Understanding the root causes of the problem, the design basics and main considerations are of utmost importance.
Main chute problems
Chutes are frequently overlooked during the design stage of the conveying systems, which can lead to the following issues:
- Material build-up and blockages causing unscheduled stops
- Material spillage and degradation
- Health and safety risks for operators when deblocking or housekeeping spilled material and carryback
- Dust emission causing unacceptable contamination or environmental conditions
- Premature belt wear, misalignment and damage in the downstream conveyor
- Premature lining wear
- Higher power consumption
Main chute design considerations
Inadequate cross section of the drop chute may lead to material blockages. As a rule of thumb, this cross section should be between three to four times the cross section of the material fed into the chute to avoid these blockages.
Final sizing will depend not only on the material characteristics, such as flowability and lump size, but also on past field experience with similar materials.
Material flow assessment and control
Chute geometry, material trajectory, material impact angle, liners, starting/entry and exit velocities, accelerations and impact pressures should be designed, calculated, and evaluated to:
- avoid any plugging due to very low material velocities at the impact points or too shallow surfaces and valley angles.
- establish proper/smooth change of flow directions, stream control and adequate material accelerations, avoiding stagnant points and dust generation.
- establish rock boxes or liner type needs.
- give a velocity magnitude and a direction to the exit stream of material as close as possible to the travel direction and velocity magnitude of the downstream or receiving conveyor belt, thus avoiding premature belt wear and damage.
- centre the exit stream of material on the belt of the downstream or receiving conveyor to avoid belt misalignment.
Dust and spillage control
Dust is inherent to solids due to the fracture of material when handled and processed. When disturbed by impact and air streams, dust becomes airborne with consequences that include contamination, safety risks and health issues.
Dust control is improved with proper design and by implementing active or passive dust control measures.
The main measures used to minimize dust generation, emissions and spillage are:
- Reduce the free fall to minimize dust generation at the impact zone.
- Minimize air passing through the chute by installing rubber curtains at the material entrance, seals at the belt return and seals and curtains in the skirtboard area.
- Minimize stream disturbance by keeping loading and receiving conveyors as aligned as possible and by aligning the material onto the receiving belt in the belt travel direction.
- Design the skirtboard cross section and length to keep air velocity low (˂ 1m/s, 200fpm), thus allowing airborne dust and material to settle down before leaving this area.
- Design the loading point and tailbox to adequately absorb impact energy, while preventing seal loss against the belt.
- Install, if necessary and possible, dust suppression and dust collection systems.
- Implement, if adequate and possible, a hood and spoon chute design.
- Install adequate belt cleaners.
When dusts are determined to be explosive, specific NFPA standards and local regulations, as per the authority having jurisdiction (AHJ), must be applied to prevent and mitigate the risks of fires and explosions.
Discrete element modelling applied to chutes
Discrete element modelling (DEM) is a methodology used to compute the movement and interaction of a large quantity of particles. The development of powerful computational systems has allowed easy access to DEM software that are currently used for diverse applications in various fields and industries, including that of solids handling.
In the case of chutes, DEM provides a preview of chute performance and material behaviour when passing through, thus giving the designer the opportunity to adjust the design and avoid any of the above-mentioned problems.
There is a caveat: the material shape, properties and their calibration are crucial to getting accurate results.