Aug 10

Design of Screw Conveyor | Size of Screw Conveyor | Screw Conveyor Capacity Calculation | Screw Conveyor Design Calculation

Design of Screw Conveyor:

The size of screw conveyor depends on two factors

1. The capacity of the conveyor

2. The lump size of the material to be conveyed (Maximum dimensions of the particle)

Usually there are three ranges of lump sizes which are considered for selection of screw size. These are:

· 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.

· A mixture of lump 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.

· A mixture of lump 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 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 the lumps in the mixture.

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

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

Capacity of Screw Conveyor:

The capacity of a screw conveyor depends on the screw diameter, screw pitch, speed of the screw and the loading efficiency of the cross sectional area of the screw. The capacity of a screw conveyor with a continuous screw:

Q = V. ρ

Q = 60. (π/4).D2.S.n.ψ.ρ.C

Where,

Q = capacity of a screw conveyor

V = Volumetric capacity in m3/Hr

ρ = Bulk density of the material, kg/m3

D = Nominal diameter of Screw in m

S = Screw pitch in m

N = RPM of screw

C = Factor to take into account the inclination of the conveyor

Screw Pitch:

Commonly the screw pitch is taken equal to the diameter of the screw D. However it may range 0.75 – 1.0 times the diameter of the screw.

Screw Diameter:

 Nominal Size D Trough height from center of screw shaft to upper edge of the trough Trough width C Thickness of Tough Tubular shaft (d1 * Thickness) Outside diameter of solid shaft Coupling diameter of shaft Heavy Duty Medium Duty Light Duty 100 63 120 – 2 1.6 33.7*2.5 30 25 125 75 145 – 2 1.6 33.7*2.5 30 25 160 90 180 5 3.15 1.6 42.4*2.5 35 40 200 112 220 5 3.15 2 48.3*3.5 40 40 250 140 270 5 3.15 2 60.3*4 50 50 315 180 335 5 3.15 – 76.1*5 60 50 400 224 420 5 3.15 – 76.1*5 60 75 500 280 530 5 3.15 – 88.9*5 70 75

RPM of Screw:

The usual range of RPM of screw is 10 to 165. It depends on the diameter of screw and the type of material (Max RPM of screw conveyor is 165)

The value of loading efficiency should be taken large for materials which are free flowing and non abrasive, while for materials which are not free flowing and or abrasive in nature, the value should be taken low:

Ψ = 0.12 to 0.15 for abrasive material

= 0.25 to 0.3 for mildly abrasive material

= 0.4 to 0.45 for non abrasive free flowing materials

Inclination Factor:

The inclination factor C is determined by the angle of screw conveyor with the horizontal.

 Angle of screw with the horizontal 0° 5° 10° 15° 20° Value of factor C 1 0.9 0.8 0.7 0.65

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Screw Conveyors | Screw Conveyor Working Principle | Screw Conveyor Systems

Screw Conveyors

Screw conveyors were used as low height lift pump to irrigate the lands. It comprises of an inclined rotating casing with an internal helical screw thread, called flight. When such a casing is rotated with its lower end submerged in water, the water entrained in the pockets, formed by the pitches, is raised. The performance of such screws is limited by the angle of inclination of the casing. A modified version of such screws have been tried with a rotating shaft having helical screw thread moving inside a stationery casing for lifting fluids like sewage sludge. But due to various limitations the process achieved limited success in the field of transportation of liquid. However when a rotating screw shaft in a stationery casing is used to convey solids, it has commendable success.

Floor mill industry was probably the first to employ horizontal screw conveyor to convey corn and flour. These screws were made by fitted wood blades to a central shaft rotating inside a cylindrical casing. Even now such screw conveyors enjoy popularity in agro based industries, handling grains and flours, because of low friction, negligible cohesive strength of the material.

A screw conveyor consists essentially of a shaft mounted screw rotating in a trough and a drive unit for running the shaft. The material is moved forward along the axis of the trough by the thrust of screw thread or flight. The trough is usually of the U-shape.

A helical blade is attached to a drive shaft which is coupled to a drive unit. The shaft is supported by two end bearings and intermediate bearings. The U-shaped trough has a cover plate with an opening for loading the conveyor. A discharge opening is provided at the bottom of the trough. The loading and discharging points can be located anywhere along the trough. More than one feed hopper and discharge hopper may be fitted according to the necessity.

The principle of material movement along the trough is similar to the sliding motion of a nut along a rotating screw when the nut is not allowed to rotate. The weight of the material and the friction of the material against the wall prevent the load from rotating with the screw.

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