11/29/2012 | Author / Editor: Joseph A. Dos Santos* / Marcel Dröttboom
Sandwich belt high angle conveyor technology provides an economic solution for high volume steep and vertical conveying. A comparison of four alternate conveying paths shows of different heights shows the advantages of the system in terms of cost as well as land area and spatial volume.
The rationalisation of sandwich belt high angle conveyor technology is fundamental to the long term technological and economic success of high volume steep and vertical conveying. The introduction of this article retraces briefly the development of sandwich belt high angle and vertical conveyors and cites the common denominators for success. The main focus however is in developing and comparing the economics of four alternate conveying paths to silos, a conventional conveyor at 15 degrees slope and three variations of the Dos Santos International Snake Sandwich Belt High Angle Conveyor at 45, 60 and 90 degrees (vertical). The economic comparison is made at silos of increasing height from 17.8 to 73 metres. Besides the hardware costs a strong case is made for the importance of land area and spatial volume costs and environmental impact.
The subject of this article was first published in 1998 with the emphasis on the hardware cost comparison. Other costs such as the required land area and spatial volume were mentioned but not emphasised as these were not deemed as important for new projects where the space was appropriated as required. Over the last decade these other costs have proven to be more than equal in importance. These costs are emphasised in this writing through specific accounts of actual high angle conveyor installations that demonstrate the importance and the high angle advantage. This article also provides an update on the progress in sandwich belt high angle conveyor technology.
Large volume high angle conveying by “conveyors with cover belt,” dates back to the early 1950s when in the German lignite mines, such systems were employed at the bucket wheel excavator boom conveyor to increase the depth of cut without increasing the boom length. Various systems were developed, patented and built. These systems were not a lasting success for various reasons which are noted by Rasper . In his 1958 Review of Cover Belt Systems, Rasper summarises the characteristics which are worth pursing and those that should be abandoned. The former are much akin to good conveyor engineering.
In its most rudimentary form the sandwich belt conveyor model was developed during the early 1950s. This allowed calculation of the required hugging pressure to develop the bulk material’s internal friction and preclude material slideback. A modern, accurate version of the sandwich belt conveyor model is presented in Fig. 1. This model depicts accurately a de-rating of the material cross-section allowing an ample belt edge distance that can tolerate normal belt misalignment without material spillage. The calculations must thus recognise that only part of the hugging pressure is imparted onto the material with the remainder transferred belt edge to belt edge affecting a material seal. For mathematical development of the sandwich belt model, the reader is referred to Dos Santos and Frizzell .
The first lasting success and arguably the most important technological development in sandwich belt high angle conveying came in the early 1970s in the Loop Belt Elevator. The Loop belt, a continuous elevator of C-shaped profile, consists of an inner belt, which is supported on troughing idlers along a convex curve and an outer belt that imparts radial pressure, due to belt tension on the conveyed material onto the idler supported inner belt.
Interestingly the loop belt was first conceived as a high speed centrifugal belt, requiring neither inner belt nor sandwiching. Centrifugal acceleration of the material held it positively against a continuously curving outer belt until flung into free trajectory at the discharge. It was the practical consideration of an emergency stop under load that resulted in the design as we know it today.
Limited to approximately semi-circular conveying paths, the Loop Belt could not solve the problems of high volume conveying along a most direct path from loading point A to discharge point B. Nevertheless, this system gained great success in self unloading bulk carrying ships where the semicircular path is ideal. This system is the first to utilise all conventional conveyor componentry and smooth surfaced belts to achieve unlimited conveying rates of widely varying materials.
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