Pressure on the Base of a Tall Cylindrical Bin

A range of devices designed to facilitate flow of powders from silos and hoppers are commercially available. These are known as discharge aids or silo activators. These should not, however, be employed as an alternative to good hopper design. 

Discharge aids may be used where proper design recommends an unacceptably large hopper outlet incompatible with the device immediately downstream. In this case the hopper should be designed to deliver uninterrupted mass flow to the inlet of the discharge aid, i.e. the slope of the hopper wall and inlet dimensions of the discharge aid are those calculated according to the procedure outlined in this chapter. 

If the stress exerted on the wall by the powder in the slice is and the wall friction is tan then the friction force (upwards) on the slice is 

If the slice is in equilibrium the upward and downward forces are equated, giving 

If we assume that the horizontal stress is proportional to the vertical stress and that the relationship does not vary with depth. 

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