Vertical screw conveyors
A vertical screw conveyor conveys material upward in a vertical path. It requires less space than some other types of elevating conveyors. Vertical screw conveyor can handle most of the bulk materials provided there is no large lump. The maximum height is usually limited to 30m.
A vertical screw conveyor consists of a screw rotating in a vertical casing. The top bearing for the screw shaft must be designed to stand against radial and thrust loads. A suitable inlet port at the lower end and a discharge port at the upper end of the casing are provided. Feeding a vertical screw conveyor deserves careful consideration. Most materials are fed to the vertical conveyor by a straight or offset horizontal feeder conveyor. The ideal operation of a vertical screw conveyor is to have a controlled and uniform volume of material feeding.
Uneven feeding and start stop operation may adversely affect the performance of the vertical screw conveyor in terms of speed, capacity and horse power.
Average capacities and speeds of vertical conveyor
| Nominal diameter of screw in mm | Capacities in m3/hr | Speed of screw |
| 150 | 10 | Up to 400 RPM |
| 250 | 35 | 300 RPM |
| 300 | 75 | 250 RPM |
| 400 | 170 | 200 RPM |
Vertical screw conveyors or some special design of vertical screw conveyor finds wide application in ship unloading.
Practical experience with these conveyors has shown that the resistance factor for vertical conveyors is higher than those of the horizontal conveyors. Resistance factor λ may be taken as 5.5 to 7.5 for grains. 6.5 to 8.3 for salt.
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)
Tags: bulk materials, careful consideration, casing, coefficient, consideration, conveyor, discharge, discharge port, feeder, flow, friction, grains, height, horizontal conveyors, horse power, inclination, inlet, kilo watts, lump, mass, maximum, maximum height, meter, operation, power, practical experience, progress, resistance, rpm, screw conveyors, speed capacity, thrust loads, Uneven, value, Vertical, vertical conveyor, vertical conveyors, vertical path, vertical screw conveyor
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