Funnel-flow effects

Incomplete and/or inefficient discharge of material from each blow tank mainly due to rat-holing or funnel-flow effects promoted by... 

Often there are situations in which mass-flow bins cannot be installed for reasons such as space limitations and capacity requirements. Also, sometimes the product to be stored has an FF flow function that lies below the flow factorj, bridging takes place, and unassisted mass flow is not possible. To handle these situations, a number of flow assisters are available, the most desirable of which use a feeder and a short mass-flow hopper to enlarge the flow channel of a funnel-flow bin. The choice of feeder or flow assister should always be made as part of the storage-vessel analysis. The resulting systems are then usually as effective as the mass-flow types. 

Figure 10 Mass flow/funnel flow design chart for a conical hopper handling a bulk material with a 40 effective angle of internal friction.

The relation between mass and funnel flows for conical bins is shown in Fig. 21-20. The angle of kinematic friction which is a measure of the friction coefficient between the solid and the material of construction used for the conical-shaped hopper, is measured with the flow-factor tester. The degree of finish of the metal surface can have a large effect in determining whether the vessel will function in mass or funnel flow. Finer degrees of finish are being used more fre-... 

For each of these different design concerns, we will review the key equipment parameters and flow properties. Note that these common flow concerns (arching, ratholing, adverse two-phase flow effects) and flow patterns (mass flow vs. funnel flow)... 

Use a different interior surface finish with better wall friction properties (lower friction). Conduct wall friction tests on alternative wall surfaces to assess if changing the surface finish while still using the existing bin geometry (e.g., electro-polishing an exiting 2B finish) will convert the bin from funnel flow to mass flow. This is often one of the most cost-effective modifications to obtain mass flow. 

The mass-flow and funnel-flow limits in silos are well known and have been used extensively in proper design. The limits for conical hoppers and plane hoppers depend on the hopper half-angle 9, the effective angle of internal friction 5 and the wall friction angle ( ). Once the wall friction angle and effective angle of internal friction have been determined by experimental means, the hopper half angle 0 may be determined. In function form it can be expressed as... 

If the bulk solid s characteristics are such that a mass flow pattern is not required, a funnel flow bin should be designed. Not only will this save on headroom and maximise storage volume for a given headroom, it will also minimise the effects of sliding abrasion on the bin walls. 

The best known and the most applied method to design silos for flow is the method developed by Jenike [1]. He distinguishes two flow patterns, mass flow and funnel flow, the border tines of which depend on the inclination of the hopper, the angle [Pg.18]

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