Design of Screw Conveyor:
The size / design 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)
For the transportation and dispensing of bulk materials, screw conveyors are one of the most often used machinery. The size of a screw conveyor must be calculated in numerous projects in order to attain the required capacity. An existing screw conveyor’s capacity might need to be checked in the event that material handling or capacity requirements change. From this brief engineering reference, some few formulas are explained.
Screw conveyors are a convenient, economical, and extremely efficient means of transporting the goods, when the conveyor is properly constructed or designed, according to the consumer requirements. Material flow rate, material dispensing, and overworking of the motor are all governed by the screw conveyor’s design and construction. Consider the following methods and techniques for screw conveyor design, in more detail.
Screw Conveyors Have a Long History
The screw conveyor was invented by Archimedes in the 3rd century B.C. The first screw conveyor was used to irrigate crops and remove water from ships. Spiral flights were attached to the inner wall of a hollow cylinder and were powered by a central shaft in the initial model. Water was transported and raised from one point to the next as the assembly revolved. The spiral project is focused on the inclined plane principle. In contemporary times, the screw conveyor has evolved and is currently utilised in virtually every major sector.
Selection of Screw Conveyor
Screw conveyors are constructed with a central shaft encircled by screw flights as its fundamental design. This mechanism is enclosed by an enclosure that is open on both ends, allowing materials to be collected and discharged from it. While screw conveyors and screw feeders have a similar architecture, screw conveyors are intended to move material from one area of the process to another, while screw feeders are intended to measure the material as it passes through the process.
Many different screw conveyor designs are available for customization. The following are a few examples of how screw conveyor designs may be tailored to meet the specific requirements of a particular application or material. Please keep in mind that this is not a comprehensive list; there are additional methods to modify the screw conveyor to suit your specific material handling requirements.
- Pitch is the distance between the screw flights on a given screw.
- Shaft refers to the size of the central shaft, or the lack of a central shaft.
- Construction: the materials used to construct the enclosure, shaft, flights, and other components.
- Capacity is measured by the amount of material that can be moved via a screw conveyor in a given amount of time.
- The speed at which the screw conveyor travels is referred to as the screw conveyor speed.
- The conveyor’s entire length is referred to as its length.
- Incline: whether or not the conveyor must travel a vertical distance, and how much distance it must travel.
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?
Material Characteristics of Screw Conveyors
The properties of the materials being carried have a major effect on the design of the screw conveyor. Various material qualities may have an effect on the design in a variety of ways, the most significant of which are listed below:
Materials that flow smoothly will need less energy to move and will move more quickly. The power required to move materials that are thick and sluggish will be greater, and the materials will move more slowly.
It is possible for corrosive elements to eat away at a screw, shaft, and trough enclosure when they’re not constructed from a resistant material. When dealing with corrosive compounds, some stainless steel grades or nickel alloys are the best options available.
Friction may eventually wear out at the screw, shaft, and enclosure as a result of the use of abrasive materials. Screw conveyors will benefit from the use of abrasion-resistant metals, which will assist to strengthen them.
The Angle of Repose
At different inclination levels, the angle of repose has an effect on the flow of the material. This will be especially essential if the screw conveyor is responsible for delivering goods across a vertical and horizontal range of distances.
Features that distinguish them from the other Material properties
Many specific features, such as levels of moisture or oil, agglomeration, toxicity, adhesion, dustiness, flammability, explosiveness, and other attributes, may have an impact on the functioning and design of a screw conveyor.
Flight Pitch, Number, Paddles, and Other Details
Pitch on the screw conveyor flights is an essential factor to take into account. Assuming the feeder is fully extended, the distance between each flight is equal to the diameter of the feeder. When the pitch is half pitch, the distance between the flights is equal to half the diameter of the feeder’s diameter. If the pitch of the flights is the same throughout the board, pressure may build up towards the end of the conveyor’s length. The pitch may also have an effect on how much material flows through the feeder at any one time.
There are a variety of options for customizing the screw conveyor pitch in order to accommodate the material properties, including the options listed below:
Progressive or Variable Pitch
A progressive or variable pitch is usually used for pulling items out of a hopper in an even and consistent manner. It is necessary in this situation to extend the distance between the flights in order to pull the material out of the hopper.
A double flight is an additional spiral-shaped component that is added around the shaft to enhance the conveying power or quantity even more. Double flight screw conveyors are also available in progressive or variable pitch configurations.
The use of paddles between screw flights may aid in the creation of a more gentle mixing and conveying action on the screw. This is particularly beneficial for materials that are brittle or fibrous, as well as those that cluster easily.
The difference between a normal screw flight and a ribbon flight is that a typical screw flight is solid all the way through, while a ribbon flight contains gaps between the flight and the shaft. This makes it easier for sticky or viscous items to pass through the screw conveyor with less difficulty.
Screw conveyor design may be tailored to your specific needs to help prevent material from being trapped in the conveyor, avoid overworking the motor, and prolong the conveyor’s usable life. Consider your materials, processes, and facility layout before collaborating with a seasoned manufacturer to create the perfect screw conveyor for your application.
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