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The difference between 120mm fans and 80mm fans?

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May 23, 2007 12:12:11 AM

Hey guys,

I have a few questions about fans and their sizes, here it goes,

1) If a 120mm (12cm) fan produces more air, but is bigger which limits the airflow a bit, would it be better then a 80mm (8cm) fan? Because the 80mm is a little smaller so im guessing less air produced, but more airflow. And the 120mm produces more air because it has bigger fan blades but limits the airflow because it is bigger. So if you can think of the 2 best fans for the sizes 80mm and 120mm, which would you think cools the computer better?

2) What is an Air duct? One of the computer boxes im planning on getting says it has a built in air duct on the side panel to funnel cool air passively without a fan. Does that mean that even with less fans it will still be a little cool?

3) I'm currently looking for a new computer case and when i see that i can fit 4 fans in an ATX case everywhere, does that mean it will fit a maximum of 4 fans? or only 4 fans of a certain size. Because one of my friends says most ATX cases can fit 6 fans but everywhere i look, the computer cases never say it can fit more then 4. So how many fans will go into an average sized ATX case, both in 80mm size and 120mm size?
May 23, 2007 12:22:50 AM

As Prozac says, the 120 is better. Since the larger blades move more air, they can run much slower - therefore, a lot quieter.
May 23, 2007 12:23:53 AM

Size matters not. What matters is the cubic feet per minute (CFM) rating on the fan. Just because a fan is bigger doesn't mean the airflow is greater... and vice versa. I personally recommend a mixture of both. If the desired airflow is in a small area, then obviously a smaller fan is recommended. Putting a big ass fan in front of a bunch of cables or blocked drive bay isn't exactly ideal. A small fan at a very high RPM can move just as much air as a big fan w/ a low RPM.

As for airFLOW, that doesn't have a whole lot to do w/ the fans. It has more to do w/ the layout of your case and how components and cables are positioned. Tie down all cables and move them away from the central airflow area. Make sure heatsinks are positioned directly in the airflow.
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May 23, 2007 12:26:01 AM

Quote:
As Prozac says, the 120 is better. Since the larger blades move more air, they can run much slower - therefore, a lot quieter.


I would generally agree. However, if the fan is too big much of the airflow will be inherently blocked by components and cables.

Just something to think about...
May 23, 2007 12:31:06 AM

Quote:
Hey guys,

I have a few questions about fans and their sizes, here it goes,

1) If a 120mm (12cm) fan produces more air, but is bigger which limits the airflow a bit, would it be better then a 80mm (8cm) fan? Because the 80mm is a little smaller so im guessing less air produced, but more airflow. And the 120mm produces more air because it has bigger fan blades but limits the airflow because it is bigger. So if you can think of the 2 best fans for the sizes 80mm and 120mm, which would you think cools the computer better?

2) What is an Air duct? One of the computer boxes im planning on getting says it has a built in air duct on the side panel to funnel cool air passively without a fan. Does that mean that even with less fans it will still be a little cool?

3) I'm currently looking for a new computer case and when i see that i can fit 4 fans in an ATX case everywhere, does that mean it will fit a maximum of 4 fans? or only 4 fans of a certain size. Because one of my friends says most ATX cases can fit 6 fans but everywhere i look, the computer cases never say it can fit more then 4. So how many fans will go into an average sized ATX case, both in 80mm size and 120mm size?
As already said,120mm are better/quieter than 80mm fans. One drawback to 120mm fans, is the enormous hub(center of the blades). It's much bigger than on an 80mm fan, and no air comes from the hub area, so sometimes a dead spot will sit below the hub.....meaning the area of the heatsink below the hub is warmer, than the area below the blades. To elliminate this minor problem, i like to put the fan in a duct, that's aimed at the heatsink. The difference though, is i mount the fan closest to the intake of the duct, then the airflow swirls around as it travels through the duct, and doesn't have a "hole" in it anymore...when it reaches the heatsink. Works very nicely. :) 
May 23, 2007 12:33:09 AM

Quote:
Hey guys,

I have a few questions about fans and their sizes, here it goes,

1) If a 120mm (12cm) fan produces more air, but is bigger which limits the airflow a bit, would it be better then a 80mm (8cm) fan? Because the 80mm is a little smaller so im guessing less air produced, but more airflow. And the 120mm produces more air because it has bigger fan blades but limits the airflow because it is bigger. So if you can think of the 2 best fans for the sizes 80mm and 120mm, which would you think cools the computer better?

The airflow depends of the diameter cubically, while it depends of the rotation speed linearly. So if you have two fans one 80mm and other 120mm, rotating at same speed, the 120mm will cause 3.375 more airflow.
May 23, 2007 12:37:49 AM

Quote:
Hey guys,

I have a few questions about fans and their sizes, here it goes,

1) If a 120mm (12cm) fan produces more air, but is bigger which limits the airflow a bit, would it be better then a 80mm (8cm) fan? Because the 80mm is a little smaller so im guessing less air produced, but more airflow. And the 120mm produces more air because it has bigger fan blades but limits the airflow because it is bigger. So if you can think of the 2 best fans for the sizes 80mm and 120mm, which would you think cools the computer better?

2) What is an Air duct? One of the computer boxes im planning on getting says it has a built in air duct on the side panel to funnel cool air passively without a fan. Does that mean that even with less fans it will still be a little cool?

3) I'm currently looking for a new computer case and when i see that i can fit 4 fans in an ATX case everywhere, does that mean it will fit a maximum of 4 fans? or only 4 fans of a certain size. Because one of my friends says most ATX cases can fit 6 fans but everywhere i look, the computer cases never say it can fit more then 4. So how many fans will go into an average sized ATX case, both in 80mm size and 120mm size?


In terms of airflow, the standard by which fans are measured is CFM.

CFM stands for "Cubic Feet per Minute." This is equivalent to the volume of air that a fan will move in one minute at 0 (zero) static presuure.

Newegg et al provide this info. Just check out the specs.

In general, larger fans move more air at lower RPMs (and with less noise).

I like big fans for case cooling.
May 23, 2007 12:46:24 AM

I think it's about 40mm :lol: 
May 23, 2007 12:54:57 AM

Why does everyone have the same DP???
Anonymous
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May 23, 2007 1:10:51 AM

I knows a fair amount (and have forgotten a fair amount) about axial fan design, having worked with large fans on heat exchangers.

If you have the space, always go with a larger fan.

Larger fans have the following advantages over smaller fans:
1) Higher resistance to static back pressure - in other words, if you have a highly resistive case air flow, a larger fan will more easily pull the pressure, because the blade chord (the width of the blade, essentially) is bigger.
2) CFM at 0" H2O column tells you jack about how the fan will perform in a real application. You need a CFM vs. Inches H2O column chart to REALLY tell if a design is good. That is not typically given anywhere - stick with bigger fans.
3) Tip speed of a fan is the chief overall indicator of noise. Larger fans spin more slowly to generate the same CFM at a given pressure than small fans do. Unless you LIKE a noisy case.
4) Hub ratio. Keep the overall diameter to the hub diameter ratio as high as possible. 3:1 or 4:1 is most preferable. Avoid the "hub and stubs" concept. A shorter fan blade can be stressed more and push higher pressure, but at a poor airflow distribution and hellacious noise.
5) Venturi. I've noticed that the 80, 90, 120 mm fans commonly available have a nice looking Venturi on them. This smoothed inlet really helps with fan efficiency. Don't modify it.
6) When you operate in "suction" mode, the dead air spot in front of the fan is a non-issue. The principal benefit the case fan has on the back side of the case is that it pulls air uniformly out of the case. A "blower" fan, on the other hand, will tend to be influenced by blockages in front of it (cables, cards) and the identical fan in "blower" mode will actually push less air. 6a) Blower in duct - a rule of thumb to eliminate the "hub shadow" is at least five fan diameters for a duct...I doubt anyone could construct this inside a case but at least one fan diameter duct length would minimize this problem.
7) A common rule of thumb to to divide your case up by sides and have as many sucker fans as blowers (front - blower, back - sucker, side - blower, top - sucker). This is a good rule of thumb in my opinion as you do not fan fans to "fight" each other.

I have not done any experiments, but at some point additional case fans will not provide any additional cooling airflow as back-pressure will be far too high and the fans will go into stall, meaning that no additonal fans will produce any more CFM thru case. Then it's time to move to centrifugal "squirrel cage" fans that can handle far more back pressure than our trusty axial fans.

FWIW.
May 23, 2007 1:27:43 AM

The difference between 120mm and 80mm fans is 40mm! :lol: 

I have all Antec TriCool fans in my case, a 120mm fan at the front, and dual 80mm exhaust fans at the back. All three are set to the lowest speed setting and you can barely hear them. Good stuff. Now I just have to replace that noisy Intel fan. :roll:
May 23, 2007 2:28:22 AM

Anecdotally, about a year ago I replaced a Lian Li PC65 case equipped with 4 x 80mm fans (2 intake, 2 exhaust). The new case was a Lian Li 7B, which is virtually identical except that it has 1 x 120mm intake (front) and 1 x 120mm exhaust (back). In the new case, I got ambient temps ~ 4 C lower. Significant improvement in airflow going with the bigger (quieter) fans.

Btw, don't forget that PSUs do more than convert current to DC. They also act as big exhaust fans, and some of them (for instance the Seasonic M12 series) exhaust enormous amounts of air out of the case. That would have to be factored into the equation too if a builder is trying to achieve neutral case pressure.
May 23, 2007 10:19:07 AM

Thanks alot guys, you helped me out alot. :D 
May 23, 2007 10:47:00 AM

Quote:
I knows a fair amount (and have forgotten a fair amount) about axial fan design, having worked with large fans on heat exchangers.

If you have the space, always go with a larger fan.

Larger fans have the following advantages over smaller fans:
1) Higher resistance to static back pressure - in other words, if you have a highly resistive case air flow, a larger fan will more easily pull the pressure, because the blade chord (the width of the blade, essentially) is bigger.
2) CFM at 0" H2O column tells you jack about how the fan will perform in a real application. You need a CFM vs. Inches H2O column chart to REALLY tell if a design is good. That is not typically given anywhere - stick with bigger fans.
3) Tip speed of a fan is the chief overall indicator of noise. Larger fans spin more slowly to generate the same CFM at a given pressure than small fans do. Unless you LIKE a noisy case.
4) Hub ratio. Keep the overall diameter to the hub diameter ratio as high as possible. 3:1 or 4:1 is most preferable. Avoid the "hub and stubs" concept. A shorter fan blade can be stressed more and push higher pressure, but at a poor airflow distribution and hellacious noise.
5) Venturi. I've noticed that the 80, 90, 120 mm fans commonly available have a nice looking Venturi on them. This smoothed inlet really helps with fan efficiency. Don't modify it.
6) When you operate in "suction" mode, the dead air spot in front of the fan is a non-issue. The principal benefit the case fan has on the back side of the case is that it pulls air uniformly out of the case. A "blower" fan, on the other hand, will tend to be influenced by blockages in front of it (cables, cards) and the identical fan in "blower" mode will actually push less air. 6a) Blower in duct - a rule of thumb to eliminate the "hub shadow" is at least five fan diameters for a duct...I doubt anyone could construct this inside a case but at least one fan diameter duct length would minimize this problem.
7) A common rule of thumb to to divide your case up by sides and have as many sucker fans as blowers (front - blower, back - sucker, side - blower, top - sucker). This is a good rule of thumb in my opinion as you do not fan fans to "fight" each other.

I have not done any experiments, but at some point additional case fans will not provide any additional cooling airflow as back-pressure will be far too high and the fans will go into stall, meaning that no additonal fans will produce any more CFM thru case. Then it's time to move to centrifugal "squirrel cage" fans that can handle far more back pressure than our trusty axial fans.

FWIW.
Nice info. :) 
May 23, 2007 3:03:59 PM

I prefer 120mm fans due to their quieter running speed. In my situation, the three 120mm fans on my Antec tower are more than adequate. It's very nearly silent and the interior temperature is only a couple of degrees warmer than room temperature. What more can you ask?
The point here is to have adequate cooling on the CPU. You need to remove the heat from the CPU, and usually this heat is emptied out into the interior of the case. The heat will build up to the point of destroying components unless you remove it from the case. For that, you need good airflow.
I've looked at several combinations. My best results have come from the setup I have now. I'm running a 3800+ X2 939 system that I built early last year. Slightly overclocked.
The Zalman (also a 120mm fan on full copper heatsink) does a great job of cooling the CPU. That heat is removed by two fans: 1) 120mm exhaust fan on the back of the case; 2) 120mm fan on the power supply.
I also have a 120mm fan at the front of the case, at the bottom, behind a built-in washable plastic dust filter.
The intake pulls in fresh cooler air through the filter; the power supply and exhaust fan remove the air; and the whole thing runs quietly even under full load.
Others have their preferences, but I find this setup to be elegantly simple.
May 24, 2007 3:54:28 AM

If you can fit them in, 120 mm fans are always better.
They can be run at lower speeds and still move as much air as an 80 mm fan, so they can be quieter, or they can be run at the same speed as the 80 mm they replace and move lots more air.
May 24, 2007 7:06:27 AM

Would a thin piece of plastic string ( very light ) be a good device to test the air flow? The only problem i have to watch out is not to get it tangled into the fans. Otherwise, it will show how the air is moving throughout the case. If the airflow is enough in the case, the string might actually float.
!