Screw Conveyor Power Calculation | Screw Conveyor Capacity Calculation | Screw Conveyor Formulae

Power requirement of Screw Conveyors:

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The driving power of the loaded screw conveyor is given by:

P = PH + PN + Pst

Where,

PH = Power necessary for the progress of the material

PN = Driving power of the screw conveyor at no load

Pst = Power requirement for the inclination of the conveyor

Power necessary for the progress of the material PH:

For a length L of the screw conveyor (feeder), the power PH in kilo watts is the product of the mass flow rate of the material by the length L and an artificial friction coefficient λ, also called the progress resistance coefficient.

PH = Im.L. λ.g / 3600 (kilowatt)

= Im.L. λ / 367 (kilowatt)

Where,

Im = Mass flow rate in t/hr

λ = Progress resistance coefficient

Each material has its own coefficient λ. It is generally of the order of 2 to 4. For materials like rock salt etc, the mean value of λ is 2.5. For gypsum, lumpy or dry fine clay, foundry sand, cement, ash, lime, large grain ordinary sand, the mean value of λ is 4.0.

In this connection it should be noted that the sliding of the material particles against each other gives rise to internal friction. Other resistance due to grading or shape of the output discharge pattern contributes to the resistance factor. That is why the parameter λ is always higher than that due to pure friction.

Drive power of the screw conveyor at no load, PN:

This power requirement is very low and is proportional to the nominal diameter and length of the screw.

PN = D.L / 20 (Kilowatt)

Where,

D = Nominal diameter of screw in meter

L = Length of screw conveyor in meter

Power due to inclination: Pst

This power requirement will be the product of the mass flow rate by the height H and the acceleration due to gravity g.

Pst = Im.H.g / 3600

= Im.H / 367

H should be taken positive for ascending screws and will be negative for descending screws.

Total power requirement:

The total power requirement is the sum total of the above items

P = (Im (λ.L + H) / 367) + (D.L /20) (Kilowatt)

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

Design of Screw Conveyor:

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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:

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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

Ψ = Loading efficiency of the 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.

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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)

Loading efficiency:

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

10°

15°

20°

Value of factor C

1

0.9

0.8

0.7

0.65