Design Size Function of Screw Conveyors

A screw conveyor or auger conveyor as it is also known is a mechanism that uses a rotating helical screw blade, viz. fighting, generally within a tube, to move liquid or granular materials

The size of screw conveyor is based on two factors

1. Capacity of the conveyor
2. Lump size of the material to be conveyed

The size allowable for 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 the lumps in the blend.

The lump size of the material affects the choice of screw diameter which should be at least 12 times more than the lump size of a sized material and four times more than the largest lumps of an un-sized material.

Example, if a screw diameter is given 250mm, that means radial clearance is 105mm, & Maximum lump size is 60mm of 10% lumps.

Capacity of Screw Conveyor:

The capacity of a screw conveyor is based on the screw diameter, pitch of the screw, screw speed and the loading efficiency of the cross-sectional area of the screw.

Mathematically

Q = V.?

Q = 60. (?/4).D2.S.n.?.?.C

Where,

Q = capacity of screw conveyor

V = Volumetric capacity in m3/Hr

? = Bulk density of the material, kg/m3

D = Diameter of Screw in m

S = Screw pitch in m

N = RPM of screw

? = Loading efficiency of the screw

C = Factor that takes into account the inclination of the conveyor

Screw Pitch:

Generally, the screw pitch is taken equal to the diameter of the screw D. It might range 0.75 – 1.0 times the diameter of the screw.

RPM of Screw:

The common range of RPM of the screw is 10 to 165. It depends on the diameter of the screw and the type of material used.

Loading efficiency:

The value of loading efficiency should be taken mainly for materials which are free flowing and nonabrasive, while for materials that are not free flowing and abrasive, the value should be considered low:

? = 0.12 to 0.15 for abrasive material

= 0.25 to 0.3 for mildly coarse material

= 0.4 to 0.45 for non-abrasive materials

Inclination Factor:

The angle of the screw with the horizontal, determines the inclination factor C :

Angle of screw with horizontal

0°5°10°15°20°Factor C value10.90.80.70.65

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