By Jeff Checchi posted Jul 11th 2012
We recently released a new lineup of Air Series case fans, which come in both AirFlow (AF) and Static Pressure (SP) versions. In this blog I will define the terms that are traditionally used to describe the characteristics of fans, and talk about the difference between "airflow" and "static pressure." I will also show you how we test the fans and how we were able to accurately determine the specifications for each fan. Let’s start with a quick explanation of the terminology which is used when describing the characteristics of fans:
- Fan Size — The fan size is usually measured in millimeters or centimeters, and the most common fan sizes used in modern systems are 120mm and 140mm, with some cases accepting 200mm or larger fans. Fan size is closely related to airflow, so that typically larger fans spin more slowly while still being able to push more air than a smaller fan that spins at the same RPM.
- dBA (Decibel Level) — This is a unit of measurement for loudness, the lower the number the more quiet the fans will be.
- CFM — Cubic Feet / Minute tells us how much air the fan is capable of moving. For a typical case fan, this is the most telling spec when it comes to the fans cooling performance.
- mmH20 — This is a measurement of static pressure. The higher this number is, the more force the fan can exert on an object. Static pressure is most important when you are mounting your fan to something that will block some of its airflow, such as a water cooling radiator, CPU heat sink, or an HDD cage where the fan will be obstructed by the HDDs.
- RPM — Revolutions Per Minute tells us how many times the fan will make a complete rotation in one minute. The higher the RPM the faster the fan is spinning, and in most cases, the louder the fan is going to be.
- Power Draw — Measured in Amps (A), tells you how much power the fan will use. To convert this figure to maximum wattage, you will want to multiply it by the maximum voltage the fan supports. For example our AF120 Performance Edition fan has max power draw of .13A and max operating voltage of 12v. — .13a x 12v = 1.56 Watts
Now that you know what the terms mean, here is how our Air Series AF120 Performance Edition is rated, compared with our SP120 Performance Edition:
|AF120 Performance Edition|
|Fan Size||120mm x 25mm|
|Noise (dBA)||30 dBA|
|Airflow (CFM)||63.47 CFM|
|Static Pressure (mm/Aq)||1.1 mm/Aq|
|Power Draw (@ +12V)||.13A|
|SP120 Performance Edition|
|Fan Size||120mm x 25mm|
|Noise (dBA)||35 dBA|
|Airflow (CFM)||62.74 CFM|
|Static Pressure (mm/Aq)||3.1 mm/Aq|
|Power Draw (@ +12V)||.18A|
You can see that the overall CFM is pretty close, however where they differ is static pressure, RPM, and the noise level. With 3x the static pressure of the AF120, the SP120 is the optimal choice for any application in which the fan is pushing air through another object, such as a water-cooling radiator, CPU heat sink, or even a hard drive cage. If you do not require high static pressure, then you are better off going with the standard AF120 Performance edition, or if silence is your goal, you could go with one of our Quiet Edition variants of either AF120 or SP120.
So where did these numbers come from? There are ways to estimate the performance of a fan, but you will always get the most accurate information from real world testing in an enclosed chamber or wind tunnel. When we decided that we wanted to design our own fans, the first thing we did was invest in one of these machines. The test rig we went with is a LW-9266 Fan PQ Performance Measurement Apparatus made by LongWin. This machine is able to accurately measure CFM and mmH20 at variable RPM. This was very helpful when testing different fan blade designs and helped us see where the RPM sweet spot was for both static pressure and airflow.
To give you an example of the data this machine gives us, I am posting some of the data generated from our AF120 Performance Edition fan. The graph shows us 3 parameters, PS(mm/Aq), Q(CFM) and N(RPM). PS is effective static pressure, Q is our overall airflow measurement and N is the speed of the fan measured in RPM.
This graph shows us how CFM is related to static pressure. With this information we are able to see how the CFM is affected while the fan is pushing out various levels of static pressure. This fan is rated for 1.1mm/Aq, but you can see that it is actually capable of slightly more at its maximum. The real usefulness of this graph is realized once we begin comparing and contrasting many different runs of the test with different fans. Changing fan blades and rerunning the test would show us real world differences and give us hints as to what types of fan blades may be more beneficial to pure CFM compared to static pressure.
Luckily you do not have to worry about fan blade designs, you can focus strictly on the specifications, which have been verified in a real world environment. Picking the right Air Series fan is as easy as first picking between Static Pressure versus AirFlow, and then Performance versus Quiet Edition.