Air curtains and air doors also utilize this effect to help retain warm or cool air within an otherwise

The Dyson Air Multiplier fans, and the Imperial C2000 series range hood fans,[7] have no exposed fan blades or other visibly moving parts. The airflow is generated using the Coanda effect; a small quantity of air from a high-pressure bladed impeller fan, contained in the base rather than exposed, drives a large airmass via a low-pressure area created by the airfoil.[8][9][10] The US Patent & Trademark Office initially ruled that Dyson's patent was not an improvement on the Toshiba patent on a nearly identical bladeless desktop fan granted in 1981.[8]
Air curtains and air doors also utilize this effect to help retain warm or cool air within an otherwise exposed area that lacks a cover or door. Air curtains are commonly used on open-face dairy, freezer, and vegetable displays to help retain chilled air within the cabinet using a laminar airflow circulated across the display opening, usually generated by a fan in the base of the cabinet.
Convective[edit source | editbeta]
Differences in air temperature will affect the density of air and can be used to induce air circulation through the mere act of heating or cooling an air mass. This effect is so subtle and works at such low air pressures that it does not appear to fit the definition of a fan technology. However, prior to the development of electricity, convective airflow was the primary method of inducing airflow in living spaces.
Old fashioned oil and coal furnaces were not electric and operated simply on the principle of convection to move the warm air. Very large volume air ducts were sloped upwards away from the top of the furnace towards floor and wall registers above the furnace. Cool air was returned through similar large ducts leading to the bottom of the furnace.
Older houses from before electrification often had open duct grilles leading from the ceiling of a lower level to the floor of an upper level, to allow convective airflow to slowly rise up the building from one floor to the next.
Outhouses commonly rely on a simple enclosed air channel in a corner of the structure to exhaust offensive odors. Exposed to sunlight, the channel is warmed and a slow convective air current is vented out the top of the building, while fresh air enters the pit through the seat hole.
Electrostatic[edit source | editbeta]
An electrostatic fluid accelerator propels airflow by inducing motion in airborne charged particles. A high voltage electric field (commonly 25,000 to 50,000 volts) formed between exposed charged anode and cathode surfaces is capable of inducing airflow through a principle referred to as ionic wind. The airflow pressure is typically very low but the air volume can be large.
However, a sufficiently high voltage potential can also cause the formation of ozone and nitrogen oxides, which are reactive and irritating to mucous membranes.
Noise[edit source | editbeta]

Fans generate noise from the rapid flow of air around blades and obstacles, and sometimes from the motor. Fan noise has been found to be roughly proportional to the fifth power of fan speed; halving speed reduces noise by about 15dB.[11]
Fan drive methods[edit source | editbeta]



Building heating and cooling systems commonly use a squirrel cage fan driven by belt from a separate electric motor.


Internal combustion engines sometimes drive an engine cooling fan directly, or may use a separate electric motor.


Large electric motors may have a cooling fan either on the back or inside the case. (Shown with black rear cover removed.)


Dual shaft fan motor in a window air conditioner.
Standalone fans are usually powered by electric motors, often attached directly to the motor's output with no gears or belts. The motor is either hidden in the fan's center hub or extends behind it. For big industrial fans, three-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, 12, or 5 V. Cooling fans for computer equipment always use brushless DC motors, which generate much less electromagnetic interference than other types.
In machines with a rotating part, the fan is often connected to it rather than being powered separately. This is commonly seen in motor vehicles with internal combustion engines, where the fan is connected to the drive shaft directly or through a belt and pulleys.
A 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. Window air conditioners commonly use a dual-shaft fan to operate separate blowers for the interior and exterior parts of the device.
Where electrical power or rotating parts are not available, fans may be drive by other methods. High-pressure gases such as steam can be used to drive a small turbine, and high-pressure liquids can be used to drive a pelton wheel, which can provide the rotational drive for a fan.
Large, slow-moving energy sources such as a flowing river can also power a fan using a water wheel and a train of gears or pulleys.