Screw conveyor capacity calculation: formula, example and calculator

The variables behind capacity

The hourly capacity of a rigid screw conveyor (tubular or U-trough) is set by:

• Screw diameter (D): the outside diameter of the flight. Capacity rises with the square of the diameter.
• Shaft diameter (d): the central shaft takes up part of the cross-section.
• Pitch (S): the distance the flight advances per turn.
• Speed (n): revolutions per minute.
• Fill level (φ): how much of the casing cross-section runs full of material.
• Inclination factor (C): as the incline rises, material slips back and capacity falls.
• Bulk density (ρ): converts volume flow into mass flow.

Formula

Volume flow (m³/h) ≈ 60 × (π / 4) × (D² − d²) × S × n × φ × C

Mass flow (kg/h) ≈ volume flow × bulk density (kg/m³)

D, d and S are in metres and n in revolutions per minute. If the shaft diameter is unknown, set d = 0 for a rough upper figure.

The fill level is chosen by material type. Approximate values commonly used in the literature:

Common approximate inclination factors: 1.0 horizontal; 0.9 at 5°; 0.8 at 10°; 0.7 at 15°; 0.65 at 20°. Steeper inclines and vertical screws need a separate design.

Calculator

Enter your own values in the screw conveyor capacity calculator at the top of this page to see volume and mass flow instantly. The calculator uses the formula above; the result is approximate and does not replace proper sizing.

Worked example

To show the arithmetic, take hypothetical values (they are not the specification of an Erk product):

1. Cross-section: (π / 4) × (0.20² − 0.06²) ≈ 0.0286 m²
2. Theoretical volume: 60 × 0.0286 × 0.20 × 60 ≈ 20.6 m³/h
3. With fill level: 20.6 × 0.30 ≈ 6.2 m³/h
4. Mass flow: 6.2 × 550 ≈ 3,400 kg/h

At a 10° incline (C = 0.8) the same screw would convey about 4.9 m³/h, or about 2,700 kg/h.

Incline and vertical conveying

As the incline rises, material slips back between the flights and less of it advances at the same speed. Inclined lines therefore need either a larger screw or a higher speed. In vertical screws the material rises through a different mechanism, so the formula above does not apply directly.

How is flexible screw capacity determined?

The formula is for rigid screws turning on a shaft. In a flexible screw conveyor a shaftless spiral turns in a flexible tube; capacity depends on the spiral profile, tube diameter, speed, the horizontal and vertical sections of the route and the material’s behaviour. So flexible screw throughput is set more by the manufacturer’s experience and material tests than by a formula. For a flexible screw the calculator gives only a rough idea; Erk confirms the exact figure for your material.

What does motor power roughly depend on?

The power a screw needs depends on the quantity conveyed, the line length, the height difference and how much friction and resistance the material creates in the screw. Abrasive, sticky or dense materials need more power at the same throughput. Start-up, running fully loaded and drive losses must also be allowed for. We select the motor at quotation stage from your material and route details.

What to send for a quotation

• The material and its approximate bulk density
• Required hourly throughput (kg/h or m³/h)
• Line length, incline and any vertical sections
• Inlet and outlet points, and the silo, hopper or machine they connect to
• Abrasiveness, moisture condition and temperature of the material

Erk PVC Makine builds tubular, U-trough and flexible screw conveyors and the screw feeder in its own factory. Which type suits you is explained in types of screw conveyors.