Screw feeders, designed to meter bulk material, are generally located at the beginning of the discharge process. Available at varying feed rates, sizes, materials, and configurations, your needs will dictate the specifics of the system you set up. Read on to see how they work, the different designs and types in existence, and the difference between screw feeders and screw conveyors.
What is a Screw Feeder and What is Its Process?
A screw feeder is an essential equipment used to deliver fine or granular substances based on your needs. The feed rate or capacity of bulk material can be controlled to a more accurate degree with the help of a screw feeder.
The process of a screw feeder is as described below:
A screw feeder helps to move bulk material from one flight to the next in line with each revolution it makes. When the screw is loaded at 100% capacity and the amount is the same for each flight, it’s only possible to add new material to the first flight under the device used for the storage of bulk material.
A well-constructed screw feeder, therefore, is one in which each flight increases in available volume as the screw progresses to the discharge point.
When the screw feeder is designed with a uniform outside diameter and constant pitch, the material will fill the screw from the end of the inlet. It will lead to stagnant material and rat-holing, even bridging material above the screw feeder.
The Difference between Screw Feeders and Screw Conveyors
While these two types of equipment may sound similar and do a similar job as well, they have three main distinctions.
Screw conveyors generally run at less than full capacity, ranges between 15% and 50% full capacity. Screw feeders run at 100% capacity, mainly because they can alter their operational speeds.
The second difference between screw feeder and screw conveyor pertains to the volumetric rate of material flow. Screw conveyors can handle free-flowing material, but they cannot regulate the speed at which they discharge it. Screw feeders, on the other hand, can control the speed of the bulk material flowing through them.
The third and final difference is that the diameter and pitch, or flighting, can be adjusted on screw feeders while it cannot be adjusted on screw conveyors. It depends on the type of bulk material being transported for added efficiency.
What Are the Different Designs of Screw Feeders?
Design of a Screw feeder mainly depends on:
● The maximum and minimum feed rates of the process
● The full length of the screw feeder
● The flow characteristics of the material being metered and stored
● Length and width of the screw feeder’s inlet opening
● The density of the bulk material
● Height of bulk material in the screw feeder hopper, silo, or bin
● The size of the bulk material
With these considerations in mind, the basic components of a typical screw feeder are the screw itself, a trough, trough ends, discharge, shroud, inlet, drive unit, and cover. And based on the composition of these components, there are three main screw feeder designs that are as outlined:
Tapered Outside Diameter Design
For this design of the screw feeder, the screw’s outside diameter is tapered from the rear end of the inlet’s opening to the shroud. It creates a large volume in the screw feeder for adding bulk material from the hopper.
Stepped Pitch or Variable Design
This design features a screw pitch that varies from a short length to a longer one as it progresses towards the screw feeder’s discharge. For the variable pitch design, the length of every pitch increases in the inlet, creating more volume to which bulk material can be added. For the stepped pitch design, flight pitch changes in increasing lengths. For instance, there may be two feet of one-third pitch each, then two feet of two-thirds pitch, and so on.
Mass Flow Design
This screw feeder design is a combination of tapered inside diameter and variable pitch. From the rear of the inlet opening of the screw’s center pipe to the center of the inlet’s opening is a tapered cone. Flights with short pitches are mounted on this tapered cone, availing more volume for the addition of bulk material. Variable pitch is then introduced to the screw where the cone ends and continues to the discharge.
Different Types of Screw Feeders
There are three main screw feeder types, namely;
Inclined Screw Feeders
These screw feeders elevate bulk material from bins, silos, or hoppers just like a horizontal screw feeder. Two factors are considered in the design of an inclined screw feeder to determine the maximum angle of recline, which are flow-ability of bulk material, and angle of repose. To establish the best design, Sodimate first tests the bulk material to be moved.
Live Bottom Screw Feeders
These are designed for large hoppers, bins, and silos or those with large discharge openings. Multiple feeder screws are used simultaneously, creating the aptly-named “live bottom” which prevents bridging of bulk material. It is, therefore, the best screw feeder to use for material that bridges or packs easily.
Multiple Diameter Screw Feeder
These have a screw feeder that has an extension conveyor. A screw feeder with a smaller diameter located under a hopper, bin, or silo is flood loaded and then feeds the extension conveyor with a larger diameter. The trough loading decreases when bulk material reaches the extension conveyor and is conveyed to the discharge.
Sodimate aims to ensure proper screw feeder design for efficient and successful bulk material processing. Consult to get advice on the best parameters and install the most efficient unit for your process.
