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)
Selection of Screw Conveyor
There are many reasons and applications which require variation to standard screw conveyors. These are just some of the factors and conditions that will influence the design and construction of a screw conveyor.
- What material will be conveying?
- What are the material characteristics?
- What is the bulk density of the material?
- What through put capacity is the screw to be designed for and at what housing loading?
- Where will the material be coming from?
- Is the screw to be a feeder or a conveyor?
- Is the screw horizontal, incline or vertical?
- Is the screw reversing?
- How long will the screw be centreline of the inlet to the centerline of the discharge?
- Are intermediate hanger bearings for the screw acceptable?
- Housing construction information
- Material of construction
- Dust tight construction
- Pressure tight construction (i.e. Will the housing have to hold pressure)
- Will the housing requires a ‘code stamp’?
- Will the equipment be in wash down service?
- Hinged covers?
- Tubular or “U” trough housing?
- Type of shaft seals?
- Food grade polish
- Welding requirements
- How is the housing supported?
- What is the inlet size that we are connecting to and how many inlets are there?
- What is the discharge size that we are to connect to?
- Is the discharge a drop out, injector style or positive feed?
- Is there more than one discharge called out and do they have slide gates?
- What style are the slide gates and how are they operated?
- Is the screw service continuous, 8 hour a day, 24 hour a day, batching, intermittent etc?
- If there is more than one screw to a housing, how many drives?
- Is the screw to be portable?
- Will the screw be required to dewater or drain product?
- Will the screw be required to mix product while conveying?
- Will the product be abrasive?
- What material of construction in product contact?
- What type of welding in product contact?
- What finish or polish is required in product contacts?
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
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
Design of Screw Pitch for Screw Conveyor:
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.
Design of Screw Diameter for Screw Conveyor:
|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|
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
The inclination factor C is determined by the angle of screw conveyor with the horizontal.
Angle of screw with the horizontal
|Value of factor C||1||0.9||0.8||0.7||0.65|