The loading and discharge of conveyor belts is the area where many, if not most, of the problems in solids conveying occur. In most cases, inadequate design of the transfer chutes is the main cause of such problems.
Although the concept of a chute may look simple, proper geometric design is very important to ensure that flow conditions in the chute remains stable and appropriate. In addition, there are some critical factors which are paramount in chute design. These are:
- Chute is not prone to blockage.
- Reduction of impact on the chute faces to minimize wear of the chute surfaces.
- Reduction of impact on downstream belt to minimize belt wear.
- Centralised loading onto the downstream belt.
- Minimum material degradation.
- Minimum dust production.
In order to achieve the above objectives, several geometric rules-of-thumb have been developed over the years. However, in many cases such design exercise is affected by additional factors such as limitations on head room, variations in lump size or other material’s properties. The designer is often forced to compromise on certain design criteria which in turn can lead to serious performance problems.
With the tremendous advances in computing power over the past two decades, mathematical modeling of granular flow using the Discrete Element Method (DEM) has become a safer way to ensure that a chute geometry will be right under the various expected flow conditions.
DEM Bulk Flow Simulation
Engineered-flow transfer chutes are developed using a 3-D parametric design procedure that starts with a precise definition of the conveyed material. Testing samples of the bulk material provides important data characteristics such as density, abrasiveness, moisture content, lump size, fines size, levels of cohesion and adhesion, and the material’s angle of repose.
The second step is to build a 3-D parametric model of the transfer point. Some of this data can be determined from a review of the site plans and conveyor specifications or in the case of a retrofit project, from a 3-D laser survey of the facility (Figure 1).
Fig.1: Point cloud survey
Such data is used to create the bounding geometry for the bulk-flow simulation. Typically this includes a feed conveyor belt, a receiving belt, and the transfer chute (Figure 2).
Fig. 2: Bounding geometry 3-D model
Computing a DEM simulations of the 3-D model require anywhere from 10 minutes, to several days. However, a typical workstation computer is capable of solving a reasonable flow simulation within a few hours.
The output of the calculation is post-processed into a 3-D animation showing the flow of material in the chute system (Figure 3). A typical analysis will simulate the transient period where the empty chute is filling and the steady-state flow that follow. Other transient simulations are possible such as emptying of a silo, belt acceleration, etc.
Fig. 3: Computer generated material flow
Because of the way it is programmed, the DEM method lends itself very well to predicting all important results of the simulation, including particle velocity, particle-to-particle force, particle-to-surface force, flow data, wear on transfer chute liners, conveyor belts, and other surfaces.
Using DEM, the engineer can qualitatively compare different transfer chute designs and select the chute geometry that provides the minimum amount of wear, dust generation, and material abrasion to optimize the entire design.