Sunday 11th Apr, 2021

TUNRA’s 10 Commandments for reliable transfer chute design

A collaboration of Newcastle businesses, including TUNRA Bulk Solids, HIC Services, and Lindsay Dynan, worked together to improve the throughput of a transfer chute at a prominent Hunter Valley coal mine by using 3D modelling technology.

Shaun Reid, Dr Jens Plinke and Priscilla Freire offer their top 10 tips on how to design transfer chutes that improve plant effectiveness and reduce downtime.

One of the most popular topics in the bulk solids handling world is the appropriate design of transfer chutes for effective plant operation and reduced downtime. Transfer chute problems are by far the most common we deal with in our engineering projects, which involve the design or re-design of transfers and associated work, including material characterisation tests, calibration tests and computational modelling. Our engineers have put together a summary with considerations for transfer chute design which was presented at our last intensive course on bulk solids storage and handling, earlier this year.

Commandment #1: Thou shalt give adequate attention to the bulk solid flow properties in view of the transfer configuration

According to Alan Roberts’ Chute design considerations for feeding and transfer, chute design has been the object of extensive research over the past decades, but attention to the properties of materials being handled and their flow dynamics is often overlooked.

Material properties of relevance for transfer chute design include bulk material specific parameters such as the internal friction angle, unconfined yield strength, cohesion and bulk density, as well as properties concerning the interaction of the bulk material with chute surfaces such as wall friction or wall adhesion.

Considerations such as boundary (or wall) friction, cohesion and adhesion are of special concern with so-called wet and sticky materials, and may change drastically with factors such as particle size distribution, consolidation pressure and moisture. It is therefore important to address the requirements of specific applications when performing material characterisation.

Figure 1 demonstrates wall friction test results for a below-water table ore at two moisture contents paired with a range of wall liners. As it becomes clear from the test results, liner performance from a friction point of view may be dependent on the moisture content of the material.

Figure 1 – Wall friction test results for different liners at 21 per cent and 12 per cent moisture contents

Another important point to note is that material characterisation tests are usually conducted across a range of consolidation (pressure) conditions. Different pieces of equipment will be subject to different loads, but, generally in the case of transfer chutes, flow is characterised by lower stresses, usually in the order of 0-5 kPa.

Commandment #2: Chutes shalt be symmetric, located centrally to the outgoing belt and direct the bulk solid material onto the belt in the direction of travel.

Although not always possible, transfer chutes should be designed with the simplest configuration possible, aiming at a symmetrical geometry. This is especially challenging for brownfield designs, where structural constraints are often in place. In all cases, however, material should be centralised to the receiving belt as early as possible in the transfer. Decentralised loading conditions often cause belt mistracking, one of the most common causes of plant down time, and a major contributor to excessive belt wear, idler failures and material spillage, as discussed by Scott and Choules in Mining industry belt conveyor transfer station design. Non-central discharge often also contributes to lateral size segregation on the receiving belt, which may present challenges in product quality control.

Commandment #3: The in-line component of the stream velocity at the discharge point shalt match, within ±10 per cent, the outgoing belt velocity.

Handling issues such as excessive abrasive wear and spillage often arise from large gradients in velocities between the incoming bulk material stream and the outgoing belt. Therefore, aiming at matching these velocities as closely as possible helps to minimise these issues, and reduces power requirements to accelerate the material to the outgoing belt velocity.

In the case of transfers between a feeder and a conveyor for example, transfer chutes act as an acceleration device to transfer from a slow-moving feeder to a high-speed belt conveyor. Transfers may also be used to change stream direction or, in very long conveyor systems, to address terrain constraints. Those situations present challenges related to both Commandment #2 and Commandment #3, but, as a general guide, the chute should be designed so that the velocity component of the bulk material in the direction of travel of the outgoing belt matches that belt’s velocity as closely as possible. This will ensure that there is little relative motion between the belt surface and the material stream, which will, in turn, reduce belt wear.

Commandment #4: The normal component of the stream velocity at the discharge point shalt be as low as possible.

Minimising the normal component of the impact velocity at the loading point on the outgoing conveyor is necessary to prevent impact wear and damage to the belt and idlers. Doing so also reduces dust generation and spillage caused by material re-bounding. In the case of a typical bulk material that exhibits non-negligible (and often quite high) friction and adhesion characteristics, discharge with some normal vector to the receiving belt is generally required so that the demands of Commandment #5 and #6 are met. It is therefore important that this tradeoff be carefully considered and mitigated by appropriate impact support and interfacing design.

Commandment #5: The slope and cross-section of the chute shalt be sufficient to achieve accelerated flow with appropriate volumetric clearance at the design throughput.

Transfer chutes are volumetrically limited devices and thereby the maximum mass throughput that may be achieved is dependent on the bulk density of the material being transported. By application of a chute analysis technique, such as the continuum mechanics method or DEM modelling, the cross section of the transfer may be designed to provide appropriate clearance for a given stream velocity. As a guideline, the ratio between chute width (B) and material stream thickness (H) shall be smaller than unity, as shown in Figure 2.

Figure 2 – Relationship between stream thickness (H) and chute width (B) for accelerated flow

Figure 2 – Relationship between stream thickness (H) and chute width (B) for accelerated flow

Commandment #6: Chute slopes shalt be at least equal to the wall friction angle plus five degrees.

Meeting all design criteria simultaneously is often a challenge, but as a general guidance the chute slope should be calculated based on the wall friction angle with a safety margin of about five degrees for the chute to be self-cleaning. As stated previously, the wall friction angle is often measured at low consolidation pressures in the order of about 0-5 kPa, but this requires selection on a case by case basis. A well-designed transfer chute will generally involve a progressively changing wall angle to ensure accelerated flow and minimise wear throughout the system.

Commandment #7: Impact angles within the transfer chute shalt be minimised.

Although Commandment #4 addresses impact specifically onto the receiving belt, this is a consideration that can be translated into other sections of the transfer as well.

Maintaining low impact angles within the transfer (15-20 degrees or lower) will reduce wear, minimise material dispersion and dust generation, as well as reduce any losses of momentum in the material stream which can lead to unfavourable flow patterns.

Commandment #8: Precautions must be taken in the discharge zone to minimise spillage.

Material spillage is a serious issue not only from a production point of view but is also a safety hazard and an environmental issue.

Ensuring that the chute and receiving belt are interfaced correctly to satisfy Commandment #3 is key to avoiding a material deceleration zone with material ‘choking’ at the exit, while minimising impact according to Commandment #4 is also imperative.

Lastly, the appropriate use of skirt plates (including selection of skirt material and configuration) can go a long way in preventing material spillage issues.

Commandment #9: Free-fall zones, or zones of high acceleration, in the chute configuration shalt be kept to a minimum.

Controlling free-fall heights leads to lower impact pressures, which assists in minimising impact wear, degradation and dust. Additionally, higher normal stresses caused by impact affect material behaviour and may lead to operating conditions for which the chute is not appropriately designed.

Another point to note is that impact pressure may also lead to consolidation of the stream, potentially causing material build-up and a reduction in effective cross-sectional area. In cases that free-fall cannot be avoided, care must be taken in design to properly consider impact angles to efficiently redirect a high-speed stream.

Commandment #10: The slope of the fines (scraper/dribble) chutes shalt be of adequate inclination to prevent any material hang-up occurring i.e. dribble chutes must be self-clearing.

The material that is captured by the belt scraper is very different from the mainstream being conveyed. This is mostly due to the different particle size: it is a much finer and wetter material, which often exhibits significant cohesion.  Therefore, in line with Commandment #1, the inclination of the dribble chute shall be determined based on the flow properties of that finer material. The dribble chute may be lined with materials with the lowest friction coefficients at lower pressures, especially considering that wear is not generally a concern in this region.