How silo capacity is calculated
Most bulk silos are a cylinder on top of a conical hopper. The calculator adds up both parts:
hhopper = (D − d) ÷ 2 · tan α
Vhopper = π · h ÷ 12 · (D² + D·d + d²)
Vcylinder = π ÷ 4 · D² · H
Capacity = (Vcylinder + Vhopper) · ρ
Worked example
A silo with 4 m diameter, 10 m cylinder height and a 60° hopper down to a 0.3 m outlet has a hopper 3.20 m tall holding 14.5 m³. The cylinder holds 125.7 m³. Together that is 140.2 m³, or 105 t of wheat at 0.75 t/m³.
Bulk densities of stored materials
| Material | Bulk density (t/m³) |
|---|---|
| Wheat | 0.72 – 0.82 |
| Maize | 0.68 – 0.75 |
| Barley | 0.60 – 0.70 |
| Cement | 1.1 – 1.4 |
| Fly ash | 0.7 – 0.9 |
| Wood pellets | 0.60 – 0.70 |
| Plastic granulate | 0.5 – 0.7 |
Design notes
Mass flow or funnel flow. In a steep, smooth hopper all material moves when the outlet opens: first in, first out. In a flat hopper only a channel above the outlet flows, and material along the walls can sit for months. For food, feed and materials that cake, mass flow is usually worth the extra height.
Top cone. Filled from a central inlet, material forms a cone at the top at its angle of repose. Depending on the silo, this adds or wastes a few percent of capacity.
Loads. Silo walls carry high and uneven pressures, especially during discharge. Structural design follows EN 1991-4 and is a job for a specialist.