Tubular drag conveyors: cable vs chain systems
Although similar in the functionality for food processors, tubular chain and cable drag conveyors have distinct differences in components and operation. A closer look at these systems will help food manufacturers better understand which system would best support their dry food processing requirements.
Tubular drag conveyors are mechanically driven conveyors that drag material along the inside of a tube. They are designed to transfer flowable bulk materials from in-feed points to discharge points through a totally enclosed conduit. Using either a cable or a chain, with close-running discs spaced along its length, the ends of the tubular drag conveyor are connected to each other to form an endless loop, which is pulled by a motor-driven sprocket within an enclosed tube.
Changes in direction are facilitated by bends in the tubing or by corner housings for tighter turns. Discharge of the product is achieved through outlet boxes.
Tubular drag conveyors are flexible and designed for versatility in gentle material handling operations. Modular components allow changes in conveyor routing, length and the number and position of inlets and outlets. They can move material vertically, horizontally, around corners and at an angle at conveying speeds of up to approximately 42.4 m3/h.
These tubular conveyors have been used successfully throughout a range of industries for over 50 years. One of these industries is in the processing of food products, gently conveying friable materials including powders, pellets, flakes, granules, chunks, shavings, prills, parts, crumbles, fluff, regrind and dust. In the process, blended materials containing particles of disparate sizes, shapes and bulk densities are transferred without separation.
Food safety and product reliability, escalating concerns in the food processing sector with more stringent track and trace requirements, have increased the need for product safety procedures to eliminate food contamination and cross-contamination of allergens. Tubular drag conveyors, with their completely enclosed tubing, have proven to be one of the most reliable methods for conveying dry food products safely. Consequently, the demand for these systems in the conveying of food products has continued to escalate.
Tubular drag chain conveyors gently move food products through a sealed tube with a drag chain pulled through it on a loop. Solid circular discs (flights), which push the food through the tube, are attached to the chain. This system can move up to 36,287 kg/h throughput of dry food product.
Since the chain is the defining characteristic of this type of conveyor, a little history might be in order. Tubular chain conveyors were first introduced in the mid-1940s and have since grown in usage in manufacturing. The use of steel bushings in chain, however, was first introduced in the late 1880s as a revolutionary development because the bushing acted as a bearing, giving chain, at the time, greater wear resistance. Chain with bushings subsequently showed up in bicycles, automobiles and airplanes. Bushings are still used today in some tubular drag chain conveyor systems. But even before the advent of steel bushings, as early as the 16th century, Leonardo da Vinci made sketches of what appears to be the first steel chain. Problems in the manufacturing and processing of steel prevented chain growth until the 19th century, when new technologies made steel chain a reality.
Two types of chain can be used in tubular chain conveyors, link chain or roller chain. Link chain can act as a universal joint during movement of the product in the tube, which enables a single conveyor to achieve directional changes in multiple planes. Roller chain, consisting of interconnected plates, held in place with pins and bushings, is capable of one-dimensional movement. Both chains are typically manufactured from type 316 stainless steel, rated for acid and corrosion resistance, but carbon steel is also an option.
Because chains exhibit friction at points of direct contact, most noticeably at system start-up, and particularly in applications of food processing where lubricants are not being used, wear can occur at these contact points throughout the length of the chain.
As with all tubular drag conveyors, the chain-drive components need to be regularly adjusted to keep the system in registration, and minimise the wear and tear of the conveying discs, chain and tube. A tension turnaround unit is used to maintain the proper chain tension.
Similar to tubular drag chain conveyors, tubular cable conveyors gently move food products through a sealed tube, but instead using a coated, flexible stainless steel drag cable pulled through on a loop. Solid circular discs (flights) are attached to the cable, which push the food through the tube. Tubular drag cable conveyors can transport up to 36,200 kg of product per hour, at low speed — similar to chain conveyors — and with product degradation practically eliminated.
The cable used in most tubular drag conveyors is WSC (wire strand core), also known as aircraft cable because it is used in control functions for aircraft. Under normal conditions, these high-tensile aircraft cables will not break. The core is type 316 stainless steel. Surrounding it are stainless steel stranded ropes, an assembly of several strands of steel laid helically in one or more layers around the core. The very high strength of the rope wires enables these cables to support large tensile forces with relatively small diameters.
Historically, wire rope (cable) evolved from wrought iron chains, which had a record of mechanical failure. While flaws in contemporary chain links can lead to catastrophic failure, flaws in the wires making up a steel cable are less critical, as the other wires easily take up the load. While friction between the individual wires and strands causes wear over the life of the cable, it also helps to compensate for minor failures in the short run.
Wire cables were first used starting with mining hoist applications in the 1830s, and not too long after formed the basis for success in suspension bridges in America, such as the support structure for the Brooklyn Bridge in 1870. Cables have since been used for virtually all suspension bridges built in the last 150 years. Elevators, aerial tramways, deep mining shafts and aircraft control systems, all critical applications, use cable. The reliability of cable, and especially WSC cable, has proven to have the required tensile strength and durability needed for any application, including transporting food products in tubular drag cable conveyors.
The tubular cable conveyor operates on low horsepower, utilising energy-efficient variable-speed motors of 1–5 HP each, effectively consuming minimal power compared to other conveyor systems used in food processing. The system’s production flow can be adjusted to variable speeds to modify throughput. A drive unit provides power to move the cable-and-disc assembly through the conveying tubes. The system components are connected with compression couplers and gaskets, making the system totally enclosed and dust-free.
Similar to tubular drag chain conveyors, maintaining the required cable tension by the installer and operator is necessary to proper function of the system, and to eliminate the remote possibility for cable strand breaks at cable connection points. A tension turnaround unit maintains the proper cable tension while the system is in operation. The entire cable is totally nylon sealed, ensuring that no food directly comes in contact with the stainless steel cable, and ensuring that no potential strand breaks come in contact with the food products being conveyed.
This conveyor system has the unique flexibility to integrate with any food production processes: from receipt of raw materials, to weight and fill, grinding and packaging. They are critically applicable where food components can become damaged, where contamination would be prevalent, or where dust accumulation can be excessive, such as in filling and packaging.
System sanitation and food safety
Product contamination is a key issue influencing conveying in food production. In every step of the process, from receipt of raw materials through packaging, keeping foreign matter from entering the process stream, maintaining system sanitation and keeping the system free from unwanted of allergens are critical objectives.
The importance of product contamination is magnified with increasingly stringent governmental and industry product track and trace mandates, and consumer demands for maintaining product integrity and safety.
Line changeovers have become a focal issue in food production, relative to both cleanliness and speed of changeovers. Companies are increasingly running different product lines within a single shift or day. Despite these changeovers, processing plants are expected to maintain stringent levels of sanitary and allergen-free operation.
Every minute spent disassembling a conveyor system for cleaning consumes valuable production time. Yet, if not cleaned properly, that batch of food product being conveyed that needs to be discarded because of contamination is lost profit. Or worse, consumers could be negatively impacted, resulting in potential injury, costly recalls and impacted brand reputation. To resolve these issues, food processors are charged with administering changeovers as quickly as possible while maintaining 100% system cleanliness.
Tubular drag conveyors, both chain and cable systems, transport dry bulk food products gently to discharge points in totally enclosed, dust-free conveying tubes. This prevents foreign substances from contaminating the product stream and keeps dust from the transported product from escaping into the production environment, reducing both the incidence of health hazards and the potential for dust explosions.
Tubular chain conveyor systems are designed to meet sanitation specifications as mandated by the FDA. At the core of these systems is the polished stainless steel conveyor link chain or roller chain, which is rated acid and corrosion resistant.
Because the entire chain is directly immersed within the food being conveyed, as is with tubular cable conveyors, cleaning the chain — quite in addition to the tubing itself — requires more care than what is needed for the cable in a tubular cable conveyor. This is particularly true with roller chain, where food particles can pack spaces between plates, pins and bushings, and where water can remain trapped after cleaning, resulting in potential product contamination.
Notwithstanding, cleaning mechanisms are available that minimise or eliminate the build-up of food particles from the chain, conveying tubes and associated equipment. These mechanisms include brush boxes, chain knockers and wet clean-in-place (CIP) systems.
Dry cleaning mechanisms available for tubular chain conveyors:
- Brush Box — a line insert that mechanically brushes accumulated fines off discs.
- Chain Knockers — a line insert that dislodges particles from the chain.
Wet cleaning CIP (self-cleaning) mechanism available for tubular chain conveyors consists of a 3-step internal tube cleaning process:
- Foam Cleaning/Sanitising — foam cleaning agent is introduced into the tube environment to cleanse and sanitise all internal parts of the system.
- Rinse — a water rinse is run through the system to flush out debris and the foam cleaning/sanitation agent.
- Dry Air — warm, dry air is pumped throughout to dry the system and make it ready for resumption of operation.
Tubular cable conveyors for food-grade applications are better designed, compared to tubular chain conveyors, to eliminate places for fines to accumulate. Solid discs are used, with no screws or bolts, and attached directly to the cable. Stainless steel connectors are used throughout the system. And equipment is removable for easy cleaning.
One of the key advantages of tubular cable conveyors over tubular chain conveyors is the significantly reduced possibility of trapping food residue on the cable, compared to a chain. As with both link chain and roller chain, the cable is fully immersed within the food being transported through the tube. But the chain has significantly more surface area exposed to the food. The cable has only approximately 20% of the surface area exposed to food, compared to the surface area of the chain.
Further, the cable is completely uniform and smooth throughout its entire route within the tube. The chain itself, however, has many 90-degree turns and connections link to link, where food residue can more easily collect. The most hygienic food processing equipment designs minimise or eliminate 90-degree angles where food contact is present.
Supporting this is the stainless steel cable used in tubular cable conveyors, which is totally nylon sealed, ensuring that no debris accumulates within the strands of the cable.
Compared to tubular chain conveyors, tubular drag cable conveyor systems offer more options for dry and wet tube conveyor cleaning. These include brush boxes, urethane wipers, air knives, inline sponges, inline bristle brushes, and multi-step CIP (self-cleaning) wet cleaning.
Dry cleaning mechanisms available for tubular cable conveyors:
- Brush Box — a line insert that mechanically brushes accumulated fines off discs.
- Urethane Wiper Disc — a disc insert that wipes away accumulated fines and particles while the system is in progress.
- Air Knife — strategically located at product discharge locations in the line, air knife inserts use compressed air to blow accumulating fines off discs and the cable.
- Sponge Disc and Sanitising Cleaner — a disposable sponge disc that is run through the system with an applied cleaning agent.
- Bristle Brushes — a brush that is run through the system to remove accumulated fines off the tubing.
Wet cleaning CIP (self-cleaning) mechanism available for tubular cable conveyors consists of a 3-step internal tube cleaning process:
- Foam Agent — foam cleaning agent is introduced into the tube environment to cleanse all internal parts of the system.
- Sanitising Rinse — a sanitising rinse is run through the system to flush out debris and foam agent.
- Water Rinse — a second rinse of hot water flushes out sanitising rinse, rendering the tube environment-production ready.
Choosing a system
When it comes to assessing what a tubular conveying system would be best suited for a specific food processing application, the following aspects should be considered:
- Can the conveyor be configured to operate within the facility’s physical layout? Make sure the conveyor system selected can manoeuvre the turns, inclines, declines, flights and lifts needed for the plant’s processes.
- Does the conveyor system permit flexibility for changes in configuration to accommodate production requirements? Systems can be designed to be highly modular, with the expectation of easy reconfiguration. But what about systems that are designed to be relatively fixed, how easy or difficult would it be to modify their configuration, and at what cost?
- Is the conveyor designed for quick cleaning and line changeovers? Every minute spent disassembling a conveyor system for cleaning consumes valuable production time. Yet, if not cleaned properly, product contamination can occur from debris build-up and allergens. Those systems that permit changeovers to be performed as quickly as possible, while maintaining 100% system cleanliness, will provide a more productive return on investment.
- What are the standard preventative maintenance requirements for the conveyor system? Frequency of maintenance, how extensive and how easy the system is to access are all factors that need to be considered. A system that requires frequent maintenance and part replacements will affect production throughput and cost of operation.
- How easy and timely is it to have repairs made in the event of a system breakdown? System failures need to be planned for, even with the most reliable automation. Getting it back up and running quickly is critically important to maintain throughput. Assessing the probabilities of what could fail, how long it would take to get technicians and replacement parts onsite, repairs made and bring the conveyor back up to full production levels should be considered when reviewing systems.
Tubular drag conveyor systems that were installed in facilities 10 to 15 years ago may have been adequate at that time, but now better technology in conveying system design, controls and automation has brought into place a whole new generation of conveyors for use in food processing, with resultant improved efficiency.
Safer, cleaner processes that reduce waste and deliver cost, labour and energy savings are increasingly being factored into equipment selection. Such tubular drag conveying systems are having a critical impact on food processors’ operational costs and plant ROI.
Benefits of cable versus chain
- Increased potential for system uptime
Cable has no moving components like links, plates, pins and bushings integral to chains, that cause friction and wear — if just one component breaks, complete system shutdown occurs; alternately, excessive system stresses on cables result in localised strain breaks — loads are redistributed, maintaining cable integrity and system uptime.
- Minimised direct contact with food
Nylon-coated cable prevents direct contact of the stainless steel cable with transported food. The nylon’s smooth surface is void of crevices or sharp-angled components where food particles could be collected, such as is the case with chain links, plates, pins and bushings where food particles are packed in.
- Improved performance for food safety, sanitation and changeovers
In addition to a 3-step, clean-in-place wet-cleaning process for internal tubing, cable systems provide a considerably more comprehensive set of five mechanisms for internal tube dry cleaning, compared to chain systems. These processes speed cleaning, ensure sanitation and enable faster changeovers compared to chain systems, which must contend with trapped water deposits within components subsequent to cleaning. Not the case with cable systems, which, because of their smooth, costed-cable design, have no trapped water deposits.
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