Thread: AF120 vs SP120
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Old 04-06-2016, 09:27 AM
c-attack c-attack is offline
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The AF (Airflow) and SP (Static Pressure) have two different blade designs that result in different performance. An AF fan or any other with many, thin, steeply raked blades, is the most efficient for moving air. This is what why we use them. The SP fan by comparison has fewer blades (usually 7) with a flatter and fatter blade. This is a less efficient design and at any given RPM, the AF style blade will move more air. However, the SP style blade has the ability to focus the airflow into a tighter stream and is able to push more air when faced with moderate resistance. This can make it the better choice for areas where you need a focused, direct cooling source or when faced with resistance, like a radiator.

For the most part, you always want to use the AF style fans unless something forces you to use the other. The flatter, thick blades tend to make more air noise. More importantly, an AF120 moves 63 CFM at 1650 RPM. An SP120 would need to run 2350+ to move that same amount of air. That is a big difference in fan noise.

However, when we start adding resistance to the fan, the table starts to shift. Any fan specification you see shows a volume of air (CFM or other) the fan can move when unhindered, in free air sitting on your desk. There is always some resistance. The moment I put an AF fan on my case, it is no longer capable of moving 63 CFM at 1650 RPM. It will move less. How much less? No way to know exactly without some precise equipment. Every case and environment is slightly different, but for the most part the volume is not substantially less. Your front grill provides some measure of resistance and the AF fan may only move 57 CFM at 1650 RPM. However, that is certainly more than the 40 CFM an SP120 can move at the same 1650 rpm. The SP fan is less affected by the resistance, but the airflow on the AF is still so much higher and it is the better choice.

Where it starts to change is when the resistance is large, like a thick radiator. Now the AF fan may only move 35-40 cfm at 1650 rpm. This is a substantial reduction. The SP fan can move the same 35-40 cfm at 1650. So now it's even. So why would you not take the quieter AF series if everything else is the same? It's not. When at high or maximum speeds, AF series fans are able to overcome more of the resistance. As we lower the RPM, the AF series will lose more and more ground to the SP fan. The SP fan is much better at moving air through resistance at low fan speeds compared to an AF fan. The overwhelming majority of people do not want to run their fans at max speed on a radiator. It makes a lot of noise. So, for the 99% of the time when your fans are running at low to medium speeds, the SP style fan can move air through the radiator.

Sometimes there are unique circumstance, but usually you want AF style fans everywhere you can. Save the SP fans for a radiator, if you have one.
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