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mounting two 80mm fans on top of each other

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October 15, 2006 5:01:06 PM

Anyone see a problem with this, with the fans blowing in the same direction, of course.

More about : mounting 80mm fans top

October 15, 2006 5:03:01 PM

other than additional noise, none...
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October 15, 2006 5:46:37 PM

I tried this with two 120mm fans. The noice increased a lot and the airfllow only increased slightly.
a b K Overclocking
October 15, 2006 5:49:34 PM

I've seen this on 1U servers before, using 40mm fans. The P4 CPUs had large heatsinks on them, and were cooled by the 40mm fans. Even though they were only 40mm fans, they moved a LOT of air. As others have pointed out however, they were also the noisest thing I've ever heard on a computer, and my brother owned a tornado HSF...
October 15, 2006 6:14:58 PM

Have you ever blown into a floor fan? You end up sounding like darth vader. You're doing the same thing with one fan on top of another, blowing air into a moving blade. If you could somehow link the two fans together as to make one big blade, you might be able to reduce the noise and increase airflow.
October 15, 2006 6:56:15 PM

Seems to me that if the blades weren't aligned properly, air pushing out from the 1st fan might be impeded or encounter turbulence when running across blades from the 2nd fan hence the increase in noise. :?:
October 15, 2006 7:01:18 PM

Are you trying to increase air flow for your cpu cooler?
October 15, 2006 7:41:11 PM

I have an aquamate mini on one of my computers with one 80mm on the top and one on the bottom (both blowing in the same direction) and it makes a dramatic difference in my temps. 32c idle vs. 38c. However you will need to factor in the noise issue because just like the cooling the difference is noticible.
October 15, 2006 8:09:37 PM

i did that with the radiator of my system, 1 for pull the other for push. it did work better and moved more air. think of it as an SLI type solution. even though you will have twice the power it will only be at best 70% better, more often then not about 30~50% more air pushed. also if they are different fans that move different amounts of air the more powerful one will stress the motor of the less powerful one as it tries to push more then the less powerful can handle.
October 15, 2006 8:18:07 PM

Can you get at least one fan with a manual speed control twist style control, you would then be able to fine tune the speed until the noise was reduced, and air throughput was good.... Otherwise you may end up with some very noisy harmonics.
October 15, 2006 8:32:22 PM

I've tried it with 2 80mm fans, it works, but like others have said, it inceases the noise quite a bit.

What I found tho is if you have a spacer, something like those fan adaptors (the 120mm to 80mm, ect...) but it doesnt change the size, then it quiets only a bit, but the airflow increase enough so that its worth it, only problem is that it takes up more space.

Sorry if it sounds a bit messed up.

HTH
October 15, 2006 9:03:45 PM

Quote:
Anyone see a problem with this, with the fans blowing in the same direction, of course.


What are you doing with 2 fans, one on top of another? What are you cooling?

If both fans push air out, they'll have to be in sync or you're going to create turbulance. Same goes if one is pushing air out while the other pulls air in. The fins can be deleterious to your CFM since its just one more thing to get int he way.

Maybe a 120mm fan is a better option if possible?

It seems to me that you're just trying to increase air flow (DUH). If your case is too hot, you'd do better cutting a hole and adding a 2nd fan. Its more important to have more area to blow air out of then to force it out faster through less space. Its like chasing your tail when you can barely rearch it.
October 15, 2006 9:12:58 PM

Engineers once considered this approach for airplane engines... the idea had a quick death.

Better find a taller/larger fan. Larger blades will be more effective in all aspects.
October 15, 2006 9:28:14 PM

also, are you planning on having the fans counter-rotate or rotate in the same direction?
October 15, 2006 9:49:38 PM

You will get 3 times the noise and 1.5 times the flow a single fan.
You could achive the theoretical 2x noise and flow only if the blades speeds and phase angles were exactly the same.
October 15, 2006 11:43:19 PM

Take a close look at a turbine engine and what it does. And the russians have some big turbo props with dual propellers on each engine, they spin in different directions to help reduce noise.
October 16, 2006 12:00:28 AM

Wow, a lot of feed back here. This is for a blow hole on the top of my case about in inch away from the PSU. I have mounted a green led fan that came with the case there, but it sucks and does not push out much air. Being cheap, a led fan is a led fan, so I want to keep it there, but if by adding a second fan underneath I could increase the airflow, I thought it might be kind of cool--and I have lots of non led fans around to play with, even some clear ones that the green light can shine through.

I suspect my temps will be very good with just the one cheapo fan, and I have been talked out of the experiment. I have ordered two other green led fans, one for the side and one for the back. There is also one in the front, non led, so the system will have two blowing in and two blowing out, plus the PSU. No, temps will be fine, but the top fan could push more air, that is all. It will be nowhere near as cool as my computer, but my nephew will be happy in the end.

I wish some of my other dumb posts were given this much attention. :) 
October 16, 2006 12:39:08 AM

My money would be on one good fan vs. 2. By having 2 fans in series with each other, you will not be able to move more air, unless there is a lot of back pressure.

In general the more air you want to move with a single fan, the louder it will be. A larger fan will move more air than a smaller fan for a given noise level.
October 16, 2006 12:48:35 AM

Quote:
Engineers once considered this approach for airplane engines... the idea had a quick death.

Better find a taller/larger fan. Larger blades will be more effective in all aspects.


Exactly. When you run a fan you create a wind vortex (ever see a windtunnel in action?) With fans mounted directly behind each other, the second fan is "fighting" the vortex created by the first, hense more noise and reduced efficiency. Unless of course you can get them both synchronized, but this is highly unlikely without an extremely sensitive speed controls and monitors.

Your better off mounting them in parallel.
October 16, 2006 4:24:23 AM

It is a complete and utter waste of time. Please believe me!

You see, it is simple math: throughput is a function of fan capacity. If you have 1, 35 CFM fan and put another 35 CFM fan on top of it - how much air can potentially be pushed? Yep - 35 CFM!

In fact though, it will be far less than this, so two fans atop each other will produce less than a single fan. This because of the turbulence between the fans, and the fact that fan blades are designed to pick up air which is essentially laminar.

No - you're wasting your time, and hurting performance.

The easiest way to confirm for yourself that you are not correct is to imagine a little thought experiment. You say "If 1 is good, then 2 is twice as good, or almost". To see if you are correct, lets extrapolate your idea:

Lets take 1000, 35 CFM 80mm fans and put them on top of each other. If you are correct, then 1000 of these fans in series should push around 35,000 CFM of air - or quite a strong thrust from the end of our device. Clearly, this won't happen: If it did, the principle of adding fans would already be used in multiple locations.

I think 1000 of these fans added together might equal almost a quarter of the total output of a single fan, due to inefficiencies, pulsating back pressures, and turbulence.
October 16, 2006 4:26:17 AM

Quote:
It is a complete and utter waste of time.


Not necessarily, could conceive getting a shiny new 40mm fan from that coupling... :wink:
October 16, 2006 9:13:32 AM

Quote:
It is a complete and utter waste of time. Please believe me!

You see, it is simple math: throughput is a function of fan capacity. If you have 1, 35 CFM fan and put another 35 CFM fan on top of it - how much air can potentially be pushed? Yep - 35 CFM!

In fact though, it will be far less than this, so two fans atop each other will produce less than a single fan. This because of the turbulence between the fans, and the fact that fan blades are designed to pick up air which is essentially laminar.

No - you're wasting your time, and hurting performance.

The easiest way to confirm for yourself that you are not correct is to imagine a little thought experiment. You say "If 1 is good, then 2 is twice as good, or almost". To see if you are correct, lets extrapolate your idea:

I think that judging by the responses on here 2 35 CFM fans seem to push more air then one.
and I have tried it, and it does allow for higher CFM.
and the turbulence? thats compression of the air between the 2 fans which allow for higher through put.
also the arguement that 2 35 CFM on top of each other will only push 35 CFM is flawed because the 35 CFM is based on the fans being in still air, if you force air towards it, it will pass thru more air...its simple maths
October 16, 2006 11:16:49 AM

Looks like we have a project for TG on our hands.
October 17, 2006 7:11:03 PM

Are you really suggesting that in an airplane engine there are two fans of the same size, side by side, operating towards the exact same immediate goal :roll: ? Because that’s the point of this thread.

If the Russians had such a success on this type of engine, how come we don’t see them more often?
October 17, 2006 7:49:57 PM

I'd think... in theory... if the second fan were of higher cfm capacity than the first.. And there was some amount of distance between the two, so the turbulence could even out to a more linear airflow, then the two could potentially have higher output.

The first fan would work as a pre-stage for the second. Get the air moving, then the second fan would be more efficient, it would accelerate the air flow even further.

If fan cfm is based on move air that is at rest, accelerating it. Then I'd think the two stage approach could potentially do the jump.

Similar to multi-stage high pressure air compressors.

Just my $0.02.
October 17, 2006 8:11:19 PM

Quote:
Are you really suggesting that in an airplane engine there are two fans of the same size, side by side, operating towards the exact same immediate goal :roll: ? Because that’s the point of this thread.

If the Russians had such a success on this type of engine, how come we don’t see them more often?

first of all, are you talking about turboprops or turbofans?
and they have had many successes with that form of engine but there are 3 main reasons
1 the technology has been supersided by turbofans and turbojet technology which have a better fuel efficiency, more range, and better thrust to wieght ratio.
2. It was really really f***ing noisy...apparently even noisier then a turbofan.
3 cold war's only been over 20 years...I dont think that the US wants to take advice from its former enemy so quickly
also...i dont think he's talking about side by side, i think he's talking about one on top of another, think counter-rotating blades. side by side is a bit misleading
October 17, 2006 8:30:21 PM

Quote:
It is a complete and utter waste of time. Please believe me!


sorry no its not a complete waste of time
its not as efficient as a larger fan

quick example - the window in my room is a ways away from my bed - when i want it cool in my room i put a fan in front of the window.
now sitting on my bed i cannot feel the air - it just so happens i have 2 identical box fans, when i stack them directly next to each other i can feel a very good amount of air flow - can i equate this? no.
can i say 2 fans directly on top of each other move more air? yes.

ranger90 had it down pretty good - 1 fan accelerates the stagnant air into the second fan hence making the second fan more efficient.

back pressure from to many fans?????? comon guys cases arent air tight, mabey a few are, i would prolly say 90% arent even close to air tight, so there is no worries about back pressure inside the majority of cases. besides the guy said he wants exit and entrance fans...

regardless of all this - even slightly warmer moving air is better than no moving air - since when did the air flow nazzis get let out????????
October 17, 2006 8:52:46 PM

Problem? no benafit? no.
October 17, 2006 9:04:33 PM

Well that's not totally correct. You will get some benefit because like it was stated earlier, you'll pre-pressurize the air for the second fan with the first one. There's always a pressure rise across a fan, that's what they do. You're not going to get 2x the airflow, but you will get 2x the noise. The blades just aren't designed for it, they're made to sit there by themselves and pull ambient air.

If noise isn't an issue, then do it. But if its going to drive you nuts to listen to it, just grab a higher flow 80mm fan.

You could up the efficiency (and probably drop the noise) by having a stator in between (like cooling fins, something similar to that) but it isn't really cost effective at that point.

And yes, I've had some schooling in blade aerodynamics.
October 17, 2006 9:26:10 PM

Hom much do you know about the dynamics of turbine engines and propellers? I have to know about them because I use them. A turbine engine uses blades to compres the air that it takes in to increase velocity.
October 17, 2006 10:29:24 PM

Using a turbine engine in relation to electric fans is just wrong (IMHO) (and I doubt anyone uses counter-rotating props to reduce noise). In a turbine the fuel burning in the combustion chamber drives the shaft that turns compressor blades.
October 17, 2006 10:31:32 PM

Using a turbine engine in relation to electric fans is just wrong (IMHO) (and I doubt anyone uses counter-rotating props to reduce noise). In a turbine the fuel burning in the combustion chamber drives the shaft that turns compressor blades.
October 17, 2006 11:14:17 PM

Quote:
If the Russians had such a success on this type of engine, how come we don’t see them more often?


Because Russia is bankrupt, How often does anyone see Russian hardware. In addition prop planes were made mostly obsolete by the high bypass turbine engine. Prop planes only have a place at relatively low speed, <<Mach 1.

Jet turbines have more than one stage to increase the pressure. This is what cascading fans will do.

From a different perspective, Most cases have ~2 fans blowing into the case with the same number blowing out. This is essentially the same thing as having 2 fans back to back, only they are separated by the case. If not for back pressure due to flow resistance you could 'double' your air flow by having all fans blowing in or out (in parallel of course) But air resistance would cause an excessive amount of backpressure, reducing the airflow.

Most fans are rated by CFM and pressure (inches of H20). If you dig around MFG. sites you will find pressure/flow curves that relate the two. For a given working pressure you will get a given airflow.

If you have a poorly designed system, it is conceivable that you could end up blowing air backwards through a fan.

The difference between a Turbine engine and a case fan is that the turbine has turning vanes to eliminate the vortices between stages. In addition all the disks turn on a common shaft eliminating some of the noise of the fans fighting each other. Additionally note that the stages get progressively smaller then bigger, air is compressible.
October 17, 2006 11:44:27 PM

Regarding turbines blades: Those blades are all on the same shaft and the blades are aligned properly.

Here, we are talking about 2 separate fans on 2 separate shafts and no alignment of the blades from the first fan to those of the 2nd.
October 18, 2006 1:20:29 AM

Ok. This is going to be a little off topic.

Before anything else; just for the record, I did not tell these types of engines were never constructed.
Performance-wise they were quite powerhouses pushing a lot of air.

Now the reasons why they were left behind:

1. The blades had to be different (from each other and from a single fan design) so the first would not cause the second to stall. Counter-rotative helps, but does not solve the problem.

2. Each engine (yes, these designs use one for each fan) have to be carefully and individually calibrated for precise RPM, as to prevent the same stalling problems mentioned in the item 1.

3. The axis of the first fan, have to rotate inside the axis of the second fan, generating problems with lubrication, heat, and possible instability (as the were quite long).

4. Since it is an in-line design, there were severe problems with cooling of the second engine as it would inevitably receive a grate deal of hot air coming from the first engine.

5. Just for the sake of it… fuel consumption. (Duh!)


Summarizing: too much effort without the corresponding performance.

Them not being widely adopted, has nothing to do with cold-war and the US not wanting to “take advice from its former enemy”. At the end of the day cost-benefit is what really matters.

And when I wrote side by side, I meant stacking them. Sorry for that.


Now, back to topic:

Because of the reasons listed above, I stick with what I said before: Go for a larger fan.

EDITED for typing
October 18, 2006 1:39:22 AM

You got on your nerves for nothing. I’m talking apples and you are talking oranges. I’m sorry I’ve just noticed it; otherwise I would have pointed it out already and saved both our times.

I actually used an example that applied to the case at hand. You not only took what I wrote out of context, but completely misinterpreted it.

If you don’t believe me, than answer this question: How exactly does your picture of a compressor applies to the question that generated this thread?
October 18, 2006 5:11:26 AM

Quote:
You got on your nerves for nothing. I’m talking apples and you are talking oranges. I’m sorry I’ve just noticed it; otherwise I would have pointed it out already and saved both our times.

I actually used an example that applied to the case at hand. You not only took what I wrote out of context, but completely misinterpreted it.

If you don’t believe me, than answer this question: How exactly does your picture of a compressor applies to the question that generated this thread?
Just showing that a succesion of fan blades will create an increase of airflow, I'm not saying that putting two case fans together will work, but the principle does. You might be able to create a good wind tunnel that would provide great airflow, but the device would be to big to be of any use in a case.
Here is a pic of that russian turboprop with twin props on each engine.


The blades counter roatated on the axis of the turboshaft
a b K Overclocking
October 18, 2006 5:55:46 AM

Quote:
I'd think... in theory... if the second fan were of higher cfm capacity than the first.. And there was some amount of distance between the two, so the turbulence could even out to a more linear airflow, then the two could potentially have higher output.

The first fan would work as a pre-stage for the second. Get the air moving, then the second fan would be more efficient, it would accelerate the air flow even further.

If fan cfm is based on move air that is at rest, accelerating it. Then I'd think the two stage approach could potentially do the jump.

Similar to multi-stage high pressure air compressors.

Just my $0.02.

Perhaps, but it would only work to a certain point where the air is constantly going at the same speed, ie. up to a certain number of fans. Beyond that point would make no difference if the fans are all identical.
If the two fans spun at exactly the same speed, started at exactly the same time, stopped at exactly the same time so as to keep the exactly same positions of the blades relative to the other fan, and had blades positioned in such a way that the air flowed off one and onto the blade on the other fan with almost no impedence (by hitting the back of another blade), then maybe you could increase your air speed. But the chances of correctly setting these parameters is low to nil because all fans have unique motors in one way or another, just like every cpu is unique.
October 18, 2006 8:45:45 AM

Quote:
Problem? no benafit? no.

how is it that people still dont think that there is any benefit?!? do they bother reading other posts?!? i think that we have well established that it DOES offer benefit of higher through flow, but at higher noise.
a b K Overclocking
October 18, 2006 9:53:52 AM

Quote:
Take a close look at a turbine engine and what it does. And the russians have some big turbo props with dual propellers on each engine, they spin in different directions to help reduce noise.


Good point, though a turbine engine is basically a huge compressor, the multiple bladed thing can be made to work very effectively. There are also dual props for boat motors, called duo-props. Although it's in water, not air, I would suppose the dynamics would be close to the same. The second prop spins backwards from the first, and is also a different size (smaller) and lesser pitch, and spins about 30% faster than the main prop, so that it actually accents and enhances the flow of water from the first prop.

It simply boils down to if the blades are designed to work together, or not.
And most single fans are in no way designed to work with another fan directly in front or behind them.
October 18, 2006 10:39:22 AM

Yay, first ever post.

I have to point out that if you use 2 fans that are exactly the same the benefits will be very small. The 2nd fan needs more pitch on the blades than the first to accelerate the air further than the first set of blade could.

When ever you see a twin prop aircraft the 2nd set of propellors not only rotate in the opposite direction to the first but have alot more pitch on the blades.

In short unless you can get a 2nd fan with greater pitch on the blades that runs at the same rpm the benefits are going to be so marginal I would forget about it.

My 2ps worth anyway :lol: 
October 18, 2006 11:25:36 PM

i read the posts but that doesnt mean anything. I can read posts all day on why hte sky is red but that doesnt make it so. Blindly blowing air onto the back of another fan doesnt mean your going to get more air flow there is alot more to it then hey here have some more air. Two computer case fans back to back on each other with no distance between them isnt going to do much more then anything but make alot of noise.
a b K Overclocking
October 19, 2006 5:11:45 AM

In conclusion, dont bother unless you want more noise. Just buy a larger fan or forget about it altogether.
October 19, 2006 5:29:46 AM

Don't piggy back fans. It makes them work harder and you don't get double gains.
October 19, 2006 1:10:06 PM

The general consensus, little more air but a lot more noise is the correct one, but I read through this thread wondering if anyone would give you the right reason.

Different fans have different advantages. Ever wonder why almost all wall fans, circulating fans, or on this case computer fans are axial, and air moving equipment (like car ac, home ac, etc.) are centrifugal (squirrel cage)?

Well axial fans can move a lot of air at speeds that do not become audibly intolerable, only they have piss poor static pressure gain when limited to “acceptable” sound levels. In other words, in a pressure static environment, there is very little pressure gain across your typical axial fan.* Centrifugal fans can increase pressure significantly greater at the same volume of air, which is needed in ducted systems.

So in your computer, if you have a lot of congestion impeding your airflow, the radial fans you are using just don't have the gusto to get it through the tight spaces (duh). Stacking the fans does not increase the airflow per se, but it does decrease the pressure differential each fan sees, moving the fan curve just a bit, where only a marginal increase in airflow is seen.

*The exception to axial fans having piss poor pressure gain is when noise is no longer a concern. Then you can create a huge pressure gain, provided you spin the fan fast enough. In a turbine engine, this is exactly what they do. The limit here then becomes the material strength, given these components are spinning 10x faster than a Hayabusa motor. Where it is NOT a marginal pressure gain across the "stage", but rather significant, adding many stages is the best way to significantly increase the pressure on massive amounts of air - one of 2 things you need to have a jet (lots of compressed air & fuel). However, even in the turbine jet, the multiple stages are not there to increase airflow, but increase the pressure.
October 19, 2006 1:33:42 PM

I am first wondering WHY you would want to do this. First, if all the companies that deal in cooling have not marketed this then maybe there is a reason. The closest you see are some CP coolers that have a push-pull setup but there is a deal of space between the 2 fans (the fins) so the flow is not restricted by being one directly on top of the other. Second, after reading all of these post I am surprised that no one has offered another solution, simply use a larger fan. IF the mounting holes are for 80mm there is a short funnel adapter I saw at Frys that will allow you to use 120MM fans with 80MM mounting holes. Get one of these and a silverstone FM121 w/120 MM fan and you can have the best of cooling and quiet. The silverstone moves up to 110 CFM with rpm ranging from 800-2400 dBA from 17 to 39.5 http://www.silverstonetek.com/products-fm121.htm

If space is an issue use their 80MM FM81. It is also adjustable and will move upto 73CFM but at a higher noise level yet 48dBA @ 5500RPM at the higest settings. http://www.silverstonetek.com/products-fm81.htm

Good luck
October 19, 2006 1:57:27 PM

I've never personaly tried it,but a friend of mine has done exactly that.He actually managed to lower his cpu temps by about 5 degrees celcius.Yes it is a bit noisier,but the benefit of lower cpu temps I think does make for sound reasoning.Although this will not work with all heatsinks,(due to mounting issues),it will work with most.Goodluck.

Dahak

AMD X2-4400+@2.4 S-939
EVGA NF4 SLI MB
2X7800GT IN SLI
2X1GIG DDR IN DC MODE
WD300GIG HD
EXTREME 19IN.MONITOR 1280X1024
ACE 520WATT PSU
October 19, 2006 2:26:14 PM

Stacking fans does not increase airflow, it only increases pressure. Applying air to a densely finned heatsink is a good application for stacking, as pressure is required more than volume.



Regards,

Rick
July 27, 2011 9:13:10 AM

I did this yesterday. I have a pretty old PC case (from like 2003) it has only place for two 80 mm fans. One on the front and one on the back. I put two 80 mm fans on top of each other on the front and back. CPU/GPU temperatures dropped dramatically with about 10-15° Celsius less. Noise level is up of course but it seemed to work. The fans are from the same manufacturer so maybe this is why it works so well. I'm going to use this method only for the summer and unplug it in the winter or when I'm not overclocking. If you put your hand in front of one these double-80mm fans you can really feel how fast it's moving the air.
!