Courtesy: Trans Tech Publications - Bulk Solids Handling Journal

1. Definition and General Background

This air-cushion conveyor is used to convey bulk materials in much the same way as a conventional idler-supported conveyor. A continuous belt is driven around a head and tail pulley using the normal motor drive, however, with one major difference. A trough section is used as the structural support of the belt and the trough section replaces the normal idler supports.

Air, a natural resource used successfully as a substitute for idlers implies substantial benefits and advantages to the end user and has created a conveyor of economical and competitive significance.

Air used in the conveyor much in the same way as for a hovercraft, i.e. an air cushion as a means of transport, was experimented with by JAMES DODGE in the USA a hundred years ago, but was only brought to fruition in this century by a Dutch professor, hence extensive development work perfecting this system was dictated to by a European industry.

The company, Aeroconveyors, of Boksburg North was established in 1981 and has gained 11 years' experience in designing, manufacturing, supplying and erecting air-supported belt conveyors and related equipment. The product is completely locally manufactured and has proved to be both cost-effective and a long-term investment, continuing to achieve good competitive standards in the market place, with versatility and low-maintenance programmes being key to its success.

2. Development in South African Market Place

The development of the air-cushion principle conveyor in South Africa has been largely dictated to by standards parallel to the European industry as well as different facets peculiar to its own industries. Aspects influencing its development here have been affected by the following:

1. Materials, minerals, ores, vegetation and products specially found in South Africa.

2. Diverse African climatic, environmental and topographic conditions so different to Europe.

3. General conservatism in the acceptance of the air principle concept.

4. The need for a security-tight materials handling system, e.g. for diamond tailings, etc.

5. The ups and downs of the South African economy (mostly influenced by the sociopolitical climate).

Apart from number 3, the aeroconveyor has proved to meet all the demands these conditions have imposed upon the South African industry. Even in economically difficult times, the installation of the conveyor has been seen as a financially viable proposition once realised as possessing a low-maintenance programme and being a long-term investment.

However, over the years it has taken much persuasion to convince clients of its potential, and the realisation of the conveyor actually functioning in various applications, i.e. "seeing is believing", has proved to be the aeroconveyor's most effective marketing tool.

But much water has flowed under the bridge since then and in 1993 Aeroconveyors can boast that their influence has been widespread and that they have established themselves in many diverse industrial fields.

3. Applications

3.1 Woodchips - Mobile Plough

The very first application for the aeroconveyor was in the woodchips industry in 1981. After 12 years, the conveyors are still running well and its total enclosure has ensured a great reduction in dust pollution in its immediate environment. The ease of installation and easy enclosure due to self-supporting structures was impressive and the saving of costs on support structures was evident to this customer.

Soon afterwards, Aeroconveyors supplied two 40 m conveyors to a manufacturer stockpiling large quantities of wood chips within an enclosed area. The manufacturer required a compact system allowing for reduced head room, increased storage capacity and a clean and smooth operation. Two aeroconveyors below the roof line were suspended from the roof trusses. Both conveyors were fitted with a mobile plough (Fig. 1).

Fig. 1: An aeroconveyor mobile plough in operation

The aeroconveyor mobile plough has a low profile as the track section for the plough is secured to the standard box section. The sideskirts of the plough ensure that the material is carried free of the box section as these extend past the conveyors preventing build-up of material on the conveyor structure. The material is removed from the belt by the floating plough and discharged off either side. An even distribution of the material is achieved by a bed of rollers in the section where the belt is flattened. Longitudinal travel of the plough is obtained through its own drive motor enabling continuous stockpiling of 36 m. The ploughs run parallel to one another and are manufactured from steel to take the brunt of the wear. The actual bearing surfaces against the belt, however, are made of Vulealon, a low friction material normally used on belt cleaners. Also, a special seal beneath one plough controls the air escape for a smooth operation of the remainder of the aeroconveyor, after it has moved through the plough.

Eight years later, 10 more conveyors were supplied to this wood board manufacturer which are incorporated with the existing aeroconveyors, improving and extending the running of this product. A feature of this installation is the tripper discharge running at a speed of 1 m/min which functions for the distribution of the product over a 40 m bunker.

3.2 Chemical Industries: Glassworks and Dust-Proofing

Several applications have taken place in the chemical industry, amongst others, at a glass-making industry where soda-ash, feldspar and calcite are handled. (These chemicals are used in the glass-making process). The system starts with the underground hopper where bottom-opening railway trucks dump the products. A cross-feed arrangement takes the products to the main conveyor which car- ries it to 138 m high silos. Here a special cross feed shuttle arrangement transports the product to one of a number of silos. The system can carry up to 120 t/h. There is a convex curve as well (with a few rollers on the curved section only) to take the conveyor into the penthouse at the top of the silos.

As a result of the good performance and iow maintenance costs that these conveyors have incurred, the manufacturer was recently prompted to place another order for 3 more conveyors and a complex shuttle conveyor arrangement.

Probably the most valued asset of the aeroconveyor here is its ability for total enclosure since there are several food processing plants in this area and the products being conveyed are powdery. This protection offered by the covered conveyors, i.e. the protection of the environment from the product or, vice versa, the product from the elements, has made a profound impression on clients and has played a major role in influencing their decision in choosing the aeroconveyor as opposed to conventional or other conveyors.

Environmental conditions and climate are particularly dusty and dry in the Northern part of South Africa where most of the general industry and mining is to be found and there has been a substantial increase in the demand for dust preventive measures and general increased interest in environmental friendly products (for which the aeroconveyor certainly does qualify).

Aeroconveyors has also found a solution for extreme dustiness including a high-rised cover on which a dust-extractor may be superimposed positioned at a prescribed distance form the loading point.

Fig. 2: An aeroconveyor significantly inclined, feeding a set of silos

3.3 Diamond Industry

Four fully enclosed aeroconveyors were made from 3CR12 (a corrosion resistant steel) for diamond mines on the West Coast of Namibia (Fig. 3). Three conveyors in excess of 70 m and a short cross conveyor of 13 m are transporting diamond tailings at a rate of 60 t/h. A minor modification to the top covers of the aeroconveyors ensures restricted access to the product by unauthorised personnel, reducing theft and increasing plant security.

Fig. 3: Totally enclosed aeroconveyor with a convex curve transporting diamond trailings in Namibia

The conveyors were manufactured in 3CR12 steel because of the corrosive environment of the West Coast (a combination of severe dustiness emanating from sandstorms from the desert coupled with wind and humidity from the sea). The entire installation has been provided with a 4-coat chlorinated rubber paint system to increase corrosion resistance. The conveyors form part of a reclamation system reworking waste dumps as modern efficient methods make this an economically viable proposition.

3.4 Furnace-Mix

Furnace-mix was the material to be handled at Middelburg Steel where an urgent changeover deadline was met in changing the existing mechanical method of transporting furnace-mix to aeroconveyor's materials handling system. No production time was lost due to ease of installation and the system was able to circumnavigate the existing plant as required by the client.

On more recent applications for chemicals, an installation was provided at Rand Refinery where a prerequisite was to supply a fully enclosed materials handling system. Fumace-mix making up the chemicals for the refining of gold is transported here and a toxic lead dust coming off this mix determined the necessity for total enclosure which aeroconveyors could easily comply with. The mix is conveyed on a 500 mm wide belt at a speed of 1 m/sec. Inspection hatches were also installed along the carrying length as well as rotary brush scrapers for belt cleaning.

3.5 Vermiculite

In the conveying of vermiculite in the Northern Transvaal, the idea of one fan supplying the necessary support to 31 conveyors was introduced. The company was requested to investigate the feasibility of this new idea, of which the outcome was favourable. The air-flow network was simulated by creating a computer model. Accurate predictions of performance under load were determined allowing the designer to optimise the ducting size an ultimate design of the conveyors. An important outcome of this project was the Aerovalve, a valve which efficiently seals off the non-operating fan from the system. The operating mechanism is the air pressure itself. The valve ensures a smooth transit between the decaying pressure of one fan and the increase in pressure from the standby. This ensured the uninterrupted air supply at a steady pressure in the event of a fault arising with the first supply fan.

3.6 Nampak and Environmental Benefits

Apart from the aeroconveyor's dust covers its use of a natural resource, i.e. air, and diminished noise pollution - all contributing factors in environmental cleanliness, the aeroconveyor itself, in its entirety was specifically used in the quest for environmental friendliness. It was awarded this opportunity by Nampak for the recycling of waste paper.

Chosen primarily for increasing production by eliminating unnecessary handling of product, its faculty for easy installation and a low-maintenance programme was also recognised. The flat-belt configuration was used here.

3.7 Port Off-Loaders

Eleven conveyors were installed at Durban Bulk Shipping establishing a new market place for aeroconveyors by revealing system suitability for the loading and discharge of ships in harbours (Fig. 4).

Fig. 4: A ship loader incorporating aeroconveyors within its structure

Further suitability for such an application arises from the ability to convey at high speed. Accelerated importation of maize resulting from the drought throughout the Afdcan continent was the issue two years ago and the air principle system proved to be highly efficient with the off-loading of this material and at a speed faster than other materials handling systems previously used. Capacity for the DBS conveyors is 500 t/h and a 750 mm wide conveyor belt was selected, running at a belt speed of 4.2 m/sec. Top conveyors were an easy addition, reducing dusting, providing weatherproofing and preventing spillage. Sight-glasses were also fitted in some of the conveyors to allow the operator to see the material should regulating or blending of materials be requested.

A 70 m long, 1,200 mm vyide belt conveyor situated inside the gantry of South Afdca's newest ship loader at the time, was also commissioned. This conveyor, along with the loader, is hinged in the middle and can load vessels efficiently at a rate of 1,500 t/h and at a belt speed in excess of 4 m/see. The result is that the time needed to load vesseis is greatly reduced. At the time that these aeroconveyors were utilised, it was evident that they had formed one of the most vital links between the importation of maize and the drought-stricken continent of Africa.

3.8 Sugar Mills

At present, bagasse, sugar cane and other sugar-related products, are by far the most important items on Aero-conveyors' agenda.

The sugar projects began when the company was recently awarded contracts to supply cane-yard conveyors to a new Transvaal sugar factory in the Eastern Transvaal.

The conveyors embrace the following:

1. The main cane conveyor which conveys the freshly cut cane up to the cane-knives (the cut cane lies 2 m deep in the conveyor belt).

2. The cane-knives then cut the cane stalks into billets which are fed onto the shredder feed-conveyor which in turn conveys the billets to the shredding machine.

3. Another conveyor is situated below the shredder conveyor which conveys the shredded cane into the diffuser.

4. Finally, the last aeroconveyor handles the diffused cane to the dewatering mill.

These conveyors have a belt width of 2.1 m and are designed to handle cane at a rate of 225 t/h. Unlike the normal aeroconveyor which is troughed, these have a flat-belt configuration.

The introduction of these wide conveyors heralds a first for South Africa, making them the widest air-supported conveyors Aeroconveyors have yet manufactured.

Running concurrently with this project are several other conveyors being built for the sugar industry. These contracts came about after supplying 4 conveyors for a new depithing station. The conveyors handle bagasse pith at 40 t/h. The bagasse pith is a dry, stringy fibre resulting from the sugar making process and is primarily used for cattle feed. A feature of one of the conveyors situated within the roof trusses of the bagasse store is that it is reversible and can distribute material over the whole store area.

The conveyors were installed within the existing plant arrangement and special emphasis was placed on erection without affecting continuous running of the plant. This criteria was successfully met by Aeroconveyors.

4. Concluding Remarks

The aeroconveyor, an intriguing, yet simple way of materials handling, finds a market place in most industries in South Africa. Having started slowly, yet surely and not without trepidation, it now appears as if the penny has dropped and finally the demand for its utilisation in many diverse industrial fields has stepped up dramatically.