, 11:34:52 am
content: Quote from: b_force on October 06, 2017, 01:54:42 amI've always wondered about that, and suspected that the relationship between noise and speed was not linear. I am currently working on a project where we could stack two fans for more pressure but I'd rather have 3 or 4 in order to be able to slow the fans down and make the overall unit more power efficient and quieter.

author: Circlotron
author_rank: Super Contributor
timestamp: October 06, 2017, 11:42:19 am
content: Quote from: Simon on October 06, 2017, 11:32:46 amI have seen it on a number of different cars over the years. It probably does save a bit of plastic too. On the car I had there was a very strong radial airflow as well as axial. I got the idea that for maximising airflow through the radiator there was an optimum depth for the fan into the shroud.

author: Circlotron
author_rank: Super Contributor
timestamp: October 06, 2017, 11:46:14 am
content: Also, you don't see any small fans with unevenly spaced blades to reduce any siren or droning effect by a spread spectrum effect. Car alternators have unevenly spaced blades for this reason.

author: tooki
author_rank: Super Contributor
timestamp: October 06, 2017, 12:41:55 pm
content: Quote from: Circlotron on October 06, 2017, 11:46:14 amI know Apple does this in the fans in recent MacBooks.

author: b_force
author_rank: Super Contributor
timestamp: October 06, 2017, 01:03:48 pm
content: Quote from: Simon on October 06, 2017, 11:34:52 amThere are enough articles on the web about this. It always blows my mind, especially in high end products, why designers always go for a one fan solution. It saves you easily 10dB, which is quite a lot.

author: Mr. Scram
author_rank: Super Contributor
timestamp: October 06, 2017, 01:14:43 pm
content: Quote from: Circlotron on October 06, 2017, 11:29:02 amShrouded fans with a fairly tight gap between fan and shroud are more efficient than loosely shrouded fans or fans without shroud. The tigher the gap, the more efficiency, until you start binding.

author: Simon
author_rank: Unknown
timestamp: October 06, 2017, 01:16:52 pm
content: indeed, the highest performance fans we have at work are cast and machined with thou's of clearance and we indeed check them with feeler gauges. Fan blades are like wings, and on aircraft they put bent tips on wings for that very reason.

author: b_force
author_rank: Super Contributor
timestamp: October 06, 2017, 01:29:51 pm
content: The efficieny highly depends on how the fan is used.To simplify things, there are fans than can push out a lot of air, but can't handle any load, and there are fans that can be loaded pretty heavily, but (therefore) don't push out so much air.A good usage of the first, is like an outake fan. Because normally they just blow in the open space.The second is mostly being used as intake fan or on top of heatinks. In situations were there is a lot of obstruction.Unfortunately (proper) thermal design is in most cases an afterthought. Also the first thing were money is being saved. Even in high end gear.

author: Simon
author_rank: Unknown
timestamp: October 06, 2017, 01:45:05 pm
content: Sucking or blowing is the same, fans produce a difference in pressure, put them on the inlet or outlet of the enclosure, it's the same thing.

author: b_force
author_rank: Super Contributor
timestamp: October 06, 2017, 02:06:18 pm
content: Quote from: Simon on October 06, 2017, 01:45:05 pmThan you're really oversimplifying things a lot.As i said in my previous post, there is difference. Especially if the enclosure/case is small position can be a key factor.Besides, pushing away air is also easier than letting air molecules filling the gaps (which we call sucking).That's just physics.

author: Mr. Scram
author_rank: Super Contributor
timestamp: October 06, 2017, 02:09:50 pm
content: Quote from: b_force on October 06, 2017, 02:06:18 pmCan you elaborate, or maybe even link sources? I'm happy to learn something.

author: b_force
author_rank: Super Contributor
timestamp: October 06, 2017, 02:15:17 pm
content: Quote from: Mr. Scram on October 06, 2017, 02:09:50 pmI have to look it up, it was part of my study back in the days (studied applied physics).It is part of thermodynamics, so if you understand those basic concepts it's almost a logic result of, so to speak.Don't know if I still have all my papers and books.I will come back to it.