2 fans

[inveterate]

How is back pressure calculated. and does anyone here know enough physics to figure out how far i have to place 2 equally powered fans to have it work compoundedly? THX, no one gave me answer in the cpu/case section, so i felt the need to put it under "Highly Technical".. Comments THX..

 

[element]

Originally posted by: inveterate
How is back pressure calculated. and does anyone here know enough physics to figure out how far i have to place 2 equally powered fans to have it work compoundedly? THX, no one gave me answer in the cpu/case section, so i felt the need to put it under "Highly Technical".. Comments THX..

I would think you'd want as low a backpressure as possible am I correct? I'm not sure why but intuitively it seems to me as if the distance between them will make little difference.

ok let me add to what I wrote a bit. I don't think backpressure is the right word here. The problem you might encounter will be with turbulence or magnetic interference. In this sense putting them a few inches apart should suffice.

I might be wrong though, in such an event may this post serve as a bump

 

[CycloWizard]

You want to put both blowing the same direction x distance apart and achieve what, exactly? It's in a computer case? Are they aligned? I can give you a pretty good idea if you clarify a little bit.

Generally, you'll have better effects in a computer case if the fans are not aligned. This prevents stagnant regions within the case, which basically just accumulate heat. Of course, there are exceptions, since having dead zones effectively increases the velocity through the rest of the case.

 

[DrPizza]

Maybe your question is simpler than I think; I however tend to believe that the solution to it is so complicated because of chaos (need to know too many exact measurements) that you could probably come up with an approximate solution using physics, but to refine the solution, you'd probably want to do it experimentally. With all the turbulence, etc. caused by the moving fans, (I think) calculations wouldn't be worth the bother- do it experimentally. There's a reason that wind tunnels are still in use, rather than just relying on physical calculations.

 

[inveterate]

UMM, yes, i just want to put them together one in front of the other. Hopeing they'd blow more air, but i don't quite see how to determine this experimentally either. how is turbulance visible, the magnetic thing i'm not worried about since, i'm sure that moving them apart solves that, but how will i determine how far, or if i need to put them at angles?

 

[CycloWizard]

Originally posted by: inveterate
UMM, yes, i just want to put them together one in front of the other. Hopeing they'd blow more air, but i don't quite see how to determine this experimentally either. how is turbulance visible, the magnetic thing i'm not worried about since, i'm sure that moving them apart solves that, but how will i determine how far, or if i need to put them at angles?

Short of dropping radioactive tracers into the air, it won't be easy to see. You could tell by monitoring your temperatures to see which configuration is the most effective. This is probably your best bet, though I would imagine that putting them in parallel would be more effective than in series.

 

[Calin]

Forget about magnetic effects - several millimeters of distance will reduce the magnetic effects from one fan to the other to practically zero. For best effect in the case, I would suggest to use one only for moving out hot air (like pulling air from the processor). Create a fan duct for the hot air, and cool your processor with that air. The temperature in the case should drop (as the processor is the biggest heat producer in a typical configuration)

 

[MyK Von DyK]

Basically, in a computer, positive or negative air pressure areas are not a problem, much more worrying are zero pressure areas, especially those that generate a lot of heat (either no fan blows in that area or negative and positive pressures are equal EDIT: and of same direction).

I have played with my fans in my mid-tower case for quite a while to see what results are better and have come to the conclusion that three case fans are needed to have enough draft inside of case and effectively blow heated air outside of case. In the end my setup was two 12cm interior fans mounted one on top of other angeling it 90 degrees (one mounted on hdd mounting holes blowing air front-back and another mounted on top of first one angling it 90 degrees and blowing on motherboard's chipset) and one 12cm fan on the rear of the case to blow heated air out.

Also, you have to account for the fact that hot air raises - so "intake" fans should be positioned as low as possible with "outtake" fans just below your PSU.

P.S.: Two fans positioned one in front of the other won't give you results you'd like. If you'd want to "double" air pressure with such setup you'd need a way to control positions of each fan's blades relative to each other. I can imagine scenario when front fan would actually interfere with second one (resonate) - could lead to vibrations, noise, ineficiency...

 

[Gibsons]

If you put the two fans working together, you'll get a higher airflow than one individually, but the second fan loses efficiency since it's dealing with turbulent airflow from the first fan. So the total airflow in volume per time will be lower than if the two were seperated. eg, compared to one fan, two stacked fans won't give you 2x airflow, but something like 1.5 or 1.8x (I'm guessing). I've no idea how to calculate that outside of empirically with smoke or whatever.

Intuitively, I would guess the turbulence begins taking effect if the fans are say, two fan diameters apart. Just a guess though.

 

[Tiamat]

Experimentally, I have never had real great luck with the 2 fans stuff. My best cooling has come from a centralized rear 120mm fan and some sort of open vent in the top 1/3 of the case, and an open vent in the bottom 1/3 of the case.

For example, an open 5.25" bay, and a front 120mm hole without fan.

This changes slightly with harddrives. A front 120mm fan at weak voltage is plenty for cooling a harddrive that is infront of it.

So, i guess I would say that overall "negative case pressure' has worked better for me than stacked fans.

 

[MyK Von DyK]

Originally posted by: Gibsons
I've no idea how to calculate that outside of empirically with smoke or whatever.

I use "sheet of paper" technique Put a low grammature sheet of paper in front (or rather at the back?) of your fan and see how well fan(s) push it away. If you wish you could also measure pressure with simple calculation. Just take into account that pressure raises exponentially with angle the sheet of paper makes. If total pressure for a 90° angle would be p>=mass/area and for 0° angle p<=0 then end formula would be p(0-90°)=sin(angle)*mass/area (I guess, this is plain trigonometry and it has been a while )

EDIT: you also have to account for the fact that total surface that fan's covering also raises exponentially so the end formula would rather look something like this:

p(0°-90°)=sin(angle)*mass/area^

where "area" is sin(angle)*fan size^

giving us end formula: p(0°-90°)=sin(angle)*mass/(sin(angle)*fan size^^).

And even this is not a true formula that would give you any meaningful values ("p" in this case is not any standard unit), it's more for comparison between different setups than anything else...

Best, MyK

 

[MyK Von DyK]

Originally posted by: Gibsons
...two stacked fans won't give you 2x airflow, but something like 1.5 or 1.8x (I'm guessing)...

Have tested this setup (one fan stacked on top of another) with two low voltage 12cm Antec fans (some 2400rpm) and calculated values with formula in my previous post. End results are not what anyone would like to see - some 22-24% increase in air flow. Will try this with higher fan speed... Stay tuned.

 

[Gibsons]

Originally posted by: MyK Von DyK

Originally posted by: Gibsons
I've no idea how to calculate that outside of empirically with smoke or whatever.

I use "sheet of paper" technique Put a low grammature sheet of paper in front (or rather at the back?) of your fan and see how well fan(s) push it away. If you wish you could also measure pressure with simple calculation. Just take into account that pressure raises exponentially with angle the sheet of paper makes. If total pressure for a 90° angle would be p>=mass/area and for 0° angle p<=0 then end formula would be p(0-90°)=sin(angle)*mass/area (I guess, this is plain trigonometry and it has been a while )

EDIT: you also have to account for the fact that total surface that fan's covering also raises exponentially so the end formula would rather look something like this:

p(0°-90°)=sin(angle)*mass/area^

where "area" is sin(angle)*fan size^

giving us end formula: p(0°-90°)=sin(angle)*mass/(sin(angle)*fan size^^).

And even this is not a true formula that would give you any meaningful values ("p" in this case is not any standard unit), it's more for comparison between different setups than anything else...

Best, MyK

Excellent! I love home-built ingenuity. This reminds me of how they've measured very tiny biological motors in some cases - they hooked it up to a microscopic glass fiber and measured how far it could bend the glass fiber. Now you need to come up with a name for your units. "This fan pushes about 5 Von Dyks, but the larger one here pushes 7." I guess you'd need calibrated pieces of paper to really make it work like that, but that's just details.

 

[MyK Von DyK]

Originally posted by: Gibsons
Now you need to come up with a name for your units. "This fan pushes about 5 Von Dyks, but the larger one here pushes 7."

You said it, "Von DyKs" it is and depending on how you pronounce it, it can also serve as measuring unit for some other purposes

Anyway, have done tests with full rotational power of my fans (some 4500rpm) and in case of two stacked fans results are even worse (+19-24% compared to single fan), although higher value suggests that when blades of each fan rotate "in sync" results are the same. BTW, I managed to push my fans to 0,125 Von DyKs, perhaps I should consider smaller units also, like "dDyK" ->:laugh:

Have fun all, MyK.

 

[NetFiend]

I have 2 fans in a duct at the front of my main machine. They are both on rheostats and i find i have to adjust them both at once for optimum airflow and noise, i.e. if the speed ratio is wrong the whole thing vibrates and is inefficient. I might experiment with distance as well.

 

[MyK Von DyK]

What is your single : double fan setup cfm? Can you at least speculate on this or give any thermal sensors readings, please? I myself have never tested such configuration with front mounted fans so any additional input would be greatly appreciated. TXH!