mounting two 80mm fans on top of each other

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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...
 
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.
 
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. :?:
 
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.
 
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.
 
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.
 
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
 
Anyone see a problem with this, with the fans blowing in the same direction, of course.
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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.
 
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.
 
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.
 
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.
 
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. :)
 
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.
 
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.
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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.
 
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.
 
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:
Click to expand...
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
 
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?
 
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