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
The Most Efficient DistanceBetween Two Points
P-0673
The Most Efficient Distance Between Two PointsBy David R. Gill, President, Flexicon Corporation
(Fig. 1)
(Fig. 2)
When bulk material characteristics, process parametersor space restrictions call for a flexible screw conveyor
Deciding on the single most efficient method to move your material from pointA to point B is a matter of simple deduction: just match your material flowcharacteristics, process parameters and space limitations to the performancewindows and dimensional restrictions of conveyors in contention. Then factorin purchase, maintenance, labor and cleaning costs, and voilà, your conveyorchoice becomes clear. (Figs. 1, 2)
Because the performance, size and cost of conveying methods vary widely,only a general knowledge of different conveyor types is necessary todetermine which is optimum for you, assuming you possess detailedknowledge of your application.
Armed only with the fundamentals outlined below, you can therefore determinewhether your application is handled most efficiently by a conveyor thatcontains a flexible screw.
Principle of operation
In basic form, a flexible screw conveyor consists of a spring steel or stainlesssteel flexible screw that is enclosed in a flexible or rigid plastic tube, or a rigidsteel tube, and driven by an electric motor normally located at the dischargeend of the conveyor.
The intake end of the conveyor tube is typically equipped with a chargingadapter, a U-shaped trough that connects to the outlet of a hopper or directlyto the outlet of process equipment such as a bulk bag discharger, bag dumpstation, grinder, crusher, screener, blender, reactor or storage vessel of anykind (Fig. 3).
The flexible screw passes through the charging adapter trough, exposing asection of the screw to material flowing into the trough from above. Whenrotating, the exposed section of screw feeds material into the outer tube,where it is then propelled through the tube by the enclosed portion of thescrew.
A phenomenon resulting from the loose fit of a rotating flexible screw within atube filled with bulk material, is that the screw automatically self-centers withinthe tube, providing clearance between the screw and tube wall. This affordssufficient space for particles to flow without damage, depending on the fragilityand size of particles being conveyed. (Fig. 4)
A removable clean-out cap covers the intake end of the conveyor tube,permitting rapid emptying and flushing of the tube, as well as disassembly andwash-down of components.
Because the intake end of the screw requires no bearing, and since thedischarge end is coupled to the motor drive above and/or beyond the point atwhich material exits the discharge spout, material does not come in contactwith seals or bearings. The flexible screw is therefore the only moving partcontacting material. (Fig. 5)
(Fig. 3) A U-shaped trough that connects to theoutlet of a hopper or directly to the outlet ofprocess equipment
(Fig. 4)
Matching application requirements with conveyorattributes
A flexible screw conveyor is your most likely choice if its list of distinguishingcapabilities closely matches that of your requirements, or if an individualcapability exclusive to a flexible screw solves an individual problem of keyconcern.
Compare your needs with the following list of flexible screw conveyorattributes:
One moving part contacting material
The only moving part contacting material in a flexible screw conveyor is arugged flexible screw, which is directly driven by an electric motor. This simpledesign does not require the filters, cyclone separators, internal bearings, andother moving parts found on pneumatic conveyors, bucket elevators, rigidaugers, drag chains, and/or aero-mechanical conveyors— parts that can addinitial cost, require maintenance, wear out, breed contamination, and/or breakdown.
Conveys free- and non-free-flowing materials including blends
A flexible screw conveyor can transport bulk materials ranging from largepellets to sub-micron powders— both free-flowing and non-free-flowing—including difficult to handle products that pack, cake, seize, fluidize, plug, orsmear. Materials range from fine powders such as fumed silica, compressiblematerials such as titanium dioxide, and dense materials such as metalpowder, to friable materials such as fiberglass, materials that fluidize such ascarbon black, and materials comprised of particle sizes to 1 inch (25mm) suchas nuts. The gentle rolling action created by the rotating screw prevents theseparation of blended products, even those comprised of disparate particlesizes and/or densities, throughout the entire length of the conveyor. (Fig. 6)
Prevents contamination
A flexible screw conveyor can be fully enclosed to prevent contamination ofbulk products and the plant environment, while preserving the moisture andtemperature levels of materials being conveyed. All interior surfaces aresmooth, and can be easier to clean than conveyors in which material contactsinternal bearings, seams, filters or other internal components. In addition, theunits can be designed and finished to industrial and sanitary standardsencompassing chemical, pharmaceutical, food/dairy, plastics, paint, mining,packaging and other applications.
Conveys in any direction
A flexible screw conveyor can move powder and bulk solid materials vertically,horizontally, or at any angle— over, under, or around obstructions, throughsmall holes in walls or ceilings, around obstructions, over short or longdistances, to any interior or exterior plant location. This allows the user toposition the intake and discharge ends of the conveyor according to thepreferred system layout, or to awkward layouts as in frequent retrofit/upgradesituations.
Typical applications include conveying of materials between storage vessels,
(Fig. 5)
(Fig. 6)
(Fig. 7)
bulk bag dischargers, manual bag dump stations, drum dump stations,crushers, screeners, blenders, reactors, packaging equipment, injectionmolding and extrusion machines and other process equipment. (Fig. 7)
Gentle product handling
Depending on particle fragility and size, a flexible screw conveyor can movematerials with little or no breakage or degradation. Clearance affordedbetween the screw and tube wall by the rotating, self-centering screw caneliminate or minimize the grinding, crushing, and impact damage that canoccur with other conveying methods.
Rapid, thorough cleaning
A flexible screw conveyor contains no internal filters or bearings that can trapparticles or breed contamination. Cleaning consists of removing the clean-outcap and reversing rotation to evacuate any residual material prior to flushingthe smooth interior surfaces with air, water, steam, or cleaning solution. Thescrew and outer tube can also be removed rapidly for thorough wash-down ofindividual components.
High reliability, low maintenance
A flexible screw conveyor contains no internal bearings, filters, or separators towear, breakdown, or require maintenance. The wear- and fatigue-resistantflexible screw is driven directly by an electric motor. Depending on applicationparameters, a flexible screw conveyor can be started and stopped repeatedly,even under a full load, with no binding or damage to the unit.
Economical to purchase, install
The lack of internal bearings, gears, filters, or chains— and externalcompressors, blowers, and separators— can make flexible screw conveyorsless costly to build and operate than other conveyors. A flexible screwconveyor can also be economical to install, since the intake and dischargeends can be positioned where desired, without exact conveyor routing,reducing the need to reconfigure other upstream and downstream processequipment.
Mobile configurations
Flexible screw conveyors can be caster-mounted on frames with conveyorsupport booms for in-plant mobility, avoiding the cost of multiple stationaryunits. (Fig. 8)
Multiple discharge point systems
Horizontally oriented flexible screw conveyors can feed multiple packagingmachines, molding machines, and process equipment simultaneously orindividually. They can also convey bulk material over long distances, feedmultiple discharge points selectively, or top off all points on a timed cyclethrough manual or automatic slide gate valves.
Volumetric feeding
A flexible screw conveyor can simultaneously meter and convey bulk solidproducts, since the units can be equipped with AC or DC variable speeddrives and hoppers with flow promotion devices for continuous feeding of both
(Fig. 8)
(Fig. 9)
(Fig. 10)
free- and non-free-flowing materials.
Weigh batching and weigh feeding systems
"Gain-in-weight" batching systems consist of individual or multiple flexiblescrew conveyors discharging into a central weigh hopper or other processequipment, which is equipped with load cells. Gain-in-weight information istransmitted to a central programmable controller, which in turn controls eachconveyor. The batch sequence can be initiated by a start button or by externalcontact closure. The controller then activates each conveyor, in sequence, toload each ingredient into the central weigh hopper, first at high speed, then atdribble speed before stopping the conveyor when the target batch weight hasbeen reached. (Fig. 9)
After each conveyor has deposited its ingredient, the entire batch can beautomatically delivered to process equipment, storage vessels, packaginglines or process equipment.
Flexibility of the conveyor tube allows the weigh hopper to be positioneddirectly above a blender or any other process equipment.
Since the system functions as both weigh feeder and conveyor, it can eliminatethe need for separate equipment.
Specifying round versus flat screw profiles
The rounded flight surface of round wire screws imparts the material withdirectional forces that propel it through the tube, and relatively high radialforces that create an effective bearing between the screw and tube wallthroughout the length of the conveyor. The material bearing surface causesthe screw to self-center within the tube, preventing it from contacting the tubewall throughout its entire length, even when the screw/tube assembly iscurved, assuming it has been correctly engineered. (Fig. 10)
These screws are fabricated from spring steel or stainless steel that is squareor rectangular in profile. Flat bearing surfaces impart greater directional forceand lesser radial force (and material compression) than do round wire screws.Flat wire screws are therefore frequently employed to convey products thattend to compress, as well as fragile materials, low mass materials and thosecomprised of the finest particle sizes. (Fig. 11)
Avoiding conveyor limitations
Since every mode of conveying also possesses inherent limitations, each mustbe identified and ruled unimportant, or less important than correspondingattributes, before deeming a flexible screw conveyor to be the mostcompromise-free alternative for your application.
Material evacuation
Flexible screw conveyors, like other mechanical conveyors, do not generallyevacuate material completely when operating in a forward direction. Theclean-out cap can be removed, however, and the screw rotation reversed tofully evacuate residual material for reasons of cleaning and/or materialchangeovers.
(Fig. 11)
Gentle product handling
While flexible screw conveyors can have less deleterious effect on materialsthan do methods that tend to grind, crush, or impact bulk products, not allmaterials can be conveyed damage-free. A percentage of large, brittle flakesor large, soft particles may, for example, may incur breakage, particularlythrough long runs or steep inclines in a flexible screw conveyor. Properengineering of a flexible screw conveyor system, however, can eliminate orminimize these effects.
Capacity requirements
Flexible screw conveyors are currently utilized for applications having capacityrequirements less than 50 tons per hour (per conveyor), explaining why theyare infrequently employed for off-loading of ships or railcars.
Conclusion
If the characteristics of a flexible screw conveyor align neatly with your materialcharacteristics and process parameters, your next step is to provide materialsamples and application details to a flexible screw conveyor applicationengineer, who can suggest the optimum system for integration with your newor existing process in the form of detailed CAD drawings, specifications andguaranteed performance ratings. Should your application call for capabilitiesbeyond those outlined above, a fundamental knowledge of other relevantmodes of conveying will enable you to determine which incurs the fewestnumber of compromises, or which best solves your most critical applicationproblems.
