Based on the material flow properties measured in laboratory, silos can be designed to function either as mass flow or funnel flow silos with a high degree of confidence.
In mass flow silos every particle is in motion during the discharge; silos are classified as funnel flow silos otherwise (see Figure 1).Mass-flow presents some obvious advantages. It guarantees complete discharge of the silo contents at flow rates that are generally predictable. When successfully designed, a mass-flow silo can also re-mix bulk solids which may have segregated during the filling of the silo. Prevention of segregation could be largely addressed by adoption of mass flow. Careful filling procedures are additional or alternative measures when segregation must be avoided.
Funnel flow silos are usually characterized either by their squat hopper geometry or their flat bottoms, they store more material than mass flow silos of the same overall height and diameter. The advantages of the reduced headroom and therefore the reduced capital expenditure make a funnel flow silo an attractive solution in certain circumstances, for example when segregation of particulate solids is not a pressing issue. The crucial concern in designing a funnel flow silo is to avoid erratic flow, formation of rat-hole and ensure a complete cleanout of solids during discharge.