A computer fan is any fan inside, or attached to, a computer case used for active cooling. Fans are used to draw cooler air into the case from the outside, expel warm air from inside and move air across a heat sink to cool a particular component. Both axial and sometimes centrifugal (blower/squirrel-cage) fans are used in computers. Computer fans commonly come in standard sizes, and are powered and controlled using 3-pin or 4-pin fan connectors.
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 heat sink to increase the area of heated surface in contact with the air, thereby improving the efficiency of cooling. Fan control is not always an automatic process. A computer's BIOS (basic input/output system) can control the speed of the built-in fan system for the computer. A user can even supplement this function with additional cooling components or connect a manual fan controller with knobs that set fans to different speeds.
In the IBM PC compatible 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.
Fans are used to move air through the computer case. The components inside the case cannot dissipate heat efficiently if the surrounding air is too hot. Case fans may be placed as intake fans, drawing cooler outside air in through the front or bottom of the chassis (where it may also be drawn over the internal hard drive racks), or exhaust fans, expelling warm air through the top or rear. Some ATX tower cases have one or more additional vents and mounting points in the left side panel where one or more fans may be installed to blow cool air directly onto the motherboard components and expansion cards, which are among the largest heat sources.
Standard axial case fans are 40, 60, 80, 92, 120, 140, 200 and 220 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/or 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 and clogging up the internal components. Heatsinks are especially vulnerable to being clogged up, as the insulating effect of the dust will rapidly degrade the heatsink's ability to dissipate heat.
While the power supply (PSU) contains a fan with few exceptions, it is not to be used for case ventilation. The hotter the PSU's intake air is, the hotter the PSU gets. As the PSU temperature rises, the conductivity of its internal components decrease. Decreased conductivity means that the PSU will convert more of the input electric energy into thermal energy (heat). This cycle of increasing temperature and decreased efficiency continues until the PSU either overheats, or its cooling fan is spinning fast enough to keep the PSU adequately supplied with comparatively cool air. The PSU is mainly bottom-mounted in modern PCs, having its own dedicated intake and exhaust vents, preferably with a dust filter in its intake vent.
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 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 also known as a blower, turbo or squirrel cage 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.
Fans may be mounted next to or onto a hard disk drive for cooling purposes. Hard drives can produce considerable heat over time, and are heat-sensitive components that should not operate at excessive temperatures. In many situations, natural convective cooling suffices, but in some cases fans may be required. These may include -
A case fan may be mounted on a radiator attached to the case, simultaneously operating to cool a liquid cooling device's working fluid and to ventilate the case. In laptops, a single blower fan often cools a heat sink connected to both CPU and GPU using heat pipes. In rack-mounted servers, a single row of fans may operate to create an airflow through the chassis from front to rear, which is directed by passive ducts or shrouds across individual components' heat sinks.
Fans are, less commonly, used for other purposes such as:
Due to the low pressure, high volume air flows they create, most fans used in computers are of the axial flow type; centrifugal and crossflow fans type. Two important functional specifications are the airflow that can be moved, typically stated in cubic feet per minute (CFM), and static pressure. Given in decibels, the sound volume figure can be also very important for home and office computers; larger fans are generally quieter for the same CFM.
Many gamers, case modders, and enthusiasts utilize fans illuminated with colored LED lights. Multi-colored fans are also available.
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 140 mm fan with holes for the corners of a 120 mm square fan). The width of square fans and the diameter of round ones are usually stated in millimeters. The dimension given is the outside width of the fan, not the distance between mounting holes. Common sizes include 40 mm, 60 mm, 80 mm, 92 mm, 120 mm and 140 mm, although 8 mm, 17 mm, 20 mm, 25 mm, 30 mm, 35 mm, 38 mm, 45 mm, 50 mm, 70 mm, 200 mm, 220 mm, 250 mm and 360 mm sizes are also available. Heights, or thickness, are typically 10 mm, 15 mm, 25 mm or 38 mm.
Typically, square 120 mm and 140 mm fans are used where cooling requirements are demanding, as for computers used to play games, and for quieter operation at lower speeds. Larger fans are usually used for cooling case, CPUs with large heatsink and ATX power supply. Square 80 mm and 92 mm fans are used in less demanding applications, or where larger fans would not be compatible. Smaller fans are usually used for cooling CPUs with small heatsink, SFX power supply, graphics cards, northbridges, etc.
Fan sizes and corresponding screw hole spacing:
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 the speed reduces the noise by about 15 dB. Axial fans may rotate at speeds of up to around 23,000 rpm for smaller sizes.
Fans may be controlled by sensors and circuits that reduce their 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 and at a fixed ambient temperature.
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:
Connectors usually used for computer fans are the following:
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: