Power requirement of Screw Conveyors:
The driving power of the loaded screw conveyor is given by:
P = PH + PN + Pst
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)
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)
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)