Critical rathole diameter

The area of influence of a vibrating discharger is limited to a cylinder, the diameter of which is roughly equal to the top diameter of the discharger. Hence, if a vibrating discharger is mounted onto a conical hopper section, flow is confined predominantly to a central flow channel located directly above the discharger. This is tme unless the slope and smoothness of the static cone meet requirements for mass flow, or the diameter of the flow channel exceeds the critical rathole diameter for the material. 

If the bin discharges in funnel flow, the bin outlet diameter should be sized to be larger than the critical rathole diameter (Of) to prevent a stable rathole from forming over the outlet. For a funnel flow bin with a circular outlet, sizing the outlet diameter to exceed the Of will also ensure that a stable arch will not form (.since a rathole is inherently stronger than an arch). The Of value is calculated in Equation (2), and additional details of the calculation are provided in Ref. 1. 

FIGURE 13 Plot of derived function G(ikt) u.sed to calculate critical rathole diameter for funnel flow bins. 

Reduce the material level in the bin Since the critical rathole diameter typically decreases with a reduction in the major consolidation pressure (require using multiple smaller bins to handle the bulk solid. 

In order to avoid the formation of a stable rathole, it is necessary that the size of the flow channel exceed the critical rathole diameter, DF, which is calculated as follows ... 

There are a number of considerations when sizing the bin outlet. If arching is a concern (either in mass flow or funnel flow), the discharge frequency must be considered since this sets the maximum time that the bulk solid will remain at rest within the vessel. If a funnel flow vessel is being designed, the critical rathole diameter will also be needed to set the minimum outlet dimensions. 

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