High Angle Conveying
The Cost and Value of High Angle Conveying – 2012
It is important to note that increasing belt width is required with increasing angle. Compared to the conventional troughed belt conveyor a sandwich belt high angle conveyor carries a lesser material cross-section for a given belt width, having a larger material-free edge distance and ample margin against overload. Furthermore, the material cross section at a sandwich conveyor is de-rated with increasing high angle. Accordingly, the vertical sandwich belt conveyor requires the widest belts (1400 millimetres), components and structure for the same coal throughput rate and belt speed. The cost consequence is seen at the conveyor equipment and components summary. On the other hand, it does result in heavier, possibly more durable equipment.
The drive power chosen differs from the calculated required as we selected the next commonly available motor size and drive components. This is further aggravated by our choosing equal drive units at the top and bottom belts of the DSI Snakes. The results in investment costs appear as less than smooth curves (see Fig. 6) at the conveyor equipment and components summary.
Conveying Angle and Safety against Material Slideback
Safety against material slide-back is a very important factor in any materials elevating system. Such a consideration might lead the conservative to erroneously choose the conventional conveyor as the safe solution. In fact, there are many documented cases of conventional slope conveyors at inclines of 15, 16 and 17 degrees that experience frequent material slide-back, especially when excessive moisture or frost are involved. This writer knows of no material slide-back at any of the more than 100 sandwich belt conveyor designed in compliance with the “expanded conveyor technology.” The safety factor against slide-back is well known at a sandwich belt conveyor system because it is chosen when choosing the induced hugging pressure. Indeed such a selection does not exist for conventional slope conveyors and an equivalent safety factor against material slide-back would limit conventional conveying angles to less than 12 degrees even with coal and crushed rock.
As a consequence of experiencing material slide-back with 16 and 17 degree conveyors, a study at a major coal company objectively determined and resolved that their conventional conveyor slopes should never exceed 12 degrees. Their next elevating conveyor project was designed and built in compliance with new criteria. The cost impact on this project was so high as to immediately cause reconsideration of the conservative criteria. It was then resolved that their conventional conveyor slopes should never exceed 14 degrees. Such cost driven compromises are unnecessary with the sandwich belt conveyors that are pursuant to the described development.
For the sake of a fair comparison it was important to structurally optimise the alternate elevating systems, especially the conventional 15 degree slope conveyor. It is well known that structural bents and towers become very massive and expensive with height. In order to minimise the mass and cost of structural steel, we must increase the structural spans with height so that we can minimise the number of very high and massive bents. In this case we have chosen truss spans, beginning from grade, of 30.5, 45.7 and 61 metres for the first, second and third pairs of trusses. Having established approximate structural optimisation for the conventional 15° conveyor, we have chosen silo wall locations at the bents.
Basis for and Presentation of Costs and Pricing
A fair costing comparison must be in compliance with a single and uniform design and pricing philosophy. Every buyer and every seller knows that the same inquiry and specification will result in, at times, widely varying price offers by various manufacturers. Even the same manufacturer’s offer will vary depending on the competitive situation and his desire for the work (i.e. having a high or low current work load.) Though our estimates here are in compliance with the uniform design and pricing philosophy of Dos Santos International, we present all pricing of comparison in the form of an index. Any reader can then normalise this cost comparison according to their own buying and/or selling experiences and policies.
Fig. 5 graphs the investment cost (engineering and supply) for the four elevating systems as a function of increasing silo height. This does not include civil, foundations or installation. It is not surprising that the conventional 15 degree slope conveyor proves least costly when elevating to the lowest silo of 17.8 metres height. However, the cost of the conventional conveyor system increases exponentially with height. Beyond approx. 33 metres of lift it becomes the most expensive solution. At 76 metres of lift its cost exceeds the sandwich belt solutions by a range of 60 to 88 percent. On the other hand the variation in investment cost for the sandwich belt solutions is approximately linear with silo height and quite modest with regard to each other. At the lowest silo height, a sandwich belt system of 45 degree slope has a 0.23 investment index compared to 0.24 (4 percent higher) for a 60 degree slope and 0.27 (17 percent higher) for the vertical unit. At the highest silo, the vertical sandwich belt system has the lowest investment index at 0.51 compared to 0.56 (10 percent higher) for the 60 degree slope and 0.60 (18 percent higher) for the 45 degree slope.
Figs. 6 and 7 show the relative make-up of the investment costs. Fig. 6 graphs the investment in conveyor equipment and components. This includes belting, idlers, pulleys, drives, switches etc. It can be seen that the investments increase linearly with height. Furthermore, the conventional 15° conveyor has always the lowest investment in conveyor equipment and components while the vertical sandwich belt conveyor is always the highest. This is due to the required wider belts and corresponding equipment. The great cost differentials between the conventional 15? slope conveyor and the sandwich belt conveyors are owing to the great difference in structural steel requirements, as illustrated in Fig. 7. The steel in this case includes all truss spans, bents, terminal framing, covers, access walkways and stairways, chutes, skirts etc.
Some Important Indirect Cost Factors
It is worth noting some important indirect cost factors for comparison. Table 2 shows a comparison of displaced projected (ground) areas, spatial volumes and perimeter area, above grade, for the different elevating systems. Significant indirect costs can be associated with each of these quantities. For the displaced ground area there is a real estate cost. For the spatial volume and perimeter area above grade there is a very real cost when the conveying system is part of a housed or covered facility. The cost of the building or covering is directly related to the displaced spatial volume and exposed surface area. In the case of heated facilities, required in frigid climates, heat is lost through the exposed surface area in direct proportion to that area. Also important is the minimal environmental footprint of the high angle conveyor solutions.
Latest Sandwich Belt High Angle Conveyors, Examples of Indirect Cost
Table 3 summarises some of the latest sandwich belt high-angle conveyor systems. Each has its unique project account and reason why it was the best solution. We will high light three projects which are examples of the significant indirect costs.