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Flow inserts can be used for many different reasons, such as improving flow, reducing overpressures on feeders, rectifying segregation and inhibiting ‘Flushing’. They are often installed as retrofits to overcome operating problems, but require experienced design to secure optimum benefits without introducing other problems.

The ‘Cone-in-Cone’ and ‘Bullet’ Types are amongst methods used to improve flow by converting a flow regime from Funnel to Mass flow. The question is : -which type is most appropriate for a given application?

The ‘Bullet’ insert is a central fitting of a diverging and converging cone that forms an annular flow channel in the hopper. The degree of cone convergence is chosen to ensure the material will mass flow when discharging. A drawback of this device is that the hopper capacity is reduced by the volume of the fitting.

This may be compared with the Cone-in-Cone insert, where a steeper cone is installed in the center of the hopper section to form an inner and outer flow channel. The inclination of this cone from the vertical is half that of the basic cone angle. In the case of the commonly used cone with wall angle of 60° to the horizontal, the cone-in-Cone annulus equates to a cone wall inclination of 15° to the vertical. A drawback of this method is that the inner cone has an outlet half that of the original diameter and the outer outlet an annulus of width one quarter the original cone. Applications are therefore limited to installations where the material is sufficiently free flowing to pass though these sizes of opening and the original cone is not less steep than twice the inclination from the vertical for mass flow.

In a practical situation the required angle for mass flow may be less or more than the wall inclination of an existing hopper either of which can be generated by a correctly designed ‘Bullet’ type insert. Clearly, the ‘Bullet’ type insert can be employed in applications where a Cone-in-Cone’ type is to shallow to convert the flow regime from Funnel to Mass Flow.

It is noted that the basic principle of both forms of insert can be applied to ‘V’ shaped hoppers. However, a further limitation of a ‘V in V’ type insert is that it can be awkward to reinforce the insert walls to contain flow pressures without compromising flow, whereas a modified ‘Bullet’ type is strong by virtue of curvature and can be stiffened by internal ribs.

Hopper inserts can be fitted for many other reasons, either as retrofits or part of the original design.