- THE MAGAZINE
The aero-mechanical conveyor (AMC), most commonly known as a “rope and disk” conveyor, has been around since the early 1960s. Now used in over 30 countries, the AMC has made a name for itself by conveying everything from coffee beans, tea leaves, malt and plastics to titanium dioxide, as well a variety of other products in the ceramic, chemical, mineral, food, pharmaceutical, plastics, rubber and water industries.
Principle of OperationAn AMC is a high-capacity, totally enclosed mechanical conveyor that handles powders, granules, pellets and flakes. A continuous wire rope with a series of equally spaced attached disks 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, just as dust is drawn into the slip-stream behind a fast-moving automobile.
It is essential that material is “stream-fed” into the AMC at a controlled rate. The high-speed disc mechanism fluidizes all flowable solids in a recirculating air stream, which provides for the low-shear conveying of powders or particulates typically up to 5/8 in. in diameter.
Most AMCs can be used with multiple inlets or outlets to move product vertically, horizontally or at varying angles up to distances of 60 ft, depending on a number of variables. AMCs can easily be connected to other equipment such as silos, mixers, sifters, reactors, bulk bag dischargers or bulk bag fillers. In addition, smaller units can readily be mounted on a mobile support frame so that one machine can be used for multiple applications.
AMCs have the ability to handle powders that have notoriously difficult characteristics, such as inherently sticky, cohesive materials like titanium dioxide (TiO2). An AMC mimics the fluidization process of a fully pneumatic conveyor without the potentially degrading high-speed pneumatic action and cost associated with, in particular, lean phase pneumatic equipment. The gentle fluidization process of an AMC dramatically reduces degradation. The AMCs’ success with difficult materials is a result of its stream-feeding process, which typically does not allow powders to cake up or otherwise become problematic compared to plug-fed systems.
The BasicsA typical AMC consists of six main components: an inlet housing, an outlet housing, a set of two conveyor tubes, a rope and disk assembly, an electric drive and a gravity inlet (see Figure 1). Typically, the rope and disk assembly is 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 molded onto the rope at regular spacing.
A drive at either the inlet housing or the 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 ft long, while conveyors that are over 20 ft long rely on an outlet (top) housing-mounted drive. “Turns” can be made in the conveyor by adding corner housings (see Figure 2).
The speed of the rope and disk assembly is usually about one-quarter that of the air speed in pneumatic systems, but much faster than the speed of most mechanical handling equipment. AMCs should not be confused with low-speed drag-link conveyors, some of which appear to be similar in construction but operate at much lower speeds.
A typical AMC 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.
The proper operation of an AMC requires stream feeding. An inlet baffle is often used to control the feed into the conveyor and is typically available in a manual or pneumatic setup. 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% solids and 85% air is used when the AMC is operated at full speed (approximately 702-1175 ft/minute). Full speed is ideal for correctly conveying dense and poor fluidizing products. Finer solids that fluidize more easily can be run at half speed (351-587 ft/minute), which also changes the solids/air capacity to 30% and 70%, respectively.
Component OptionsAMC options vary with manufacturers but usually include various inlet/hopper options like 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.
A dust hood can be installed over the hopper to reduce or eliminate dust when feeding an AMC. The dust hood is fitted to the facility-supplied dust collection equipment. In most cases, a grate will also be included to provide a means of preventing the operator from reaching into any moving parts and 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 functionality. Dump stations can be used with or without dust hoods.
Hopper Level Indicators
To prevent any damage to the rope and disk assembly, it is important that the AMC fully conveys and discharges the last bit of material and is not turned off with material remaining in the conveyor. Hopper level indicators can be used in conjunction with programmable logic controllers (PLCs) to convey the final amount of material in a specific time interval and then shut down the AMC motor. Running the AMC without material is permitted and will not cause any damage.
Large-capacity hoppers can be fitted to the inlet housing, depending on the application. A meter feeding device must be used.
To ensure proper feeding, many AMC users turn to a valve that regulates the material being fed into the conveyor. Rotary and butterfly valves perform well in AMC applications and are commonly used.
When a truly accurate flow of material into a process is required, AMCs can be fed through volumetric or gravimetric metering feeders.
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.
In addition to being configurable for 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 (see Figure 3).
If desired, AMCs can feature multiple tube inlets or outlets to transfer material into the conveyor. Typically, a slide gate device is used to stream-feed the material.
Recent DevelopmentsMaintaining 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 to ensure long rope life and to decrease the possibility of lost production time due to broken ropes. This is an easy maintenance issue to address as long as the AMC is readily accessible, but it may often be overlooked. For example, in some AMC systems where a vertical unit feeds into a horizontal unit operating in the roof area, access may involve the use of scaffolding or a mobile access platform.
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, and 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 in. between the rope and sprocket. At the top sprocket, users had to check for slippage between the rope and the sprocket.
The advent of automatic tensioning systems has reduced maintenance and operational costs. Upon each shutdown of the system, an integrated load cell measures the tensioning and an electric or pneumatic linear actuator sets the housing position to reach the correct predetermined tension. An automatic tensioning system increases rope life, reduces maintenance time, eliminates the need for maintenance employees to check rope tension, and provides feedback on rope wear via position control.
Other ConsiderationsWith this type of conveyor, maintenance needs are moderate to high, depending on the amount of time the conveyor runs and the type of material that is conveyed. The rope must be tensioned periodically, and 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 problem can be eliminated by the use of an auto-tensioning device.
Being a series of strands, the rope is not the easiest component to clean. However, this is usually only an issue when cross-contamination cannot be tolerated between frequent batch changes. AMCs can be dry-cleaned, but more often they are washed through with a suitable cleaning fluid and then dried by running them empty for a period of time. Most manufacturers 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, as with most coatings, they inevitably break down and create a greater problem in the long term.
Go with the FlowThe 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 dust-free sealed operation for contaminant-free delivery. The capacity of material handled varies but can reach 120 tons per hour.
An AMC can usually convey up to 60 ft without any problem. 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). In addition, the aero-mechanical conveyor does not need a cyclone or filter system to separate the product from the air. This eliminates the cost of purchasing and running a filtration system, as well as the possibility of environmental contamination. AMCs normally convey product with virtually no losses.
Consistent performance and operational reliability have endeared aero-mechanical conveyors to a wide audience of industries. AMCs are well equipped to provide the ideal solution for a broad spectrum of bulk solids.
For more information about aero-mechanical conveyors, contact Spiroflow Systems, Inc. at (704) 291-9595, fax (704) 291-9594, e-mail email@example.com visit www.spiroflowsystems.com.