Closed Belt Conveying for Overland High Lift Applications
Lennart Tingskog, Sweden
Courtesy : Trans Tech Publications - Bulk Solids Handling Journal
The introduction of the closed belt conveyor U-CONQ has opened new possibilities to create economical closed belt conveying over long distances and overcoming big height differences. In the article the influence of having a closed system and how other desired features could be met, is discussed.
We can begin by formulating an "ideal Specification" for an overland system:
Closed, no spillage or losses from wind and belt movement.
Free choice of route.
High degree of safety.
Low investment cost.
Low maintenance cost.
2. The U-CON System
The key part in the U-CON system is a patented closed belt. The belt has a carcass that takes up the dragging force only in the mid zone. The rest of the belt can be elongated 10- 15%, also in fatigue, to allow bending over drums etc. in closed shape. Further, the belt has variable cross stiffness over its width. The belt is manufactured and supplied flat but when folded it forms a well-defined stable flat package. The folded belt can be kept folded by rollers on top. When released it goes back to flat. In one version the belt is fitted with a locking system (U-CONQL) that keeps the belt folded without rollers. This device also means an extremely good sealing, good enough for vertical transport of materials like alumina and cement. It also keeps the material well protected against water, etc.
A consequence of the locking system is that the needed number of rollers is considerably reduced. A U-CON conveyor means a universal system. The conveyor can negotiate very narrow curves and go uphill at unlimited angles.
Another member of the U-CON family is the TWIN system. Here the functions of dragging and enclosing are divided on two separate belts (Fig. 1) e.g., one normal U-CONQ and one flat belt. The two belts follow each other in the curves etc. but when an intermediate drive is wanted the flat belt makes a loop over a driven drum (Fig. 4).
Fig. 1: U-Con Twin system
3. Closed Conveying
Overland conveying is today mostly carried out using open belt, in a few cases with open belt in galleries or covers. Covers are used to reduce material losses caused by wind and as a weather protection, ventilation losses remain. What is really wanted is an enclosing that moves with the material, is wrapped around the material and gives full view to belt and rotating parts. U-CONQ is such a system.
If a weather protection is needed it should preferably be built to allow inspection without opening covers etc. The ultimate protection against weather, hostile actions etc. is to have a closed gallery with a gangway on one side. The cost for this can be kept down by using a compact conveyor like the U-CONQ (Fig. 3).
Fig. 3: A compact enclosure. All details open for inspection
4. Free Choice of Route
Horizontal and vertical curves should be possible. In addition, steep inclines should also be able to be accommodated for. Horizontal curves have been effected by means of banking, using a very large radius of several kilometres. Pipe conveyors and swinging type arrangements can incorporate smaller radii. All have in common that a large part of the conveyor needs special frames and extraordinary care to achieve correct angles etc.
The U-CON conveyor uses sharp curves and only a very short distance is influenced. Belt stretch, load distribution etc. have no influence on belt behaviour. Steep inclination can also be used to overcome hills.
5. High Degree of Safety
High safety level is of the utmost importance for an overland system. Matters that influence the safety are
good control of belt independent of speed, filling degree and load distribution. The belt should not be allowed to twist.
low stress level. A high amount of energy stored in the belt is a danger.
the transported material should not be allowed to damage the force carrying part of the belt.
the number of moving or rotating parts should be low.
The U-CONQL TWIN is a favourable system in all these respects.
The flat belt package configuration with good stiffness sideways gives full control under all circumstances. Should the belt be cut off it has stiffness enough to stay in the path, also in inclined parts.
Intermediate drive stations will keep the stored energy down to a minimum. Motor sizes can be a convenient 300 kW or lower. Energy is transported to the mid part of the conveyor in electric wires instead of mechanical in shape of extreme force in the belt.
Using the twin system means that the transported material has no contact at all with the force carrying belt. The enclosing U-CON belt on the other hand is very insensitive to damage.
The low stress level means that cuts etc. do not propagate.
U-CONQL TWIN has only two rollers in each frame, one for loaded, one for return belt.
With 2.5 m frame distance we get only 0.8 rollers per meter conveyor (Fig. 4).
Fig. 4: Intermediate drive station also arrange for two-way transport
6. Low Investment Cost
Investment costs will differ, depending on whether or not environmental aspects need to be taken into account. The enclosed system, however, also includes features that lower the cost. Compactness for example means lower freight, less steel and, generally, lower erection costs.
The recommendation is therefore: study in any case also an enclosed transport alternative.
7. Low Maintenance Cost
Maintenance cost is influenced by:
Roller replacement, labour and material.
Roller replacement cost is likely to be proportional to the installed number of rollers. Essential is also a design that makes inspection and replacement easy.
The belt is an essential part of the investment. The flat belt in the twin system is not exposed to the transported material and is only bent in one direction. It can therefore be expected to have a longer life than a conventional belt it also costs less.
The U-CONQ part of the system is, on the other hand, not exposed to a dragging force (apart from that keeping it in stretch) and can also be expected to have at least as a long life as a conventional belt.
The cost for belt repairs and power consumption is not likely to show any big differences for the different conveyor systems.
8. Use of Intermediate Drives
For mainly horizontal transport and up to 2 - 3 km lengths it is not likely that a system with intermediate doves is competitive. In the region 5 - 50 km intermediate drives give definite advantages.
If level differences are involved intermediate drives could be considered when the power needed is 400-500 kW.
Note, however, that the U-CONQL TWIN system can be considered also at shorter distances for important transport to obtain a higher safety level. (The cargo has no contact with the force carrying belt.)
9. Two Way Transport
The new U-CON QL TWIN system can be used for simultaneous transport in both directions. An example of use might be coal to a cement factory and cement/clinker back. It could be iron ore in one direction and iron ore pellets back (Fig. 5). Heavy industry could, with advantage, be placed, for example, 40 - 50 km away from the big port, take the raw material to the factory and send the ready product back.
Fig. 5: Example of lifeline to satellite factory, to avoid heavy concentration to port areas, etc.
10. High Lift Conveying
Continuous vertical transport in the range higher than 100 m is limited by the high forces needed. In the end the weight of the combined useful load per meter and the force carrying member itself will make the system impractical. Inclined conveying on belt is possible if transfers are introduced at certain levels. U-CON QL TWIN opens a new possibility. Continuous unlimited lift with intermediate drive stations with, for example, height differences giving motor sizes limited to about 300 kW. The closed conveyor makes steep angles possible but the limitation is instead the friction between the two twin belts. Without artificial means 20 degrees should be possible in most cases. Curves can be used if necessary to direct the transport to the wanted end station (Fig. 6).
Fig. 6: Example of high lift transport from 500 m deep mine
11. Concluding Remarks
Closed belt conveyors are also likely to be wanted for long distance conveying. A suitable system for that purpose is the U- CON QL for medium lengths. For longer distances a TWIN belt system allows convenient intermediate drives and a high safety level. The intermediate drive system can also be of interest for up-from-mine transport.
Mr. Lennart Tingskog, Managing Director, Conveytech AB,
La Cours gata 4, S-25231, Helsingborg, Sweden.
Tel.: +46 42 11 03 20; Fax: +46 42 11 03 25.