Todd Smith and Kevin Solofra, Kansas State University’s Bulk Solids Innovation Center

Industrial processing companies such as those that manufacture food, animal food, pharmaceuticals, chemicals, and plastics handle an immense variety of powders and bulk solids. Each application and material has its own needs and requirements. A bewildering number of conveyor types are available to meet those needs, making it difficult to know which one to apply in which application. This article evaluates one particular conveyor--the cable-type tubular drag conveyor--describing how it works and explaining its advantages, disadvantages, and appropriate applications.

Explanation of the Technology

Many types of conveyors are used to move dry materials from one location to another. For example, traditional drag conveyors achieve this by mechanically pulling a series of plates or paddles through a trough; conveyance occurs as the paddles drag the bulk solid materials along the conveying path.

Tubular drag conveyors refine this method by using smooth disc-shaped paddles to move the material through an enclosed tube or pipe. The refined benefit is clean and gentle conveying when the discs are attached to a continuous smooth cable, and the cable is mechanically pulled through the tube at slow speeds.

Discs are evenly spaced along the cable, and the ends of the cable are connected, forming a continuous loop. Bulk solid materials are introduced at the source and gently conveyed through the enclosed tube to the destination. Then the empty discs and cable travel back through a parallel return tube, returning back to the source to pick up more material.

The cable and disc system is powered by a motor-driven sprocket. Since pulling a cable is much more effective than pushing it, the powered sprocket is located after the destination, so that it can pull the loaded cable from the source to the destination. Another sprocket is used at the other end to make the loop. This lower sprocket is not powered, but it typically includes a tensioning device to maintain the desired tension on the cable.

The entire system is impressively simple, clean, and reliable. These attributes are not easy to achieve individually, let alone at the same time, making it clear that much engineering effort has gone into the component and system designs.

Advantages of Tubular Drag Conveyors

Most advantages of tubular drag conveyors center around the gentle handling and the clean system provided by this type of conveyor.

Product integrity, especially maintaining the desired particle size and shape, is critical for many ingredients and finished products. Gentle handling to prevent product degradation is essential, and many conveyors struggle to meet this requirement.

Within many other types of conveyors, velocity of the conveyed material is the most significant cause of particle degradation--at high velocities, particles slam into each other, causing them to fracture or rounding-off corners of the particles. In addition, in many conveyors, the particles impact elbows, chutes, buckets, or the receiver at high velocity, striking these hard surfaces at speeds of 300 to 6,000 ft/min. In contrast, tubular drag conveyors traverse at a slow speed of 20 to 140 ft/min. This slow speed allows them to handle even challenging friable materials such as breakfast cereals, chips, crackers, roasted nuts, and delicate agglomerates with minimal breakage.

In addition, tubular drag conveyors offer cleaner conveying than many other types of conveyors. The conveying occurs within a closed system. There is little residual material left after the transfer, which reduces cross-contamination and the need for cleaning. Slow velocity means less dust is generated, and any dust that might be present isn’t trying to escape the system due to pressure or Bernoulli/venturi aspects. Finally--at the request of end customers who require regular cleaning to reduce the chance of contamination or pathogens--unique and thorough methods have been developed for cleaning tubular drag conveyors. The cleaning steps are somewhat intensive, but they meet the end-users’ rigorous requirements, and automation of the cleaning process can eliminate most of the labor.

Summary of Advantages

* Slow conveying velocity results in gentle conveying. This reduces product degradation, and it reduces abrasive wear of the conveying tubes and bends.

* Systems are almost totally enclosed. Therefore, minimal contamination can come into the system from the plant environment.

* Mixed materials stay mixed, without segregation during conveyance

* Little of the conveyed material or dust will escape from the system into the plant. Dust collection requirements are lower than for many other conveying options.

* Washing is a viable option, when needed to eliminate residue or pathogens.

* Washing can be automated.

* Energy efficient, with low horsepower requirements

* System layout has good flexibility, with direction changes, multiple inlets, and multiple outlets all possible. These options provide more layout flexibility than is available with most other types of mechanical conveying. Unlike pneumatic conveying and most mechanical conveyors, it can be installed at any angle.

* System design is achieved by combining a few basic modular parts in a variety of combinations. This means that individual parts can be stocked at the manufacturer, then combined in many ways by the system design engineer to achieve the desired layout and throughput needs. Modularity keeps system engineering requirements and lead times small and reasonable.

* The systems are simple and do not require other significant infrastructures such as control feedback, sophisticated controls or instrumentation, or dust collection.

* System supports can be simple since the equipment is relatively light and does not generate dynamic loads or vibration.

* Low noise level

Disadvantages of Tubular Drag Conveyors

The mechanical nature of tubular drag conveyors--which provides the advantages described above--also result in this type of conveyor's disadvantages. For example, the cable must return from the destination back to the material inlet; therefore, a closed loop of tubing is required. And every direction change adds friction and stress to the moving parts, so layout is limited to a few direction changes, and length is limited as well.

Furthermore, mechanical devices can stretch, break, or wear out, but the equipment engineers have developed nice improvements that allow equipment to last a long time, and sensors are readily included to provide an alarm if anything goes wrong. The authors asked end-users what they liked about this type of conveyor, and the answers often centered around reliability. For example, “Reliability has been great. We were able to install it and forget it, with no issues or problems during several years of operation.”

Summary of Disadvantages

* The cable system makes a loop. After the material is discharged, another tube is required to close the loop and return the cable back to the source.

* Layout is limited regarding distance and number of bends. Generally, no more than six bends are recommended (e.g. three on the transfer line and three on the return). Like all conveyors, fewer bends is better.

* Wear and maintenance are consistent with other types of mechanical conveyors.

* Product residue can remain in the system, therefore if cross-contamination is critical, then each product needs a dedicated conveyor, or a wash sequence is needed between changeovers.

* A controlled infeed such as a vibratory feeder is normally required to control the feed rate and thus prevent overfilling at the material source into the conveyor. This also helps prevent material breakage at the infeed point since it can eliminate pinching of material between the disc and tube at the source opening.

Summary and Best Practices

The tubular drag conveyor outperforms many other conveyor options when product degradation of friable products is a top concern. This makes it especially desirable for transfer of friable ingredients and products such as nuts, coffee, pet food, crackers, snack food, cereal, delicate ingredients, and pelletized products such as animal food.

These conveyors are cleaner and generate less dust than most other types of conveyors. Furthermore, washing is feasible and can be automated. Maintenance and reliability aspects are also good, especially for the light process industries and applications described in this article.

Layout flexibility and limitations make this type of conveyor especially suitable for relatively short, simple configurations with a couple of direction changes.

When choosing the best conveying option, the project engineer must consider requirements of the particular application, material, and regulations. No conveying option is the best for every application, which is why there are so many choices. As described, tubular drag conveyors can provide the best option when its strengths and advantages meet the challenging needs of these difficult applications.

The K-State Bulk Solids Innovation Center (BSIC) offers material properties testing, education classes, product and equipment testing, research, and consulting services related to powder and bulk solids materials. For more information, visit bulk-solids.k-state.edu.