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المراوح الطاردة المركزية Centrifugal fans radial fans
المراوح, المركزية, الطاردة, centrifugal, fans, radial
Centrifugal fans also known as radial fans are the most common type of fan in use today. The principle of operation, albeit relatively simple, has evolved over the years with machines now capable of high aerodynamic efficiency and with significant powers.
In a centrifugal (or radial) fan the rotation of the impeller causes the air to travel through it in a radial direction, developing pressure as it does so. The impeller, which does most of the work within the fan, and located in the centre of the casing, are all of a similar shape. A sketch of a large aerofoil bladed centrifugal fan is shown below.
Centrifugal fans may be single or double inlet. A double inlet impeller consists of two single inlet impellers back to back on the shaft sharing a common backplate, providing almost double the flow of a single inlet fan.
Output can be controlled by variable vanes in the casing close to the impeller inlet - inlet vane control, or by a louvre damper in the fan inlet box - box vane control.
A cooling fan is a specialised axial fan used to supply large flows of air to cooling towers, heat exchangers and steam condensers in industrial plant. As the pressure drop across the cooling system is very low, the fans operate at low speeds and for large installations can be up to 25m in diameter.
The blades are manufactured from Fibre Reinforced Polyester (FRP), and in many cases glass is used as the fibre, or from aluminium. The lightweight construction of the blades, combined with the relatively low speed means that the hub is a simple disc and blades are attached to it by U-bolts
A common example of an axial fan is a typical desk fan. The name "axial" comes from the fact that the air passing through the fan does not change direction and flows parallel to the fan axis. An axial fan is normally used when the flow requirements are high and the pressure demand is low.
An axial fan blade works in a similar manner to an aircraft wing. However, whilst in the latter case the lift force acts upwards on the wing and supports the weight of the aircraft, the axial fan is fixed in position and causes the air to move.
Axial fan blades are generally aerofoil in cross-section. The blade can be fixed in position or it can rotate around its longitudinal axis. The angle of the blade to the airflow, or blade pitch, can be fixed or adjustable. Changing the blade angle, or pitch, is one of the major advantages of an axial fan. Small blade pitch angles produce lower flows while increasing the pitch produces higher flow.
Sophisticated axial fans are capable of having the blade pitch changed when the fan is running, much like a helicopter rotor, changing the flow accordingly. These are called variable pitch (VP) axial fans
|مواقع النشر (المفضلة)|
|الكلمات الدلالية (Tags)|
|المراوح , المركزية , الطاردة , centrifugal , fans , radial|
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