by David Boger, Sales Manager and Allen Powell, Regional Applications Engineer
The choice between dilute phase pneumatic conveying and flexible screw conveying is not always clear
Everyone has rules of thumb when it comes to choosing a conveying system—rules based on bulk material characteristics, required flow rates and distances, source and destination, plant configuration, and other variables. But those with experience in all types of systems will tell you that the only real rule for choosing a conveying technology is that there are no hard-and-fast rules.
The fact is, most materials can be conveyed using any of several methods. No one can say with certainty, for example, that positive pressure pneumatic conveying is always the better choice for conveying high-temperature materials. 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 you select the optimum method for conveying your bulk solid products.
The first rule
The first rule is, ask the experts. While the science of conveying has advanced significantly over the past two 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 you.
A corollary to the first rule is, test before you buy. Testing will assure that the specified system will, in fact, convey your powder or bulk material the required distance without degradation or undesirable changes in product characteristics. A fully equipped, state-of-the-art testing facility will contain full-size systems that are easily reconfigured, as well as a full range of accessories and peripheral equipment (Fig. 1). It will contain both pneumatic and mechanical bulk handling equipment to produce an objective recommendation based on your actual material. By verifying performance prior to fabrication, you can avoid costly misjudgments and delays in getting your system up and running.
These test laboratories for mechanical conveying systems (top) and pneumatic conveying systems (bottom) contain full size equipment that can be reconfigured to simulate equipment and systems under consideration, verify performance prior to fabrication, and demonstrate the performance of newly constructed equipment.
A flexible screw conveyor (Fig. 2), 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. Materials are specified according to application, with the screw fabricated of spring steel or stainless steel, and the outer tube being of plastic or steel. 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 fluidize, as well as fragile or brittle materials prone to breakage or crumbling.
Plant diagram showing flexible screw conveyor configurations, Sanitary/mobile flexible screw conveyor, High capacity flexible screw conveyor, Flexible screw conveyor with motor drive at intake end to reduce head space requirements, and Automated weigh batching/blending/filling system with flexible screw conveyors under automated control.
Pneumatic conveyors (Fig. 3) are also generally custom-engineered for each application and will satisfy a wide range of requirements. They 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.
Plant diagram showing pneumatic conveyor line routing, Filter receiver above a bulk bag filler, Combination loss-of-weight and manual batching system with multiple pneumatic conveyor intake points, Filter receiver above a ribbon blender.
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
One of the most important factors to consider is 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 flexible screw conveyors will handle a wide range of products, from fine powders to large particles. Both can be designed to move materials that are friable or fragile, as well as temperature-sensitive materials. Fig. 4 illustrates some of the different types of materials that can be conveyed.
Moist, sticky materials, Friable material, Large particle, Blends of dissimilar ingredient, Abrasive materials, Free-flowing materials, Hazardous Materials, Contamination-Sensitive products, and Products that pack, cake, or smear. Collectively, flexible screw and pneumatic conveying systems can transport virtually any bulk product including: heavy materials; free-flowing or non-free-flowing powders and agglomerates; sub-micron powders to large pellets; difficult-to-convey materials such as abrasives, moist, sticky materials or products that pack, cake, smear or plug; fragile or friable products; blends of dissimilar ingredients; hazardous materials, contamination-sensitive products; and materials that fluidize or liquefy.
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. Specially engineered flexible 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 uniquely designed conveyors have specially engineered screws, tight tolerances, and straight conveyor tubes to efficiently handle a broad variety of non-free-flowing products. Examples include materials that are moist such as brown sugar, materials that tend to cake, stick, or clump like TiO2 and other pigments, and products like cake mix that have high fat or oil content.
Where it is important to maintain temperature and moisture content, exposure to large volumes of air can rule out pneumatics. While it is possible to condition pneumatic conveyor air for temperature and moisture, this adversely impacts the economics, adding considerably to the costs of installing and running the system. These factors may cause a flexible screw conveyor to be a more desirable choice.
Extremely fine (submicron) powders are best conveyed with a 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 make it difficult to keep the filters clean in a pneumatic conveying system, requiring greatly increased area of filtration media, which can add cost and require larger space for installation. Flexible screw conveyors are also the technology of choice when dealing with blended materials, because unlike pneumatic conveyors, they prevent the separation of blends throughout the entire length of the conveyor, regardless of differences in the flow characteristics, bulk density or particle size of ingredients.
The scale tilts toward pneumatic conveying when handling hazardous materials that require inert gas blanketing to prevent explosions, oxidation, or other changes in product characteristics, although it is also possible to blanket a flexible screw conveyor. Where a completely airtight system is necessary, pneumatic conveying is the method of choice. Likewise, with very-high-temperature material, a positive pressure pneumatic conveyor has the advantage. A vacuum system can’t be ruled out, but measures must be taken to protect the air mover and filter media from the heat.
If degradation of product during transport is a concern, either system must be properly designed to minimize 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. Pneumatic conveying has been used to move products as diverse as bran flakes, ice cream sprinkles, bottle caps, capsules and tablets without damage. But experience with certain materials demonstrates that pneumatic conveying can significantly alter its bulk density, much more than with a flexible screw conveyor. A test program will ensure that, whatever the bulk product, it will reach its destination with properties intact.
Abrasive materials are a unique situation, and personnel accustomed to 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. Dilute phase pneumatic systems can also handle abrasive materials if other parameters favor this conveying method, but will require periodic replacement of elbows, rotary valves and other components. This requires an operator with a higher level of mechanical competence, plus longer shutdowns for maintenance. Proper design and specification of the system, e.g., layouts that minimize 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 are the better choice since separate flexible screw conveyors may be required for each source, increasing cost. But since pneumatic conveyors require separate receiving equipment at every destination, such as filter receivers, weighing valves or rotary airlock valves, flexible screw conveyors can be the lower cost alternative when delivering to multiple discharge points, which are in close proximity.
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 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 form bulk bags when the bag is properly suspended above a receiving hopper that is equipped with the appropriate intake adapter.
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.
Since 1990, the capacity of flexible screw conveyors has climbed more than tenfold. However, other parameters being equal, mechanical conveyors are better suited for capacities that correspond to in-plant processing. 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 utilizes large diameter conveying lines.
Where distances are short, either mechanical or vacuum conveying may be feasible, but vacuum units tend to be more expensive. As distance increases, you reach the limits of a single flexible screw conveyor and may require several conveyors in series. At some point the multiple mechanical conveyor transfer system becomes more costly than a comparable pneumatic system. Conversely, as conveying distance increases, pneumatic conveying systems become more practical.
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 multiple types of bulk solids, the versatility of a pneumatic conveyor in handling diverse products may be the deciding factor, since flexible screw conveyors can potentially require screws of different geometries to handle different materials.
Cleanability may also drive your conveyor choice, particularly for food processing or pharmaceutical/biotechnology applications. Flexible screw conveyors are easier 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 allow the smooth interior surfaces to be flushed with air, water, steam or cleaning solution. A pneumatic conveying system must be 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 conveyance of a batch is essential, positive pressure or vacuum pneumatic conveyors hold the advantage over flexible screw conveyors, which contain residual 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 sanitized.
The plant layout, routing requirements, and space considerations are all important when comparing conveying options. Where the conveying route is straight or can be gently curved, a flexible screw conveyor will get the job done. Where there are numerous changes in direction or turns in a short space, a pneumatic system has the advantage, although in designing the system the number of bends should be minimized and their spacing should be well planned. Pneumatic conveyors are best suited to straight horizontal or vertical routing, and should not be routed at other angles because the combination of gravity and friction may negatively affect performance of the system. Where conveying at an angle or in a curve is desirable, a flexible screw conveyor is the better choice.
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 can handle right-angle bends.
The filter receivers of pneumatic systems may require more headroom than the ceiling permits, swinging the pendulum in favor of a flexible screw. At floor level, however, the charging adapter of a flexible screw conveyor is often oriented to accommodate an inclining conveyor, thus requiring more space than a horizontal flow-through or non-flow-through pick-up adapter of a vacuum system, swinging the choice back to pneumatics.
When an application requires multiple types of conveying, both flexible screw and pneumatic conveyors can be employed, eliminating compromise, as shown in Fig. 5.
This automated weigh batching system conveys material from bulk bag dischargers to a central weigh hopper using flexible screw conveyors, and discharges the batch through a rotary airlock valve into a vacuum conveyor system.
It comes down to economics
The preceding discussion has outlined the most common factors influencing the selection of flexible screw and pneumatic conveyors. While the characteristics of your material and the requirements of your process may clearly dictate one technology over the other, both flexible screw and pneumatic conveyors are suitable for most bulk conveying applications. The decision then comes down to economics, with flexible screw conveyors offering lower capital and operating costs, especially over shorter distances involving lower capacities, and pneumatics, although higher in initial cost and power consumption, offering greater cost effectiveness when conveying in higher capacities and/or over longer distances.
In the final analysis, an expert, unbiased opinion and full-scale testing should confirm the proper conveyor choice for your need.