Computer fan

A computer fan is any fan inside, or attached to, a computer case used for active cooling, and may refer to fans that draw cooler air into the case from the outside, expel warm air from inside, or move air across a heat sink to cool a particular component.

Usage of a cooling fan
As processors, graphics cards, RAM and other components in computers have increased in speed and power consumption, the amount of heat produced by these components as a side-effect of normal operation has also increased. These components need to be kept within a specified temperature range to prevent overheating, instability, malfunction and damage leading to a shortened component lifespan.

While in earlier personal computers it was possible to cool most components using natural convection (passive cooling), many modern components require more effective active cooling. To cool these components, fans are used to move heated air away from the components and draw cooler air over them. Fans attached to components are usually used in combination with a heatsink to increase the area of heated surface in contact with the air, thereby improving the efficiency of cooling.

In the IBM compatible PC market, the computer's power supply unit (PSU) almost always uses an exhaust fan to expel warm air from the PSU. Active cooling on CPUs started to appear on the Intel 80486, and by 1997 was standard on all desktop processors. Chassis or case fans, usually one exhaust fan to expel heated air from the rear and optionally an intake fan to draw cooler air in through the front, became common with the arrival of the Pentium 4 in late 2000. A third vent fan in the side of the PC, often located over the CPU, is also common. The graphics processing unit (GPU) on many modern graphics cards also requires a heatsink and fan. In some cases, the northbridge chip on the motherboard has another fan and heatsink. Other components such as the hard drives and RAM may also be actively cooled, though this remains relatively unusual. It is not uncommon to find five or more fans in a modern PC.

Case mount
Used to aerate the case of the computer. The components inside the case cannot dissipate heat efficiently if the surrounding air is too hot. Case fans move air through the case, usually drawing cooler outside air in through the front (where it may also be drawn over the internal hard drive racks) and expelling it through the rear. There may be a third fan in the side or top of the case to draw outside air into the vicinity of the CPU, which is usually the largest single heat source. Standard case fans are 80 mm, 92 mm, 120 mm, 140 mm, 200 mm, 230 mm in width and length. As case fans are often the most readily visible form of cooling on a PC, decorative fans are widely available and may be lit with LEDs, made of UV-reactive plastic, and covered with decorative grilles. Decorative fans and accessories are popular with case modders. Air filters are often used over intake fans, to prevent dust from entering the case.

A power supply (PSU) fan often plays a double role, not only keeping the PSU itself from overheating, but also removing warm air from inside the case. PSUs with two fans are also available, which typically have a fan on the inside to supply case air into the PSU and a second fan on the back to expel the heated air.

CPU fan
Used to cool the CPU (central processing unit) heatsink. Effective cooling of a concentrated heat source such as a large-scale integrated circuit requires a heatsink, which may be cooled by a fan; use of a fan alone will not prevent overheating of the small chip.
 * See computer spot cooling.

Graphics card fan
Used to cool the heatsink of the graphics processing unit or the memory on graphics cards. These fans were not necessary on older cards because of their low power dissipation, but most modern graphics cards designed for 3D graphics and gaming need their own dedicated cooling fans. Some of the higher powered cards can produce more heat than the CPU (dissipating up to 289 watts ), so effective cooling is especially important. Since 2010 graphics cards have been released with either axial fans or a centrifugal fan commonly known as a blower or squirrel cage fan.

Chipset fan
Used to cool the heatsink of the northbridge of a motherboard's chipset; this may be needed where the system bus is significantly overclocked and dissipates more power than as usual, but may otherwise be unnecessary. As more features of the chipset are integrated into the central processing unit, the role of the chipset has been reduced and the heat generation reduced also.

Other purposes
Fans are, less commonly, used for other purposes such as:


 * PCI slot fan: A fan mounted in one of the PCI slots, usually to supply additional cooling to the PCI and/or graphics cards.
 * Hard disk fan: A fan mounted next to or on a hard disk drive. This may be desirable on faster-spinning hard disks with greater heat production. less expensive drives rotated at speeds up to 7,200 RPM; 10,000 and 15,000 RPM drives were available but generated more heat.
 * CD burner fan: Some internal CD and/or DVD burners included cooling fans.
 * Memory fan: Modern computer memory can generate enough heat, especially when overclocked, that active cooling may be necessary, usually in the form of small fans positioned above the memory chips.

Appearance
Many gamers, case modders, and enthusiasts utilize fans illuminated with colored LED lights.

Physical characteristics
Most fans used in computers are of the axial-flow type; centrifugal and crossflow fans are sometimes used.

Two important functional specifications are the airflow that can be moved, typically stated in cubic feet per minute (CFM), and static pressure.

The dimensions and mounting holes must suit the equipment that uses the fan. Square-framed fans are usually used, but round frames are also used, often so that a larger fan than the mounting holes would otherwise allow can be used (e.g., a 120 mm fan with holes for the corners of a 90 mm square fan). The width of square fans and the diameter of round ones are usually stated in millimeters; common sizes include 40, 60, 80, 92, 120 and 140 mm. Heights are typically 10 or 25 mm, but this is usually not an important dimension as it does not affect mounting holes or apertures in the case.

The speed of rotation (specified in revolutions per minute, RPM) together with the static pressure determine the airflow for a given fan. Where noise is an issue larger, slower-turning fans are quieter than smaller, faster, fans that can move the same airflow. Fan noise has been found to be roughly proportional to the fifth power of fan speed; halving speed reduces noise by about 15dB. Axial fans may rotate at speeds of up to around 8,000 rpm; fans may be controlled by sensors and circuits that reduce fan speed when temperature is not high, leading to quieter operation, longer life, and lower power consumption than fixed-speed fans. Fan lifetimes are usually quoted under the assumption of running at maximum speed.

A fan with high static pressure is more effective at forcing air through restricted spaces, such as the gaps between a radiator or heatsink; static pressure is more important than airflow in CFM when choosing a fan for use with a heatsink. The relative importance of static pressure depends on the degree to which the airflow is restricted by geometry; static pressure becomes more important as the spacing between heatsink fins decreases. Static pressure is usually stated in either mm Hg or mm H2O.

The type of bearing used in a fan can affect its performance and noise. Most computer fans use one of the following bearing types:


 * Sleeve bearings use two surfaces lubricated with oil or grease as a friction contact. They often use porous sintered sleeves to be self-lubricating, requiring only infrequent maintenance or replacement. Sleeve bearings are less durable at higher temperatures as the contact surfaces wear and the lubricant dries up, eventually leading to failure; however, lifetime is similar at relatively low ambient temperatures. Sleeve bearings may be more likely to fail at higher temperatures, and may perform poorly when mounted in any orientation other than vertical. The typical lifespan of a sleeve-bearing fan may be around 30,000 hours at 50 °C. Fans that use sleeve bearings are generally cheaper than fans that use ball bearings, and are quieter at lower speeds early in their life, but can become noisy as they age.
 * Rifle bearings are similar to sleeve bearings, but are quieter and have almost as much lifespan as ball bearings. The bearing has a spiral groove in it that pumps fluid from a reservoir. This allows them to be safely mounted with the shaft vertical (unlike sleeve bearings), since the fluid being pumped lubricates the top of the shaft. The pumping also ensures sufficient lubricant on the shaft, reducing noise, and increasing lifespan.
 * Ball bearings: Though generally more expensive, ball bearing fans do not suffer the same orientation limitations as sleeve bearing fans, are more durable at higher temperatures, and are quieter than sleeve-bearing fans at higher rotation speeds. The typical lifespan of a ball bearing fan may be over 60,000 hours at 50 °C.
 * Fluid bearings have the advantages of near-silent operation and high life expectancy (comparable to ball bearings), but tend to be the most expensive.
 * Magnetic bearings or maglev bearings, in which the fan is repelled from the bearing by magnetism.

Air pressure and air flow
Air pressure is most important for cooling. It indicates cooling power per unit area. If the diameter of a fan is constant, then more air pressure indicates more air flow. A simple relation of air flow with air pressure is $(fan diameter)^{2} * (air pressure)^{2} / (100 * 28.51875) = air flow$, where the diameter of the fan is in millimeters, air pressure is in mmH2O, and air flow is in cubic feet per minute.

Fan sizing
Fans are available in many sizes and capacities. Typically square 120 mm case and power supply fans are used where cooling requirements are demanding, as for computers used to play games, and for quieter operation at lower speeds, but 140 mm and larger fans also exit. 80 mm fans are used in less demanding applications, or where larger fans wouldn't be compatible. Smaller fans are usually used for cooling CPUs, graphics cards, northbridges, etc. For a given physical size, airflow capacity increases with power consumption.

Fan connector
Connectors usually used for computer fans (there is no agreed standard) are
 * 3-pin Molex connector KK Family: This connector is used when connecting a fan to the motherboard or other circuit board. It is a small, thick, rectangular in-line female connector with two polarizing tabs on the outer-most edge of one long side.  Pins are square and on a 0.1 inch (2.54 mm) pitch. The three pins are used for ground, +12 V power, and a tachometer signal. The Molex Part number of receptacle is 22-01-3037. The Molex Part number of the individual crimp contacts is 08-50-0114. The matching PCB header Molex part number is 22-23-2031.


 * 4-pin Molex connector KK Family: This is a special variant of the Molex KK connector with four pins but with the locking/polarisation features of a 3-pin connector. The additional pin is used for a pulse-width modulation signal to provide variable speed control. These can be plugged into 3-pin headers, but will lose their fan speed control. The Molex Part number of receptacle is 47054-1000. The Molex Part number of individual crimp contacts is 08-50-0114. The Molex Part number of the header is 47053-1000.


 * 4-pin Molex connector: This connector is used when connecting the fan directly to the power supply. It consists of two wires (yellow/12V and black/ground) leading to and splicing into a large in-line 4-pin male-to-female Molex connector.  This is the same connector as used on hard drives before the SATA became standard.


 * Dell proprietary: This connector is an expansion of a simple 3-pin female IC connector by adding two tabs to the middle of the connector on one side and a lock-tab on the other side. The size and spacing of the pin sockets is identical to a standard 3-pin female IC connector and 3-pin Molex connector.  Some models have the wiring of the white wire (speed sensor) in the middle, whereas the standard 3-pin Molex connector requires the white wire as pin #3, thus compatibility issues may exist.

Alternatives
If a fan is not desirable, because of noise, reliability, or environmental concerns, there are some alternatives. Some improvement can be achieved by eliminating all fans except one in the power supply which also draws hot air out of the case.

Systems can be designed to use passive cooling alone, reducing noise and eliminating moving parts that may fail. This can be achieved by: Other methods of cooling include:
 * Natural convection cooling: carefully designed, correctly oriented, and sufficiently large heatsinks can dissipate up to 100 W by natural convection alone
 * Heatpipes to transfer heat out of the case
 * Undervolting or underclocking to reduce power dissipation
 * Submersive liquid cooling, placing the motherboard in a non-electrically conductive fluid, provides excellent convection cooling and protects from humidity and water without the need for heatsinks or fans. Special care must be taken to ensure compatibility with adhesives and sealants used on the motherboard and ICs. This solution is used in some external environments such as wireless equipment located in the wild.
 * Water cooling
 * Refrigeration, e.g. by Peltier effect devices
 * Ionic wind cooling is being researched, whereby air is moved by ionizing air between 2 electrodes. This replaces the fan and has the advantage of no moving parts and less noise.