Moving bulk solids: comparing pneumatic and mechanical conveying
By David Boger, Allen Powell and Paul Sattler*
Thursday, 03 October, 2019
The choice between dilute phase pneumatic conveying, tubular cable conveying and flexible screw conveying is not always clear.
Most materials can be conveyed using any of several methods. The final selection comes down to balancing the pros and cons of each type of system for the particular application, with all its variables. That said, the following guidelines will help in selecting the optimum method for conveying different types of bulk solid products.
The first rule
The first rule is: ask the experts. While the science of conveying has advanced significantly over the past several decades, there is still an art to selecting the best overall system and engineering it to meet individual needs. The final analysis requires an intimate understanding of the material and process as they relate to the strengths and limitations of each conveyor technology. Consult with a specialist who does not have a vested interest in selling only one type of equipment. An expert will weigh each parameter and recommend the best solution for each application.
A corollary to the first rule is: test before buying. Testing will assure that the specified system will, in fact, convey the subject material the required distance without degradation or undesirable changes in product characteristics. A fully equipped testing facility will contain full-size systems that are easily reconfigured, as well as a full range of accessories and peripheral equipment. It will contain both pneumatic and mechanical bulk handling equipment to produce an objective recommendation based on the actual material to be conveyed. By verifying performance prior to fabrication, costly misjudgements and delays can be avoided in installing and commissioning the system.
A flexible screw conveyor (Figure 1), also known as a spiral conveyor, helical conveyor or centerless auger conveyor, consists of a flexible screw contained in a flexible or rigid tube that is driven by an electric motor. It is a relatively simple design, and generally the most economical choice, with efficient performance, high reliability, and low capital and operating costs. When properly engineered and tested, it will provide excellent performance across a broad range of applications. There are also systems specifically designed to convey difficult-to-handle materials that tend to pack, cake, smear or fluidise, or break apart.
Pneumatic conveyors move bulk materials that are suspended in a gas stream (most often air, but sometimes an inert gas) introduced by either a positive pressure blower upstream of material intake points, or by a vacuum pump downstream of material discharge points. Product is separated from the gas stream at the end of the line by filter receivers or cyclone separators or sent directly into process vessels. These systems, which may be more complex than mechanical conveyors, can be integrated into process or production lines and will readily handle diverse products in the same equipment. Positive pressure pneumatic conveying is generally used to convey materials from a single source to one or multiple destinations, over relatively longer distances and with greater capacity than vacuum systems with similar size conveying lines. Vacuum systems allow easy pick-up of materials from open containers using wands, so are better suited to transport material from multiple sources such as storage vessels, process equipment and rail cars to single or multiple destinations.
Tubular cable conveyors (Figure 2), also known as drag or disc conveyors, consist of low friction polymer discs attached to a steel cable. The discs and cable are driven by a wheel at one end of the circuit, while a second wheel maintains the cable’s tension. The entire circuit is enclosed inside steel tubing. Tubular cable conveyors gently slide bulk solid material at relatively slow speeds through this tubing in the space between the discs. A truly ‘modular’ system, tubular cable conveyors can have multiple inlets and outlets (which may be added or moved). The conveyor can be routed at nearly any angle, and pass through small openings in walls or ceilings.
Choosing a system
The factors to evaluate when selecting a type of conveying system are:
- Material characteristics
- Material source and destination
- Conveying parameters
- Plant conditions
First, consider the properties of the material to be conveyed, including bulk density, flow properties, temperature, moisture content, inherent hazards and allowable degree of degradation. Both pneumatic and mechanical conveyors will handle a wide range of products, from fine powders to large particles. Both types can be designed to move materials that are friable or fragile, as well as temperature-sensitive materials.
Individual parameters or a combination of requirements can swing the advantage to one conveyor or other. Pneumatic conveying systems are best suited for dry, free-flowing to semi-free-flowing bulk products. Highly engineered screw conveyors are available for moving more difficult materials that might cause a pneumatic conveyor to plug, and a general-purpose screw conveyor to bind or seize. These conveyors have specially designed screws, tight tolerances and straight conveyor tubes to efficiently handle a broad variety of non-free-flowing products such as brown sugar, TiO2 or products that have high fat or oil content. With their relatively slower speed, tubular cable conveyors are highly regarded for gently moving shape sensitive, fragile or friable materials with minimal damage.
Where temperature and moisture content must be maintained, exposure to large volumes of air can rule out pneumatics. While it is possible to condition pneumatic conveyor air for temperature and moisture, this adds considerably to the costs. In such cases either a flexible screw conveyor or a tubular cable conveyor is more effective.
Flexible screw conveyors and tubular cable conveyors are also a better choice when dealing with blended materials because they prevent the separation of blends throughout the entire length of the conveyor, regardless of differences in the flow characteristics, or bulk density.
Extremely fine (submicron) powders are best conveyed with an enclosed flexible screw system because the amount of dust created by the process is minimal and requires little or no air filtration at the discharge point. Fine particles can clog the filters in a pneumatic conveying system, adding to maintenance cost and requiring larger space. Fine particles tend to migrate along the edges of the discs and the inner wall of the conveyor tubing in a tubular cable conveyor, which can cause additional drag and increased component wear.
Pneumatic conveying is usually best when handling hazardous materials that require inert gas blanketing to prevent explosions, oxidation or other changes in product characteristics. When complete containment of a material is necessary, vacuum pneumatic conveying is the method of choice since any breaches in the system will generally leak air into the conveyor, not material into the plant environment. Likewise, with very-high-temperature material, a positive pressure pneumatic conveyor has the advantage.
If degradation of product during transport is a concern, all three systems must be properly designed to minimise damage. This is where testing becomes particularly important, especially if there are no fixed, measurable criteria for the permissible degree of degradation. Testing can determine how much degradation occurs with each technology at varying flow rates and operating conditions. A test program will ensure that, whatever the bulk product, it will reach its destination with properties intact.
Plant personnel handling abrasives expect to maintain equipment and replace components. With flexible screw conveying, the inner screw may need to be replaced periodically, but downtime is minimal. Tubular cable conveyors are more likely to suffer component damage and downtime from the handling of abrasive materials due to their design. Dilute phase pneumatic systems can also handle abrasive materials if other parameters favour this conveying method, but will require periodic replacement of elbows, rotary valves and other components. Proper design and specification of the system, eg, layouts that minimise impact points and the use of wear-resistant elbows, can increase the viability of pneumatic systems.
Material source and destination
Material sources include process equipment, small containers (bags, drums or boxes), bulk bags or bulk transportation vehicles (trucks, rail cars, and ships/barges).
When materials are introduced from multiple sources, either sequentially or simultaneously, pneumatic conveyers or tubular cable conveyors are the better choice. However, as pneumatic conveyors require separate receiving equipment at every destination, tubular cable conveyors can be the lower cost alternative when delivering to multiple discharge points.
For material in bags, drums or boxes, a vacuum conveying system with a pick-up wand can pull material directly from the container. Use of a flexible screw conveyor or tubular cable conveyor requires that the containers be dumped or discharged into a hopper fitted with an intake adapter. Either technology is suitable for conveying products being discharged from bulk bags.
Emptying larger volumes of material from trucks, rail cars and ships is best accomplished with a pneumatic conveying system due to the configurations of these larger containers.
Where distances are short, pneumatic units tend to be more expensive. As distance increases, the limits of a single flexible screw conveyor are reached and may require several conveyors in series. At some point, the multiple flexible screw conveyor transfer system becomes more costly than a comparable tubular cable or pneumatic system. The higher capacities associated with moving large volumes of materials in a relatively short time, as in unloading ships or barges, are better suited to a pneumatic system that utilises large diameter conveying lines.
Where equipment mobility is a requirement, either flexible screw conveyors or vacuum conveyors can be designed as self-contained modular units on caster-mounted frames. However, if the same equipment is used to move varied bulk solids, then a pneumatic conveyor is better, since flexible screw conveyors may require screws of different geometries to handle dissimilar materials.
Where cross-contamination is a concern, flexible screw conveyors are easy to clean because they lack internal seals, crevices or joints that can trap particles or breed contamination. Simply reversing the screw rotation will evacuate residual material and the interior surfaces can be flushed with air, water, steam or cleaning solution. Wet or dry cleaning accessories can be attached to the cable of a tubular cable conveyor to minimise downtown between changeovers. In contrast, a pneumatic conveying system must be specially designed to decrease cleaning difficulty, with conveying lines broken into detachable sections, and supports that allow removal of heavy rotors from rotary airlock valves, increasing cost significantly.
If complete discharge of a batch is essential, positive pressure or vacuum pneumatic conveyors and tubular cable conveyors hold the advantage over flexible screw conveyors, which will retain material after the conveyor has stopped discharging. This is why the lower end cap of a flexible screw conveyor must be removed and the screw reversed to evacuate residual material before the conveyor can be sanitised.
The plant layout, routing requirements and space considerations are all important when comparing conveying options. Pneumatic conveyors are well suited to straight horizontal or vertical routing, but should not be routed at other elevations because gravity and friction may negatively affect performance of the system. When conveying to an elevated point in a straight or curved path, a flexible screw conveyor is the better choice. Where there are numerous changes in direction, or turnings in a limited space, either a pneumatic or tubular cable conveyor has the advantage.
Physical limitations such as floor space and ceiling height also impact the choice of conveyors. Flexible screw conveyors require a larger bend radius and cannot make tight turns, while pneumatic conveying lines and tubular cable conveyors can handle right-angle bends. If filter receivers of pneumatic systems require more headroom than the ceiling permits, one may need to consider a mechanical conveyor.
At floor level, a wide range of factors may come into play when determining how much vertical space is required including the material source and whether the conveying equipment requires a metered feed of material or can been flood-fed.
When an application requires multiple types of conveying, all three conveying technologies can be employed, eliminating compromise.
It comes down to economics
Ultimately, the decision then comes down to economics, with flexible screw conveyors offering lower capital and operating costs, especially over shorter distances involving lower capacities. Pneumatic conveyors, although higher in initial cost and power consumption, offer greater cost-effectiveness when conveying in higher capacities or over longer distances. The initial tubular cable conveyor investment usually falls somewhere between the costs of a flexible screw conveyor and a pneumatic system.
In the final analysis, an expert, unbiased opinion and full-scale testing should confirm the proper conveyor choice for individual applications.
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