The character of the lump also is involved. Some materials have hard lumps that won't break up in transit through a screw conveyor. If that is the case, provision must be made to handle these lumps. Other materials may have lumps that are very hard, but degradable in transit through the screw conveyor, thus really reducing the lump size to be handled. Still other materials have lumps that are easily broken in a screw conveyor and therefore impose no limitations.
Three classes of lump sizes apply as follows:
Class I - A mixture of lumps and fines in 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 fines in which not more than 25% are lumps ranging from the 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 in which 95% or more 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.
Table III shows the recommended maximum lump size for each customary screw diameter and the three lump classes. The ration, R, is included to show the average factor used for the normal screw diameters which then may be used as a guide for special screw sizes and constructions. For example:
Radial Clearance, Inches
Ratio, R = Lump Size, Inches
The allowable size of a lump in a screw conveyor is a function of the radial clearance between the outside diameter of the central pipe and the radius of the inside of the screw trough, as well as the proportion of lumps in the mix. The following illustration shows this relationship:
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