A fan is a device to agitate or move air or gas. It is basically a device for creating a current of air by movement of a surface or surfaces. It is used to move air or gas from one location to another, within or between spaces, for industrial reasons, or for residential use, for ventilation purposes or to increase the circulation of air in a living space. Fans have broad surfaces that usually revolve in motion.
Leaves or flat objects, waved to produce a more comfortable atmosphere, have not entirely given way to variable-speed electromechanical devices. In the course of their development, fans have exhibited a great variety of materials, a richness of decorative artwork, and associations with sophisticated milieus that belie the origins of this humble and apparently universal tool.
Applications include ornamental decorations, climate control, cooling system, refreshing air, personal wind-generation (e.g., an electric table fan), ventilation (e.g., an exhaust fan), winnowing (e.g., separating chaff of grain), removing dust (e.g., sucking as in a vacuum cleaner), cuttings, and to provide draft for a fire.
A mechanical fan is an electrically powered device used to produce an airflow for the purpose of creature comfort (particularly in the heat), ventilation, exhaust, cooling or any other gaseous transport.Mechanically, a fan can be any revolving vane or vanes used for producing currents of air. Fans produce air flows with high volume and low pressure, as opposed to a gas compressor which produces high pressures at a comparatively low volume. A fan blade will often rotate when exposed to an air stream, and devices that take advantage of this, such as anemometers and wind turbines often have designs similar to that of a fan.
Typical applications include climate control, cooling systems, personal comfort (e.g., an electric table fan), ventilation (e.g., an exhaust fan), winnowing (e.g., separating chaff of cereal grains), removing dust (e.g. sucking as in a vacuum cleaner), drying (usually in addition to heat) and to provide draft for a fire. It is also common to use electric fans as air fresheners, by attaching fabric softener sheets to the protective housing. This causes the fragrance to be carried into the surrounding air.
In addition to their utilitarian function, vintage or antique fans, and in particular electric fans manufactured from the late 19th century through the 1950s, have become a recognized collectible category, and in the U.S.A. an active collector club, the Antique Fan Collectors Association, supports the hobby.
Types of fans
Mechanical revolving blade fans are made in a wide range of designs. In a home you can find fans that can be put on the floor or a table, or hung from the ceiling, or are built into a window, wall, roof, chimney, etc. They can be found in electronic systems such as computers where they cool the circuits inside, and in appliances such as hair dryers and space heaters. They are also used for cooling in air-conditioning systems, and in automotive engines, where they are driven by belts or by direct motor. Fans create a wind chill but do not lower temperatures directly.
There are three main types of fans used for moving air, axial, centrifugal (also called radial) and cross flow (also called tangential).
The axial-flow fans have blades that force air to move parallel to the shaft about which the blades rotate. Axial fans blow air across the axis of the fan, linearly, hence their name. This type of fan is used in a wide variety of applications, ranging from small cooling fans for electronics to the giant fans used in wind tunnels.
Examples of axial fans are:
- Table fan - Basic elements of a typical table fan include the fan blade, base, armature and lead wires, motor, blade guard, motor housing, oscillator gearbox, and oscillator shaft. The oscillator is a mechanism that motions the fan from side to side. The axle comes out on both ends of the motor, one end of the axle is attached to the blade and the other is attached to the oscillator gearbox. The motor case joins to the gearbox to contain the rotor and stator. The oscillator shaft combines to the weighted base and the gearbox. A motor housing covers the oscillator mechanism. The blade guard joins to the motor case for safety.
- Electro-mechanical fans, among collectors, are rated according to their condition, size, age, and number of blades. Four-blade designs are the most common. Five-blade or six-blade designs are rare. The materials from which the components are made, such as brass, are important factors in fan desirability.
- Ceiling fan - A fan suspended from the ceiling of a room is a ceiling fan.
- In automobiles, a mechanical fan provides engine cooling and prevents the engine from overheating by blowing or sucking air through a coolant-filled radiator. It can be driven with a belt and pulley off the engine's crankshaft or an electric fan switched on/off by a thermo switch.
- Variable Pitch Fan A variable pitch fan is used where precise control of static pressure within supply ducts is required. The fan wheel will spin at a constant RPM (1780 in this case). The blades follow the control pitch hub. As the hub moves toward the rotor the blades increase their angle of attack and an increase in flow results.
Centrifugal fan
Often called a "squirrel cage" (due to its similarity in appearance to exercise wheels for pet rodents), the centrifugal fan has a moving component (called an impeller) that consists of a central shaft about which a set of blades, or ribs, are positioned. Centrifugal fans blow air at right angles to the intake of the fan, and spin the air outwards to the outlet (by deflection and centrifugal force). The impeller rotates, causing air to enter the fan near the shaft and move perpendicularly from the shaft to the opening in the scroll-shaped fan casing. A centrifugal fan produces more pressure for a given air volume, and is used where this is desirable such as in leaf blowers, hair driers, air mattre
ss inflators, inflatable structures, and various industrial purposes. They are typically noisier than comparable axial fans.
Cross flow fan
a cross flow fan, is a centrifugal fan in which the air flows through the fan, rather than through an inlet. The rotor of a cross flow fan is covered to create a pressure differential. When used in household fans, cross flow fans have a smaller opening on one side and a larger opening on the other. The resultant pressure difference allows air to flow straight through the fan, even though the fan blades counter the flow of air on one side of the rotation. Cross flow fans give airflow along the entire width of the fan. Cross flow fans are noisier than ordinary centrifugal fans presumedly because the fan blades fight the flow of air on one side of the rotation unlike normal squirrel cage fans. Cross flow fans are often used in air cond itioners, automobile ventilation systems, and for cooling in medium-sized equipment such as photocopiers. The action of a fan or blower causes pressures slightly above atmospheric, which are called plenums. 
There have been applications of cross flow fan in aircraft propulsion systems, in close coupling with wings. For a wing leading edge application, see the FanWing, for a trailing edge application see external links section Fan motor
A stand alone fan is typically powered with an electric motor. Fans are often attached directly to the motor's output, with no need for gears or belts. The electric motor is either hidden in the fan's center hub or extends behind it. For big industrial fans, 3-phase asynchronous motors are commonly used, placed near the fan and driving it through a belt and pulleys. Smaller fans are often powered by shaded pole AC motors, or brushed or brushless DC motors. AC-powered fans usually use mains voltage, while DC-powered fans use low voltage, typically 24 V, 12 V or 5 V. Cooling fans for computer equipment exclusively use brushless DC motors, which produce much less electromagnetic interference.
In machines which already have a motor, the fan is often connected to this rather than being powered independently. This is commonly seen in cars, boats, large cooling systems and winnowing machines, where the fan is connected either directly to the driveshaft or through a belt and pulleys. Another common configuration is a dual-shaft motor, where one end of the shaft drives a mechanism, while the other has a fan mounted on it to cool the motor itself.
Solar Powered Fan
Electric fans used for ventilation may be powered by solar panels instead of mains current. This is an attractive option because once the capital costs of the solar panel have been covered, the resulting electricity is free. In addition, electricity is always available when the sun is shining and the fan needs to run.
A typical example uses a detached 10 watt, 12x12 inch (30x30 cm) solar panel and is supplied with appropriate brackets, cables, and connectors. It can be used to ventilate up to 1250 square feet (100 m²) of area and can move air at up to 800 cubic feet per minute (400 L/s). Because of the wide availability of 12 V brushless DC electric motors and the convenience of wiring such a low voltage, such fans usually operate on 12 volts.
The detached solar panel is typically installed in the spot which gets most of the sun light and then connected to the fan mounted as far as 20 to 25 feet (6 to 7 m) away. Other permanently-mounted and small portable fans include an integrated (non-detachable) solar panel.
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