A New Way of Testing Fan Airflow

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EXPERIMENT #2 PROBLEMS

Why is the difference between the old results and the new ones greater with the smaller fans? We're not positive about the answer. Fan aerodynamics is a complex field, and what we can offer here are observations and hypotheses.

Our old technique measured the airspeed directly in front of the fan, without worrying about capturing all of the air flowing through the fan. The airspeed was assumed to be more or less constant across the whole area of the fan, and that speed (in linear feet per minute) was multiplied by the area of the fan (in square feet), to produce a final flow volume (in cubic feet per minute). Our readings were affected by extra turbulence caused by placing the anenometer so close to the fan blades themselves; with most fans, the effect was to raise the CFM readings.

With the new technique, because the test box is sealed, every bit of air pushed into the box by the fan will eventually pass through the anemometer and contribute to the LFM measurement. The anenometer impeller measures 68mm (2.67") in diameter, which is a lot smaller than 120mm. It is also smaller than 92mm or 80mm.

FANS versus ANEMOMETER IMPELLER
Radius
Anenometer
120mm
92mm
80mm
Area*
36cm²
113cm²
66cm²
50cm²
*This calculation includes the central hub area.

The airspeed through the anemometer is not the same as the airspeed right at the fan. Instead, it is governed by a complex formula involving the difference in size between the fan and the anemometer, the speed of the fan's rotation, and the impedance of the test box, which includes the anenometer vane. We are no longer measuring unimpeded airflow; our test box brings pressure into the equation as well.

What effect does this have on our measurements?

The instruction manual of most handheld anemometers contain this information:

To determine the volume of air flowing through a duct, take the area of the duct in square units (like square feet) and multiply the value by the measured linear velocity (ft/min).

In the airflow measurement technique we've been using all along, the area of the test fan's impeller opening (diameter of the blades minus the diameter of the center hub) is considered to be the duct. In the airflow test box, this can no longer be valid. It is the anenometer impeller which becomes the duct. This means the fan diameter no longer has any bearing on the relationship between the measured LFM and calculated CFM. It also means that the relationship between measured LFM and calculated CFM is now linear regardless of fan size; the area of the anenometer impeller is used for every fan.

Now compare the LFM results between the various size fans:

LFM measurements in Airflow Test Box
Fan
rated CFM
12V
9V
7V
5V
Arctic 3 (80mm)
28
410
320
260
160
Scythe 80
27
420
310
200
100
Nexus 92
27
410
320
250
150
Fander FX92-W
35
580
460
360
250
Scythe S-Flex SFF21E
49
600
430
310
170
Arctic Fan 12L
37
390
300
230
140
Noctua NS-S12-1200
48
590
480
390
280
Antec Tricool 120
79
810
650
530
370

Let's accept the manufacturers' CFM claims for the time being. The discrepancies between the claimed CFM and our measured LFM are dramatic in some cases, and consistent:

1) The Antec Tricool 120 has the highest rated airflow of 79 CFM. But its measured LFM is only double that of the Scythe 80mm or Nexus 92, both rated at less than a third of the Tricool's CFM. Its rated CFM is more than double that of the Fander 92, but the measured LFM is only about 35% higher.

2) The Noctua 120 and Scythe 120 are rated at 48 and 49 CFM, while the Fander 92 is rated at 35 CFM. Yet the measured LFM of the Fander is the same as the bigger fans.

3) The Arctic Fan 12L, rated at 37 CFM, has slightly lower measured LFM than the Arctic and Sycthe 80mm fans and the Nexus 92, all three of which are rated at 27 or 28 CFM.

The consistent part of the above data is that it's almost always the larger higher CFM fans that don't seem to have high enough measured LFM. This suggests that there is higher airflow restriction at the anenometer for the bigger fans at higher speeds. In other words, the pressure in the box rises for the bigger fans at higher speeds, resulting in a compression of the airflow measurements.

If we take the point of view that to minimize pressure and airflow restriction, the exhaust vent (the area of the anemometer impeller) should be at least as big as the intake vent (fan impeller area), then our test box with this anenometer should be limited to fans no bigger than about 70mm diameter.

In conclusion, we can probably say the airflow box of Experiment #2 provides...

  1. a fairly low resistance condition for 80mm and 92mm fans spinning not too fast; we don't know how fast is too fast.
  2. a higher impedance to almost any larger diameter fan even when it's spinning quite slowly.

This means data between smaller and larger fans, and perhaps between faster and slower fans, cannot be compared fairly due to the increasing pressure that prevails with higher airflow.

In the end, we had to call Experiment #2 an interesting failure.



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