There are many parameters that describe a fan’s performance. Here is a brief explanation of what these numbers mean:

Airflow: measured in CFM (cubic feet per minute) or in metric units m3/h (cubic meters per hour). This refers to the volume of air moved by the fan – a higher number is normally better and indicates higher performance. The airflow number is always measured without any resistance. A radiator or heatsink with a high fin density will resist airflow which in return will lower the airflow. Some manufacturers provide graphs that show the decrease in airflow per resistance (which is called static pressure). An average 80mm fan provides about 28CFM.

RPM: Revolutions per minute is a unit of frequency, commonly used to measure rotational speed, in particular in the case of rotation around a fixed axis. It represents the number of full rotations something makes in one minute. This is how fast the fan blades are turning. Normally a higher number would mean a noisier fan. An average 80mm fan spins at 2000RPM.

Starting voltage: This is the lowest voltage that is enough to make the fan start spinning. This number is important to people who use fan controllers and want their fans to be as quiet as possible and provide a lower voltage to the fan in order to decrease the noise. Many fans will not start spinning with a voltage of less than 7V.

Bearing type: there are two types of bearings that are more common in fans: ball bearings and sleeve bearings. Ball bearings are normally considered of higher quality but cause more noise. Sleeve bearings are quieter but are considered less reliable. There are other types of bearings but they are less common.

Noise: Measured in db. This is a logarithmic unit so a 23db fan makes twice the noise of a 20db fan. There are many characteristics to noise. The most common types are wind noise, bearing noise and motor noise. Each type of noise sounds differently and some people dislike certain types of noise more than other type. The noise is measured at a certain distance from the fan, each manufacturer measures at a slightly different distance so two fans with the same number may not sound the same. Thermaltake are notorious for their optimistic noise numbers. Nexus and Yate-Loon are pessimistic about their numbers. I would consider a true 23db fan to be quiet and anything under 17db to be considered “silent”.

Air pressure: measured in mmH2O – the height of a column of water in mm. This number reflects the increase in air pressure provided by the fan. Normally this number is considered less important than CFM from a PC cooling perspective, but if you have a choice get the one with the higher air pressure, especially if you are cooling a radiator - they have high air resistance.

Additional information about fans:
The three most common types of fan are axial, centrifugal and cross flow.
Cooling fans for computer equipment exclusively use brushless DC motors, which produce much less EMI (Electromagnetic interference).
The standard sizes for DC fans are 80mm 92mm and 120mm. There are special fans that have different sizes than these but are not as common. A few examples are the Silverstone 92mm fan that uses 80mm fan mounting holes, the Aerocool 140mm and the Scythe 100mm with 100mm and 92mm mounting holes.

This could prove useful for someone somewhere in the future.

Sticky?

Definitely a good article, short sweet, and very to the point, good for begginers looking for help finding a fan, some examples for pressure such as .13 or .072 would be nice to add, the aerocool 120mm 3000 turbine has a static pressure in H20 of .072, while the NMB-MAT 120mm has a static pressure of .13, with a lower CFM of about 10.

Another characteristic would be width, typically the 25mm are more widely used, while 20mm, 15mm, and 38mm are available. Fins/blades per fan varies, and typically makes things change such as airflow (more blades higher airflow at lower speeds, less blades more noise but bigger angles give more pressure)

I'm sure others could help on adding some info to the post, i second that sticky though.

great help for me I must say
THX for this post

Good post, but I think you skimped on the detail when it comes to bearings. And the bearing type is probably one of the most important considerations in a fan's construction.

Ball-bearing fans are much, MUCH better. They last several years, instead of maybe 1 or 2, or perhaps even only months under high-stress conditions, when compared to sleeve "bearings" (which aren't even bearings at all, they're just bushings cut out from a single tubular piece of slippery plastic). Bushings wear out a helluva lot faster than ball-bearings do. And when they wear out, they get VERY noisy, start moaning and groaning and making scritchy-scratchy noises. Well, I guess a badly worn ball-bearing fan will too, but it'll last several years longer than any bushing fan will.

In my teen days of high-performance R/C off-road dune buggies, the ones that cost over a thousand dollars to buy and you build the kit yourself, the good ones always included ball-bearings for the wheels, motors, chain or shaft drives, etc. and the cheap ones always only used bushings. You could always buy ball-bearing upgrades for those cheaper models. The point being the same as when we're talking about fans.

I'll NEVER put a sleeve "bearing" fan in any case I'm working on. Ball-bearings all the way. If it says "Sleeve" on it, or has "SL" somewhere in the name on the round label (often they don't want you to know there's no ball-bearings in it, so they just put "SL" somewhere on the label or product number), or it says "Bushing" on it, it's of inferior quality, will wear out much faster, make more noise, and usually cost less.

All the best case fans use ball-bearings.

Depending on the amount of heat a sleeve may last 80% to 20% as long as a ball bearing.

This handy dandy PDF gives a better pro to con. The only pro to sleeve is quiet operation. I personally would use sleeves for case if quiet was my ultimate main goal because no fan is used nearly as long as it's useful life in my rigs. But all most built to match their own goals.

ADD: Also should point out sleeve fans should be mounted vertically, and they're only quiter in low RPM operation scenarios (example 120mm in sub 1,400 rpm range.) So it's truly a if you want QUIET QUIET sleeves are worth looking into still. Blow hole in your system? Forget it. Using anything producing more 30dBA on it's own, forget it too. The gain will not be worth in, it's very limited scenario where sleeve is worthy of use.

I think you're probably right, when both a sleeved fan and a bearinged fan are both new, and running at low RPM, the sleeved fan will be quieter...until it begins to wear out. And it'll wear out a lot sooner. So when I said sleeved fans tend to be louder, it's because usually they're already wearing out their bushings. I still would rather buy a low-RPM high-quality BB fan than any kind of sleeved fan.

I'm truely surprised Anonymous hasn't had a word in this thread.

(not to argue the length of use) Especially with the new panflo sleeve bearing "hydro wave," the "high-end sleeve bearing" is what i go for anymore, although the length of time my case fans are in use is normally years.

With my general rig the cheap xoxide blue LED 120mm fans sleeve bearings, still running after two years of almost constant use, the rig is on steady and everyday of the year (downtime maybe 5%). With that i have no reason to complain about sleeve bearings, and the NMB-MAT fan that i have is quiet as ever, and has run for nearly three months steady already (will be running as a steady server fan).

I prefer sleeve bearings only for the noise level (and yes low rpm is the best bet with sleeve bearing). I only use ball bearing fans in my high performance (100cfm or more) fans that are so loud there no use in classifing it as anything but annoying.

As for the article from purple diamond, a little too little detail was thrown into it. Maybe some links (kinda like you fan article before) and a more detailed list of components.

Quote: Originally Posted by s1ugh34d As for the article from purple diamond, a little too little detail was thrown into it. Maybe some links (kinda like you fan article before) and a more detailed list of components.

Well, I can add links to articles and some examples of fans and reviews but I am not going to be able to maintain another price list...

The whole idea was to provide a basic intro to fans. You have probably noticed that I did not talk about case airflow, heatpipes or any of the more advanced air cooling related topics.

Third option. Magnetic bearing. ENERMAX Marathon Enlobal Fan  80mm UC-8EB or 120mm UC-12EB (+$2.00)

Quote: Originally Posted by Seyeklopz Third option. Magnetic bearing. ENERMAX Marathon Enlobal Fan* 80mm UC-8EB or 120mm UC-12EB (+$2.00)

I love me my enlobals!

I just read about Noctua using a similar but different named bearing "Self Stabilizing Oil-pressure bearing". Read it here

Quote: Originally Posted by Seyeklopz Third option. Magnetic bearing. ENERMAX Marathon Enlobal Fan* 80mm UC-8EB or 120mm UC-12EB (+$2.00)

There are other bearing types in addition to the ball and sleeve - like fluid in the Scythe S-Flex. I was just trying to cover the basic stuff that everybody should know about fans, I was not trying to write the definitive fan encyclopedia...

I am gathering some more details about fans and plan on editing the original post in a couple of days - stay tuned.

I'm in the market for a meter to do some air flow testing. Preferably something
not to expensive. I've looked around a little but I'm really not acquainted with
the various manufacturers and features. Any help would be appreciated :-)