I see all kinds of refuse piled at curbs on my daily walks. Today, it is an apparently non-working box fan. Although air conditioners are much better at cooling the air — especially when there is high humidity like there often is in Texas — electric or battery-operated fans can provide some comfort. From the tiniest ones like those you can wear around your neck while standing in line at a sweltering amusement park to the expensive but efficient Vornado to giant fans in warehouses or industrial settings, fans can make a big difference in your level of comfort, thus affecting your productivity. Many of the less expensive fans are made in Asian countries. Their included instruction leaflets are often poorly translated. I once bought a fan with a leaflet that said, “Please do put finger in fan.” One summer I attended a Christian women’s conference in Lafayette, Indiana. We stayed in dorm rooms at Purdue University. Unfortunately, there was an unseasonable heat wave, and the dorms were not air-conditioned. Organizers scrambled to accommodate us, providing a large electric fan on a stand for every room. I was very grateful to receive this magic appliance which brought balmy breezes to my room on the sixth floor. Let’s learn more about these wonderful inventions that can do so much to soothe us.
According to Wikipedia, a fan is a powered machine used to create a flow of air. A fan consists of a rotating arrangement of vanes or blades, which act on the air. The rotating assembly of blades and hub is known as an impeller, rotor or 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, handcranks and internal combustion engines.
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 — although higher than ambient pressure, as opposed to compressors which produce high pressures at a comparatively low volume. A fan blade will often rotate when exposed to an air-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., sucking as in a vacuum cleaner), drying (usually in combination with a heat source) and providing draft for a fire.
While fans are often used to cool people, they do not cool air — electric fans may 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. A fan blade is generally made of wood, plastic or metal.
Fans have several applications in the industries. Some fans directly cool the machine and process and may be indirectly used for cooling in the case of industrial heat exchangers. These are critical machines responsible for operating the entire plant, which may shut down without the proper fan. In mines and tunnels, fans are also used as safety equipment.
History
The punkah fan was used in India about 500 BCE. It was a handheld fan made from bamboo strips or other plant fiber, that could be rotated or fanned to move air. During British rule, the word came to be used by Anglo-Indians to mean a large swinging flat fan, fixed to the ceiling and pulled by a servant called the punkawallah.
For purposes of air conditioning, the Han Dynasty craftsman and engineer Ding Huan (fl. 180 CE) invented a manually operated rotary fan with seven wheels that measured10 feet in diameter; in the 8th century, during the Tang Dynasty (618–907), the Chinese applied hydraulic power to rotate the fan wheels for air conditioning, while the rotary fan became even more common during the Song Dynasty (960–1279).
In the 17th century, the experiments of scientists including Otto von Guericke, Robert Hooke and Robert Boyle, established the basic principles of vacuum and airflow. The English architect Sir Christopher Wren applied an early ventilation system in the Houses of Parliament that used bellows to circulate air. Wren's design would be the catalyst for much later improvement and innovation. The first rotary fan used in Europe was for mine ventilation during the 16th century, as illustrated by Georg Agricola (1494–1555). According to the article “The advent of mechanical air movement using ‘air pumps’ and fires” at fenkoil.ru/1894, he described the use of bellows and crude fans in German underground metal mines in a manner which makes one assume that they were then well established. These early fans were, of course, made of wood with radial paddle vanes fitted to a spindle which rotated in a casing. Thus, they were the first centrifugal fans and were rotated by animals, men or water mills. His illustration on the left is labeled:
A–Prom. B–Box and casing. C–Blow-holi. D–Sicojcd aati.
EóCow doit. F–Axl. G–LtTi or axle. H–Rodv.
According to Wikipedia, John Theophilus Desaguliers, a British engineer, demonstrated the successful use of a fan system to draw out stagnant air from coal mines in 1727 and soon afterward he installed a similar apparatus in Parliament. Good ventilation was particularly important in coal mines to reduce casualties from asphyxiation. The civil engineer John Smeaton, and later John Buddle installed reciprocating air pumps in the mines in the north of England. However, this arrangement was not as ideal as the machinery was liable to break down.
In 1849 a 6m, radius steam-driven fan, designed by William Brunton, was made operational in the Gelly Gaer Colliery of South Wales. The model was exhibited at the Great Exhibition of 1851. Also in 1851 David Boswell Reid, a Scottish doctor, installed four steam-powered fans in the ceiling of St George’s Hospital in Liverpool, so that the pressure produced by the fans would force the incoming air upward and through vents in the ceiling. Improvements in the technology were made by James Nasmyth, Frenchman Theophile Guibal and J. R. Waddle.
Between 1882 and 1886 Schuyler Wheeler invented a fan powered by electricity. It was commercially marketed by the American firm Crocker & Curtis electric motor company. In 1882, Philip Diehl developed the world's first electric ceiling fan. During this intense period of innovation, fans powered by alcohol, oil or kerosene were common around the turn of the 20th century. In 1909, KDK of Japan pioneered the invention of mass-produced electric fans for home use. In the 1920s, industrial advances allowed steel fans to be mass-produced in different shapes, bringing fan prices down and allowing more homeowners to afford them. In the 1930s, the first art deco fan — the "Silver Swan" — was designed by Emerson. By the 1940s, Crompton Greaves of India became the world's largest manufacturer of electric ceiling fans mainly for sale in India, Asia and the Middle East. By the 1950s, table and stand fans were manufactured in eye-catching bright colors.
Window and central air conditioning in the 1960s caused many companies to discontinue production of fans. But in the mid-1970s, with an increasing awareness of the cost of electricity and the amount of energy used to heat and cool homes, turn-of-the-century styled ceiling fans became immensely popular again as both decorative and energy-efficient units.
In 1998 William Fairbank and Walter K. Boyd invented the high-volume low-speed or HVLS ceiling fan, designed to reduce energy consumption by using long fan blades rotating at low speed to move a relatively large volume of air.
Axial-flow fans
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 wind tunnels. 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 to 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 to 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 impeller, while kitchen extractor fans typically use a six-inch impeller, as the room itself is often bigger. Axial fans with five-inch impellers are also used in larger bathrooms, though are much less common. Domestic axial extractor fans are not suitable for duct runs over 3 or 4 meters, depending on the number of bends in the run, as the increased air pressure in longer pipework inhibits the performance of the fan.
Electro-mechanical fan: Among collectors, electro-mechanical fans are rated according to their condition, size, age and several 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. Most ceiling fans rotate at relatively low speeds and do not have blade guards. Ceiling fans can be found in both residential and industrial/commercial settings.
Automobile mechanical 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.
Audio power amplifier fan helps to draw heat away from the electrical components.
Variable-pitch fan: A variable-pitch fan is used where precise control of static pressure within supply ducts is required. 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 fans
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, blow dryers, 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.
Cross-flow fans
The cross-flow or tangential fan, sometimes known as a tubular fan, was patented in 1893 by Paul Mortier and is used extensively in heating, ventilation and air conditioning or HVAC, especially in ductless split air conditioners. The fan is usually long concerning the 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: vortex region near the fan discharge called an eccentric vortex, through-flow region and 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 cross-flow 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 flowrate 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 both enters and exits the impeller radially, the cross-flow fan is well suited for aircraft applications. Due to the two-dimensional nature of the flow, the fan readily integrates into a wing for use in both thrust production and boundary-layer control. A configuration that utilizes a cross-flow fan located at the wing leading edge is the fanwing. This design creates lift by deflecting the wake downward due to the rotational direction of the fan, causing large Magnus force, similar to a spinning leading-edge cylinder. Another configuration utilizing a cross-flow fan for thrust and flow control is the propulsive wing. In this design, the cross-flow fan is placed near the trailing edge of a thick wing and draws the air of the wing's suction or top surface. By doing this, the propulsive wing is nearly stall-free, even at extremely high angles of attack, producing very high lift. The external links section provides links to these concepts.
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. Cross-flow fans are made to have a double circular arc rear wall with a thick vortex wall that decreases in radial gap. The gap decreases in the direction of the fan’s impeller rotation. The rear wall has a log-spiral profile while the vortex stabilizer is a horizontal thin wall with 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, presumably because the fan blades fight the flow of air on one side of the rotation, unlike typical centrifugal fans. Cross-flow fans are often used in ductless air conditioners, air doors, some types of laptop computers, automobile ventilation systems and medium-sized equipment such as photocopiers.
Bellows
Bellows are also used to move air, although not generally considered fans. A hand-operated bellows is essentially a bag with a nozzle and handles, which can be filled with air by one movement, and the air expelled by another. Typically, it would comprise two rigid flat surfaces hinged at one end, where a nozzle is fitted and with handles at the other.
The sides of the surfaces are joined by a flexible and air-proof material such as leather; the surfaces and joining material comprise a bag sealed everywhere but at the nozzle. The joining material typically has a characteristic pleated construction that is so common that similar expanding fabric arrangements not used for moving air, such as on a folding camera, are called bellows. Separating the handles expands the bag, which fills with air; squeezing them together expels the air. A simple valve — e.g., a flap — may be fitted so that air enters without having to come from the nozzle, which may be close to a fire.
Bellows produce a directed pressurized stream of air; the airflow volume is typically low with moderate pressure. They are an older technology — used mainly to produce a strong and directed airflow, unlike non-electric bladed mechanical fans — before the introduction of electricity.
- A single-acting bellows will only produce airflow during the exhaust stroke.
- A double-acting bellows is a pair of bellows capable of blowing out air from one while inhaling air into the other, but airflow still temporarily ceases when the stroke direction is reversed.
- Combining multiple bellows at third-cycle or quarter-cycle arrangements on a crank arm allows for nearly continuous airflow from several bellows at once; each is in a different phase of inhaling and exhausting during the cycle.
Coandă effect
The Dyson Air Multiplier fans and the Imperial C2000 series range hood fans have no exposed fan blades or other visibly moving parts except their oscillating and tilting head. The airflow is generated using the Coandă 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. The U.S. 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. 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 hidden in the base of the display cabinet.
Convective
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 fan technology. However, before 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 raise 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 slow convective air current is vented out the top of the building, while fresh air enters the pit through the seat hole.
Electrostatic
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
Fans generate noise from the rapid flow of air around blades and obstacles causing vortexes and from the motor. Fan noise is roughly proportional to the fifth power of fan speed; halving speed reduces noise by about 15 dB.
The perceived loudness of fan noise also depends on the frequency distribution of the noise. This, in turn, depends on the shape and distribution of moving parts — especially of the blades and stationary parts, struts in particular. Like with tire treads and similar to the principle of acoustic diffusors, an irregular shape and distribution can flatten the noise spectrum, making the noise sound less disturbing.
The inlet shape of the fan can also influence the noise levels generated by the fan.
Fan motor drive methods
Standalone fans are usually powered by an electric motor, 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, may be placed near the fan and drive 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 typically use low voltage, typically 24V, 12V or 5 V.
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, large cooling systems, locomotives and winnowing machines, where the fan is connected to the drive shaft 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. Window air conditioners commonly use a dual-shaft fan to operate separate fans for the interior and exterior parts of the device.
Where electrical power or rotating parts are not readily available, fans may be driven 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, either of 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 series of step-down gears or pulleys to increase the rotational speed to that which is required for efficient fan operation. Anderson Mill of Texas is a neighborhood in Austin and is undershot, backshot and overshot using two sources of water. This allows the direction of the water wheel to be reversed.
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. Also, electricity is always available when the sun is shining, and the fan needs to run.
A typical example uses a detached 10-watt, 12-inch × 12-inch solar panel and is supplied with appropriate brackets, cables and connectors. It can be used to ventilate up to 1,250 square feet of area and can move air at up to 800 cubic feet per minute. 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 sunlight and then connected to the fan mounted as far as 25 feet away. Other permanently mounted and small portable fans include an integrated non-detachable solar panel.
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