How the calculation works
- Capacity = 60 × screw cross-section × pitch × speed × filling degree × inclination factor, in m³/h.
- Cross-section = π/4 × (D² − d²). For a shafted screw the calculator assumes a shaft of 0.3 × D; a shaftless spiral uses the full trough cross-section.
- Inclination factor: the capacity falls by about 2 % for every degree of inclination.
- Mass flow = capacity × bulk density.
Typical values for wastewater plants
| Material | Bulk density | Filling degree | Usual screw |
|---|---|---|---|
| Screenings | 700–900 kg/m³ | 20–30 % | Shaftless spiral |
| Dewatered sludge cake | 900–1,100 kg/m³ | 25–40 % | Shaftless spiral |
| Grit | 1,400–1,700 kg/m³ | 15–30 % | Shafted or shaftless, wear-resistant |
| Dry bulk solids such as lime | 500–1,000 kg/m³ | 30–45 % | Shafted screw |
Limits of this calculator
The filling degree and the permitted speed depend on how the material behaves: sticky, fibrous and abrasive materials need lower values. The calculator does not check the drive power or the torque. Our screw and belt conveyors are sized for the material, the conveying length and the inclination of your plant.
Frequently asked questions
How is the capacity of a screw conveyor calculated?
Multiply the free cross-section of the screw by the pitch and the speed, then by the filling degree and a factor for the inclination. The result is the volume conveyed per hour.
When is a shaftless screw conveyor used?
For sticky, fibrous or lumpy materials such as screenings and dewatered sludge. Without a centre shaft there is nothing the material can wrap around or block.
How fast should a screw conveyor run?
Shaftless spirals for screenings and sludge usually run at 10–30 rpm to limit wear on the liner. Shafted screws for free-flowing bulk solids run faster, typically 30–80 rpm.