There are two major flow patterns that can develop in a bin, funnel flow and mass flow.

The definition of funnel-flow is, some material moves while some material remains stationary or stagnant This type of flow pattern occurs because of shallow and/or rough hopper wall surfaces. Mass-flow develops when all the material in a bin is in motion whenever any material is withdrawn. This type of flow pattern is indicative of uniform, first-in-first-out flow.

What is expanded-flow?

The definition of expanded-flow is that it uses a funnel-flow hopper below which is attached a mass-flow hopper. The mass-flow pattern created by the mass-flow hopper expands into the upper funnel-flow portion, creating a useful, yet efficient flow pattern.

This allows use of the best characteristics of funnel-flow and mass-flow. Namely, the low headroom required to develop a funnel-flow pattern and the many flow benefits of mass-flow.

Typically, shallow cones (60 deg or less), pyramidal hoppers, and flat bottomed bins exhibit a funnel-flow pattern. Most solids are not capable of overcoming the friction that develops between the shallow/rough walls and themselves. Mass-flow however, uses its "first-in-first-out" flow sequence to minimize bridging problems, eliminate ratholes, and minimize segregation and flooding.

How do I design an expanded-flow bin?

First, you need to know your material flow properties. The reason being is that you are going to design a funnel-flow bin and a mass-flow bin combination.

The upper portion of our expanded-flow bin will exhibit a funnel flow pattern and unfortunately be prone to ratholing unless we know the ratholing diameter of the material to be handled. This is determined by flow properties tests which can predict the opening size required to make a rathole unstable and promote flow. For example, if your rathole dimension is 6 ft., then an opening greater than 6 ft., will not result in stable rathole formation. The material will still flow in funnel flow, but not rathole.

Attached at the outlet of your funnel-flow portion is a mass-flow hopper.

Obviously, the mass-flow hopper design (opening size, wall angles, etc.) are set by your flow properties tests, as was the funnel-flow design. A conical or slotted configuration can be used to provide the mass-flow pattern.

It is not recommended to use an expanded-flow design for bin diameters less than 25 ft. It is not cost effective. Also, as I always say, the feeder design at the outlet of the mass flow portion must withdraw material over it's entire outlet cross-section. If not, you have gone to great lengths to create another funnel-flow bin!