Determining the most efficient method to move material from point A to point B is a matter of simple deduction. Just match the material flow characteristics, process parameters and space limitations to the performance windows and dimensional restrictions of different conveyors. Then, factor in purchase, maintenance, labor and cleaning costs, and the conveyor choice is clear.

Because the performance, size and cost of conveying methods vary, only a general knowledge of the different types is necessary to decide which is best. Armed only with the fundamentals, anyone can determine whether or not a flexible screw conveyor can handle an application most efficiently.

In its basic form, a flexible screw conveyor consists of a spring-steel or stainless-steel, flexible screw that is enclosed in a flexible or rigid plastic tube, or a rigid steel tube. A small-diameter, plastic inner tube may also run the length of the screw. The conveyor is driven by an electric motor normally located at the discharge end.

The intake end of the conveyor tube is typically equipped with a charging adapter. This U-shaped trough connects to the outlet of a hopper or directly to the outlet of process equipment such as a bulk bag discharger, bag dump station, grinder, crusher, screener, blender, reactor or storage vessel of any kind.

The flexible screw passes through the charging adapter trough, exposing a section of the screw to material flowing into the trough from above. When rotating, the exposed section of screw feeds material into the outer tube. It is then propelled through the tube by the enclosed portion of the screw.

Because of its loose fit, the screw automatically self-centers within a material-filled tube, providing clearance between the screw and tube wall. This clearance between the screw and tube wall can eliminate or minimize grinding, crushing and impact damage.

Flexible screw conveyor attributes

Because the intake end of the screw requires no bearing and the discharge end is coupled to the motor drive above and/or beyond the point at which material exits the discharge spout, material does not come in contact with seals or bearings. The flexible screw is the only moving part contacting material.

This design does not use filters, cyclone separators, internal bearings and other moving parts found on pneumatic conveyors, bucket elevators, rigid augers, drag chains and aero-mechanical conveyors. These parts can add to initial cost, require maintenance, wear out or break down.

A flexible screw conveyor can transport bulk materials ranging from large pellets to sub-micron powders — both free-flowing and nonfreeflowing — including difficult to handle products that pack, cake, seize, fluidize, plug or smear. These materials include fine powders, compressible materials, dense materials and materials with particles as large as 1 in. The rolling action created by the rotating screw prevents the blended products from separating, even those with disparate particle sizes and densities, throughout the conveyor's length.

They can move powder and bulk solid materials vertically, horizontally, or at any angle; over, under, or around obstructions; and through small holes in walls or ceilings. This allows for positioning the conveyor's intake and discharge ends to accommodate the preferred system layout or awkward layouts with retrofits and upgrades. These conveyors can be fully enclosed to prevent product and plant contamination, while preserving the materials' moisture and temperature levels.

For easier cleaning, all interior surfaces are smooth, and there is a removable clean-out cap covering the intake end of the conveyor tube. Cleaning consists of removing the clean-out cap and reversing rotation to evacuate any residual material prior to flushing with air, water, steam or cleaning solution. The screw and outer tube can also be removed to wash individual components.

Reliability and maintenance

A flexible screw conveyor contains no internal bearings, filters, or separators to wear, breakdown or require maintenance. Depending on application parameters, the drive motors can be started and stopped repeatedly, even under a full load, with no binding or damage to the unit.

The lack of internal components — and external compressors, blowers, and separators — can make the flexible screw less costly to build and operate than other conveyors. It also can be economical to install, since the intake and discharge ends can be positioned where desired, without exact conveyor routing. This reduces the need to reconfigure other upstream and downstream process equipment.

They can be caster-mounted on frames with conveyor support booms for in-plant mobility, avoiding the cost of multiple stationary units. Horizontally oriented flexible screws can feed multiple packaging machines and process equipment simultaneously or individually. They also can convey bulk material over long distances, feed multiple discharge points selectively, or top off all points on a timed cycle through manual or automatic slide gate valves.

Since the units can be equipped with AC or DC variable-speed drives and hoppers with flow promotion devices for continuous feeding of both free- and nonfree-flowing materials, they can simultaneously meter and convey bulk solid products.

Beyond conveying

Gain-in-weight batching systems have individual or multiple flexible screw conveyors discharging into a central weigh hopper or other process equipment equipped with load cells. Gain-in-weight information is transmitted to a central programmable controller, which in turn controls each conveyor. The batch sequence can be initiated by a start button or by external contact closure. The controller then activates each conveyor, in sequence, to load each ingredient into the central weigh hopper, first at high speed, then at dribble speed before stopping the conveyor when the target batch weight is reached.

After each conveyor has deposited its ingredient, the entire batch can be automatically delivered to process equipment, storage vessels, or packaging lines. The conveyor tube allows the weigh hopper to be positioned directly above a blender or other process equipment. Because the system functions as both weigh feeder and conveyor, it can eliminate the need for separate equipment.

Limitations

Because every conveying possesses has inherent limitations, each must be identified and ruled unimportant or less important than corresponding attributes, before deeming a flexible screw conveyor to be the best alternative for an application.

Screw conveyors, like other mechanical conveyors, do not generally evacuate material completely when operating in a forward direction. Residual material can be fully evacuated, however, by removing the cleanout cap and reversing the screw rotation.

Although these conveyors tend not to grind or crush, not all materials can be conveyed damage-free. Some large, brittle flakes or large, soft particles may break, particularly through long runs or steep inclines in a flexible screw conveyor. But, proper system engineering can eliminate or minimize these effects.

Flexible screw conveyors are often used where capacity requirements are less than 50 tph (per conveyor). Thus, they are infrequently employed for off-loading of ships or railcars.

If a flexible screw conveyor's characteristics align neatly with material characteristics and process parameters, then the next step is to provide material samples and application details to an application engineer. The engineer can suggest the optimum system for integration with a new or existing process in the form of detailed CAD drawings, specifications and guaranteed performance ratings.

Should the application call for capabilities beyond those outlined above, a fundamental knowledge of other relevant modes of conveying can determine which incurs the fewest number of compromises, or which best solves the most critical problems.

David R. Gill is Vice President of Flexicon Corp.

Typical applications

• Conveying between storage vessels
• Bulk bag dischargers
• Manual bag dump stations
• Drum dump stations
• Crushers, screeners and blenders
• Reactors and packaging equipment
• Injection molding and extrusion machines