Maximum lump size limitation in a screw conveyor

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Lump size limitation

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 the maximum dimension it has. A closer definition of the lump size would be the diameter of a ring through which the lump would pass. However, if a lump has one dimension much longer than its transverse cross-section, the long dimension or length would determine the lump size. The character of the lump also is involved. Some materials have hard lumps that won’t break up in transit through a screw conveyor. In that case provision must be made to handle these lumps. Other materials may have lumps that are fairly 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 lumps of these materials impose no limitations.

Three classes of lump sizes apply as follow:

  • Class 1
Lump size class 1.png

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 2
Lump size class 2.png

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 3
Lump size class 3.png

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.

The Maximum lump size table shows the recommended maximum lump size for each customary screw diameter and the three lump classes. The ratio, 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.

Archimedes screw conveyor cross section.jpg

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 illustrates this relationship.

Table in mm

The Maximum lump size table with R=Radial clearance(mm)/Lump size (mm)

Screw
Diameter mm
Pipe O.D Radial
clearance mm
Class 1

10% Lump
Ratio R=1.75
Max. lump, mm

Class 2

25% Lump
Ratio R=2.5
Max. lump,mm

Class 3

95% Lump
Ratio R=4.5
Max. lump,mm

50,8 24 13 6 3.8 2.5
76,2 30 25 13 6 2.5
101,6 39 33 17 13 6
150 57 49 31 19 13
228,6 57 86 57 38 19
304,8 60 122 70 50 25
356 140 103 82 63 32
406 140 138 95 70 38
508 180 194 108 76 45
557 180 170 120 89 50
609 225 197 152 95 63


Table in inches

The Maximum lump size table with R=Radial clearance(inch)/Lump size (inch)

Screw
Diameter inch
Pipe O.D Radial
clearance inch
Class 1

10% Lump
Ratio R=1.75
Max. lump, inch

Class 2

25% Lump
Ratio R=2.5
Max. lump,inch

Class 3

95% Lump
Ratio R=4.5
Max. lump,inch

(2'') (0.94'') (0.51'4) (0.24'') (0.15'') (0.1'')
(3'') (1.18'') (0.98') (0.51'') (0.24'') (0.1'')
(4'') (1.53'') (1.30'') (0.67'') (0.51'') (0.24'')
(6'') (2.24'') (1.93'') (1.22'') (0.75'') (0.51'')
(9'') (2.24'') (3.38'') (2.24'') (1.49'') (0.75'')
(12'') (2.36'') (4.80'') (2.75'') (1.96'') (0.98'')
(14'') (5.52'') (4'') (3.25'') (2.5'') (1.25'')
(16'') (5.52'') (5.4'') (3.75'') (2.75'') (1.5'')
(18'') (7.08'') (7.6'') (4.25'') (3'') (1.75'')
(20'') (7.08'') (6.7'') (4.75'') (3.5'') (2'')
(24'') (8.86'') (7.8'') (6'') (3.75'') (2.5'')