The path the bulk material takes as it is discharged from the delivery conveyor is called the trajectory. Trajectory is affected by the speed of the belt, the angle of inclination of the discharging belt and the profile of the material on the belt. In conventional transfer-chute design, the trajectory is plotted and used as a starting point for estimating where the material stream will first impact the head chutewall. From there, the material stream is assumed to be reflected from the chutewall much like a light beam being bent with a series of mirrors. CEMA's Belt Conveyors for Bulk Materials, Sixth Edition, provides a detailed discussion of calculating and plotting material trajectories.
The most common mistakes made at this stage of design are developing an incorrect initial material trajectory and failing to consider the effects of friction when plotting subsequent reflections of the material stream from the transfer chutewalls.
The current thinking in transfer-chute design is to control the stream of bulk material and not allow it to free-fall from the discharge to the receiving belt. With this controlled approach, the designer assumes the material cross section does not fan out or open up significantly. Drop heights are minimized to help reduce material degradation, dust creation and wear on the receiving belt.
This approach requires some knowledge of the friction values between the bulk material and transfer chute materials. Discrete Element Modeling method is being used in conventional chute design as an aid to the designer in assessing the effects of changing properties, such as the coefficient of friction. There are several DEM software packages on the market designed for this purpose.
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