Design of a High Speed Transfer Chute in a confined Space – A DEM Case Study

By A. Edilbert , R. Spaargaren, C. Geijs, J. Ruijgrok, G. Lodewijks, D. Schott This paper aims to redesign a transfer chute as used in a Eurosilo system specifically for handling coal. The velocity of material vertically entering the chute was estimated at 15 to 34 m/s depending on the fill height of the silo. A three-stage approach was taken. First the existing situation with the highest velocity was modelled with DEM (Discrete Element Simulations), and the results of the wear profile on the existing chute were compared with practice. Secondly, three configurations were designed and the performance of these configurations in terms of wear volume were tested by simulation. Thirdly, and lastly, the performance of the configuration in terms of ensuring material flow through the chutes for a cohesive coal was evaluated. Based on the results of the simulation, two chute designs showed an improved performance where less wear was observed while securing flow through the chute.
(From the archive of ”bulk solids handling“, article published in Vol. 37 (2017) No. 4 , ©2017 bulk-online.com
)

1. Introduction
Fig. 1: Eurosilo system. (Picture: © Eurosilo b.v.)

In a Eurosilo system handling coal (Fig. 1), the coal is fed into the silo by means of infeed conveyors on the top of the silo (the direction of coal flow is shown in Fig. 1 with red arrows). The conveyor at the top transfers the coal on to a transfer chute, down through a telescopic chute and is stacked by means of screw augers. These screw augers along with their frames (in green) can be lifted or lowered by a winch system. The auger frame is rotated about the silo axis by rotating the swivelling bridges below the silo roof.

The transfer between the telescopic chute and the screw conveyor is achieved by means of a stone box chute with a wear plate, which changes the direction of the coal towards the screw auger (see Fig. 1). The material stream needs to be controlled and directed to flow towards the zone between the screw augers. The reason is that the screw augers acting over a free surface need a dead zone of material (shaded lines in Fig. 2) between them to propel the material (indicated in large dots) for stacking.

Fig. 2: Material dead zone between screw augers. (Picture: © Eurosilo b.v.)

The current transfer chute design in the Eurosilo is a rockbox chute wherein the material forms a bed of particle with a slope and the material starts sliding beyond the slope. The rockbox chute is preferred when the material is very abrasive or for transfers with high rate of discharge when the chute material is expected to wear off quicker. The highest velocity for which any transfer chute has been designed is 15 m/s [1]. In the Eurosilo application the velocity of the materials varies between 15 m/s to 34 m/s at the retracted and expanded position, respectively. No studies have been found on design and analysis of chute design such a high velocity transfer in the bulk material handling field.

The wear-plate-design and the wear plate after a number of years are presented in Figs. 3 and 4. Improved design of this wear plate would eradicate the downtimes due to replacement of the plate and could increase the performance of the machinery inside the silo.

Fig. 3: Drawing of the wear plate. (Picture: © Technical University Delft)

Fig. 4: Photo of failed wear plate. (Picture: © Technical University Delft)

The objective of this study is to improve the design of the wear plate chute that facilitates the transfer of coal from the telescopic chute to the screw auger. To achieve this, available literature for designing a new chute is studied and a virtual environment is developed to simulate the interaction between coal and the wear plate. Three chute designs are developed and the performance of these chute designs in terms of wear and flow are analysed using DEM.

2. Literature
2.1. Wear

In general, wear is defined as the removal of material from a solid surface as a result of mechanical action [2]. Archard derived that the total wear volume Vw in the form of a generalized equation applicable for all available wear mechanisms (adhesive, abrasive, corrosive and fatigue wear) and is given by [3]

(1)

where:

K = wear coefficient between the sliding surfaces [-]
H = hardness of the softer material at the contact [-]
Fn = normal contact force [N]
L = sliding distance [m]

From a thermodynamic point of view, the resultant wear due to the sliding and non-sliding contact is mainly due to the frictional energy dissipated. This is mainly proportional to the applied load and the velocity in case of non-sliding contact and the applied load and sliding distance in case of a sliding contact. The material characteristics, relative velocity, size and shape of the materials also influence the distribution and the dissipation of energy leading to wear. The works of Mindlin and Deresiewicz [4] provided a major advancement in the study of energy dissipation under non-sliding contact. A lot of research has been done to experimentally confirm Mindlin and Deresiewicz’s theory of surface damage due to energy dissipation. It was concluded that the wear volume varied linearly with the energy dissipation during a non-sliding contact (Fig. 5) [5].

Fig. 5: Wear volume as a function of cumulative dissipated energy [5]. (Picture: © Technical University Delft)
Fig. 6: Wear volume as a function of work done by contact force due to sliding contact [6]. (Picture: © Technical University Delft)

For sliding contact, a similar energetic approach to wear established the correlation between energy dissipated by friction force and the wear volume. The experimental results as shown in Fig. 6 [6] are based on the extension of Archard’s wear theory. It shows the linear relation between wear rate and work done by contact force. The tangent of the line gives the estimation of the wear coefficient.

2.2. Chute Design

Conventional chute designs are mainly performed with the considerations of accelerated flow assuming that the material is always in contact with the chute bottom and side walls [7]. Continuum approach has been used to model the flow of bulk particles within the coal and bulk solid motions are described by a lumped parameter model [8]. For any chute design for transfer of bulk materials under gravity, the basic design principles to be considered are as follows [7].

  • Prevent plugging at impact points.
  • Ensure that the bulk material accelerates or decelerates sufficiently to match the receiving equipment’s speed.
  • Control stream of particles.
  • Minimizing abrasive wear on the chute surface.
  • Control generation of dust.

For a rockbox chute design specifically, only a few design principles are shallowly discussed in an online forum [9].

Design of transfer chutes using the lumped parameter model, as developed by Roberts [10, 11, 12, 8, 13] has been widely used for different case scenarios. Modelling the particle stream using a continuum approach is on a macroscopic scale in steady state, which makes it difficult for the engineers to conceptualize or visualise complex situations. The properties of bulk solids tend to vary throughout the process which makes it difficult to analyse the design for situations such as cohesion, wear, handling multiple commodities etc. Hence defining very general continuum models applicable for many situations is tedious [14].

3. Method

In recent times, Discrete Element Method has become a widely accepted technique for addressing engineering problems related to the flow of bulk solid materials. The limitation of continuum methods to account for local variation in particle concentration and localized flow behaviour can potentially be modeled using DEM [15]. By now, a vast number of researchers have recommended and used DEM for optimizing and improving design of bulk material handling equipment [15]. In this work a three-stage approach is used:

  • Stage 1: Modelling As-Is situation and comparing wear patterns found from DEM simulations with practice.
  • Stage 2: Design of three configurations and test their performance against wear in the scenario with the highest material velocities (when the telescopic chute is in extracted position).
  • Stage 3: Test the best performing configurations on flow performance in the scenario with the lowest material velocities (when the telescopic chute is in retracted position).
3.1. System Model

The infeed conveyor to the telescopic chute has a capacity of 1600 t/h, with a bulk density of 800 kg/m3 and a belt speed of 2.6 m/s. In reality the wear plate and chute box are attached to the auger frame, and rotate about the silo axis to spread the coal inside the silo. The distribution of particles over the cross-section of the telescopic chute is not homogeneous as the particles are fed by a belt conveyor at the top from one side. Therefore the simulation is done for 4 discrete orientations (0°, 90°, 180° and 270° about silo axis) to understand the wear pattern in the chutes at different positions.

The retracted height of the telescopic chute is 10 m, and the longest (extracted) height is 61 m. The respective maximum particle velocities in vertical direction are 15 and 35 m/s.

3.2. Stage 1: Modelling As-Is Situation

The Hertz-Mindlin (no-slip) contact model was used with EDEM 2.6.1 for particle-particle contact (p-p) and extended with Archard wear for the contact between particles and geometry (p-w). The latter involves both the concepts of Mindlin and Deresiewicz and Archard (Eq. 1 in section 2) and records the energy dissipation at the surface and the wear volume.

Table 1: DEM input properties – Coal & Steel, partially based on [16][17][18].
The input properties used are given in Table 1. The particles are spherical and the distribution in size has a minimum diameter of 5 and a maximum of 50 mm. Both these sizes present 20% of the number of particles. The wear pattern varies with every position of the auger frame, but by overlapping the pictures of each of the positions the concentration of wear in the simulations becomes alike the real wear plate (Figs. 7 and 8).

Fig. 7: Simulated wear (superposed from 0, 90, 180 and 270 deg). (Picture: © Technical University Delft)
Fig. 8: Real wear profile work done by contact force due to sliding contact [6]. (Picture: © Technical University Delft)
3.3. Stage 2: Three Transfer Chute Designs

The limitation in space is given by the auger and auger frame dimensions which are optimised for capacity. Therefore the maximum chute height can be 2.54 m. The three designed concepts are shown in Fig. 9. Their performances, simulated with identical settings as Stage 1, are discussed in Section 4.

Fig. 9: Transfer chute concepts. (Picture: © Technical University Delft)

Evaluation of the concepts will be done by assessing the three key performance indicators: Wear volume, discharge velocity and the totalled normal and tangential cumulative energy.

3.4. Stage 3: Material and Contact Model to assess Flow

One of the key aspects of chute design, to have a smooth material flow without blockage, should never be overlooked regardless of the nature of flow of bulk materials. To test the flow of a truly cohesive material through the chute Hertz-Mindlin with JKR contact model was used. This contact model enables to model a material with a strong cohesive behaviour at low contact forces.

Since there are no determined methods to calibrate the cohesive energy and the translation to the flowability of bulk solids, a qualitative calibration method is followed. A very cohesive material was modelled according to Hill’s classification [16] by trial and error. A poured angle of repose of 60° was achieved with a cohesive energy of 35 J/m2. Other material properties remained unchanged. First the particle bed formation in the stagnant zones of the designs is created by assigning high cohesion energy (500 J/m2) to the particles in rest and 400 J/m2 to the coal–chute interaction. Then a new simulation is started with cohesive energy of 25 J/m2 with the particles in rest and 35 J/m2 with the newly generated particles.

4. Results
4.1. Performance of designed Chutes

Wear profiles of the three designs are shown in Fig. 10. Horizontally the three designed concepts are shown with vertically the four simulated orientations. It can be clearly seen that different wear zones exist with different orientations of the wear plate under the telescopic chute.

Fig. 10: Wear profiles of the three transfer chute designs for 4 different rotations. (Picture: © Technical University Delft)

Table 2 shows that all three chute designs experience less wear compared to the current design. A variation in the different orientation of the telescopic chute can be seen; at 90° the reduction compared to current design is the least. It is found that the chute with modified wear plate with ribs exhibits reduced wear because of the formation of material bed between ribs and material on material interaction, while in the chute with multiple rockboxes, energy is dissipated over a wider surface area rather than material on material interaction.

Table 2: Maximum wear depth compared to existing chute.

The total energy dissipated (Table 3) is considerably reduced at the modified wear plate design. This is mainly due to the formation of the particle bed between the ribs. In case of chute with central opening the total energy dissipated at the surface is not as good when compared to the results of the modified wear plate. Whereas with the chute with multiple rockboxes, the total energy dissipated is much higher than other designs.

Table 3: Total energy dissipated at the geometry surface compared to existing chute.

From Table 4, it can be seen that the discharge velocities of modified wear plate and the chute with multiple rockboxes are also considerably lower when compared to the current design and the chute with central opening. The low velocities of the particles in the chute with mini rockboxes can be attributed to the total energy dissipated in the chute by the particles sliding horizontally after the first impact and dissipating low energies multiple times as they slide down the rockboxes for positions 0 and 180°.

Table 4: Discharge velocity compared to existing chute.

Velocities are considerably higher at 90° and 270° positions because at 90°, the particles fall closer to the outlet of the chute. In case of the modified wear plate, the velocities are normalized to a range of 3.5 m/s for 0°, 180° and 270° positions due to the material on material impact while at 90° the velocity is higher at 4.93 m/s owing to the proximity towards the outlet. Based on the chosen KPI, the maximum wear depth, the chute with modified wear plate and the chute with multiple rockboxes are most promising. However, the performance of the chutes with respect to flowability when the telescopic chute is in retracted position needs to be investigated.

4.2. Stage 3: Flow

Fig. 11 shows the difference in spatial distribution of the particle flow in extracted and retracted position of the telescopic chute. The bulk material flow is concentrated on one side inside the telescopic chute and the impact location on the rock box chute varies with the telescopic chute rotation of 360°.

Fig. 11: Topview on chute with spatial distribution of particle stream for retracted and extracted telescopic chute. (Picture: © Technical University Delft)

In Figs. 12 and 13 the flow through the chute with mini rockboxes and the modified wear plate with ribs is illustrated for cohesive material. Overall, it can be seen that the particle flow is ensured even if there is an agglomeration happening as much as the particle build up. For transfer of materials at high velocities (>15 m/s), chutes with rockbox concepts seem to be a best option. In Eurosilo application, where the impact pressure and the energy of the particle stream is very high, the material flow can be maintained if there is enough cross-sectional area at the point of lowest velocity of the bulk material and maintaining particle-particle contact to reduce wear.

Fi. 12: Flow in retracted position of the telescopic chute with mini rockboxes. (Picture: © Technical University Delft)

Fi. 13: Flow in retracted position of the telescopic chute with modified wear plate. (Picture: © Technical University Delft)
5. Conclusions

The aim of this study was to improve the design of the wear plate/chute in the Eurosilo. Two designs have shown significant improvement based on DEM simulations of a single type of coal. These designs have lesser wear rates at the most extreme situation (extracted telescopic chute) while maintaining flow when the incoming energy is the lowest (retracted chute). Further work is required on testing different kinds of materials with the design in the DEM simulation environment. However, to ensure realistic results quantitative calibration of the material properties is required. 

Acknowledgement

This article is based on a presentation of the authors during Schüttgutfördertechnik – (Markt-)Platz für Innovationen, 27./28. September 2017 in Magdeburg, Germany

References:
  1. Phoenix Conveyor Belt Systems GmbH. (2016) Fastest Conveyor Belt. Retrieved November 15, 2016, from Phoenix Conveyor Belts Website: http://www.phoenix-conveyorbelts.com/pages/world-records/fastest/fastest_en.html
  2. Rabinowicz, E.: Wear; Material Science and Engineering, Vol 25 (1976), pp. 23-28.
  3. Archard, J.: Wear Theory and Mechanism. In: W.M.B. Peterson (Ed.), WearControl Handbook. New York, (1980).
  4. Mindlin, R.D., and H. Deresiewicz: Elastic Spheres in Contact under varying Oblique Force; Trans. ASME, J. Appl. Mech. 20, (1953), p. 327.
  5. Fouvry, S., P. Kapsa, H. Zahouani, and L. Vincent: Wear Analysis in Fretting of hard Coatings through a dissipated Energy Concept; Wear, 203–204 (1997), pp. 393-403.
  6. Ramalho, A., and J. Miranda: Relationship between Wear and dissipated Energy in sliding Systems; Wear, (2006), p. 361.
  7. Stuart Dick, D., and T. Royal: Design principles for chutes to handle bulk solids; Bulk Solids Handling 12, No. 3, p. 447.
  8. [8] Roberts, A.: Chute Design Considerations for Feeding and Transfer; Beltcon 10, International Materials Handling Conference, Johannesburg, South Africa (2001).
  9. The Powder/Bulk Portal – Forum. Thread: Rockbox parameters. Retrieved October 2016, from Bulk-Online: The Powder/Bulk Portal: http://forum.bulk-online.com/showthread.php?12853-Rockbox-Parameters. (Feb. 2008).
  10. Roberts, A.W.: An Investigation of the Gravity Flow of Noncohesive Granular Materials Through Discharge Chutes; J. of Engineering for Industry (68-MH-5), (1969), pp. 373-381.
  11. Roberts, A., and P. Arnold: Discharge-Chute Design for Free-Flowing Granular Materials; Transaction of the ASAE 14 (1971) No. 2, pp. 304-312.
  12. Roberts, A., and O. Scott: Flow of Bulk Solids through Transfer Chutes of Variable Geometry and Profile; bulk solids handling Vol. 1 (1981) No. 4, pp. 715-727.
  13. Roberts, A.: Chute Performance and Design for rapid Flow Conditions; Chemical Engg. & Techn., 26 (2003) No. 2, p. 163.
  14. Orlando, A.D., and E.P. Maynard: Using Discrete Element Method Software to design Bulk Solids Handling Equipment; Powder and Bulk Engineering, June 2016.
  15. Grima, A.P.: Quantifying and Modelling Mechanisms of Flow in cohesionless and cohesive granular Materials; PhD-thesis. Univ. of Wollongong, Wollongong, NSW, Australia (2011)
  16. Hose, R.S.: RBCT Phase V expansion project case study and focus on belt feeder using DEM; BeltCon 15, Johannesburg, South Africa (2011).
  17. ASM (2016). AISI Type 304 Stainless Steel Datasheet. Retrieved September 2016 http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MQ304A.
  18. Teffo, V., and N. Naudé: Determination of the Coefficients of Restitution, static and rolling Friction of Eskom-grade Coal for Discrete Element Modelling; The Journal of The Southern African Institute of Mining and Metallurgy, 113 (2013), pp. 351-356.
  19. Hill, L.: Bulk Solids Handling: an Introduction to the Practice and Technology. New York: Chapman and Hall, (1987).
About the Authors
Antony Edilbert
Faculty of 3mE, Dept. of Marine & Transport Technology
Delft University of Technology, The Netherlands
Richard Spaargaren
Sales Director
ESI Eurosilo B.V., The Netherlands
Chris Geijs
Project Engineer
ESI Eurosilo B.V., The Netherlands
Jaap Ruijgrok
Manager Business Development
ESI Eurosilo B.V., The Netherlands
Gabriel Lodewijks
Faculty of 3mE, Dept. of Marine & Transport Technology
Delft University of Technology, The Netherlands
Dingena Schott
Faculty of 3mE, Dept. of Marine & Transport Technology
Delft University of Technology, The Netherlands

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