Fan Flow

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This is really the heart of the article and will be broken into a few separate parts. To begin, let’s dig a little deeper into the term “CFM”.

CFM is an often-confusing term because it has no single definition that applies to all instances. In the most basic sense, CFM means cubic feet per minute. Sounds simple enough, doesn’t it? Unfortunately, air is a compressible gas. This means that if you put one cubic foot of air into a balloon at sea level and then go climb a mountain, you’ll have more than one cubic foot of air atop the mountain. Sound confusing yet?

To further confuse the issue, fans tend to be constant CFM devices. This means that provided the fan speed remains constant it will pump a constant volume of air. This is not the same as pumping a constant mass of air. One cubic foot of air at sea level weighs about 0.075 lbf. A cubic foot of air in Denver weighs about 0.062 lbf. If you filled a plastic bag with 38 actual cubic feet of air in Denver and took it to sea level, the bag would actually contain only 31 standard cubic feet of air.

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Sea Level Pressure ~ 101.3kPa (14.7 psia) Denver Pressure ~ 83.7kPa (~12.1 psia) Adding a final bit of confusion, fan flow depends on static pressure. When tested, a fan is placed in what amounts to an ideal situation with minimal flow resistance. In the real world, heat sink fans have to either push or pull air though a heat sink. Case fans must push or pull air through fan grills and bezels. This added resistance lowers the actual fan flow.

This final note on resistance can be very important to computer cooling, especially CPU cooling. The reason is some fans handle high flow resistance better than others. As a general rule, a fan of equal size but higher rpm will handle higher flow resistance than a lower rpm fan.

Let’s take a look at a typical fan performance graph.

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There’s a lot of information on this curve that you won’t see when you compare typical computer fans. Let’s investigate a little to determine what the various lines mean. First of all, the X-axis is critical. It is the percentage of rated flow delivered by the fan. If a fan is rated for 38 CFM, then 75% delivery would equal 28.5 CFM. The Y-axis scales the values of power, static pressure, static efficiency, and total efficiency.

“Power” is simple enough. The fan label typically shows a power rating, so 100% power equals the value on the fan label. For a typical fan, power equals 100% only when flow is somewhat lower than the rated capacity. “Static Pressure” is the pressure measured on the outlet side of the fan. If you deadhead a fan allowing zero flow, the static pressure reaches its maximum value. As you reduce flow restriction, flow will increase as static pressure decreases. At low flow values, the fan stalls. As the fan recovers from the stall condition, around 40% in the above example, static pressure actually increases a little. It again tapers as flow continues toward 100% of rated capacity. “Static Efficiency” is a measure of static pressure multiplied by flow rate and divided by input power. “Total Efficiency” is a measure of static pressure multiplied by flow rate plus the momentum of the air stream divided by input power.

Key Point: No fan will pump its rated CFM when flow resistance is present.

Key Point: Each fan handles flow resistance according to its performance curve. In general but not always, fans with a higher CFM rating will pump more air than fans with a lower CFM rating mounted in the same conditions.

Key Point: The higher the peak static pressure a fan will generate, the higher flow resistance it will tolerate before stalling.

Key Point: Peak static pressure is normally related to fan rpm with faster fans having a higher peak static pressure.

Key Point: A properly mounted fan normally operates at about 80% of its rated flow, not at 100% of rated flow. Due to higher resistance, heat sink fans may operate at an even lower percentage.

OK, so what does a fan CFM rating really mean? It means that the fan will pump a specific volume of air in a given amount of time when the fan speed and flow resistance matches the test conditions. It also means that the fan will pump the same volume, though not mass, at any other air density. Since our concern is computer cooling of the case and CPU, how does a change in air density affect cooling?

In order to investigate the impact of air density, we need to consider how air transfers heat. Heat is transferred from a heat sink or motherboard to an air stream via convection. Air convection depends on a temperature differential between the heat source and air stream as well as a convection coefficient.

The convection coefficient is essentially an efficiency rating for heat transfer to air. One of the largest determinants of the coefficient is air velocity. Recall that fans pump a fixed air volume for a given fan speed regardless of air density. This means that the air velocity is the same even though mass flow rate through the fan is not. The single largest determinant of convection efficiency is not impacted by air density changes.

The lesser factors involved in convection do change, however. The decreased density results in lower thermal capacity per volume, for example. These changes do lower the ability to transfer heat via convection. Within normal operating ranges, however, convection efficiency is not affected significantly by elevation and the resulting changes in air density. What is crucial is the temperature differential between the air and the object the air cools.

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