Fan implement: Encyclopedia II - Fan implement - Mechanical devices

Fan implement - Mechanical devices

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, as opposed to a gas compressor which produces high pressures at a comparatively low volume. Fans are useful for moving large quantities of air, which is suited for applications such as winnowing grain or blowing a fire, cooling and ventilation purposes, and in conjunction with a heat source for heating and drying. A fan blade will often rotate when exposed to an air stream, and devices that take advantage of this, such as anemometers and wind turbines often have designs similar to that of a fan.

Mechanical revolving blade fans are made in a wide range of designs. In a home you can find fans that can be put on the floor or a table, or hung from the ceiling, or are built into a window, wall, roof, chimney, etc. They can be found in electronic instruments such as computers where they cool the circuits inside, and in appliances such as hair dryers and space heaters. They are also used for cooling in air-conditioning systems, and in automotive engines, where they are driven by belts or by direct motor. Fans create a wind chill but do not lower temperatures directly.

Fan implement - Types

There are three main types of fans used for moving air, axial, centrifugal (also called radial) and cross flow (also called tangential). The axial-flow fans have blades that force air to move parallel to the shaft about which the blades rotate. Axial fans blow air across the axis of the fan, linearly, hence their name. This is the most commonly used type of fan, and is used in a wide variety of applications, ranging from small cooling fans for electronics to the giant fans used in wind tunnels. The centrifugal fan has a moving component (called an impeller) that consists of a central shaft about which a set of blades form a spiral pattern. Centrifugal fans blow air at right angles to the intake of the fan, and spin (centrifugally) the air outwards to the outlet. An 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, air mattress inflators, and various industrial purposes. They are typically more noisy than comparable axial fans. The cross flow fan has a squirrel cage rotor (a rotor with a hollow center and axial fan blades along the periphery). Tangential fans take in air along the periphery of the rotor, and expel it through the outlet in a similar fashion to the centrifugal fan. Cross flow fans give off an even airflow along the entire width of the fan, and are very quiet in operation. They are comparatively bulky, and the air pressure is low. Cross flow fans are often used for cooling in photocopiers. The action of a fan or blower causes pressures slightly above atmospheric, which are called plenums.

Fans usually use electric power. Electric fans generally consist of a set of rotating blades that are placed in a protective housing that permits air to flow through it. The blades are rotated by an electric motor, for big industrial fans, 3-phase asynchronous motors are commonly used. 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 24V, 12V or 5V. Cooling fans for computer equipment exclusively use brushless DC motors, which produce much less EMI. In machines which already have a motor, the fan is often connected to this rather than being powered independently. This is commonly seen in cars, large cooling systems and winnowing machines.

Fan implement - 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 axle comes out on both ends of the motor, one end of the axle 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 to the weighted base and the gearbox. A motor housing covers the oscillator mechanism. The blade guard joins to the motor case for safety.

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.

Fan implement - Ceiling fan

A fan suspended from the ceiling of a room is a ceiling fan. It usually has a light associated with it to replace any displaced light. These devices are generally used in homes without central air conditioning, or in conjunction with air conditioning to lower energy bills. Ceiling fan controls usually include one for speed (slow, medium, and fast), one for the light (on and off), and one for directional control of the fan blades (clockwise and counterclockwise). Ceiling fans can be used as a cooling device in warm months (pushing air down, thereby creating a wind chill effect) and a heat transferrer (pulling air up, thereby pushing the heat that stratifies by the ceiling, down along the walls so as not to create a wind chill) in colder months. For example in the winter, the fan should be rotating in a clockwise direction, with the leading edge of the fan blades being the downward edge (when standing below and looking upwards at the fan.)

Fan implement - 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. In addition, electricity is always available when the sun is shining and the fan needs to run.

A typical example uses a detached 10 watt, 12x12 inch solar panel and is supplied with appropriate brackets, cables, and connectors. It can be used to ventilate up to 1250 square feet (100 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 sun light and then connected to the fan mounted as far as 20 to 25 feet (6 to 7 m) away. Other permanently-mounted and small portable fans include an integrated (non-detachable) solar panel.

Fan implement - Gas turbine fan

The low pressure compressor in a turbofan engine is often called a fan. Typically, these units absorb thousands of horsepower, the power being provided by the expansion of hot combustion gases through the low pressure turbine.

Fan implement - Aft fan

Several turbofans feature an aft fan, where the fan rotor blades are mounted radially outwards of the (LP) turbine rotor blades. This dispenses with the need for an (LP) shaft. In an early example, General Electric bolted a fan/turbine unit to the rear of a J79 turbojet, to convert it into the CJ805 turbofan.

The GE36 UDF Demonstrator used a similar arrangement to convert an F404 mixed exhaust turbofan into a propfan.

Fan implement - Supersonic fan

Early gas turbine fans rotated at subsonic tip speeds, to avoid the generation of shock waves in the airflow. Modern fans, however, often rotate at supersonic tip speeds, and exploit the shock waves. Some advanced designs can generate a pressure ratio of more than 2.2:1 in a single stage, although 1.8:1 is more typical.

Fan implement - Supersonic through-flow fan

Although supersonic fans rotate at a supersonic tip speed, the axial flow is subsonic. However, some experimental devices have demonstrated supersonic axial flow. All the speed lines on the resulting fan map (or characteristic) are virtually horizontal, unlike those of more conventional units.

Fan implement - Variable pitch fan

Several ultra-high bypass ratio turbofan demonstrator engines (e.g. Rolls-Royce/SNECMA M45SD-02) have incorporated variable pitch fans, much like the variable pitch propellers on a turboprop engine. Varying the pitch of the rotor blades improves the low flight speed handling of the low pressure ratio fan unit, without the need to resort to a variable area cold or mixed flow nozzle. Reverse thrust down to zero aircraft speed is also practical.

Fan implement - Variable geometry fan

Some multi-stage, high pressure ratio, fans on military turbofan engines incorporate variable geometry, (e.g. F404). The variability is usually confined to the inlet guide vanes. Although the leading edge of the vane is static, a piano-type hinge allows the trailing edge to be adjusted in pitch, to redirect the airflow onto the first rotor. VIGV's enhance the surge margin of the fan in the mid-flow region.

Fan implement - Propfan

Some ultra-high bypass ratio turbofans dispense with the fan nacelle and have an unducted fan rotor. The fan blades, which resemble scimitars, are especially shaped to work efficiently at flight speeds up to about Mach 0.75. General Electric demonstrated a propfan engine, called the GE36 UDF, in the 1980's.

Fan implement - Overhung fan

Turbojets and early turbofans used the inlet guide vanes to support the front bearing of the (LP) compressor/fan rotor assembly. Today, the fans used in turbofan engines are often to an overhung design, where the fan rotor is cantilevered out forward, beyond the front bearing. This facilitates the removal of the inlet guide vanes. Consequently, the fan rotor blades are the first aerofoils encountered by the engine airflow.

Fan implement - Snubbered fan

Prior to the introduction of wide chord fan blades, fan blades fitted to turbofan engines often featured snubbers. These are protuberances that stick-out at right angles to the fan aerofoil, somewhere between mid-span and blade tip. The snubbers on adjacent fan blades butt-up against each other, in a peripheral sense, and improve the vibration characteristics of the blade.

Wire lacing (e.g. Pegasus) is an alternative approach.

Fan implement - Wide chord fan

As might be expected, snubbers reduce the aerodynamic efficiency of fan aerofoils. Rolls-Royce pioneered a more efficient alternative: wide chord fan blades. The increased blade chord (i.e. width) is used to enhance the vibration characteristics.

Wide chord first went into service in the RB311-535E4 for the Boeing 757 in 1984 and have been a feature of the RB211/Trent/V2500 engine family ever since. Potential weight increases are usually offset by making the blades hollow. Other engine manufacturers have now introduced wide chord fans.

Fan implement - Swept fan

Engine manufacturers are beginning to introduce so-called swept fan blades, which should yield benefits in aerodynamic efficiency and noise.

Fan implement - Other meanings

• In a fan heater, a fan (or blower) blows cool air past a heating element, heating the air (forced convection). It has a fan wheel with vanes fixed on a rotating shaft enclosed in a case or chamber, to create a blast of air (i.e., the fan blast) for forge purposes.
• In automobiles, a mechanical fan, driven with a belt and pulley off the engine's crankshaft, or an electric fan switched on/off by a thermo switch is used to blow or suck air through a coolant filled radiator, to prevent the engine from overheating.
• A fan is also a small vane or sail that is used to keep the large sail of a smock windmill always in the direction of the wind.
• A fan vault is a type of roof used in church architecture.
• Fan death is an urban legend common in South Korea.
• A fan can also refers to an ardent admirer. This meaning derives from the word fanatic, as opposed to other meanings above. See Fan fiction and Fandom for more information.

See also

• heat exchanger
• turbine
• wind turbine
• windmill
• Fan death
• The Fan Museum in Greenwich (Greenwich, London)

Adapted from the Wikipedia article "Mechanical devices", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki