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What blows more: 120mm 80 cfm, or TWO 80mm 40 cfm?

V

VBboy

Diamond Member
It's not a math puzzle.

I can put either ONE 120mm 80cfm fan, or TWO 80mm 40cfm fans into my PC, in the front of my case. Other than noise, what are the advantages/disadvantages of either setup? For example, as long as I get 80 total cfm in either case, is that really the same 80 cfm?

You know, how 2 light bulbs don't produce twice the light of 1 light bulb? Is this something similar?

Thanks in advance!
 
I'm not sure if this is entirely correct, but when I design the combustion system for an industrial furnace I have used to blowers to get the same amount of cfm that one could give me. I'm pretty sure that cooling fans aren't that different because it's still air. I know that when using the two blowers that I can add up my total cfm, just not my pressure. (I think, its sunday my brains on vacation)
 
use the 120 mm.... same amount of air... but it'll be quieter b/c it's a larger fan(it wont be as high pitched a whine as the 80's)

zs
 


<< It's not a math puzzle.

I can put either ONE 120mm 80cfm fan, or TWO 80mm 40cfm fans into my PC, in the front of my case. Other than noise, what are the advantages/disadvantages of either setup? For example, as long as I get 80 total cfm in either case, is that really the same 80 cfm?

You know, how 2 light bulbs don't produce twice the light of 1 light bulb? Is this something similar?

Thanks in advance! >>




It depends on what kind of static pressure you place on the fan and the engineering spec of the fan that shows airflow vs. static pressure graph.


CFM ratings are open air ratings. When you hang them hanging from the celing, one 120mm 80cfm is the same as two 80mm 40cfm fan.
I can not tell you the specifics without seeing the data for both fans.
 


<< It depends on what kind of static pressure you place on the fan and the engineering spec of the fan that shows airflow vs. static pressure graph.

CFM ratings are open air ratings. When you hang them hanging from the celing, one 120mm 80cfm is the same as two 80mm 40cfm fan.
I can not tell you the specifics without seeing the data for both fans. >>


this is actually very important to consider. also, some fans handle static pressure better than others... one of my 80mm fans almost stops moving air when slightly distrubed where as another is hardly affected. anyway, for a large, fairly open case, probably the single 120... but for a case full of messy cables, the two 80s might do better. the 120 will be MUCH quieter.
 
Thanks, guys!

I am deciding which case to buy, that's why I needed to know which type of fan is better.
I keep wires very neat and well-cabled, so there is almost nothing hanging in the way of the air.

If you can give me more information after seing the fan specs, please have a look:
I'm considering Panaflo fans. Both are quiet:

Panaflo 120 mm x 38 cm - FBA12G12L1A:
Sleeve-Hydro 30.0 db, 68.9 cfm, 2.29 ratio, 1700 rpm

Panaflo 90 mm x 25 mm - FBA08A12L1A:
Sleeve-Hydro 21.0 db, 24.0 cfm, 1.14 ratio, 1900 rpm

Need more info?
 


<< Thanks, guys!

I am deciding which case to buy, that's why I needed to know which type of fan is better.
I keep wires very neat and well-cabled, so there is almost nothing hanging in the way of the air.

If you can give me more information after seing the fan specs, please have a look:
I'm considering Panaflo fans. Both are quiet:

Panaflo 120 mm x 38 cm - FBA12G12L1A:
Sleeve-Hydro 30.0 db, 68.9 cfm, 2.29 ratio, 1700 rpm

Panaflo 90 mm x 25 mm - FBA08A12L1A:
Sleeve-Hydro 21.0 db, 24.0 cfm, 1.14 ratio, 1900 rpm

Need more info? >>


You're missing the most critical info.

Take a look at this data sheet.
You need a graph like that, xCFM@0mm aq and 0CFM@x mm aq at LEAST. The pressure vs. CFM is not linear and it is impossible to know which will perform better in certain situation unless the operating condition in specific situation is known.

There are many other key infos you're missing and aerodynamic engineering is such a complex thing that requires alot of data collected through difficult data collection process.

With the case placed on the computer, it is necessary to know the static pressure resistance(which varies with flow)and the degree of vacuum created inside the case(anytime air is being pulled from the back and there is even smallest resistance, negative pressure is generated).

The whole setup required for this will probably go for at least $50,000
 
I'd say that they are the same since the different would be negligible.
120mm will be quieter.

This question sounds like asking: which one is heavier? one pound of steel or one pound of cotton.
 


<< I'd say that they are the same since the different would be negligible.
120mm will be quieter.

This question sounds like asking: which one is heavier? one pound of steel or one pound of cotton. >>


did you read the thread at all???
 
I'd go for the 120mm fan. It'll be a bit more quiet and installation will be easier. Mouser sells some wire mesh grills that would actually make it look clean and they keep dust out (clean with a damp terrycloth rag). The 120mm high speed panaflow that I have (105CFM) isn't too noisy and the sound is quite tolerable.
 


<<
You're missing the most critical info.

Take a look at this data sheet.
You need a graph like that, xCFM@0mm aq and 0CFM@x mm aq at LEAST. The pressure vs. CFM is not linear and it is impossible to know which will perform better in certain situation unless the operating condition in specific situation is known.

There are many other key infos you're missing and aerodynamic engineering is such a complex thing that requires alot of data collected through difficult data collection process.

With the case placed on the computer, it is necessary to know the static pressure resistance(which varies with flow)and the degree of vacuum created inside the case(anytime air is being pulled from the back and there is even smallest resistance, negative pressure is generated).

The whole setup required for this will probably go for at least $50,000 >>



Agree entirely; however, this is much simpler for most case designs due to the exhaust fans in back. I'm assuming you are putting an equal number of fans in the back? If so, the static pressure on the fan(s) becomes negligible. If your fans aren't balanced front to back, then ignore this post and spend $50K 🙂
 


<< Agree entirely; however, this is much simpler for most case designs due to the exhaust fans in back. I'm assuming you are putting an equal number of fans in the back? If so, the static pressure on the fan(s) becomes negligible. If your fans aren't balanced front to back, then ignore this post and spend $50K 🙂 >>




It isn't about "balanced" and while we're at it, will you please define balanced?

Open air operation and two identical fans at both ends in an enclosed space is not the same. There's many many many other factors to be considered.

Obstruction

convection current

venturi effect

turbulence

and many more I don't know off top of my head.

 
Balanced = negligible pos/neg pressure inside case. Most people aim for slightly positive pressure to help keep dust out. In reality, most people don't achieve such a condition due to the small confined space of a case. If the case air is balanced, then the intake fans are pushing against the same static pressure as the rear fans are pulling against. One common mistake is to neglect the PSU exhaust fan. It is very easy to create a negative pressure case if the PSU fan is not accounted for.

As far as bringing in many complex variables such as convection currents, venturi effect, turbulence, eddies, laminar air flow, obstructions, .... its just not practical. We are talking about a 2 foot long box here.

In a properly built system (wire management within case, balanced air pressure, proper placement of hardware, etc..) these variables are just not going to have an effect.

Venturi effect? Lets hope that we have all learned not to place box fans outside our cases to help cool them.
Turbulence, obstructions? All avoidable with proper rig building practices.

I think we are really approaching analysis paralysis on this issue.

To keep it simple, install more cfm of intake that you have exhausting (per open air fan specs). The following factors will bring the difference down to a slightly positive box (ideal)
1. Intake filters: reduces the intake cfm
2. Venturi effect: any cross drafts (cd) present will decrease the pressure within the box. CDs on intake side will reduce air flowing in, CDs on exhaust side will increase air flowing out.

$.06
 
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