Static Pressure vs High CFM questions

mecra

Limp Gawd
Joined
Apr 7, 2006
Messages
173
Hey guys. I've been reading about the two terms for a bit now but I'm still kinda confused.

So I hook up my 1900 rpm slipstream to my H50 and I've noticed a significant increase in airflow on the other side of the rad. (More than the stock fan.)

So my question is, if I'm feeling good airflow, will I feel that much better airflow if I say go to a GT that's running half of the CFM?

I'm trying to wrap my head around the concept that double CFM with a noticable air flow increase is bad compared to a half CFM but in theory high static pressure fan. Some concept is not jiving in my brain. :(

So a 50+ CFM GT is noticably better than a 110 CFM SS? (Aside from teh fact that you can't find the 50+ CFM GT anywhere.) Also, is there a 120mm high static fan that people can recommend outside of the GT?
 
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RevMen

Limp Gawd
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Feb 22, 2004
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460
So my question is, if I'm feeling good airflow, will I feel that much better airflow if I say go to a GT that's running half of the CFM?
CFM and airflow are the same thing. The more air you move across your radiator, the better. More CFM will always mean better cooling. CFM across/through the radiator, that is, not necessarily what the fan is rated for.

As for static pressure, the idea is to match the pressure increase created by the fan with the pressure drop across the radiator, so a higher static pressure isn't necessarily better. Higher pressure fans are noisier, so that's bad. You don't want any more pressure than you need. But you don't want too little pressure or else you won't get as much airflow.

If the actual terms are confusing for you, I'll try to explain.

CFM is short for cubic feet per minute. The fan's rated CFM means the number of 1'x1'x1' chunks of air that it can push in one minute.

The fan's rated static pressure is the maximum amount of pressure it's capable of creating as it blows air through a system that's resistive to airflow (like a whole bunch of metal fins).

They're separate ideas, but their result is related. Think of a garden hose with water flowing through. If you just let the hose flow freely, the pressure in the hose is low but the flowrate out the end is high. Lots of water just pouring out the opening. If you cover up the opening partially with your thumb the pressure inside the hose goes up and the water that does make it past your thumb shoots out with high velocity, but the overall flow rate is still much lower than when you just let it flow freely.

Your radiator is similar to your thumb in the hose example. It's harder for air to flow through fins than if the fan was just sitting in an open room, so there's a higher pressure drop across the radiator, and to get a higher flow rate you need a fan that's capable of creating higher pressure. But the fan also has to move enough air to begin with. So the ideal situation is a high flow rate with a static pressure that's just high enough to match the pressure drop across the radiator.
 
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Tudz

Supreme [H]ardness
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Jun 15, 2008
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7,434
The CFM rating tells you how much air a fan can move if unobstructed. As mentioned, its cubic feet per minute, so what volume of air can be moved per minute.

The pressure of a fan will tell you how well it will push through an obstruction, such as a fan filter, a radiator, a heatsink, a fan grill or even just the case itself. The higher the pressure of the fan, the closer it will be to its rated CFM when placed near an obstruction. A high CFM fan with no obstruction, like a Noctua S12, wont run anywhere near its rated CFM when placed near an obstruction like a filter or a heatsink.

But I've always wondered how do they measure static pressure of a fan? I would have thought the stagnation pressure of an obstructed fan would be more relevant given fans are typically high flow rate devices.

EDIT: To answer one of your specific questions, what you "feel" when you place your hand in front of a fan, but not so close to the fan to choke the fan is both the airflow ("cfm") and the static pressure. What you're feeling is the "stagnation pressure" which is the sum of the static pressure and the pressure due to the airflow. It depends how the static pressure rating is measured as to which one will be more dominant.
 
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Skoobs

Weaksauce
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Oct 4, 2010
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90
The slipstream has much higher static pressure than the GT, but it's also much louder.
There is a good review of fans here:
http://www.overclock.net/air-cooling/724577-well-dressed-megahalems-65-fans-112-a.html
this is an AMAZING thread!

i wish there was a way to differentiate between different sounds though... i HATE the sound of a high RPM PWM fan on low, the way it growls. the sound of a motor can be super ugly. but, the sound of wind is good =)

assuming the OP is looking to push air through a radiator, what is the best solution, fan-wise, for that? i have Scythe S-Flex SFF21G 120mm Case Fan and i love the noise level and the amount of wind i can feel coming out of my comp, but im not sure if they have the static pressure needed for a thick radiator, which i am about 5 days away from installing.

fan insight and recommendations appreciated!
 

Archmage

2[H]4U
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Jul 13, 2000
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3,023
http://www.xtremesystems.org/forums/showthread.php?t=259802 - Martin JUST started doing some sound-sample tests again. This will be of use.

Summary: Among the fans tested, the Scythe GT 1850 produced the lowest noise levels for any given CFM. More CFM was achieved via PULL vs PUSH. The Scythe GT emitted some whining noise in the middle-voltage range, and thus Martin preferred the louder Noctua P12 in the ~20cfm range.

Watch and listen to the clips to decide for yourself. Pay close attention to the CFM reading, db-meters, and voltage.

On Static Pressure vs CFM: For our purposes, we can think of pressure as an expression of the force per unit of area required to STOP a fluid (gas) from expanding.

If you've ever looked at the P/Q curves in Martin and Skinnee's pump reviews, the total head pressure in PSI (generally 3-7 PSI) is rated at a flow rate of ZERO GPM. That is - 3-7PSI was required not to impinge, but to completely stop the flow of water given a tested pump. At the other end, the maximum flow-rate is achieved at as close to zero applied pressure as possible. Everywhere in-between describes the flow-rate characteristics of that pump. For each component included in a cooling loop, a pressure-drop can be measured, and these are additive. To estimate the flow of a loop, one need only add the pressure-drop from each component, and find the result on the pump's P/Q curve (That pressure-drop value will have a corresponding flow-rate), perhaps accounting a little for gravity and extraneous friction losses.

Back to AIR: The concept is the same. The pressure is generally measured in mmH20 or inH20 (pressure equivalent to a column of water of height specified by the measurement).

I don't think we can really trust manufacturer ratings. For my purposes, I use the test data and noise samples from Martin, Vapor, ehume, xbit, etc + personal experience.

If we were to trust manufacturer specs (and if they were more complete), and assume that all sound signatures are alike (which, as seen in Martin's latest video, and as we've no doubt heard for ourselves, they are not), then I suppose it'd be a simple matter to choose our fans: Obtain cumulative pressure drop for each isolated system in question, and find the fan that will perform best in that range given our personal criteria.
 

mecra

Limp Gawd
Joined
Apr 7, 2006
Messages
173
Excellent info from all the thread contributors. :) My thanks to you.

One thing that caught me off guard was that it was said that the Slipstream has high static pressure. However, I've seen a number of threads say that it's terrible for static pressure. Are they staying it's terrible because of its noise to CFM/pressure ratio?
 

audirs

Limp Gawd
Joined
Oct 2, 2007
Messages
260
the slip stream 1900 compared to an sflex G series @ 1900, is considered bad. IIRC the slipstream 1900 has 2.4mm/h20 static pressure where the G series sflex is almost 4mm/h20. that being said the sflex is noticeably louder then my slipstreams, only because the sflex make a weird humming noise.

the slipstream is a better case fan, it moves more air and is a bit more quiet, but the sflex g is a better rad/heat sink fan at the cost of some noise
 
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