A New Way of Testing Fan Airflow

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RPM VS. AIRFLOW VS. COOLING

This long study of axial fan airflow led us inexorably to the question of the relationship between fan speed, airflow and cooling. We've stated in the past that all other things being equal, higher airflow usually means better cooling. This statement is a simplification. The reality is that airflow is one of many factors of cooling in electronics.

Surprising though it may seem, increased airflow does not automatically or always mean improved cooling. One of the closely related factors is the rate at which heat is brought to the surface where airflow can carry it away. Looking specifically at CPU heatsinks, given a certain amount of heat generated by the CPU, a heatsink conducts heat at a certain rate to the fins where it can be dissipated into the air. If the flow of air across the fins remains below the rate at which heat is conducted from the CPU to the fins, then increasing airflow improves cooling and lowers CPU temperature. But once the rate of airflow matches the rate of heat conduction to the fins, further increases of airflow will not improve cooling.

Airflow Vs. RPM

The first matter is to look at the close relationship between RPM (fan speed) and cooling with a given fan size. All of the fans in the next table are 120mm fans. They were used with a Scythe Ninja (original version) heatsink on our current socket 775 heatsink test platform. The data in the table speak clearly enough.

Thermal Test Results: 120mm Fans mounted on Scythe Ninja
Fan Speed
Nexus 120
Antec TriCool
Scythe S-Flex SFF21E
Noctua NS-S12-1200
SilenX IXP-74-11
Arctic Fan 12L
500 RPM
49°C
50°C
49°C
48°C
50°C
48°C
800 RPM
46°C
45°C
46°C
44°C
45°C
43°C
1,100 RPM
41°C
42°C
41°C
42°C
41°C
42°C
25 CFM*
45°C
46°C
46°C
45°C
45°C
45°C

Test Platform Details:
Intel Pentium D 950. TDP of 130W. Under test load, it measures 78W including efficiency losses in VRMs.
ASUS P5LD2-VM microATX motherboard with integrated graphics.
Hitachi Deskstar 7K80 80GB SATA hard drive.
1 GB stick of Corsair XMS2 DDR2 memory.
FSP Zen 300W fanless power supply.

At the same RPM, the various fans achieved CPU cooling within a 3°C range. At 1100 rpm, the range dropped to just 1°C; the CPU temperature was 41°C or 42°C with every fan.

The last line shows the results with every fan set to the RPM at which it delivered 25 CFM. As expected, the temperature variance was a mere 1°C again. This helps to verify that our testing is giving us accurate airflow readings.

What all of this data tells us is that airflow is very closely related to RPM and fan diameter. This is a basic fan law. Given the same RPM, most fans of the same size provide about the same airflow. There are no miracles, despite extensive technical marketing talk about blade design and geometry, which seem more or less irrelevant.

Airflow Vs. Cooling

The above table shows a steady drop in CPU temperature as the fans were sped up, from ~50°C at 500 RPM to ~41°C at 1100 RPM. It might be tempting to conclude that this supports the idea that higher airflow results in better cooling. But does this hold true even as we keep increasing airflow? To answer this question, a high speed 120mm fan was brought into play. We feel that the result is important enough that it deserves a highlighted text box.

JMC model 1225-12HB
rated current draw of 0.55A at 12V
Measured at 12V: 2700 RPM, ~80 CFM, 49 dBA@1m

At 2700 RPM, this fan spins some 2.5 times faster than the Nexus 120 at 12V (and most of the other 120mm fans in the table). It also generates more than double the CFM, according to our measurements. It makes enough noise to give most SPCR enthusiasts a headache in a couple of minutes.

What was the CPU temperature achieved by this high airflow fan?

41°C. Exactly the same temperature reached by most of the 120mm fans at 1100 RPM.

What this tells us is that for this combination of CPU, load, and heatsink, the airflow of an 1100 RPM 120mm fan matches the rate of thermal conduction from CPU to heatsink fins. This is why increasing the airflow does not decrease the CPU temperature. The increased fan speed does not change the ability of the cooling system to conduct the heat any faster to the fins. Only if the heat of the CPU was increased or the fin area of the heatsink expanded would the increased airflow result in a change.

So what practical implication does this have on the quiet-seeking PC builder or modder? What we've been saying all along for years:

  • Choose the quietest fan with the best noise signature and adjust its speed to give a balance of cooling and noise you can live with.
  • Don't worry about CFM. Just go for the quietest fan and the best low-airflow performance heatsink, and minimize airflow impedances in your case.

It's funny, but true: We went through this entire rigamarole of establishing an accurate system of measuring axial fan CFM only to tell you it doesn't really matter, and not to worry about it. It may seem a waste of time, effort and money to some, but this type of process has always been an integral part of empirical, scientific exploration at SPCR.

CONCLUSIONS

The airflow measurement system we described for Experiment #3 with the test box and the hot wire anemometer has become our new fan airflow test methodology. The resulting CFM data makes sense for lower and higher airflow fans of all the standard sizes (80mm to 120mm diameter). It is also repeatable and consistent with all the fans we've retested. The fact that it represents flow into some kind of impedance rather than free air makes the results more relevant for practical purposes.

Our CFM data is not directly comparable to manufacturers' specifications, which are purported in free air, without any load. They may never have been directly comparable anyway.

Keep in mind the comment made recently by Russ Kinder in the SPCR forums:

"General rule (about fan specs):

"The people who actually make the fans tend to have reliable specs, for legal and liability reasons. If an engineer spec's a fan for a piece of equipment, and that equipment then dies a fiery death due to bogus fan specs from the fan manufacturer, you can guarantee that the engineer's company lawyers will be very interested.

"But the people who resell the fans are free to lie, cheat, and steal to their hearts' content, because there are no consumer protection labeling laws regarding fans or PC components."

We will be updating the airflow information for all the fans in all the roundups thus far. Yes, we're busy collecting this data now. The airflow information will be the only data in the reviews that will be changed. In many cases, the airflow data will not change by much. There will be some changes in the text and analysis for the unusual fans whose odd characteristics prompted these airflow measurement experiments in the first place. Neither noise nor RPM data will be changed. Most significantly, our assessments of the noise characteristics of each fan will stay unchanged.

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SPCR Articles of Related Interest:
SPCR's Fan Roundup #3: 92mm Fans
SPCR's Fan Roundup #2: 120mm Fans

SPCR's 80mm Fan Roundup #1
SPCR's Fan Testing Methodology
Anatomy of the Silent Fan
Simple Fan Controllers from Zalman
Get 5V, 7V, or 12V for your Fans

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Discuss this article in the SPCR Forums.



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