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News Spiral Conveyors

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Vibrating spiral conveyors for bulk materials

Vibrating spiral conveyors are increasingly used in mechanical and thermal processes to convey bulk material upwards on an inclined plane by means of microscopic throwing movements. In contrast with linear vibrating conveyors, the path of a spiral conveyor is not straight but helical. Torsional vibrations are accordingly produced about the center axis of the spiral tower at a defined slope angle. These cause the load to be moved spirally upwards. Whereas with linear vibrating conveyors, the acceleration acting on the product is normally identical at every point on the conveyor deck, the magnitude of the acceleration of a spiral conveyor varies proportionally to the path radius. The operating conditions are therefore crucially influenced by the conveying conditions at the inner radius of the spiral path, in other words in the zone with the greatest inclination and the least acceleration.

Figure 1: Final assembly of a spiral conveyor

   
   

In a theoretical examination of the dynamic conditions, the mass and acceleration of a linear conveyor are replaced by the moment of inertia and the angular acceleration of a spiral conveyor. The design of all components that are subjected to heavy loads is verified by means of computational stress analyses based on the finite element method. Figure 3 shows the structure of a spiral conveyor model. It is dimensioned according to the conveyed mass flow rate, the heat transfer surface required for process applications and the specified or available conveying height. Particular attention must be devoted by design engineers to the torsional strength and bending stiffness of the spiral tower. The most commonly used variants have a diameter between 500 and 1500 mm and a conveying height between 1 and 7 m. Mass flow rates of up to 30 m³/h are possible with a diameter of 1500 mm. The importance of vibrating spiral conveyors for modern bulk handling technology is explained firstly by the fact that they are hygienic and easy to clean and secondly by their ability to combine conveying action with the intended action of the process.

Figure 3: Structure of a spiral conveyor model

Figure 2: Production of a 1400 mm spiral conveyor

Since relatively large exchange surfaces can be implemented for process-related tasks, e.g. for drying or cooling bulk materials, on a comparatively small floor space, the potential for vibrating spiral conveyors is enormous. Unlike spiral conveyors used purely for conveying tasks, spiral conveyors for thermal processes feature decks in the form of heat exchanger plates. The ends of the heat exchanger consist of two sheets, usually with different wall thicknesses and joined together by means of a grid arrangement of welding spots. High hydraulic pressure causes the gap, which is hermetically sealed by inner and outer welds, to swell like a pillow, enabling a heat transfer medium to be conducted through the resulting pressure-resistant conduit. Laser welding is today the preferred method for manufacturing heat exchange area. The main advantage of this method compared to the formerly predominant resistance welding process (seam or spot welding) is that the surface of the weld around the circular anchor points of the pillow remains notch-free even after inflation, making it considerably more resistant to crevice corrosion than resistance welds. Figure 2 shows the underside of laser-welded heat exchange aera. The heat transfer medium is usually water, steam or thermal oil, depending on the application. It should be mentioned for the sake of completeness that slight cooling or drying effects can also be achieved without heat exchange area, i.e. simply as a result of convection or of blowing air onto the product stream. The possibility of heat transfer both within the product bed and between this bed and the heat transfer medium has a crucial influence on the dimensioning of the heat transfer surface, in addition to the specific heat capacity and the required product temperature difference. The heat transfer coefficient which must be applied for this purpose is dependent on the grain size distribution and the bed thickness, so that an experimental calculation is often indispensable. The heat transmission coefficient can be determined extremely accurately on insulated test conveyors.

Figure 4: Cooling spiral conveyor in a plastics factory

Vibrating spiral conveyors are a standard solution for handling bulk materials in many branches of industry. Plastics granules, rubber granules, chemical and pharmaceutical intermediates, metal salts, welding powder, glass batch, catalysts, abrasives and ash as well as milk powder, powdered coffee, tea, nuts, nutrients and cereals are conveyed, dried and cooled. Owing to the diversity of the products and their properties, the conveyor design is almost always project-specific, although of course fixed steps are normally adhered to for the spiral diameter. Figure 1 shows fifteen spiral conveyors for cooling and drying, with different diameters and heights, at the final assembly stage. In many applications the spiral tower is required to be dustproof or even gas-tight. Proven solutions are available here, such as covibrating, easy-to-remove jackets made of metal or rubber and a fixed enclosure with the necessary inspection and cleaning covers. Various models are shown in Figures 5 and 6. Spiral conveyors with a tubular cross-section can be offered for very low mass flow rates. The range of special models also includes multiple-track spiral conveyors and conveyors with an integrated screening section. Finally, it should be mentioned that spiral conveyors can of course optionally be designed for conveying from top to bottom. This principle is applied, for instance, whenever bulk materials need to be conveyed gently downwards or products which would otherwise be unsuitable for conveying have to be treated thermally.

 

Figure 5: Dust cover consisting of stainless steel elements

Figure 6: Spiral conveyor with a dust cover / noise reducing enclosure. The enclosure consists of two halves which move on rails and can be separated for cleaning

 

 

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