High Angle Conveying
The Cost and Value of High Angle Conveying – 2012
The Expanded Conveyor Technology
In 1979, a “High Angle Conveyor Study” was funded by the U.S. Department of the Interior, Bureau of Mines and executed by Dravo Corporation. The target applications in open pit mining, required high volume conveying rates of the coarsest materials along the most direct steeply inclined paths (along the pit wall.) This precluded the C-Shaped Loop Belt profile but not the principles of operation. In depth research sought to unify the sandwich belt technology with the conventional conveyor technology, thus expanding the conventional conveyor technology and ensuring that none of the equipment is subject to adverse loading conditions beyond its intended use.
A study of all methods of steep angle conveying found the sandwich belt approach to be the most promising but also the least understood technologically. Basic research of sandwich belt conveyors began with the study “Cover Belts of the 1950s” and continued with study of all developments and variations through the 1960s and 70s. Like the 1958 Rasper review it was sought for characteristics and features that produced success in order to create a basis, complying to the findings, that would ensure success.
That rationalisation is revealed in detail in the 1982 article “Evolution of Sandwich belt High Angle Conveyors” by Dos Santos and Frizzell . The basis to ensure success is complete adherence to the rules of conventional conveyor technology while ensuring complete continuity of hugging on the material regardless of the conveying rate (from empty, through overload, with continuous and discontinuous material flow) and material size distribution. The former will ensure long life of the belting, components and the equipment while the latter ensures frictional development of the material within the sandwich and precludes any material slide-back.
Though the Loop Belt displayed the key characteristics, it failed to offer the desired profile and did not adhere to all design rules of conventional conveyors. Rather, it featured exceptions of convenience. The latter are to the detriment of the system.
“Evolution of Sandwich Belt High Angle Conveyors” has expanded the conventional conveyor technology into sandwich belt conveyors ensuring that such systems will have all of the operating and maintenance characteristics of conventional conveyors and will be as widely applicable. With emphasis on the technology rather than particular manifestation that writing  goes on to propose some five different sandwich belt conveying methods that when executed according to the guidelines established will result in success. Since the 1980s, there have been numerous successful commercial installations.
The Snake Sandwich Belt High Angle Conveyor
The DSI Snake Sandwich Belt High Angle Conveyor, evolved from the Snake Sandwich Conveyor, which was first introduced to the industry in 1982. The Snake system uses all (and only) conventional conveyor components and equipment and smooth surfaced belts (that are continuously cleaned by scraping.) Hugging pressure on the conveyed material is imparted by a radial pressure which is induced by the inherent belt tension on an engineered curving profile. This is the most positive and gentle form of hugging pressure.
Possible Snake configurations are illustrated in Figs. 2 and 3. The most basic configurations are of simple C-shape and S-shape as shown in Fig. 2. Many such systems have been built and are in successful operation. Extended C and S-shaped profiles (Fig. 3) are facilitated by a multitude of profile curves with inflection zones between adjacent curves. Based on this writer’s experience, to meet the industry requirements, about half of the Snake units will be of simple profile as shown in Fig. 2.
The principles of sandwich belt high angle conveying are not new and reached technological maturity in the period 1979-1982. These principles have been demonstrated successfully in nearly 200 installations worldwide. Many of these installations, which are of Loop Belt systems, preceded 1979. Many more sandwich belt systems have been built for the low tonnage and duties of municipal waste and chemical sludge handling and other industries. Though these smaller systems are admirable, they are not suitable for the high volume requirements of open pit mining, transfer yard and dock and power plant applications. Despite the numerous successes, sandwich belt conveyor systems have not been generally embraced as main stream conveyor technology rather these have been viewed as a specialised technology. The DSI Snake represents an expansion of the mainstream conventional conveyor technology, subject to all pre-established rules of good conveyor design. This will ensure long life of the equipment and belts, and low operating and maintenance costs. Applicability is as wide ranging as for conventional conveyors. Mainstreaming of the sandwich belt high angle conveyor technology is long overdue and this will come with wider and routine use.
The Cost/Value of High Angle Conveyors
The favourable economics of materials haulage by belt conveyors has long been acknowledged. The economics become even more favourable with increased volumetric rates and lifts. The superior reliability and availability of belt conveyors along with lower operating and maintenance costs are factors generally acknowledged. These favourable characteristics have also been demonstrated at numerous installations of sandwich belt high angle conveyors. Indeed, such systems, in compliance with the expanded conveyor technology, have equalled or outperformed the conventional conveyors at the same job site subject to the same or similar duty. This article will not compare the economics of belt conveying against other haulage methods (such comparisons can be found in various references [3-7]). We will not compare the operational and maintenance characteristics and costs of conventional and sandwich belt conveyors. These are demonstrated to be equal in numerous applications. Instead, a comparison of the investment costs (engineering and supply) of four conveying paths to silos of various heights will be given. The four conveying paths, shown in Fig. 4 are by a conventional conveyor and three variations of the Snake sandwich belt conveyors as follows:
1. Conventional conveyor at 15° slope.
2. Snake at 45° slope.
3. Snake at 60° slope.
4. Snake at 90° vertical.
The silo heights vary from 17.8 to 73 metres with the system lifts being an additional 3 metres. Besides total investment costs we will look at cost breakdown. Such breakdowns reveal economic sensitivity and imply operating and maintenance characteristics. Some important non-cost factors will also be analysed.
Fig. 4 shows the configurations of the conventional and high angle conveyor elevating options to silos of increasing height. Overall dimensions at the illustrate the significant differences in the foot print required for the alternate profiles. Table 1 summarises the technical basis for the design and estimating of the various elevating options.