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Fans are the "workhorses" of the ventilation systems. To have an effective ventilation system, fans must be of the appropriate size and type. They must provide enough speed (air movement) to capture contaminants at the source, draw them through the hood, and carry them through the duct system, through the air cleaning devices and exhaust to the outdoors.
There are two main types of exhaust fans:
Figure 1Types of fans
There are three basic types of axial fans: propeller, tubeaxial, and vaneaxial. Propeller fans are most commonly used for dilution ventilation or cooling. These fans have a propeller-shaped blade and a drive motor mounted on a flat frame. They are often mounted on a wall or ceiling. Common examples are your automobile radiator fan or a free-standing room fan. The basic characteristics of these fans include:
Tubeaxial and vaneaxial fans are propeller fans made to fit in a duct.
The tubeaxial fans have the propeller and the drive motors mounted in tubes. The clearance between the propeller and tube is very small to improve airflow efficiency.
The vaneaxial fans are tubeaxial fans with air straightening vane before and behind the propeller.
These fans usually limited to "clean air" applications such as exhaust ducts going through the roof.
Where the exhaust air is at a high temperature or contains contaminants (e.g., grease, corrosive, etc., such as in kitchens or paint booths) that could damage the drive motor, a bifurcated axial fan could be used. The design of this axial fan keeps out the drive motor of the air stream.
There are three types of centrifugal fans determined by the type of fan blades:
The fans in your home furnace, vacuum cleaner and hairdryer are examples of centrifugal fans. They can operate against a high resistance and are typically used in local exhaust ventilation systems. The rugged radial blade centrifugal fans are the best type for exhausting heavy amounts of dust because they are less likely to become clogged or abraded by the dust.
Selection of the proper fan can be complicated and should be done by a ventilation or fan expert. However, you can make the following observations to determine if the fan selected is appropriate:
Fans that handle high-temperature exhaust air must be made from materials and parts resistant to high temperatures.
You may or may not know how much air has to be moved by the fan. You may also not know the amount of resistance in the exhaust system that the fan has to overcome and what is the fan's efficiency. However, the following general information may be helpful:
Safety guards are required for all danger points such as the inlet, outlet, shaft, drive, and cleanout doors. Construction should comply with applicable jurisdictional governmental safety requirements.
Except for low-speed fan units, fans usually are noisy. Noise can be distracting, irritating, or damaging to the ear. Fan noise can be a problem both in the plant and to neighbours outside. Most fan manufacturers publish sound ratings for their products and these levels should be considered when selecting a fan.
Common components of most fans that affect performance are listed below:
Bearings: Fan shaft bearings are often the single greatest source of trouble. Bearing life is reduced by overly tightened or excessively loose fan belts, fan vibration, uneven loading on the fan blades, high ambient operating temperatures, and improper (over or under) lubrication.
Belts (V belts): Improper belt tension can affect the fan performance. Audible belt squealing during start-up is a sign of insufficient belt tension. As a rule of thumb, belt tension should be tight enough so that the centre of the span will move 2.5 cm (1 inch)when moderate finger pressure is applied.
Blades and Housing: A buildup of solid material (dust, fumes, particulate matter) on the fan blades or the housing causes imbalance, vibration, and loss of capacity (reduced airflow).
Connectors and isolation foundations: Flexible connectors and isolation foundations are used to isolate fan vibrations from the building and the rest of the ventilation system. Flexible connectors attach the ventilation system duct to the fan while eliminating fan vibration that may travel through the ventilation system duct. If they are torn or corroded, the fan performance will be affected.
Louvers and dampers: Some fans have inlet or outlet louvers or dampers to adjust airflow. They may be manually, pneumatically or electronically controlled. If the louver linkage connections are not tight and secure, they may affect the fan performance.
Motors: Motor operating voltage must be maintained within 10% of the recommended voltage to ensure proper fan performance. Most motors are permanently lubricated for life and require no further maintenance.
Fans can go "out of balance" because material builds up on the fan blades, or because of wear. Imbalanced fans will vibrate and may cause damage to various parts of the fan (blades, housing, motor, etc.). It is important to keep fans clean and properly balanced, particularly if the air being removed contains abrasive, sticky or wet materials. Scheduled maintenance should check items including:
Air discharged from a fan should be kept away from inlets (intake) of the make-up air system. In this way, the make-up air system will draw only clean, outdoor air into the workplace.
The discharge exhaust stacks should be high enough from the roof so that contaminants do not re-enter the workplace. Generally, they should be located no closer than 15.24 m (50 feet) from the inlet to prevent recirculation of contaminants. Stacks work best when they are tall, usually at least 3 m (10 feet) above the roofline.
This article is about mechanical fans. For hand-powered fans, see Hand fan . For other uses, see Fan (disambiguation)
A typical electric fan A table fanA fan is a powered machine used to create a flow of air. A fan consists of a rotating arrangement of vanes or blades, generally made of wood, plastic, or metal, 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.[1] 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. Some fans may be indirectly used for cooling in the case of industrial heat exchangers.
While fans are effective at cooling people, they do not cool air, but rather 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 less effective at cooling the body if the surrounding air is near body temperature and contains high humidity.
History
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Patent drawing for a Fan Moved by Mechanism, November 27, 1830The 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 measured 3 m (10 ft) 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).[2][3]
During the Heian period (794-1185) in Japan, fans adapted the role of symbolizing social class as well as a mechanical role. The Tessen, a Japanese fan used for the Feudal times, was a dangerous weapon that was hidden in plain sight in the shape regular fan. What started off as a mechanism to cool oneself and a symbol of respect and wealth soon became a weapon used by samurais when Katanas were not ideal. The weapon paralleled the mannerisms of the Japanese.
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).[4]
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.[5] 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 breaking down.
Steam
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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.[6][7] Improvements in the technology were made by James Nasmyth, Frenchman Theophile Guibal and J. R. Waddle.[8]
Electrical
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Two c. 1980 box fansBetween 1882 and 1886 Schuyler Wheeler invented a fan powered by electricity.[9] It was commercially marketed by the American firm Crocker & Curtis electric motor company. In 1885 a desktop direct drive electric fan was commercially available by Stout, Meadowcraft & Co. in New York.[10]
In 1882, Philip Diehl developed the world's first electric ceiling mounted 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.[11] 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 bright colors and eye-catching.
Window and central air conditioning in the 1960s caused many companies to discontinue production of fans,[12] 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 (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.[13]
Social Implications
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Before the widespread accessibility to fans, fans embodied the social divide between social classes. In Britain and China, these fans were initially only installed in the buildings of Parliament and in noble homes. Fans have symbolized of the difference between the working class and the top percent of the world. In Ancient Egypt (3150 BC), servants were required to fan Pharaohs and important figures, showing the social divide between the powerful and common class.
In parts of the world such as India, where the temperature reaches above 100 °F (38 °C), standing and electric box fans have become a vital part of the business world. They enable customer comfort and a conducive work environment. Fans have become solar powered, energy efficient and battery powered in places with unreliable energy sources. In South Korea, fans play a part in an old wives tale that has continued on for generations. Many older and elderly South Korean citizens believe in the myth called fan death. There is no scientific data or evidence supporting this myth, but many Koreans still believe in the tale. As a result, Korean companies built fans designed to turn off after several hours to protect children from fan death.
In the present, electrical fans users have reduced their use of electricity as the cost to run them is between 50 and 100 watts to run whereas an AC unit use between 500 and 4000 watts. Arguments surrounding both switching to using fans vs. switching to only using AC units are increasing. The cost to run a commercial fan is less than an AC unit on average $5.50 per month. The cost to run an AC unit runs from on average $14.40 to $211.40 per month. Commercial fans are louder than AC units and are reported to disrupt a common environment. According to the U.S. Consumer Product Safety Commission, reported incidents related to box fans include, fire (266 incidents), potential fire (29 incidents), electrocution (15), electric shock (4 incidents), and electrical hazard (2 incidents). Injuries related to AC units mostly include AC units falling from buildings.
Types
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Ceiling fan with a lampMechanical 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). They can also be built into a window, wall, roof, chimney, etc. Most electronic systems such as computers include fans to cool the circuits inside, and in appliances such as hair dryers and portable space heaters and mounted/installed wall heaters. They are also used for moving air in air-conditioning systems, and in automotive engines, where they are driven by belts or by a direct motor. Fans used for comfort 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 do cool the equipment directly by forcing hot air into the cooler environment outside of the machine.
There are three main types of fans used for moving air, axial, centrifugal (also called radial) and cross flow (also called tangential). The American Society of Mechanical Engineers Performance Testing Code 11 (PTC)[14] provides standard procedures for conducting and reporting tests on fans, including those of the centrifugal, axial, and mixed flows.
An axial box fan for cooling electrical equipmentAxial-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:
Centrifugal
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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).
A diagram of a centrifugal fan, with a top view to show airflow
Typical centrifugal fan
Centrifugal cooling fan installed within an HP inc. ZBook mobile workstation laptop. The fan displaces air over the fin-stack which contain a pair of heatpipes.
The cross-flow or tangential fan, sometimes known as a tubular fan, was patented in 1893 by Paul Mortier,[15][16] 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.[17] 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.[18] 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.[when?] 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 crossflow fan has been studied and prototyped for potential aircraft applications.[19] 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 is 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 of 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. 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 fans impeller rotation. The rear wall has a log-spiral profile while the vortex stabilizer is a horizontal thin wall with rounded edge.[20] 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
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An open-face supermarket freezer with an air curtain. Cooling air circulates across the food through the dark slot seen at the rear of the freezer, and through another grille not visible along the front.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).[21] A relatively small quantity of air from a high-pressure-bladed impeller fan, which is contained inside the base rather than exposed, induces the slower flow of a larger airmass through a circular or oval-shaped opening via a low-pressure area created by an airfoil surface shape (the Coandă effect).[21][22][23]
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 described in this article 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
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Fans may be installed in various ways, according to the application. They are often used in a free installation, without any housing of any kind. There are also some specialised installations.
Ducted fan
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In vehicles, a ducted fan is a method of propulsion in which a fan, propeller or rotor is surrounded by an aerodynamic duct or shroud which enhances its performance to create aerodynamic thrust or lift to transport the vehicle.
Jet fan
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In ventilation systems a jet fan, also known as an impulse or induction fan, ejects a stream of air which entrains ambient air along with it, in order to circulate the ambient air. The system takes up less space than conventional ventilation ducting and can significantly increase the rates of inflow of fresh air and expulsion of stale air.[24]
Noise
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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.[25]
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 of 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.[26][27][28]
The inlet shape of the fan can also influence the noise levels generated by the fan.[29]
Fan motor drive methods
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Building heating and cooling systems commonly use a squirrel cage fan driven by a belt from a separate electric motor.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 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.
Large electric motors may have a cooling fan either on the back or inside the case. (Shown with the black rear cover removed.)
Dual shaft fan motor in a window air conditioner
Solar power
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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. During the absence of electric power solar powered fan is operate the air flowing and maintain environmental condition. A typical example uses a detached 10-watt, 12 in × 12 in (30 cm × 30 cm) solar panel and is supplied with appropriate brackets, cables, and connectors. It can be used to ventilate up to 1,250 square feet (116 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 sunlight and then connected to the fan mounted as far as 25 feet (8 m) away. Other permanently-mounted and small portable fans include an integrated (non-detachable) solar panel.
See also
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References
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If you want to learn more, please visit our website Fan Manufacturers.