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 heatsink to cool a particular component.
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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.[1] 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.[1] 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 as of 2007[update] this remains relatively unusual. It is not uncommon to find five or more fans in a modern PC.
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 or 120 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.
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.
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, especially those 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[2]), so effective cooling is especially important. Passive coolers for new video cards, however, are not unheard of, such as the Thermalright HR-03.
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 is usual, but may otherwise be unnecessary.
Fans are, less commonly, used for other purposes such as:
Many gamers, case modders, and enthusiasts utilize fans illuminated with colored LED lights.
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 20 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.[3] Axial fans may rotate at speeds of up to around 6,000 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:
Fans are available in many sizes and capacities. Typically square 120mm 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; cheaper 80mm fans are used for this purpose in less demanding applications. Smaller fans are usually used for cooling CPUs, graphics cards, northbridges, etc. Within a given physical size airflow capacity increases with power consumption.
Connectors usually used for computer fans (there is no agreed standard) are
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.[7]
Other methods of cooling include: