A fan is
a machine used to create flow within a fluid, typically a gas such as air. The fan consists of a rotating
arrangement of vanes or blades which act on the fluid. The rotating assembly of
blades and hub is known as an impeller, a rotor, or a runner. Usually, it is
contained within some form of housing or case. This may direct the airflow or
increase safety by preventing objects from contacting the fan blades. Most fans
are powered by electric motors, but other sources of
power may be used, including hydraulic motors,
Fans produce
flows with high volume and low pressure (although higherthan ambient pressure), as opposed to compressors which produce high pressures at a comparatively low
volume. A fan blade will often rotate when exposed to a fluid 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 and personal thermal comfort (e.g., an electric table or floor fan), vehicle engine
cooling systems (e.g., in front of a radiator), machinery cooling systems (e.g., inside computers
and audio power
amplifiers),
ventilation, fume extraction, winnowing (e.g., separating chaff of cereal grains), removing dust (e.g. in a vacuum cleaner), drying (usually in combination with a heat source) and to
provide draft for a fire.
While fans
are often used to cool people, they do not actually cool air (if anything,
electric fans warm it slightly due to the warming of their motors), but work
by evaporative cooling of sweat and increased heat convection into the surrounding air due to the airflow from the
fans. Thus, fans may become ineffective at cooling the body if the surrounding
air is near body temperature and contains high humidity. During periods of very
high heat and humidity, governments actually advise against the use of fans.
CENTRIFUGAL
TYPICAL CENTRIFUGAL FAN
Often called a "squirrel cage" (because of its
general similarity in appearance to exercise wheels for pet rodents) or "scroll fan", 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, blowdryers, air mattress inflators, inflatable
structures, climate control, and various industrial purposes. They are typically
quieter than comparable axial fans.
CROSS-FLOW
Cross-section of Cross-flow fan, from the 1893
patent, The rotation is clock-wise. The stream guide F is usually not present
in modern implementation.
CROSS-FLOW FAN
The cross-flow or tangential fan, sometimes known
as a tubular fan, was
patented in 1893 by Paul Mortier, and is used extensively in the HVAC industry. The fan is usually
long in relation to the diameter, so the flow approximately remains
two-dimensional away from the ends. The CFF uses an impeller with forward
curved blades, placed in a housing consisting of a rear wall and vortex wall.
Unlike radial machines, the main flow moves transversely across the impeller,
passing the blading twice.
The flow
within a cross-flow fan may be broken up into three distinct regions: a vortex
region near the fan discharge, called an eccentric vortex, the through-flow
region, and a paddling region directly opposite. Both the vortex and paddling
regions are dissipative, and as a result, only a portion of the impeller
imparts usable work on the flow. The cross-flow fan, or transverse fan, is thus
a two-stage partial admission machine. The popularity of the crossflow fan in
the HVAC industry comes from its compactness, shape, quiet operation, and
ability to provide high pressure coefficient. Effectively a rectangular fan in
terms of inlet and outlet geometry, the diameter readily scales to fit the
available space, and the length is adjustable to meet flow rate requirements
for the particular application.
Common
household tower fans are also cross-flow fans. Much of the early work focused
on developing the cross-flow fan for both high and low-flow-rate conditions,
and resulted in numerous patents. Key contributions were made by Coester,
Ilberg and Sadeh, Porter and Markland, and Eck. One phenomenon particular to
the cross-flow fan is that, as the blades rotate, the local air incidence angle
changes. The result is that in certain positions the blades act as compressors
(pressure increase), while at other azimuthal locations the blades act as turbines (pressure decrease).
DRIVE METHODS
Building heating and cooling system 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 motor may have a cooling fan either on the back or inside the
case.(Shown with black rear cover removed.)
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.
WORKING