Embed Size (px)
Citation preview
TECHNICALARTICLE SERIES
SOUTH AFRICA
FLEXICON AFRICA (PTY) LTD P.O. Box 10450 Linton Grange Port Elizabeth 6015 South Africa Tel: +27 41 453 1871Fax: +27 41 453 1670E-mail: [email protected]: www.flexicon.co.za
CHILE
FLEXICON CHILE LTD Avenida Andres Bello #1051 Oficina #1602 Providencia Santiago ChileTel: +56 2 2415 1286 Fax: +56 2 2415 1393E-mail: [email protected]: www.flexicon.com
SINGAPORE
FLEXICON SINGAPORE PTE LTD27 West Coast Highway#02-22Singapore 117867Tel: +65 6778 9225Fax: +65 6690 8850E-mail: [email protected]
GERMANY
FLEXICON (EUROPE) LTD Matthäusstrasse 13 63743 Aschaffenburg GERMANY Tel: +49 170 8 187 613 E-mail: [email protected]: www.flexiconeurope.de
AUSTRALIA (SYDNEY)
FLEXICON CORPORATION (AUSTRALIA) PTY LTDSuite 5, Level 3 45-47 Hunter St.Hornsby, NSW 2077 Australia Tel: +61 (0)7 3879 4180Fax: +61 (0)2 9477 5548E-mail: [email protected]: www.flexicon.com.au
AUSTRALIA (BRISBANE)
FLEXICON CORPORATION (AUSTRALIA) PTY LTD 78 Westgate Street Wacol, QLD 4076 Australia Tel: +61 (0)7 3879 4180 Fax: +61 (0)7 3879 4183E-mail: [email protected]: www.flexicon.com.au
USA
FLEXICON CORPORATION 2400 Emrick Boulevard Bethlehem, PA 18020-8006 USATel: 1 888 FLEXICON (1 888 353 9426) Tel: 1 610 814 2400Fax: 1 610 814 0600E-mail: [email protected]: www.flexicon.com
UK
FLEXICON (EUROPE) LTD 182 John Wilson Business Park Harvey Drive Whitstable, Kent CT5 3RB United KingdomTel: +44 (0)1227 374710Fax: +44 (0)1227 365821E-mail: [email protected]: www.flexicon.co.uk
SPAIN
FLEXICON (EUROPE) LTD Méndez Núñez 3-5, Principal 08003 Barcelona Spain Tel: +34 930 020 509E-mail: [email protected]: www.flexicon.es
Art or Science? The choicebetween dilute phasepneumatic conveying andflexible screw conveying is notalways clear
Q-0950
Art or Science?
(Fig. 1) These test laboratories for mechanicalconveying systems (top) and pneumatic conveyingsystems (bottom) contain full size equipment thatcan be reconfigured to simulate equipment andsystems under consideration, verify performanceprior to fabrication, and demonstrate theperformance of newly constructed equipment.
(Fig. 2) Plant diagram showing flexible screwconveyor configurations
By David Boger, Sales Managerand Allen Powell, Regional Applications EngineerFlexicon Corporation
The choice between dilute phase pneumatic conveyingand 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 withexperience in all types of systems will tell you that the only real rule forchoosing 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. Noone can say with certainty, for example, that positive pressure pneumaticconveying is always the better choice for conveying high-temperaturematerials. The final selection comes down to balancing the pros and cons ofeach type of system for the particular application, with all its variables. Thatsaid, the following guidelines will help you select the optimum method forconveying your bulk solid products.
The first rule
The first rule is, ask the experts. While the science of conveying has advancedsignificantly over the past two decades, there is still an art to selecting the bestoverall system and engineering it to meet individual needs. The final analysisrequires an intimate understanding of the material and process as they relateto the strengths and limitations of each conveyor technology. Consult with aspecialist who does not have a vested interest in selling only one type ofequipment. An expert will weigh each parameter and recommend the bestsolution for you.
A corollary to the first rule is, test before you buy. Testing will assure that thespecified system will, in fact, convey your powder or bulk material the requireddistance without degradation or undesirable changes in productcharacteristics. A fully equipped, state-of-the-art testing facility will containfull-size systems that are easily reconfigured, as well as a full range ofaccessories and peripheral equipment (Figure 1). It will contain bothpneumatic and mechanical bulk handling equipment to produce an objectiverecommendation based on your actual material. By verifying performanceprior to fabrication, you can avoid costly misjudgments and delays in gettingyour system up and running.
Equipment comparison
A flexible screw conveyor (Figure 2), also known as a spiral conveyor, helicalconveyor or centerless auger conveyor, consists of a flexible screw containedin a flexible or rigid tube that is driven by an electric motor. Materials arespecified according to application, with the screw fabricated of spring steel orstainless steel, and the outer tube being of plastic or steel. It is a relativelysimple design, and generally the most economical choice, with efficientperformance, high reliability, and low capital and operating costs. Whenproperly engineered and tested, it will provide excellent performance across a
(Fig. 2) Sanitary/mobile flexible screw conveyor
(Fig. 2) High capacity flexible screw conveyor
(Fig. 2) Flexible screw conveyor with motor driveat intake end to reduce head space requirements
broad range of applications. There are also systems specifically designed toconvey difficult-to-handle materials that tend to pack, cake, smear or fluidize,as well as fragile or brittle materials prone to breakage or crumbling.
Pneumatic conveyors (Figure 3) are also generally custom-engineered foreach application and will satisfy a wide range of requirements. They movebulk materials that are suspended in a gas stream (most often air, butsometimes an inert gas) introduced by either a positive pressure blowerupstream of material intake points, or by a vacuum pump downstream ofmaterial discharge points. Product is separated from the gas stream at theend of the line by filter receivers or cyclone separators, or sent directly intoprocess vessels. These systems, which may be more complex thanmechanical conveyors, can be integrated into process or production lines andwill readily handle diverse products in the same equipment. Positive pressurepneumatic conveying is generally used to convey materials from a singlesource to one or multiple destinations, over relatively longer distances andwith greater capacity than vacuum systems with similar size conveying lines.Vacuum systems allow easy pick-up of materials from open containers usingwands, so are better suited to transport material from multiple sources such asstorage vessels, process equipment, and rail cars to single or multipledestinations.
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• Economics
Process & Product Variables Flexible ScrewConveyor
Dilute Phase Pneumatic Conveyor
PositivePressure
Vacuum
Material Characteristics Ultra-fine particles X
Fragile or friable X X X
Moist, packing, caking X
High fat or oil content X
Blended materials X
Heat sensitive X X
High temperature X X
Contamination sensitive X X X
Hazardous X X
Material Source & Destination Multiple sources X X
Multiple material destinations X X X
Convey directly from bags or barrels X
Large capacity source X X
Process equipment source or destination X X X
Very-high-temperature destination X
Conveying Parameters Lower volumes X X X
High volumes X X
Shorter distances X X
Long distances X X
Mobile units X X
Same equipment for diverse products X X
Ease of cleaning X
Plant Conditions Flexibility of conveyor routing X
Indirect routing required X X
Conveying at an angle X
Right-angle turns required X X
Economics Lowest initial cost X
Lowest energy consumption X
Lowest operating costs X
Large capacities, long distances X XTable 1. Parameters for different conveyor types
(Fig. 2) Automated weigh batching/blending/fillingsystem with flexible screw conveyors underautomated control
(Fig. 3) Plant diagram showing pneumatic conveyorline routing
(Fig. 3) Filter receiver above a bulk bag filler
Material characteristics
One of the most important factors to consider is the properties of the materialto be conveyed, including bulk density, flow properties, temperature, moisturecontent, inherent hazards, and allowable degree of degradation. Bothpneumatic and flexible screw conveyors will handle a wide range of products,from fine powders to large particles. Both can be designed to move materialsthat are friable or fragile, as well as temperature-sensitive materials. Figure 4illustrates some of the different types of materials that can be conveyed.
Individual parameters or a combination of requirements can swing theadvantage to one conveyor or other. Pneumatic conveying systems are bestsuited for dry, free-flowing to semi-free-flowing bulk products. Speciallyengineered flexible screw conveyors are available for moving more difficultmaterials that might cause a pneumatic conveyor to plug, and ageneral-purpose screw conveyor to bind or seize. These uniquely designedconveyors have specially engineered screws, tight tolerances, and straightconveyor tubes to efficiently handle a broad variety of non-free-flowingproducts. Examples include materials that are moist such as brown sugar,materials that tend to cake, stick, or clump like TiO2 and other pigments, andproducts like cake mix that have high fat or oil content.
Where it is important to maintain temperature and moisture content, exposureto large volumes of air can rule out pneumatics. While it is possible tocondition pneumatic conveyor air for temperature and moisture, this adverselyimpacts the economics, adding considerably to the costs of installing andrunning the system. These factors may cause a flexible screw conveyor to bea more desirable choice.
Extremely fine (submicron) powders are best conveyed with a flexible screwsystem because the amount of dust created by the process is minimal andrequires little or no air filtration at the discharge point. Fine particles can makeit difficult to keep the filters clean in a pneumatic conveying system, requiringgreatly increased area of filtration media, which can add cost and requirelarger space for installation. Flexible screw conveyors are also the technologyof choice when dealing with blended materials, because unlike pneumaticconveyors, they prevent the separation of blends throughout the entire lengthof the conveyor, regardless of differences in the flow characteristics, bulkdensity or particle size of ingredients.
The scale tilts toward pneumatic conveying when handling hazardousmaterials that require inert gas blanketing to prevent explosions, oxidation, orother changes in product characteristics, although it is also possible to blanketa flexible screw conveyor. Where a completely airtight system is necessary,pneumatic conveying is the method of choice. Likewise, withvery-high-temperature material, a positive pressure pneumatic conveyor hasthe advantage. A vacuum system can't be ruled out, but measures must betaken to protect the air mover and filter media from the heat.
If degradation of product during transport is a concern, either system must beproperly designed to minimize damage. This is where testing becomesparticularly important, especially if there are no fixed, measurable criteria forthe permissible degree of degradation. Testing can determine how muchdegradation occurs with each technology at varying flow rates and operatingconditions. Pneumatic conveying has been used to move products as diverseas bran flakes, ice cream sprinkles, bottle caps, capsules and tablets withoutdamage. But experience with certain materials demonstrates that pneumaticconveying can significantly alter its bulk density, much more than with a
(Fig. 3) Combination loss-of-weight and manualbatching system with multiple pneumatic conveyorintake points
(Fig. 3) Filter receiver above a ribbon blender
flexible screw conveyor. A test program will ensure that, whatever the bulkproduct, it will reach its destination with properties intact.
Abrasive materials are a unique situation, and personnel accustomed tohandling abrasives expect to maintain equipment and replace components.With flexible screw conveying, the inner screw may need to be replacedperiodically, but downtime is minimal. Dilute phase pneumatic systems canalso handle abrasive materials if other parameters favor this conveyingmethod, but will require periodic replacement of elbows, rotary valves andother components. This requires an operator with a higher level of mechanicalcompetence, plus longer shutdowns for maintenance. Proper design andspecification of the system, e.g., layouts that minimize impact points, and theuse of wear-resistant elbows, can increase the viability of pneumatic systems.
Material source and destination
Material sources include process equipment, small containers (bags, drums orboxes), bulk bags or bulk transportation vehicles (trucks, rail cars, andships/barges).
When materials are introduced from multiple sources, either sequentially orsimultaneously, pneumatic conveyers are the better choice since separateflexible screw conveyors may be required for each source, increasing cost.But since pneumatic conveyors require separate receiving equipment at everydestination, such as filter receivers, weighing valves or rotary airlock valves,flexible screw conveyors can be the lower cost alternative when delivering tomultiple discharge points which are in close proximity.
For material in bags, drums or boxes, a vacuum conveying system with apick-up wand can pull material directly from the container. Use of a flexiblescrew conveyor requires that the containers be dumped or discharged into ahopper fitted with an intake adapter. Either technology is suitable forconveying products being discharged form bulk bags when the bag is properlysuspended above a receiving hopper that is equipped with the appropriateintake adapter.
Emptying larger volumes of material from trucks, rail cars and ships is bestaccomplished with a pneumatic conveying system due to the configurations ofthese larger containers.
Conveying parameters
Since 1990, the capacity of flexible screw conveyors has climbed more thantenfold. However, other parameters being equal, mechanical conveyors arebetter suited for capacities that correspond to in-plant processing. The highercapacities associated with moving large volumes of materials in a relativelyshort time, as in unloading ships or barges, are better suited to a pneumaticsystem that utilizes large diameter conveying lines.
Where distances are short, either mechanical or vacuum conveying may befeasible, but vacuum units tend to be more expensive. As distance increases,you reach the limits of a single flexible screw conveyor and may requireseveral conveyors in series. At some point the multiple mechanical conveyortransfer system becomes more costly than a comparable pneumatic system.Conversely, as conveying distance increases, pneumatic conveying systemsbecome more practical.
Where equipment mobility is a requirement, either flexible screw conveyors orvacuum conveyors can be designed as self-contained modular units oncaster-mounted frames. However, if the same equipment is used to movemultiple types of bulk solids, the versatility of a pneumatic conveyor inhandling diverse products may be the deciding factor, since flexible screwconveyors can potentially require screws of different geometries to handledifferent materials.
Cleanability may also drive your conveyor choice, particularly for foodprocessing or pharmaceutical/biotechnology applications. Flexible screwconveyors are easier to clean because they lack internal seals, crevices orjoints that can trap particles or breed contamination. Simply reversing thescrew rotation will evacuate residual material and allow the smooth interiorsurfaces to be flushed with air, water, steam or cleaning solution. A pneumaticconveying system must be designed to decrease cleaning difficulty, withconveying lines broken into detachable sections, and supports that allowremoval of heavy rotors from rotary airlock valves, increasing costsignificantly.
If complete conveyance of a batch is essential, positive pressure or vacuumpneumatic conveyors hold the advantage over flexible screw conveyors, whichcontain residual material after the conveyor has stopped discharging. This iswhy the lower end cap of a flexible screw conveyor must be removed and thescrew reversed to evacuate residual material before the conveyor can besanitized.
Plant conditions
The plant layout, routing requirements, and space considerations are allimportant when comparing conveying options. Where the conveying route isstraight or can be gently curved, a flexible screw conveyor will get the jobdone. Where there are numerous changes in direction or turns in a shortspace, a pneumatic system has the advantage, although in designing thesystem the number of bends should be minimized and their spacing should bewell planned. Pneumatic conveyors are best suited to straight horizontal orvertical routing, and should not be routed at other angles because thecombination of gravity and friction may negatively affect performance of thesystem. Where conveying at an angle or in a curve is desirable, a flexiblescrew conveyor is the better choice.
Physical limitations such as floor space and ceiling height also impact thechoice of conveyors. Flexible screw conveyors require a larger bend radiusand cannot make tight turns, while pneumatic conveying lines can handleright-angle bends.
The filter receivers of pneumatic systems may require more headroom than theceiling permits, swinging the pendulum in favor of a flexible screw. At floorlevel, however, the charging adapter of a flexible screw conveyor is oftenoriented to accommodate an inclining conveyor, thus requiring more spacethan a horizontal flow-through or non-flow-through pick-up adapter of avacuum system, swinging the choice back to pneumatics.
When an application requires multiple types of conveying, both flexible screwand pneumatic conveyors can be employed, eliminating compromise, asshown in Figure 5.
It comes down to economics
(Fig. 4) Collectively, flexible screw and pneumaticconveying systems can transport virtually any bulkproduct including: heavy materials; free-flowingor 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; hazardousmaterials, contamination-sensitive products; andmaterials that fluidize or liquefy.
(Fig. 5) This automated weigh batching systemconveys material from bulk bag dischargers to acentral weigh hopper using flexible screwconveyors, and discharges the batch through arotary airlock valve into a vacuum conveyor system.
The preceding discussion has outlined the most common factors influencingthe selection of flexible screw and pneumatic conveyors. While thecharacteristics of your material and the requirements of your process mayclearly dictate one technology over the other, both flexible screw andpneumatic conveyors are suitable for most bulk conveying applications. Thedecision then comes down to economics, with flexible screw conveyorsoffering lower capital and operating costs, especially over shorter distancesinvolving lower capacities, and pneumatics, although higher in initial cost andpower consumption, offering greater cost effectiveness when conveying inhigher capacities and/or over longer distances.
In the final analysis, an expert, unbiased opinion and full-scale testing shouldconfirm the proper conveyor choice for your need.
