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Advantages of and Recent Developments in Aero-Mechanical Conveying Equipment

May 13, 2008
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The Aero-Mechanical Conveyor (AMC), most commonly known as a “rope and disk” conveyor, has been around since the early 1960’s. Now used in over 30 countries, the AMC has made a name for itself for conveying everything from coffee beans, tea leaves, malt, plastics to titanium dioxide as well a variety of other products in the chemical, mineral, food, ceramic, pharmaceutical, plastics, rubber and water industries. This article outlines the advantages and disadvantages of using AMC’s and notes some recent equipment developments that reduce maintenance cost.

An Aero-Mechanical Conveyor (AMC) is a high capacity, totally enclosed mechanical conveyor that handles powders, granules, pellets and flakes. In the AMC, a continuous wire rope, with a series of equally spaced disks attached to it, travels through a flow and return tube arrangement.

A constant speed motor moves the rope and disk assembly creating a conveying action that draws material into the slip-stream behind the disks like the way dust is drawn into the slip-stream behind a fast moving automobile! It is essential that material is ‘stream fed’ into the Aero-Mechanical Conveyor at a controlled rate. The high-speed disc mechanism fluidizes all flowable solids in a recirculating air stream, which provides low shear conveying of powders or particulates typically up to 5/8 inches in diameter.

Most AMCs can be used with multiple inlets or outlets, move product vertically, horizontally or at varying angles and up to distances of up to 60 feet, depending on variables. AMCs can easily be connected to other equipment such as Silos, Mixers, Sifters, Reactors, Bulk Bag Dischargers or Bulk Bag Fillers. Smaller units can readily be mounted on a mobile support frame so that one machine can be used for a multiplicity of applications.

One of the most noted successes of AMCs is their ability to handle powders that have notoriously difficult characteristics such as inherently sticky, cohesive materials like Titanium Dioxide (TiO2). This is due to the way that an Aero-Mechanical Conveyor mimics the fluidization process of a fully pneumatic conveyor without potentially degrading the high-speed pneumatic action and cost associated with, in particular, lean phase pneumatic equipment. The gentle fluidization process of an AMC dramatically reduces degradation.

An additional reason for most AMCs success with difficult materials is its stream feeding process, which normally doesn’t allow powders to cake up or otherwise be problematic compared to plug fed systems.

Aero-Mechanical Conveyor Basics

A typical AMC, like that of Spiroflow Systems, consists of six main components: inlet housing, outlet housing, set of two conveyor tubes, rope and disk assembly, electric drive and a gravity inlet.

The rope and disk assembly is typically completely enclosed in the conveyor tubes and is wrapped around sprockets at each end of the conveyor within the inlet and outlet housings. Typical rope and disk flights are manufactured out of stainless steel strand rope and plastic disks moulded on to the rope at regular spacing.

A drive at either the inlet housing or outlet housing drives one of the sprockets and in turn rotates the rope and disk assembly around the sprockets. The drive is typically mounted to the inlet (bottom) housing for conveyors up to 20 feet long, whereas any conveyor over 20 feet would rely on an outlet (top) housing mounted drive. By adding corner housings to some makes of AMCs, “turns” can be made in the conveyor.

Additional inlets can normally be positioned along the length of an AMC for batching applications. Product is ejected centrifugally via the outlet housing. Additional intermediate outlets, each with its own valve, can be installed along the conveyor tubes for multiple discharge points.

The speed of the rope and disk assembly is usually about one quarter of the air speed in pneumatic systems, but very much faster than the speed of most mechanical handling equipment. Aero-Mechanical Conveyors should not be confused with low speed Drag-Link Conveyors some of which appear to be similar in construction but which operate at much lower speeds.

Operation of a Typical AMC

A typical AMC like that from Spiroflow uses a motor that rotates either the outlet sprocket or the inlet sprocket. Material is stream fed (metered) into the inlet housing and picked up by the slip-stream behind the disks. The high speed of the rope and disk assembly conveys the material to the outlet where it is centrifugally separated from the air stream.

Proper operation requires stream feeding an AMC. An inlet baffle is often used to control the feed into the conveyor and is typically available in a manual or pneumatic option. Other options, such as a rotary valve or flexible screw conveyor, can also be used to control the rate of flow from the hopper to the conveyor.

Typically a mix of 15 percent solids and 85 percent air is seen when the AMC is operated at full speed of approximately 702-1175 ft./min. Full speed is ideal for correctly conveying dense and poor fluidizing products. Finer solids that fluidize more easily can be run at half speed, which is 351-587 ft./min. This also changes the solids/air capacity to 30 percent/70 percent respectively.

Component Options

AMC options vary with manufacturer, but typically include various inlet/hopper options such as dust hoods, bag dump stations with bag break devices, hopper level indicators, extended/large capacity hoppers, valves for proper stream feeding and flow aids such as vibratory pads.

Other options include multiple configurations, tube mounted inlets/outlets, manual and automatic rope tensioning devices and static bonding.

Dust Hoods

To reduce or eliminate dust when feeding an AMC, a dust hood can be installed over the hopper. The dust hood is fitted to the facility supplied dust collection equipment. In most cases, a grate will also be included with the dust hood as a means of preventing the operator from reaching into any moving parts and also to stop foreign materials such as bags from being dropped into the hopper.

Bag Dump Stations

Bag dump stations are typically installed with bag break devices, and sometimes a means of bag disposal, to ensure easy user functionality. Dump stations can be used with our without dust hoods.

Hopper Level Indicators

It’s important that the AMC fully conveys and discharges the last bit of material and not be stopped with material in the conveyor. This prevents any damage to the rope and disk assembly. Hopper level indicators can be used in conjunction with programmable logic controllers (PLCs) to convey the final amount of material in a time interval and then shut down the AMC motor. Running the AMC without material is permitted

Extended/Large Capacity Hoppers

Large capacity hoppers can be fitted to the inlet housing depending on application. A meter feeding device must be used.

Valves

To ensure proper feeding, many AMC users turn to a valve that will regulate material feeding into the conveyor. Rotary and butterfly valves are very commonly used and perform well in an AMC application.

Metering Feeders

Where a truly accurate flow of material into a process is required, AMCs can be feed by way of volumetric or gravimetric metering feeders.

Flow Aids

For conveying characteristically difficult materials, vibratory pads may be mounted to the inlet hopper to reduce the chances of material bridging, caking or rat holing.

Multiple configurations

In addition to being configurable to have angles of operation from 0º to 90º, up to two sets of corner housings can be added to provide a horizontal-vertical-horizontal configuration. Other configurations are possible and depend on the application needs and the material being conveyed.

Tube mounted inlets/outlets

If desired, AMC’s can have multiple tube inlets or outlets to transfer material into the conveyor. Typically, a slide gate device is used to stream feed the material.

Recent Developments in Aero-Mechanical Conveyor Technology

Maintaining AMCs requires periodic maintenance at regular intervals to assure proper rope and disk assembly tension and to lubricate moving parts.

One of the most important maintenance issues is properly maintaining rope tension. This will ensure long rope life and decrease the possibility of lost production time due to broken ropes. This is an easy maintenance issue to address, but may often be overlooked. That is, it is easy as long as the AMC is readily accessible. In some AMC systems where a vertical unit feeds into a horizontal unit operating in the roof area, for example, access may involve the use of scaffolding or a mobile access platform.

An automatic tensioning system is available that keeps the rope properly tensioned. Upon each shutdown of the system, an integrated load cell measures the tensioning. An electric or pneumatic linear actuator sets the housing position
to reach the correct predetermined tension. This feature increases the rope life and reduces maintenance time.

In older AMC models, the rope had to be manually tensioned. This meant checking the rope tension at 1, 4, 8 and 50-hour intervals, then every 100 hours or as needed. The process involved checking rope tension at the inlet (bottom) sprocket for excess rope sag from the sprocket. Normal clearance is 1/8-inch between the rope and sprocket. At the top sprocket, users had to check for slippage between the rope and the sprocket.

The advantages of the automatic tensioning systems include reducing maintenance and operational costs. The automatic tensioning systems eliminates the need for maintenance employees to check rope tension and provides feedback on rope wear via position control.

Advantages of the Aero-Mechanical Conveyor

The AMC has proven to be one of the most cost efficient methods of conveying materials in terms of its high productivity and dust-free operation. Features typically include total batch transfer, flexible operation at any angle without loss of capacity and a dust-free sealed system for contaminant free delivery. Capacity of material handled varies but can reach 120 tons per hour.

The AMC can usually convey up to 60 feet without any problems. A major advantage is that degradation to the material is almost negligible with this type of conveyor, since it creates a moving current of air in which the material is conveyed, similar to the effect of a vacuum or pneumatic system. However, the Aero-Mechanical Conveyor does not need a cyclone or filter system to separate the product from the air – this fact alone is a major advantage. Not only is there no initial cost of a filtration system, there are no filtration running costs or the possibility of environmental contamination. Aero-Mechanical Conveyors normally conveys product without virtually any losses.

Other than free flowing powders such as acrylics, flour and carbon black, the system can also convey difficult materials such as titanium dioxide. It addition, it has no problem with granules, flakes or chips. Unlike screw conveyors that tend to allow the material to cake and compact, the fluidizing action of the AMC efficiently moves the material without problems.

Disadvantages

With this type of conveyor, maintenance needs are moderate to high depending on the amount of time the conveyor runs as well as the material conveyed. The rope must be tensioned periodically. Rope life also depends on the conveyor length, the number of starts and stops, solids loading and whether routine inspection and tensioning are properly performed. This particular disadvantage can be eliminated by the use of an auto-tensioning device available from some manufacturers.

The only other disadvantage is the rope itself. Being a series of strands, it is not the easiest component to clean. Having said that, this is usually only an issue where cross-contamination cannot be tolerated between frequent batch changes. AMC’s can be dry-cleaned or more often they are washed through with a suitable cleaning fluid and then dried by running them empty for a period. Most manufactures offer a variety of access panels for dry cleaning or connection and drain points for an integrated clean-in-place system.

Coated or encased ropes have been tried by some manufacturers but inevitably, as with most coatings, they break down and create a greater problem in the long term.

Conclusion

Now used in over 30 countries, the AMC has made a name for itself for conveying everything from coffee beans, tea leaves, malt, plastics to titanium dioxide. Consistent performance and operational reliability have endeared aero mechanical conveyors to a wide audience of industries. From the seemingly simple task of transferring food ingredients to the abrasive and corrosive nature of conveying chemical industry products, AMCs are well equipped to be the ideal solution for a broad spectrum of bulk solids

Spiroflow is a worldwide supplier of a wide range of both standard and custom powder handling equipment, specializing in Bulk Bag Dischargers and Fillers, Mechanical, Flexible, and Pneumatic Conveyors, and Bag Packing equipment. For more information about this article, contact Spiroflow Systems, Inc at 704-291-9595, fax 704-291-9594 or email info@spiroflowsystems.com. Spiroflow can also be found on the Internet at www.spiroflowsystems.com.
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