Fan Technologies Unwound
There have been fans in computers ever since the power demanded by the components inside outstripped the cooling capabilities of passive convection. For CPUs, this occurred in 1989 with the launch of Intel’s 80486, commonly known as the 486. Discrete graphics cards were ushering in a new era of 3D aming in the late ’90s when cards like ATI’s Rage Fury Maxx, NVIDIA’s RIVA TNT2, and 3DFX’s Voodoo Banshee all launched with active cooling. PSUs have shipped with built-in fans for even longer, and at least one intake and one exhaust fan have become the minimum requirements in PC enclosures.
If you value performance, quiet operation, and want your components to run reliably for as long as you need them to, then fans are the easiest and least expensive way to do it. Liquid-cooled PCs needs fans to cool off the radiators and even LN2 overclockers looking to break records use them to keep water vapor from condensing on the motherboard. But how much do you really know about these whirling wonders? Perhaps the more obvious question is, how much is there to know about fans? It turns out, there’s quite a lot.
The Spin Zone
A vast majority of the fans used in PCs, laptops, and computing components consist of a brushless electric motor in the center of a plastic frame with a laded hub mounted to the motor’s shaft. The core mechanism that lets the fans spin is usually some type of bearing, but as we explain a little later, there are a variety of designs available depending on the type of fan in question.
Brushless DC motors rely on a switching signal to operate, and generate little to no electromagnetic interference compared to AC (alternating current) - powered fans, making them ideal for use inside PCs. The aptly named stator is the stationary portion of the motor, while the portion that rotates is called the rotor. These types of fans most commonly feature four poles on the stator, or an iron crossshaped piece of metal with copper wire wound around each of the four arms. This is considered a two-phase, four-slot motor, however some fan designs utilize threephase, six-slot motors, capable of achieving higher RPMs. EVGA’s ACX 2.0 graphics card heatsink fans are an example of a three-phase, six-slot motor-equipped fan.
The electromagnet inside the motor is capable of alternating charges on each arm, very rapidly. Another magnet is embedded in the fan’s hub with alternating north/south polarities. Because the polarity on the rotor’s magnet never changes, it is referred to as a permanent magnet. As the electromagnets in the stator are alternately charged positive and negative, the permanent magnet on the rotor moves to align its poles respectively, which results in the spinning of the fan’s rotor.
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| Most computer fans feature an electromagnet on the stator and a permanent magnet on the rotor. |
Inside The Fan
Generally speaking, there are two types of fans inside your system, axial fans and centrifugal fans. Axial fans are so named because they blow air parallel to the axis, or shaft to which the blades are affixed. This type of fan is the most common type used in computers. Case fans are a prime example of the axial fan, but many non-reference graphics cards and CPU coolers also use this type of fan.
The other, less common, type of fan found in PCs is the centrifugal fan, or squirrel cage fan. These are commonly used in closed-shroud graphics cards, particularly reference design cards from AMD and NVIDIA. These fans feature an open cylinder design in which the walls of the cylinder consist of numerous pitched blades. As the cylinder spins, air is sucked into the open portion and forced out at right angles in all directions. It’s this ability to change the direction of air that makes centrifugal fans popular in laptops. This fan type is also good for generating high levels of air pressure, which is why they’re also commonly used in leaf blowers, hairdryers, and various industrial ventilation applications.
Different Strokes
Fans come in a range of sizes, and most often are sized in millimeters measured from one side of the fan frame to the opposing side. Common case fan sizes include 40mm, 80mm, 92mm, 120mm, 140mm, 200mm, and larger. The most common intake and exhaust fan size used in cases today is 120mm, though less than a decade ago, 80mm was the de facto standard. When it comes to fan sizes, you’re typically limited by the mounts your case supports, however, some cases feature fan mounts that can handle multiple fan sizes. Thermaltake’s Core V71, for instance, has front and top panels that can support an impressive three 120mm fans, two 140mm fans, or two 200mm fans. The depth measurement is a little more standardized, with most fans measuring 25mm deep. Low-profile fans measuring around 14mm or 15mm deep are available as well, but are much less common. Highpowered fans measuring up to 38mm deep can also be found, but you may run into clearance issues when using some enclosures.
Fans can be further categorized by the amount of air they can move in a given time frame, expressed as cubic feet per minute, or CFM. Although this number is useful for determining the power of case fans, cooling power per unit area, or static air pressure expressed in mmH2O, is more useful for gauging the difference between fans used with heatsinks. Of the two specs, however, CFM is much more widely reported.
Another common number you’ll find while perusing a fan’s specifications is its maximum RPM, or revolutions per minute. For fans that have built-in rheostats for controlling speeds, oftentimes you’ll find RPM ratings for a handful of input voltages. Some fans also ship with a short 3- or 4-pin male-to-female cable adapter that has a resistor connected to the red, or positive 12V, wire. Using this adapter cuts the amount of power supplied to the fan, letting it run more slowly and quietly. Aerocool’s 120mm Dead Silence fans come with a 7V Voltage Reduction Cable, which slashes the fan’s peak 1,500rpm speed to 1,100rpm. Although the use of such adapters cuts noise output, airflow also diminishes. In the case of Aerocool’s Dead Silence fans, using the Voltage Reduction Cable cuts the fan’s sound output from 23.1dBA to 14.8dBA, and reduces the CFM rating from 81.5 to 62.5.
Generally speaking, as fan diameter increases, the revolutions per minute required to maintain a given CFM decreases. Furthermore, the noise produced also decreases. For instance, an average 80mm fan spinning at 2,500rpm might deliver around 30cfm, whereas a 120mm fan spinning at the same 2,500rpm can more than double the smaller fan’s CFM. In short, when you have the option, choose larger fans and reap the rewards of higher CFM and, typically, lower noise output.
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| Ball bearings are significantly more complex than | sleeve bearings, but they tend to last longer. |
Find Your Bearings
The bearings, or primary rotating mechanism of a fan, are largely responsible for the lifespan of the fan, the noise output, and overall performance you can expect. There are generally two types of bearings used, ball and sleeve. The fluid dynamic bearing, or FDB, is really just a variant of the sleeve bearing. The three-bearing types each have their own advantages and disadvantages, and even within each type, unique designs and (and some not so unique) can be used to minimize these drawbacks.
Ball bearings consist of a pair of rings, or races, that are separated by a series of metal balls. The inner race is the portion of the bearing that rotates, it is attached to the fan’s shaft. The outer race remains stationary, usually mounted into the hollow space at the center of the stator. Ball bearings employ a lubricant to minimize friction and occasionally a plastic retainer is used to keep the balls evenly spaced. Side plates and retention rings keep the balls, retainer, and inner and outer races together as a single unit.
Generally speaking, ball bearings tend to have a longer service life and slightly better performance in highertemperature environments compared to sleeve bearings. For example, it’s is not uncommon to find ball bearing-equipped fans with service lives in excess of 80,000 hours at 40 degrees C. As temperatures increase, however, the lubricant inside ball bearings can age more rapidly, losing its friction-reducing properties and slashing the fan’s life expectancy. At 70 degrees C, for instance, that same fan is more likely to last in the range of 50,000 hours, though this performance degradation is similar in sleeve bearing and FDB fans. Disadvantages of ball bearings include higher shock sensitivity and a tendency to generate noticeably more noise compared to sleeve bearing fans. And as ball bearing fans age, they tend to get even louder. Some fans use single-ball bearings, most higher-quality fans use two, and others still use a combination ball bearing/sleeve bearing design.
Sleeve bearings, on the other hand, use significantly fewer moving parts. These consist of a thick metal sleeve that is composed of sintered brass powder, or other alloys, formed into a cylindrical shape by the high-temperature process. The finished sleeve remains very porous, with between 15% and 30% of its structure consisting of air pockets. When impregnated with lubricant, the fan’s central shaft makes very little physical contact with the sleeve, especially as centrifugal forces distribute lubricant evenly along the length of the shaft during operation. Sleeve bearings are cheap to manufacture, they are fairly shock resistant, and fans that use them tend to emit less noise than ball bearing fans. Drawbacks include some friction occurring until a hydrodynamic state is reached, particularly as the fan gets up to speed, and a relatively short life span of between 30,000 and 50,000 hours.
Fluid-dynamic bearings are very similar to sleeve bearings except in that they tend to do a much better job of lubricating the shaft. Some FDB fans use a herringbone pattern on the inside of the sleeve, others employ grooves, and yet others allow more clearance between the shaft and bearing, filling the gap with lubricant and maintaining an airtight seal. We did a quick check of the FDB fans currently available and found more than 20 varieties with names like self-stabilizing oil-pressure bearings, Ever Lubricate, Fluid Circulative Bearing, Hydro Bearing, Hydro Wave, Hyper-spin, Oil System Bearing, Rifle, and more. ENERMAX calls its take on the fluid dynamic bearing Twister Bearing Technology, which adds a magnetic metal ball to the tip of the fan shaft in an effort to reduce friction even further. As a result of these unique designs, FDB-type bearings tend to approach ball bearing lifespans, but with much quieter operation. These designs tend to offer low shock sensitivity and very minimal noise output, but they are also more expensive to manufacture than generic sleeve bearings.
Sound Science
For most sound-emitting devices used in computing, the results are expressed as decibels (dB) weighted using the A scale, which limits the sound pressure level meter’s recording range to the 500Hz to 10,000Hz range. This is the frequency range that the human ear is most sensitive to, and it gives us a good idea how loud one fan might be compared to another. Decibels are logarithmic units, however, which means that the loudness will double every 10 increments. Effectively, a fan emitting 30dBA will be twice as loud as a fan emitting 20dBA. For reference, 20dBA is considered equivalent to the sound of a person whispering at a distance of five feet, 60dBA is the sound of a normal close proximity conversation, and the sound of a chainsaw rates at 120dBA.
In addition to speed and fan size, the type of bearing used in the fan also has a big impact on the device’s sound output. Although fans often feature a dBA number in their specifications, differing manufacturers don’t always use the same testing methods. Additionally, other conditions can affect the sound emitted by a fan, including the presence of a grille, dust filter, unique blade shapes, and resonant vibrations, which generally won’t be represented in the dBA numbers listed for each fan.
Manage Your Case Pressure
There is a real science to effectively managing the fan placement, air speed, and airflow direction throughout your case. Everyone agrees that relying on merely a CPU cooler fan and a GPU heatsink fan is a good way to burn out your parts prematurely. Where opinions diverge is whether it is better to rely on positive air pressure or negative air pressure for cooling.
A case with positive air pressure is one in which there is more air coming in than is being forced out, for instance a pair of 200mm intake fans in the front panel capable of moving air at a combined 260cfm and a pair of 120mm exhaust fans, one in the top panel and another in the rear panel, that move a combined 140cfm. Benefits of a positive air pressure environment include improved cooling for graphics cards that rely on open-shroud heatsink fans and less dust collecting on components overall. Drawbacks of a positive-pressure environment include suboptimal airflow due to the location and size of case exhaust points, less efficient performance of graphics cards that feature closedshroud heatsink fans, and slightly higher overall case temperatures compared to negative-pressure environments.
A case fan arrangement that exhausts more air than it takes in is referred to as a negative air pressure scenario. An example of this type of scenario would be a case with a 140mm top-panel exhaust fan and a 120mm rear-panel exhaust fan that have a combined 140cfm, and a single 120mm intake fan in the front panel capable of moving air at 70cfm. Benefits of such a setup include slightly better overall cooling performance compared to positive air pressure environments, enhanced natural convection, a more direct path for air to move from the front to the rear of the case, and improved performance of closed-shroud graphics cards. The downsides of a negative air pressure enclosure include less-efficient cooling for open-shroud heatsink graphics cards and a tendency to draw in dust that can collect on components, reducing the efficiency of heatsinks.
When deciding how best to cool your system, it’s important to know which direction the fan will move air. When looking at a fan, there’s what we call the open side (the side where you can touch the hub) and the back (where the fan’s motor connects to the outer frame via a quartet of struts). Air flows into the open side and out the back side, and the blades spin counter-clockwise when facing the open side. There’s no correct way to install a fan; you can install it hub-side in or hub-side out, depending on your preferences regarding air flow direction.
Given the above information, managing case air pressure may seem fairly straightforward, but there are a few things to keep in mind. First of all, fan manufacturers often advertise maximum CFM, but in some instances, you won’t be running the fans at their peak speeds all the time. Fans plugged into 4-pin headers on your motherboard are often going to be PWM (pulse width modulation)-controlled and experience speed fluctuations based on temperature data reported by the motherboard.
One good way to manage airflow would be to use a fan controller that works with all your fans, letting you change the air pressure on the fly to suit your needs. For instance, you could create a negative air pressure environment by lowering the speed of the intake fans and increasing the speed of exhaust fans when overclocking, gaming, or performing other hardware-intensive tasks, and reverting to a positive air pressure environment by increasing speed of intake fans and decreasing speed of exhaust fans when idling, to keep dust buildup to a minimum.
On the topic of dust, using dust filters on your intake fans is a good way to mitigate the major drawback of a negative air pressure environment. Keep in mind though, dust filters will slightly diminish the CFM performance of fans.
Because enclosure design factors so heavily on the cooling performance you’ll experience, we hesitate to recommend one setup over another. For instance, the presence of a mesh front panel, large vents, or empty fan ports can have a big impact on the flow of air inside your case. Whether your graphics card has an open or closed heatsink shroud will also have an impact. We recommend installing at least two or more intake and exhaust fans for the best cooling performance. Try to achieve a fairly balanced internal air pressure, with slight positive or negative air pressure, do some temperature testing, then reverse one of the fans (ideally a top- or side-panel fan) then test again. As long as the noise output is to your liking, you may find that the difference in temperatures is large or negligible. Then just make adjustments based on the performance you observed.
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| Voltage-reducing cables like this one can be used to reduce a fan’s speed. |
Speed Control
Without a speed-controlling mechanism, the DC (direct current) fans in your computer either run at their peak speeds or they won’t run at all. We’ve already touched on fan speed control via fan controllers, which let you manually adjust the voltage applied to each fan, but that’s really only one side of the speed-control coin.
The other means of controlling your fan speed is via PWM, which uses a microprocessor to very rapidly turn off and on the flow of electricity to the fan, continuously. This modulation coupled with a clocking mechanism represents the duty cycle, or the percentage of time the electric signal is on vs. off. To generate a 75% duty cycle, for example, a PWMcontrolled fan will have power supplied to it for an average of 75% of the time and power cut off for the remaining 25% of the time. When used in conjunction with software (or firmware such as in the BIOS) PWM-controlled fans can very effectively spin faster when temperatures increase, and then slow down as temperatures decline.
The fastest way to determine if your fan is PWM-capable is to look at the connector. These fans have 4 leads, one for ground, a power lead, a control signal lead, and a sense lead. This lattermost lead is the one that is lacking on 3-pin fan connectors and headers. Modern motherboards typically have at least one PWM-controlled fan header, and it is usually reserved for the most temperature sensitive component in the system, the CPU. Although you can plug a 3-pin fan connector into a 4-pin PWM header, it will not magically work like a PWM-controlled device. The control pin does allow these non-PWM fans to operate at various speeds, but they do so using a reduced but constant flow of voltage. Often these settings can be adjusted in the BIOS. In most cases, PWM will be the most efficient way to keep your vital components cool.
Exotic Extras
In a market as crowded as the case fan market is, you can bet that manufacturers are eager to create unique features that lower costs, reduce noise output, extend life expectancy, and generally make their fans more attractive to you and me. Some of the features include adding or subtracting the number of blades or changing the sweep of the blades to increase static pressure. Adjusting the angle of the struts in relation to the edge of the blades is one way Fractal Design’s Venturi fans reduce noise output.
We’ve also seen fan blades with grooves designed to change the frequency and pitch of the emitted sound. Bumps, rims, ridges, small fins, and other structures are also being added to the blades, frames, and struts of various fans to alter air turbulence to one degree or another. Covering the corners of the fan with soft rubber is becoming increasingly common in an attempt to isolate the fan and prevent vibrations from resonating throughout the case.
Of course, fans often come with aesthetic touches that can add some color, light, or added bling to your system, such as LEDs, colored blades or frames, UV reactive materials, and more. These features are more subjective, but most users want their systems to look as good as they perform.
Fan Favorite
There are a great many options available when it comes to choosing fans, graphics cards, and CPU coolers. Equipped with a working knowledge of what makes these gyrating gadgets tick, keeping your system cool—literally as well as figuratively—is easier than ever.