The size of a screw conveyor not only depends on the capacity required, but also on the size and proportion of lumps in the material to be handled. The size of a lump is determined by the maximum dimension it measures around the center of the material.

The character of the material lump classifies the material in one of the three classes;

Class I

A mixture of lumps and fine particles of which not more than 10% are lumps ranging from maximum size to one half of the maximum; and 90% are lumps smaller than one half of the maximum size.

Class II

A mixture of lumps and fine particles of which not more than 25% are lumps ranging from maximum size to one half of the maximum; and 75% are lumps smaller than one half of the maximum size.

Class III

A mixture of lumps only of which 95% or ore are lumps ranging from maximum size to one half of the maximum size; and 5% or less are lumps less than one tenth of the maximum size.

The allowable size of a lump in s screw conveyor is a function of the radial clearance between the outside diamter of the central pipe and the radius of the inside of the screw trough., as well as the portion of lumps in the mix. Table C shows the recommended maximum lump size for ech customary screw diameter and the three lump classes.