Fans

Fans are used for pneumatic transport. By creating a pressure difference between the suction and discharge sides, they are able to move airflow at a high speed within a closed system. High pressure air can be used to transport various fractions. The places where dust is formed and where it is stored are spatially separated in most cases, with the airflow being used to move the by-products of surface treatment from the extraction unit to the dust separators. Different materials react in different ways in this closed system, depending on the size, homogeneity and material of the fraction. There may be losses in pressure as a result of various bends, constrictions and flaps in the closed system, or due to the length of the duct system. This loss in pressure must be taken into consideration when designing the system to ensure that materials are transported without a hitch. Otherwise the transported material, dust, shavings, fertiliser or even hard feed may build up in various locations This leads to further losses in pressure or the system may even become entiredly clogged, which can only be fixed by mechanical means, by dismantling the affected section. After these losses have been calculated, the performance of the fan needed for generating the required airflow can be determined.

Types of fans based on operating principle:

♦ Radial or centrifugal
♦ Axial

Radial or centrifugal fans

The flow medium (air in most cases) enters the housing through an extraction vent where the centrifugal forces of the impeller push it onto the external surface of the rotor blades, changing its potential energy. The medium, which now has increased kinetic energy, exits into the duct system on the discharge side. The airflow thus enters and exits in the axial direction, in contrast to the name of the fan.

Fans may be used for different purposes: transport and ventilation. The latter are used exclusively for transporting air or gaseous substances. Their most important fields of application include providing ventilation and extracting waste gases. Material transport fans can be used to transport aerial and solid materials in combination. In contrast to axial fans which cannot move solid materials, centrifugal fans can be used for both purposes.

Fans may also be categorised by the pressure they generate:

♦ low pressure.....maximum       300 Pa
♦ medium pressure.....maximum 5,000 Pa
♦ high pressure....maximum   10,000 Pa

Radial or centrifugal fans may also be distinguished by the location of their housing. For practical reasons this takes the drive-side of the housing into account, thus differentiating between left and right-sided housings. The suction vent rotates every 45 degrees.

The noise emitted by these appliances cannot be ignored either. Besides the noise produced by the motors operating the system, the impeller is also noisy due to the inertia of the air and to centrifugal forces, though the spiral casing dampens these sounds to some extent.

Axial fans

Axial fans can only be used to transport gaseous materials and are therefore only suitable for ventilation and extraction. These appliances are able to create a vacuum, however. A good example is the fan duct used in animal farms, where an axial fan is placed horizontally in the duct, with the suction side facing towards the barn and the discharge side towards the open air.

The spent air and the ammonia released from excretions are extracted from the room by the vacuum created by the fan, with fresh air being supplied through air vents. Axial fans are also widely used for extracting gases and fumes from gluing and varnishing units. In this case, the fan is placed in a chimney or deflector and the released fumes are trapped by filter cartridges. Poisonous components are neutralised by activated carbon filters. Axial fans have direct drive in every case. The air enters axially by free suction. The impeller blades change the kinetic energy of the air, increasing the air pressure from p1 to p2. The size of the impeller and the angle of the blades are in direct proportion to the decrease and increase of the air pressure. The greatest advantage of axial fans is that they are able to transport large volumes of air at low pressure.

Both versions are available in ATEX configurations to ensure protection against explosions.