Due to their relatively low cost and uncomplicated construction, flexible screw conveyors are very popular with manufacturers. In these units, a screw (also called an auger), driven by an electric motor, rotates within a tubular housing. The screw is the only part that moves.
Despite the conveyors' simplicity and straightforward operation, many end-users overlook important factors that potentially could lead to poor performance, excessive power usage, severe wear and material degradation. To avoid such issues, it's essential to consider all system parameters when selecting conveyor options. Only then can you ensure a correct and successful installation that achieves maximum operational efficiency.
A flexible screw conveyor has a helicoid screw that rotates inside a fixed tube. Screw conveyors typically range in diameter from 2½ to 8 inches, and can be used for horizontal transports of up to 80 feet and inclines of up to 60°. An 8-in.-diameter unit can handle 1,800 ft3/h at 360 rpm at a 45° angle.
The flexible screw is made from stainless steel and comes in a variety of configurations. As the screw rotates, it creates directional force that moves the product through the tube. At the same time, it creates centrifugal force that pushes the material against the tube's wall.
The characteristics of the material being conveyed are extremely important considerations when selecting any conveyor. Flexible screw conveyors can move many types of materials, such as powders, crystals, flakes, granules, pellets and irregular shapes. However, you still must assess how the material will affect the conveyor as well as how the conveyor might affect the material.
For example, certain materials, such as sugar or salt, have the potential to create friction within the flexible screw conveyor. This can cause crushing, heating or chipping of some particles during transport. Moreover, material friction can result in overtaxing of the conveyor's motor, increasing wear and eventually leading to failure if left unchecked. Or the actual material being conveyed could build up on the screw and make it stall.
TYPES OF SCREWS
Augers come in many designs, each addressing the challenges presented by different material characteristics. To get the best results possible from your flexible screw conveyor, you must select the auger type that best suits the material you need to convey. Here's a rundown on the basic types (Figure 1):
• Flat wire. These augers are the most common type used. Made of rectangular wire about ¼-in. thick, they can handle a wide variety of powders ranging in bulk density up to 60 lb/ft3. The flat conveying surface applies a more positive forward directional force on the product being conveyed and reduces the outward force against the tube wall.
• Beveled or square bar. This type of auger is designed specifically to convey difficult-to-handle or fragile material with minimal product degradation or damage. The beveled auger must be installed straight. It rotates at a much lower rpm, typically 360–450 rpm.
• Heavy-duty square bar. Usually 3/8-, ½- or even ¾-in. thick, wound into sections and welded as needed, these screws can cope with heavy and abrasive materials and suit large hoppers.
• Round wire. Made from a coiled round bar, these screws mainly are used for heavy or highly abrasive materials. The biggest benefits of this auger design are its strength and flexibility, which minimizes the load imposed on it by material weight or size or particle shape. The design creates high slippage and stronger outward forces on the conveyed product.
• Centercore. A small tube placed in the center of the screw extends the full length of this auger (Figure 2). By restricting the space available, the centercore reduces the amount of material that can enter the conveyor, which in turn decreases the load imposed upon the screw. Use a centercore when conveying very dense products or highly aerated or fluid product.
Table 1 summarizes the advantages and disadvantages of the various types of augers. Table 2 lists some of the materials each auger type suits.
ADVANTAGES AND LIMITATIONS
The flexible screw conveyor is the best choice for many batch processing applications. However, as with any type of conveyor, when selecting equipment you should consider advantages and limitations:
• well suited for horizontal and vertical conveying;
• easily transports many free- and non-free-flowing bulk materials;
• low cost;
• takes up very little floor space;
• easy to clean;
• allows for flexible installations (conveyors 4-in. and smaller can be curved within a specific bend radius, and inlet hopper angles also can be varied);
• causes low-to-moderate material degradation; and
• ensures a dust-tight system, with hopper options to ensure that product remains within the process.
• residual product left in the casing;
• height and length limits; and
• different screws may be required when conveying multiple products with the same conveyor.
Many factors affect capacity and power requirements. The most common elements to consider are material particle size, bulk density and flowability, conveyor length, diameter and incline angle, auger rotational speed and helicoid pitch. Optimal operation will strongly depend upon the conveyed material, the rpm of the auger, and what size hopper is being used. If the conveyor is inclined, tests must be run to determine capacity and power required. Capacity decreases as you increase the incline. The power requirement rises with speed at any slope.
You can increase or decrease material flow rate by:
1. the type of auger used;
2. its rotational speed;
3. the configuration of the feed hopper and tube;
4. the material's flowability and overall characteristics; and
5. the conveyor's angle of incline.
A general rule of thumb is to set a flexible screw conveyor at no more than a 45° angle. Taking into account the material's characteristics, you may be able to convey above 45° without risking backflow; when conveying heavier materials, you may need to decrease the angle to below 45° to prevent backflow.
With the exception of initial charging and emptying, you never should operate a flexible screw conveyor empty for extended periods of time unless it's specifically designed for that. Running without material in the hopper causes excessive fatigue on the system's components. To mitigate this potential problem, use sensors at the pickup point to automatically shut off the conveyor when material isn't present.
Dry operations occur when a conveyor has been running fully loaded or at a high fill percentage and then no longer gets material to convey. This typically happens in applications exhibiting increased slippage, visual material rollback and a steady decrease in output. A conveyor can be designed to run under these conditions by using the proper outer tube, screws and slower rpm levels.
During standard operation, all flexible screw conveyors will leave residual material in the inlet tube once the hopper is empty. You can quickly extract this material by removing the end cleanout cap at the bottom of the conveyor and reversing the rotation of the screw. Further cleaning can be performed with air, water or solvents with or without disassembly of the unit; if necessary, you can remove and thoroughly clean the outer tube and flexible screw.
An option that's ideal for applications where total evacuation of conveyed materials is essential is a high/low or tilt-style flexible screw conveyor. The high/low design allows the operator to lower the conveyor to a horizontal position to fully remove the screw (Figure 3). This greatly simplifies and enhances washdowns, cleanouts, auger changeovers and regular maintenance. Operators can clean and disassemble the conveyor safely on the floor in just minutes without using lifts or ladders.
MODES OF OPERATION
A flexible screw conveyor can handle batch, intermittent and continuous services.
Batch operation. This is the easiest of all duties. When only one material is batched, it's relatively easy to maximize batch output; product left in the conveyor casing will make the first batch somewhat underweight, but successive batches will be more repeatable. When dealing with multiple materials, always consider residual product in the conveyor because it may contain various ingredients that may not completely transfer from the system as desired. By first introducing a portion of the major ingredients, followed by the minor ingredients, and then dumping in the remaining portion of the major ingredients, you can minimize loss. This is just one method of batching when delivering an exact weight isn't critical.
Intermittent operation. During this process, the conveyor starts and stops in a repetitive fashion under full load. Some heavy materials may cause startup issues. To mitigate this problem, special modifications to the conveyor can increase its structural strength or reduce the amount of material during startup.
Continuous operation. Units expected to perform nonstop for extended intervals must be specifically designed for this type of operation and should be sized to run at an average speed range.
Flexible screw conveyors are a cost-effective dependable choice for conveying a wide range of materials. Their rugged simple design fosters reliable service, provides user-friendly operation and keeps maintenance low.
By first considering several important factors — material characteristics (e.g., whether heavy granules or light powder, wet and sticky or dry and aerated, abrasive or corrosive) and process criteria (e.g., whether continuous or batch operation, necessary incline and run length) — you can ensure selection of a flexible screw conveyor that suits your application and provides proper operation and optimal service.
MIKE ZELUFF is a product manager for Hapman, Kalamazoo, Mich. E-mail him at firstname.lastname@example.org.