Screw feeders are designed to meter bulk materials and are typically located at the beginning of a process. Capacity or feed rate can be accurately controlled with screw feeders. Variable speed drives improve metering accuracy and can provide a wide range of feed rates. Screw feeders are available in a variety of sizes, lengths, configurations and materials of construction.
Most screw feeders are less than 20-feet in length because the use of internal hanger bearings is not recommended. In most applications a short screw feeder will meter a bulk material to a screw conveyor for transfer to the next step of the process.
The pitch of the screw varies from shorter to longer as the screw progresses toward the discharge of the screw feeder. With variable pitch, every pitch increases in length in the inlet section creating more available volume for addition of bulk materials from the hopper. With stepped pitch the flight pitch changes in increments. For example, a stepped pitch screw feeder may have 2-feet of 1/3 pitch, then 2-feet of 2/3 pitch in the inlet section.
The mass flow design was developed by Jenike & Johanson and is a combination of variable pitch and tapered inside diameter. A tapered cone is located on the center pipe of the screw from the rear of the inlet opening to approximately the center of the inlet opening. Short pitch flights are mounted on the cone creating available volume for addition of bulk materials from the hopper. Variable pitch is then added to the screw starting where the cone ends and continuing to the discharge.
It is not recommended to design screw feeders with uniform outside diameter and constant pitch because bulk materials will fill the screw from the rear of the inlet opening first, creating rat-holing, stagnant material and possible bridging of bulk materials above the screw feeder. To draw bulk materials evenly across the full length of the inlet each flight must increase in available volume as the screw progresses towards the discharge of the screw feeder. Variable pitch, tapered outside diameter (OD) or mass flow screw design is required.
Screw feeders must be equipped with a shroud for at least 2 pitches beyond the inlet opening to prevent flooding of the bulk material past the inlet. The shroud is a curved cover that converts a standard U-trough into a tubular housing to prevent bulk materials from flooding past the screw. Extended shrouds, tubular housings or short pitch flights can be utilized for accurate feed rate control when metering very free flowing bulk materials.
The pitch of the last screw flight going into the shroud determines the feed rate of the screw feeder and is called the Control Pitch. The Control Pitch is typically less than full pitch. The capacity of the Control Pitch is calculated in cubic feet per hour per RPM. The speed of the screw feeder can be determined by dividing the maximum screw feeder capacity in cubic feet per hour by the capacity of the Control Pitch in cubic feet per hour per RPM.
Most screw feeder speeds are lower than standard screw conveyor speeds. For example, in heavy industrial applications, screw feeders typically operate at speeds less than 20-RPM. More torque is generated at lower operating speeds ensuring the screw feeder does not stall at start-up.
The horsepower and torque requirements for a screw feeder are much higher than a comparable screw conveyor. A screw feeder must start up with a flood loaded inlet and the head load weight of the bulk material in the inlet section. Bulk materials also tend to pack when under pressure in a hopper, bin or silo. As the bulk material density increases, so do the horsepower and torque requirements.
The Material Factor or HP Factor (MF) can exceed 4.0 for some bulk materials when under pressure and packed. The start-up horsepower and torque can easily be 2.5 times the normal operating conditions. Please consult the KWS Engineering Department for proper screw feeder design.
Multiple Diameter Screw Feeder/Conveyors consist of a screw feeder with an extension conveyor. A smaller diameter screw feeder is located under a hopper, bin or silo and is flood loaded. The screw feeder meters the bulk material to the larger diameter extension conveyor. When the bulk material reaches the extension conveyor the trough loading decreases and the bulk material is conveyed to the discharge. Hanger bearings are allowed in the extension conveyor as long as the trough loading is below 45-percent.
Live bottom screw feeders are designed for use on large silos, bins and hoppers with large discharge openings. The live bottom screw feeder utilizes multiple feeder screws in tandem to create a "live bottom" to prevent bridging. Bulk materials are metered and drawn out equally from the full width and length of the inlet opening. Live bottom screw feeders are used on bulk materials which tend to pack or bridge easily.
Inclined screw feeders meter and elevate bulk materials from hoppers, bins or silos and perform the same function as horizontal screw feeders. However, special care is required when designing inclined screw feeders.
Knowledge of the flow characteristics of bulk materials is extremely important for successful inclined screw feeder design. The angle of repose and flowability of a bulk material will determine the design of the screw feeder and the maximum angle of incline. Testing of bulk materials is required for all inclined screw feeders before a proper design can be established. Bulk material samples can be sent to KWS for laboratory and field testing.
Inclined screw feeders must be designed to meter a desired capacity or feed rate and elevate a bulk material to a desired height. Screw feeders become less efficient when inclined over 5-degrees from the horizontal position. The loss of efficiency is determined based on the degree of incline of the screw feeder and the angle of repose and flowability of the bulk material. The diameter of the inclined screw feeder can be selected once the incline efficiency factor is determined.
Inclined screw feeders utilizing U-troughs are typically used on inclines up to 15-degrees and tubular housings are recommended for inclines over 15-degrees. Reducing the pitch of the screw increases the incline efficiency factor because the shorter pitch provides a better conveying surface and bulk materials do not fall back when compared to full pitch flights. Full pitch flights are the least efficient at metering and conveying bulk materials on an incline.
Inclined screw feeders typically operate at higher speeds when compared to horizontal screw feeders because additional speed is required to elevate a bulk material and overcome the forces of gravity and bulk material fall back. The desired capacity is adjusted using the incline efficiency factor calculated from testing of the bulk material. The speed of the inclined screw feeder can then be determined.
Inclined screw feeders require more horsepower and torque when compared to a horizontal screw feeder. Additional horsepower and torque is required to elevate a bulk material and overcome the forces of gravity and bulk material fall back. Bulk materials can become packed inside an inclined screw feeder, causing more demand on the drive unit.
The inlet length on an inclined screw feeder must be kept to a minimum to prevent the bulk material from falling back over the top of the flights in the inlet section. Typically, the length of the inlet should not exceed 2 times the diameter of the screw for an inclined screw feeder.
Inclined screw feeders are typically designed with multiple flight pitch changes. Shorter flight pitches are used in the inlet section to control the capacity or feed rate. Typically, the flight pitch increases beyond the inlet to reduce the trough loading to less than 100-percent. The conveying efficiency must be calculated in the longer flight pitch section to make sure the desired capacity or feed rate is met. Improper design of the flight pitches could result in the inclined screw feeder becoming plugged at the transition from shorter to longer pitch flights.
Screw feeders today play an increasingly important role in the drive towards improved quality, reduced costs, increased capacity, better working conditions, and flexibility in solids processing. Advances in control methods are being matched with improved predictability and reliability of the processes being controlled. The intensive and integrative nature of many production lines crucially depends upon each element working to its full design capability. Solid feeding operations comprise a key activity, renowned for operating difficulties out of all proportion to the cost of the equipment. This excellent book, by an acknowledged expert in the area, provides a valuable introduction to the subject together with guidance on the selection and application of a range of screw feeders.
Screw Conveyor applications are required in Food, Pharmaceuticals, Chemical industries, Packaging industries, salt, cement, coal, biomass, agriculture, gypsum and other industries where there is a need for industrial duty equipment to convey free-flowing bulk materials. Screw Conveyors are used in a wide variety of applications to efficiently convey dry, free-flowing, semi-fluid and sticky bulk materials. Screw conveyors are employed for handling a great variety of materials which have relatively good flowability. Sticky and stingy materials are unsuitable for screw conveying.
Screw conveyors have a few favourable points to its credit for recommending its application in different industries. Simplicity of design, easy construction and maintenance, are the few points which are often mentioned in favour of its application. But material characteristics often dictate the choice of its selection.
Screw conveyors are very suitable for being adopted as a device to control the volume of materials flow from the bottom of bins, hoppers, storage silos etc. when screw conveyors are used for this purpose they are termed as screw feeders. Screw feeders have wide applications for the operation of processing units like driers, hammer mills, oil expellers and other innumerable mills.
Archimedes designed the first screw conveyor in the third century B.C. It was used for removing water from ships and for irrigating farmland. The device consisted of a hollow cylinder with a center shaft and a spiral fixed to the inner wall of the cylinder and center shaft. As the assembly rotated, water was conveyed and lifted from one location to another. The spiral design is based on the theory of the inclined plane.
The screw conveyor began to evolve in the late 1800s and was used as a means of increasing feed and grain production to serve the needs of the rapidly growing American population. The first feed mills utilized screw conveyors throughout the process.Even the most modern feed mills today depend on screw conveyors for many of their material handling requirements. The screw conveyor has evolved to modern times. It is now used in almost every major industry.
The flexible screw conveyor operation begins with product entering the sealed tube as the auger rotates. The shape, design style, size, and length of the auger are factors selected for each application. The screw conveyor is typically mounted to a hopper for easy loading of material. A single motor drives the auger rotation allowing the material to be pulled through the enclosed tube. The helicoid design of the auger effectively conveys a wide range of materials from powder, to granular, to larger particles.
Screw conveyors may be used to convey materials which can be subjected to thermal operation during transit. The conveyor trough is provided with jacket through which the heating or cooling medium may be circulated to effect the thermal operation.
Screw conveyors may be operated horizontally, on an incline, or vertically. Often inclined conveyors can solve a transportation problem effectively, but otherwise a combination of horizontal and vertical units should be installed. The angle of inclination of inclined conveyors is usually 10 to 20. Vertical conveyors are usually complex in design and should be run at faster speed.
Screw conveyors do not always get favourable consideration due to high friction of the material against screw and trough, probability of material degradation due to crushing of material and due to higher power consumption resulting from higher friction. Therefore screw conveyors have limitation in capacity and distance of conveying. They are suitable for low and medium capacities for short distance. Capacity may be as high as 100m3/hr and distance of conveying is of the order of 30 to 40m typically.
Screw conveyors are bulk material transporting devices capable of handling great variety of materials which have relatively good flow ability. This characteristic is important in screw conveyor operation as the screw helix mounted on a central pipe or shaft, rotates within a fix trough or tube, pushes the material along the bottom and sides, sharing the material in the radial clearance between the helix and trough and causing the material to tumble upon itself as the moving face of the helical flight tends to lift the material.Get in Touch with Mechanic