The rationalization 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 writing briefly retraces 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° slope and three variations of the Dos Santos International snake sandwich belt high angle conveyor at 45°, 60° and 90° (vertical). The economic comparison is made at silos of increasing height from 17.8 meters to 73 meters.
The sandwich belt conveyor model
The sandwich belt conveyor model was developed in its most rudimentary form 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 Figure 1. This model depicts accurately a derailing of the material cross-section allowing an ample belt edge distance that can tolerate normal belt misalignment without material spillage. The calculations must thus recognize that only part of the hugging pressure is imparted on to 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 [1].
The loop belt
The first lasting success and arguably the most important technological development in sandwich belt 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 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.
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