Mechanical Conveying of Bulk Solids
Selection & Sizing for Screw, Belt, Vibrating & Bucket Conveyors
| Page Contents | |
|---|---|
| 1. Definition of Mechanical Conveying | 4. Aero-Mechanical Conveyors |
| 2. Screw Conveyors (with Calculator) | 5. Belt Conveyors |
| 3. Vibrating Conveyors | 6. Bucket Elevators |
1. Definition of Mechanical Conveying
Mechanical conveying is the transport of bulk solids (powders, granules, pellets) using moving physical components. This is distinct from pneumatic conveying, which uses air or gas as the transport medium.
Selection of a conveyor depends on capacity requirements, distance, material sensitivity, and dust containment needs. The table below summarizes key selection criteria:
| Equipment | Capacity | Distance | Material Fragility | Clogging Risk | Containment |
|---|---|---|---|---|---|
| Screw Conveyor | High (0-450 m³/h) | Short (<10m) | High Impact | Good | Excellent |
| Vibrating Tube | Medium | Short (<6m) | Low Impact | High Risk | Excellent |
| Aero-Mechanical | Low-Med | Medium (<50m) | Low Impact | Moderate | Excellent |
| Belt Conveyor | Very High | Very Long | Low Impact | Good | Poor |
| Bucket Elevator | Very High | Vertical (<50m) | Moderate | Moderate | Moderate |
2. Screw Conveyor Sizing & Capacity
The capacity of a screw conveyor depends on the screw diameter, pitch, shaft diameter, rotation speed, and the degree of trough loading (filling level). The theoretical volumetric capacity \( V \) is calculated as:
\[ V = 60 \cdot \frac{\pi}{4} \cdot (D^2 - d^2) \cdot P \cdot n \cdot \lambda \]Where:
\( D \): Screw Outer Diameter [m]
\( d \): Shaft Diameter [m]
\( P \): Pitch [m]
\( n \): Speed [RPM]
\( \lambda \): Filling Level [-]
Interactive Screw Conveyor Calculator
Practical Engineering Rules of Thumb
- Filling Levels (CEMA): 15% for highly abrasive materials; 30% for standard non-abrasive powders; 45% for light, free-flowing materials.
- ATEX Safety: Maintain flight tip speed below 1.0 m/s to minimize frictional ignition risks in potentially explosive atmospheres.
- Inclination: Capacity drops significantly with inclination. A 45° incline may reduce capacity by up to 75% compared to horizontal.
- Critical Speed: Do not exceed 70% of the material's critical speed to prevent centrifugal "fluidization" within the screw.
3. Vibrating Tubes
Vibrating conveyors utilize electro-mechanical vibrators or magnetic drives to move materials. They are ideal for sanitary applications as the conveying tube has no moving parts in contact with the product.
- Distance: Up to 6m per unit.
- Safety: Inherently safe due to lack of exposed rotating shafts.
- Constraint: Material must be non-clogging and have a reasonable friction coefficient with the tube wall.
4. Aero-Mechanical Conveyors
Also known as "cable and disc" conveyors, these use discs attached to a cable running at high speed within a tube to create pockets of air and material.
- Benefit: Gentle handling of fragile granules.
- Layout: Flexible, can handle vertical and horizontal runs in a single loop.
- Risk: Cable/chain wear leading to metal contamination if not monitored.
5. Belt Conveyors
Standard for high-capacity, long-distance transport. Typical in mining and heavy bulk handling.
- Speed: Typically 1.5 to 3.0 m/s.
- Maintenance: Requires tracking sensors and belt cleaners.
- Containment: Difficult to make dust-tight without expensive enclosure systems.
6. Bucket Elevators
Designed for vertical lifting of bulk solids in confined spaces.
- Capacity: High throughput in a small footprint.
- Safety: High dust explosion risk. Explosion venting and belt alignment sensors are mandatory under NFPA/ATEX guidelines.