two fans, twice the airflow?

[CyNics]

http://www.industrialtechnology.co.uk/2000/apr/papst.html

The two smaller fans placed side by side will move less than a total of 100 cfm. Two similar fans in series (stacked) will not move more air than one fan but it will increase the amount of pressure the fans generate so will support more movement into a higher pressure environment. - www.7volts.com

does that mean there is no point to place 3 120mm fans side by side since they won't move more air than one fan?
I just cut 6 120mm blowholes, 3 for intake (bottom of my case, places side by side) and 3 for exhaust (on top of my case, placed side by side)

 

[Anubis]

6 120mm fans??!?!?!?!

wow i think you computer will actually me louder then mine

 

[CyNics]

no...actually they are okay...quiet...even though there are 6 120mm fans...they aren't that powerful..

 

[drx9175l]

Larger fans are usually quieter than smaller ones because they don't need to spin as fast to move a significant amount of air. The 120mm units in my case are much quieter than any of the 80mm fans I've tried.

CyNics, I believe the article from 7volts.com you quoted is saying that two fans mounted on top of one another will not increase airflow. Your fans mounted side by side should be moving an impressive amount of air through the case. The fact that you've balanced the amount of intake with the exhaust also helps. But remember, you'll never be able to reduce the case temperature below your room temperature unless you use some device to chill the incoming air. The CFM you have may be overkill. I'd suggest you disable a pair (one intake & one exhaust) at a time and see if there in any increase in your case temp. If you find that having all six fans on doesn't change the temperature any versus, say, having only two fans on then wire in switches (baybus) to turn off the unnecessary units. Of course if the noise of the extra fans doesn't bother you then disregard. Do you use filters on the intakes?

 

[CyNics]

on top of one another will not increase airflow
what do you mean? you mean one fan blows toward another fan? or placed on top of another fan(still it's side by side right?)
I'm currently looking for some suitable dust filters
Yeah I'm planning to make a baybus and will cover up the 3rd hole if 3 fans cooling performance is same with 2 fans or maybe 1 fan

 

[MIGhunter]

Do you have all the fans yet?

Here is the deal, putting the fans side by side like this [][][] is OK Staking them won't help.

The problem is in the intake/exhaust set up. Take a look at the specs on your fans. There is a measurement on how much air each one can push or pull. That is a max. So no matter what you do you can't exceed that rating. Now by taking on fan as intake and one as an exhaust, you can't compound the rating. So the two fans are limited by the amount of air it can push or pull. The better option is to put an open vent with nothing in it. Then putting your 3 fans as all either intake or exhaust. By doing this the only thing holding back your airflow is the pressure on the vent. Eventually you will reach this threshhold and adding more fans won't do any good. This is all a matter of physics. If you want I can pull a thread from a Car Audio forum by an EE on the subject.

 

[Drewpy]

Originally posted by: MIGhunter
Do you have all the fans yet?

Here is the deal, putting the fans side by side like this [][][] is OK Staking them won't help.

The problem is in the intake/exhaust set up. Take a look at the specs on your fans. There is a measurement on how much air each one can push or pull. That is a max. So no matter what you do you can't exceed that rating. Now by taking on fan as intake and one as an exhaust, you can't compound the rating. So the two fans are limited by the amount of air it can push or pull. The better option is to put an open vent with nothing in it. Then putting your 3 fans as all either intake or exhaust. By doing this the only thing holding back your airflow is the pressure on the vent. Eventually you will reach this threshhold and adding more fans won't do any good. This is all a matter of physics. If you want I can pull a thread from a Car Audio forum by an EE on the subject.

You could go with only intake/exhaust fans, and you are correct that eventually you'd be limited by the pressure differential for your CFM's. However, this can be overcome by having both intake and exhaust fans. You are incorrect by stating that the airflow would be limited by the pressureon the vent. The flow is limited by the pressure difference across the fan. By having fans as both intake and exhaust you can overcome a greater pressure difference (similar to stacking fans). This was noted in the initial post. The other plus is that because you can overcome a greater pressure gradient, you can also use air filters on you intake to help with dust inside your case.

Further by having both intake and exhaust fans, you should have cleaner flow inside your case. The majority of the air would be entering from one location and exiting via another. Without intake fans, your exhaust would be drawing air through all the little nooks and cracks in he case, rather than just through the vent. By having intake fans pushing CFM in, and exhaust fans pulling an equal CFM out, the air moving inside your case will be less turbulent, and will result in a more direct flow from the the intake to the exhaust, as well as having greater air velocity inside your case. Greater velocity will take heat away from your components faster than slower airflow.

In short, use an equal number of fans for intake and exhaust for best results.

hope this helps somewhat

 

[CyNics]

yeah I know that... Just want to know whether is this correct or not?
The two smaller fans placed side by side will move less than a total of 100 cfm. Two similar fans in series (stacked) will not move more air than one fan but it will increase the amount of pressure the fans generate so will support more movement into a higher pressure environment. - www.7volts.com
by the way..what is stacking fans?

 

[Drewpy]

the quote is correct. Two fans side by side will move twice the amount of air that a single fan is rated at. Two fans stacked will move the same amount of air as a single fan.

side-by-side is having the fans placed adjacent to each other, mounted through different holes/cages
stacking fans is bolting through both of them to mount, both set to blow in the same direction.

 

[L00PY]

Originally posted by: MIGhunter
Here is the deal, putting the fans side by side like this [][][] is OK Staking them won't help.

I think that picture confuses things.

Stacking exhaust fans like this won't help:
Outside < [<] [<] [<] < Inside

Side by side exhaust fans like this will help:
Outside
^^^^^^
[^] [^] [^]
^^^^^^
Inside

 

[CyNics]

totally clear now thanks guys

 

[MIGhunter]

While I do agree that the intake/exhaust may help with the dust, I still disagree with the air flow.

The rating on a fan is a max rating. Lets assume that you have 2 fans rated at 100 CFM. 1 fan intake and the other exhaust. The intake fan can pull in a max of 100 CFM. The other fan is set as exhaust and can only push out 100 CFM. Now say you have 3 fans. 2 intake and 1 exhaust. Now you are pushing in 200 CFM but the exhaust fan is regulated at 100 CFM. It is like turning your water on faster than your sink can drain. It eventually fills up.

 

[keyeye]

I'm not sure the above responses are quite right. I'll give it a shot based on my understanding.
Two fans side by side (parrallel) as exhausts will move twice the amount of air a single fan will move (assuming little pressure differential- non-restrictive inflow).
Two fans in series (i.e. and intake fan and an exhaust fan for an enclosed box) will move less than twice the air of each fan. It may move more air than one fan alone. All of the CFM measurements are made without a pressure differential, so if the intake fan is able to generate pressure within the box, you will increase your CFM out the exhaust fan, even past the rated CFM. Because most cases are leaky, this positive pressure effect may be less than in an enclosed tube.

Based on those "theories", you should only use exhaust fans in parrallel to maximize your airflow through the case. If that is not sufficient, you then add the intake fans.

BTW, 3 120mm fans is unbelievable! I don't think i have more than 2 80mm case fans in a computer.

 

[Shadowgate]

this may be ot but

if you have a couple of fans blowing in clean air (i.e. filtered) and some blowing air out, if your incoming cfm is higher than your outgoing cfm then it should cause enough pressure to build up inside the case to start blowing out all the little cracks and crevices. and since you're pushing in filtered air, this should help blow dust out of your case and keep it cleaner. at least that's what i'm hoping.

 

[CyNics]

Originally posted by: keyeye
I'm not sure the above responses are quite right. I'll give it a shot based on my understanding.
Two fans side by side (parrallel) as exhausts will move twice the amount of air a single fan will move (assuming little pressure differential- non-restrictive inflow).
Two fans in series (i.e. and intake fan and an exhaust fan for an enclosed box) will move less than twice the air of each fan. It may move more air than one fan alone. All of the CFM measurements are made without a pressure differential, so if the intake fan is able to generate pressure within the box, you will increase your CFM out the exhaust fan, even past the rated CFM. Because most cases are leaky, this positive pressure effect may be less than in an enclosed tube.

Based on those "theories", you should only use exhaust fans in parrallel to maximize your airflow through the case. If that is not sufficient, you then add the intake fans.

BTW, 3 120mm fans is unbelievable! I don't think i have more than 2 80mm case fans in a computer.

it's 6 3 intakes and 3 exhausts

 

[Pul54r]

>> two fans will move twice as much air as one

Absolutely incorrect.

Air movement (ignoring fan characteristics) is severely impacted by pressure differential. Fans are rated by speed, power draw, and pressure of the area you are blow air into (back pressure).

The first fan will increase the air pressure in the case significantly. This makes each additional fan EXTREMELY less efficient, as each will add more pressure into the case, up to the point where the fan blade design is too inefficient to overcome it.

These CFM ratings people here rate fans by means crap. Those CFM ratings are done with both sides of the fan at room pressure, not enclosed in a very small case. Therefore they do not take the effiency of the blade design, pitch, etc into account.

You need to look at a fan's pressure curve at a given differential and include the power used and CFM AT THAT PRESSURE to determine a fan efficiency.

It's much like the design of submarine propellor blades for maximum efficiency at minimum noise (zero cavitation where air bubbles are formed due to a change in pressure between the front and back of the blade).

Anyway, short of educating the people here on fan physics (which from the posts I've seen would take a LONG LONG time, because only a few have a clue), I'll simply say:

You'll get maybe 80% of the rated CFM out of your first case fan. Your second will be perhaps 50%. Your third 10%. Your fourth will do nothing. Those are complete guesstimates, but you get the idea.

 

[CyNics]

yeah I get your idea..
but I placed my 3 fans in this way..[]-[]-[] at the bottom of my casing..I cut 3 blowholes there..and I placed another 3 fans on top of my casing in this way []-[]-[] too..now I think 3 of the intakes should blow the same amount of air into my casing cause they are placed in one straight line..i mean..u know..they are not like the first fan get the air first..then the rest pass to the second fan..and so on..am i right or wrong? and I think the air pressure should be in balance because I have 3 intakes and 3 exhausts.

 

[MIGhunter]

Some fickle science.

Fan thread

 

[jteef]

does anybody have any quantitative data on what actual pressures might be inside a computer chassis? Especially one with the rather large leaks etc. My guess is that unless you've got a microATX case that is well sealed, the pressures are going to be low enough that parallel operation with a strategically placed blowhole is best. My digikey catalog has a couple of graphs for the 92mm panasonic hydro wave bearing fans. The graph is fairly linear from zero output with a 4mmAg pressure to a max output(56CFM) with 0mmAg. Also included is a noise graph that could be useful. I would bet their webpage has full data for each fan size and type.

Incidently, does anybody have a better supplier for the full line of panasonic HWB fans than digikey? $12 / fan is a bit steep. Do the fans with the BX specification "2PPR Speed Sensor (TAC Output)" work with the motherboard fan speed things?

Thanks

jt

 

[sodcha0s]

The first fan will increase the air pressure in the case significantly. This makes each additional fan EXTREMELY less efficient, as each will add more pressure into the case, up to the point where the fan blade design is too inefficient to overcome it.

That is true for a case without any exhaust fans only. That is why you should always try to at least have a balanced pressure in your case, with slightly more exhaust being optimal for case cooling.

These CFM ratings people here rate fans by means crap. Those CFM ratings are done with both sides of the fan at room pressure, not enclosed in a very small case. Therefore they do not take the effiency of the blade design, pitch, etc into account.

Very true. Ratings can only be used as a guideline for an inclosed case. The perforations on most case chassis' fan mounting areas have a huge effect on CFM.

 

[keyeye]

jteef, I have a 60mm BX. It works with the motherboard sensor.
I agree with you. I'm not aware of any real measurements in this debate, though I'd be interested. I'd bet that in my Antec 1035 the pressure is closer to zero using undervolted panaflo's. Maybe it'd be different if they were high CFM fans.

Anyway, I think most people in this forum are looking for education. It would be nice it that didn't come with insults.

Bottom line is: empiric data is alot better than theory. Try it and see!