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
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.
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 | 0° | 5° | 10° | 15° | 20° |
| Value of factor C | 1 | 0.9 | 0.8 | 0.7 | 0.65 |
Types of screw flight:
The screw of the conveyor may be right hand or left hand, the right hand type being the usual design. The threads of the screw may be single, double or triple.
The flight of the screws may be made in either of the two ways:
1. As Helicoids
2. As Sectional flight
Helicoids Flight:
They are formed from a flat bar or strip into a continues helix. The threads are thinner at the outer edge and thicker at the inner edge.
Sectional flights:
Sectional flights are formed from a flat disc and the thickness of the thread is uniform throughout. A continuous helix is made by joining a number of sectional flights together on a piece of pipe and butt welded them. Various styles of screw flights are in use, depending on the service required.
Some of the typical configurations are:
1. Short pitch or continuous flight:
If the conveyor is required to handle dry granular or powdered materials that do not pack, this style of flight may be selected. It is of regular construction and recommended for inclined conveyors having a slope of 20 or more, including vertical conveyors. This style is extensively used as feeder screw.
2. Ribbon flight:
If the conveyor is to handle lumpy, clinging, sticky, gummy or viscous substances, this type flight may be selected. It consists of continuous helical flight formed from steel bar and secured to the pipe by supporting lugs.
3. Cut flight:
In this type of flight screws have notches cut in the periphery of the flight. These notches supplement the conveying with moderate mixing action. They are recommended for conveyors required to handle light, fine, granular or flaky materials.
4. Cut and folded flights:
This type of flight is characterized by notches as in cut flight, together with folded segments. This type of flight creates agitation and aeration resulting in better mixing. This type of flight is used to handle light or medium weight materials having fine, granular or flaky materials.
5. Some screw conveyors have cut flight with paddles mounted at regular intervals. The paddles counteract the flow of material past the flight resulting in greater agitation and mixing.
6. Sometimes screws are made of stainless steel to suit special requirements, like the sanitation requirements for handling food, drugs and other hygienic materials.
Mechanical Engineering Department 