Mechanical revolving blade fans are made in a wide range of designs. They are used on the floor, table, desk, or hung from the ceiling (
ceiling fan) and can be built into a window, wall, roof, etc. Tower fans tend to have smaller blades inside. Electronic systems generating significant heat, such as
computers, incorporate fans. Appliances such as hair dryers and space heaters also use fans. They move air in air-conditioning systems and in automotive engines. Fans used for comfort inside a room create a
wind chill by increasing the
heat transfer coefficient but do not lower temperatures directly. Fans used to cool electrical equipment or in engines or other machines cool the equipment directly by exhausting hot air into the cooler environment outside of the machine so that cooler air flows in. Three main types of fans are used for moving air,
axial,
centrifugal (also called
radial) and
cross flow (also called
tangential). The
American Society of Mechanical Engineers (ASME) Performance Testing Code (PTC) 11 provides standard procedures for conducting and reporting tests on fans, including those of the centrifugal, axial, and mixed flows.
Axial-flow Axial-flow fans have blades that force air to move
parallel to the shaft about which the blades rotate. 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
cooling towers. Axial flow fans are applied in air conditioning and industrial process applications. Standard axial flow fans have diameters of 300–400 mm or 1,800–2,000 mm and work under pressures up to 800
Pa. Special types of fans are used as low-pressure compressor stages in aircraft engines. 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 armature axle shaft comes out on both ends of the motor; one end of the shaft is attached to the blade, and the other is attached to the oscillator gearbox. The motor case joins the gearbox to contain the rotor and stator. The oscillator shaft combines the weighted base and the gearbox. A motor housing covers the oscillator mechanism. The blade guard joins the motor case for safety. • Domestic extractor fan: Wall- or ceiling-mounted, the domestic extractor fan is employed to remove moisture and stale air from domestic dwellings. Bathroom extractor fans typically utilize a four-inch (100 mm) impeller, while kitchen extractor fans typically use a six-inch (150 mm) impeller as the room is often bigger. Axial fans with five-inch (125 mm) impellers are also used in larger bathrooms, though they are much less common. Domestic axial extractor fans are unsuitable for duct runs over 3 m or 4 m, depending on the number of bends in the run, as the increased air pressure in longer pipework inhibits the fan's performance. • Continuous running extractor fans run continuously at a very slow rate, running fast when necessary, for example when a bathroom light is switched on. At working speed, they are just normal extractor fans. They extract typically 5 to 10 l/sec at continuous speed and use little electricity, 1 or 2 watts, for low annual cost. Some have humidity sensors to control trickle operation. They have the advantage of ensuring ventilation and preventing the build-up of humidity. Alternatively, a normal extractor fan may be fitted to operate intermittently at full power for the same purpose. In cold weather they may have noticeably cool the room they are in, or, if the door is open, the house. • 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. • A ceiling fan is a fan suspended from the ceiling of a room. Most ceiling fans rotate at relatively low speeds and do not have blade guards because they are inaccessible and unwieldy. Ceiling fans are used in both residential and industrial/commercial settings. • In
automobiles, a mechanical or electrically driven fan provides
engine cooling and prevents the engine from overheating by blowing or drawing air through a
coolant-filled
radiator. The fan may be driven with a
belt and pulley off the
engine's
crankshaft or an electric motor switched on or off by a
thermostatic
switch. •
Computer fan for cooling electrical components and in
laptop coolers. • Fans inside
audio power amplifiers help to draw heat away from the electrical components. •
Variable pitch fan: A variable-pitch fan is used to precisely control static pressure within supply ducts. The blades are arranged to rotate upon a control-pitch hub. The fan wheel will spin at a constant speed. 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 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 that form a
spiral, 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 in
air handling units and various industrial purposes. They are typically noisier than comparable axial fans (although some types of centrifugal fans are quieter such as in air handling units). Centrifugal fan.png|A diagram of a centrifugal fan, with a top view to show airflow CentrifugalFan.png|Typical centrifugal fan Centrifugal Cooling Fan.jpg|Centrifugal cooling fan installed within a laptop. The fan displaces air over the fin stack, which contains a pair of heat pipes.
Cross-flow The
cross-flow or
tangential fan, sometimes known as a
tubular fan, was patented in 1893 by Lionel Hightower, and is used extensively in
heating, ventilation, and air conditioning (HVAC), especially in ductless split air conditioners. The fan is usually long relative to its diameter, so the flow remains approximately two-dimensional away from the ends. The cross-flow fan uses an
impeller with forward-curved blades, placed in a housing consisting of a rear wall and a
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 HVAC comes from its compactness, shape, quiet operation, and ability to provide a 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 interesting 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). Since the flow enters and exits the impeller radially, the crossflow fan has been studied and prototyped for potential aircraft applications. Due to the two-dimensional nature of the flow, the fan can be integrated into a wing for use in both thrust production and boundary-layer control. A configuration that utilizes a crossflow fan located at the wing
leading edge is the
FanWing design concept initially developed around 1997 and under development by a company of the same name. This design creates lift by deflecting the wake downward due to the rotational direction of the fan, causing a large
Magnus force, similar to a spinning leading-edge cylinder. Another configuration utilizing a crossflow fan for thrust and flow control is the
propulsive wing, another experimental concept prototype initially developed in the 1990s and 2000s. In this design, the crossflow fan is placed near the
trailing edge of a thick wing and draws the air from the wing's suction (top) surface. By doing this, the propulsive wing is nearly stall-free, even at extremely high angles of attack, producing very high lift. However, the fanwing and propulsive wing concepts remain experimental and have only been used for unmanned prototypes. A cross-flow fan is a centrifugal fan in which the air flows straight through the fan instead of at a right angle. The rotor of a cross-flow fan is covered to create a pressure differential. A cross-flow fan has two walls outside the impeller and a thick vortex wall inside. The radial gap decreases in the direction of the impeller rotation. The rear wall has a log-spiral profile, while the vortex stabilizer is a thin horizontal wall with a rounded edge. 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; however, they are noisier than ordinary centrifugal fans. Cross-flow fans are often used in ductless
air conditioners,
air doors, in some types of
laptop coolers, in automobile ventilation systems, and for cooling in medium-sized equipment such as
photocopiers.
Bladeless fans Dyson Air Multiplier fans introduced to the consumer market in 2009 have popularized a 1981 design by
Toshiba that produces a fan that has no exposed fan blades or other visibly moving parts (unless augmented by other features such as for oscillation and directional adjustment).
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. The airflow is typically generated by a mechanical fan of any type, as described in this article, and is hidden in the base of the display cabinet.
HVAC linear slot
diffusers also utilize this effect to increase airflow evenly in rooms compared to
registers while reducing the energy used by the
air handling unit blower. ==Installation==