Sunday 22nd Sep, 2019

Problems with short drop height soft flow chutes

Aspec Engineering’s Paul Munzenberger investigates some of the performance problems that can result from an insufficient drop height in bulk material handling systems.

Aspec Engineering’s Paul Munzenberger investigates some of the performance problems that can result from an insufficient drop height in bulk material handling systems.

Transfer chutes are an indispensable part of bulk material handling systems. Although seemingly simple, if a comparatively blasé approach is followed in designing the chute, there is a high risk the chute could become a bottleneck and not efficiently move material from one conveyor, or process, to the next.

A common design issue with new chutes in material handling plants is the relative locations of the discharge conveyor and the receiving conveyor below it. Usually, the design position for the two conveyors will be too close vertically – in an effort to reduce plant construction costs by reducing the overall height.

In many cases, a chute will be intended to handle material from a discharge conveyor to a receiving conveyor on the next floor down with conveyor-to-conveyor drop heights as short as two or three metres. This can present certain problems that are likely to be discovered by the engineers designing the chute; however, by this time, the plant’s structural design and layout may have reached the point where only small, potentially insignificant, adjustments to the chute’s constraints are possible.

Soft flow chutes

Soft flow transfer chutes are a comparatively recent development in the materials handling industry, with their theory and design maturing throughout the past three or four decades.

Prior to the uptake of soft flow chutes, chute designs were characterised by having only flat surfaces and minimal steel work, using simple shapes to direct material from one conveyor to the next.

Soft flow chutes, as depicted in Figure 1, are characterised by curved surfaces that catch and redirect the flow of material. A soft flow chute will typically consist of two curved sections called a hood and a spoon. The hood takes the place of a traditional flat impact plate, or curtain, and is engineered to catch the bulk material stream with minimal impact, redirecting it downwards to the spoon. The spoon is positioned to catch the material stream from the hood then direct it onto the receiving conveyor.

Figure 1 Diagram of a hood and spoon chute from a stacker.

The two curved surfaces are specially engineered to gently control the material flow and their radii, relative positions, entry and exit angles, and rate of convergence are carefully calculated.

Soft flow chutes can reduce material degradation, conveyor belt wear, and dust, which has seen them become increasingly popular and routinely specified in new plant designs.

Unfortunately, the design of a soft flow chute is sensitive to the drop height from the discharge conveyor to the receiving conveyor and the chute’s performance can be greatly affected by poor placement of these items.

Chute exit velocities

One of the design goals of soft flow chutes is to accelerate the material so that its exit velocity matches the receiving conveyor’s belt speed. This is due to rubber being typically poor at withstanding the wear generated by accelerating the bulk solid up to the conveyor’s belt speed.

However, velocity matching is at odds with the use of curved surfaces which, while gentler on the bulk material, also keep the bulk material in contact with friction-generating walls for longer than in traditional chute designs.

To mitigate this, the chute should be designed with enough drop height to allow gravity to accelerate the material before it is caught by the spoon and passed on the receiving conveyor at a matched velocity.

The lack of a suitable drop height in a soft flow chute reduces the exit velocity and exposes the receiving conveyor to higher wear rates. It is particularly important that the velocity of the bulk material stream and receiving conveyor is matched for the typically short conveyors used in handling plants, as each section of the conveyor is exposed to the chute flow more often.

Using a smaller spoon radius to increase a deficient drop height will not only increase the wear on the chute’s lining through higher normal pressures but will also fail to increase the exit velocity by any appreciable amount, due to the increased resistance developed by the higher normal pressures.

Dealing with scraper fines

Poor drop heights between discharge and receiving conveyors are also a problem when dealing with scraper fines.

A well-designed chute will have two sets of scrapers installed to clean the conveyor belt: a set of primary scrapers that contact the belt on the head pulley and a second set contacting the conveyor belt a short distance after it has left the head pulley.

Primary scrapers are designed to remove most of the carry-back, which simply falls down the chute and becomes entrained into the main material stream. Handling of the remaining carry-back, collected by the secondary scrapers, is more difficult as the small amounts collected can only generate low normal wall pressures and therefore high wall friction angles when compared to the main stream.

Higher wall friction means the angle of the surfaces catching the secondary scraper carry-back must be steep. This presents a problem when trying to collect them from behind the discharge pulley and moving them forward into the main stream, within the length of a short drop height. To move this ultra-fine material, the secondary scrapers should be enveloped by the main chute and rely on induced vibrations and stray lumps of bulk solid to dislodge any build-up.

An alternative is to provide a secondary dribble chute that, with its steep sides, collects the ultra-fine material and deposits it onto the receiving conveyor up-stream of the main flow. This solution can be effective but is only really practical when the discharge and receiving conveyors are in line or nearly in line.

A further, though unrecommended alternative, is to collect the ultra-fines in a small hopper which is connected to a pipe linked to the receiving conveyor. This option is tempting if the structure hasn’t been designed with a suitably sized hole to accommodate the secondary scraper location as it only requires the boring of a 200 to 300-millimetre hole under the conveyor to pass the pipe through.

However, the hopper walls and pipe still need to be steeply inclined. This is a problem if the hopper has to fit in the space between the return strand of a conveyor and the floor beneath as shown in Figure 2. Now there is a small diameter pipe in the system which can block easily and is difficult to inspect internally.

Figure 2: An example of an inappropriate fines chute that would need flow aids to work effectively.

A blocked fines chute means the build-up can reach the underside of the discharge conveyor belt and then lift the belt off the secondary scrapers and, in the worst case, pinch the conveyor belt against the conveyor structure hard enough to retard the motion of the belt and cause a fire risk or other belt damage.

Central loading of the receiving conveyor

Insufficient drop height can also be partly responsible for poor loading of receiving conveyors.

Central loading of any belt conveyor is critical as it is the primary means by which the belt’s tracking is maintained. It is best achieved by incorporating a long, and thus high, spoon section that provides sufficient transit times for gravity to pull the material stream to the centre of the chute before depositing the material centrally on the receiving conveyor as shown in Figure 3.

Figure 3: A splitter chute with enough length to centralise the material flow.
Figure 3: A splitter chute with enough length to centralise the material flow.

Problems with central loading occur when vertical material flow exiting the hood is not directly in line with the receiving conveyor and must therefore be deflected sideways to meet the conveyor as is common in splitter, or trouser leg, chutes.

In these types of chutes, a large part of the drop height is used to shift the bulk solid stream sideways, with respect to the receiving conveyor. Unless the drop height is significant there won’t be enough height left for the spoon to collect and centralise the flow before it reaches the receiving conveyor represented by the dashed outline in Figure 3.

Conclusion

It is recommended that chute designers are consulted during the layout phase of a material handling plant so that potential problems can be discussed, and reasonable compromises can be made. This is the best way to reduce the chances of problems occurring in a plant’s soft flow chutes.