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外文翻译--简单的双螺杆挤压设备的设计制造和测试.doc

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外文翻译--简单的双螺杆挤压设备的设计制造和测试.doc

Asimplifiedtwinscrewcorotatingfoodextruderdesign,fabricationandtestingAbstractAsimplifiedcorotatingtwinscrewfoodextruderwasdesigned,fabricatedandtestedinEngland,followedbyextensivetestinginSriLanka.Itwasbuiltasamodeltomeetthespeci®cproductand®nancialconstraintsoflessdevelopedcountriesandwasexpectedtobeusedinthosecountriestowidentheproductioncapabilitiesofextrudedfoods.Themachinehadanestimateddeliveryof10kg/handwasmademainlywithmildsteel.Twotypesofscrewweremade,onewithaconstantpitchof14mmandtheotherwithvaryingpitchinsegmentsof14,12and10mm.Themachinewaspoweredbya2.2kWelectricmotorwithelectronicspeedcontrols.Themachinealsohadelectricalheatingwithatemperaturecontrollerandapressuresensingdevice.Thecostoffabricationofthemachinewasestimatedat£2000withmostofthepartsbuiltinafairlysimpleworkshop.Amixtureofriceanddriedbananawassuccessfullyextrudedasapotentialsnackfoodandonthebasisofmaximumexpansionthebestresultswasobtainedfromabarreltemperatureof120°C,screwspeed125rpm,feedmoisture15andwithadieori®cesizeof3mm.Whenthealternativecompressionscrewwastestedverysimilarresultswereachievedwithnosigni®cantimprovementinproductexpansion.Ó1999ElsevierScienceLtd.Allrightsreserved.KeywordsTwinscrewextruderDesignLowcostSnackfoodContinuouscookerLocalconstructionCerealmixturesNomenclatureADiediametermmBChannelwidthmmCScrewcircumferencemmdScrewcorediameterDOuterdiameterofscrewsmmHFlightdepthmmMMoisturecontentwetbasisnNumberoffightturnsNSpeedangularrev/minpPitchmmQDeliveryratemm3/minSTotalhelicallengthofscrewsmmtTemperatureCTResidencetimeminOverlapangleofscrewfightsdegreesCalendergapmmSideclearancemmProductdensityg/mm3Helixangledegrees1.IntroductionExtrusioncookingisfindingeverincreasingapplicationsinthefoodprocessindustry.Apartfromprovidingameansofmanufacturingnewproducts,ithassuccessfullyrevolutionisedmanyconventionalmanufacturingprocessesHarlow,1985,Frame,1994.Today,extruderscomeinawidevarietyofsizes,shapesandmethodofoperation.Therearethreetypesoffoodextruderfoundinindustryhydraulicram,rollerandscrewtypeextrudersFrame,1994.Thescrewextrudersareverydifferenttotheothertwohavingspecialfeaturessuchascontinuousprocessingandmixingability.Singleandtwinscrewtypesarebothwidelyusedinthefoodprocessindustry.Unfortunately,mostofthefoodextrudersavailableinthemarketareeithersocostlythatlessdevelopedcountriescannota.ordtobuythemexceptbysomeformofassistanceoroutsideinvestmentorelsearenotappropriateforthewidevarietyofmaterialsthatneedtobeprocessed.Asaresultthegrowthofextrusiontechnologyoffoodintothesecountrieshasbeenhindereddespiteitsmanyadvantages.Inparticularthereappearsnottobeasuitabletwinscrewextruderavailableatsuffcientlylowcostforuseindevelopingcountries.Thehighcostofthesemachinesislargelyduetothesophisticatedconstructionalfeatureswithmanycontrolsthatarenotessentialformanyapplications.Theinitialpurchasecostisthereforeamajorbarriertothewiderintroductionofextrusiontodevelopingcountrieswheremanyrawmaterialsareavailabletobeprocessedintonutritiousandpalatablefooditems.AttemptshavebeenmadetoutiliselowpressureandsimpleextrudersforbabyweaningfoodssomeofwhichhavebeenverysuccessfulwithprogramsthroughorganizationssuchasFeedtheMillionsSantaMonica,California,90406,USAandThriposhaUSDA/USAID.Theneedhaslongbeenrecognisedforsimple,yetversatilemachinesataffordablepriceswhicharecapableofexploitingthenaturalfoodresourcesSahagun,1977,JansenandHarper1980,HarperandJansen1985.Thesecouldincorporatealltheusualcerealssuchasrice,millet,maizeandsorghumwitharichvarietyofnutritionalandtastyadditivessuchasfruits,vegetablesandspices.Theobjectiveofthisstudythereforewastoeliminateorsimplifymanyoftheunnecessaryfeaturesofthecurrentlyavailablemachinessothatfabricationcouldbedoneinanunsophisticatedworkshop,whileensuringtheversatilitytopermittheproductionofacceptableextrudedfoodsespeciallyusingblendsoffruitsandcereals.ThislattertechniquewasdevelopedbyGamlath1995usingasinglescrewextrudertoproducearangeofextrudatesthatsheadjudgedtoshowgreatpromiseasasnackfoodorbreakfastcerealinSriLankainparticularbutalsoinmanydevelopingcountriesandalsoasahighvalueexport.Theseproductswerelargelybasedonmixturesoffruitandcerealssuchasmango,bananaandtomatowithmaize,riceandwheat.Attractivelytexturedandflavoureditemswereproducedwhichcouldeasilybefurtherflavouredwithspices,sugarandsalt.Manydevelopingcountriesusehomebasedbusinessestoproducesnacksandreadytoeatmealsandtheseproductwellintothiswellestablishedculture.Machinesmadeinthedevelopingcountry,however,needtobelowcostandrelativelysimple,easytomaintainandoperate.Inthisstudytheparticularfeaturesrequiredweregoodtransportcharacteristicsforstickymaterialsandgoodmixingabilitywhichareneededfortheblendedcropsselected.Shortresidencetimesandlowtomediumpressureswereacceptable.2.DesignapproachThedesignoftheextruderinthisstudywasmostlybasedontheresultsofextrusionstudiesdoneusingasinglescrewlaboratoryscaleextrudersBrabender,Warrington,Cheshire,UKaspartofthesameprogramme,andsupportedbyengineeringprinciplesandconstraintsapplicabletoadevelopingcountry.Inthedesignprocess,thelevelofpressurelikelytobedevelopedwithintheextruderbarrelbecomesthemostimportantfactorasitdeterminesthesizesofalltheimportantcomponentsoftheextrudersuchasbearings,shaftdiametersandbarreldimensions.Asthesemachinesoftenoperateunderelevatedtemperatures,pressuretendstovaryunpredictablybecauseofcomplexrheologicalpropertiesoffooddoughasrelatedtovaryingtemperaturealongthebarrel.Therefore,actualmeasurementsofextruderprocessvariables,weremadeonthechosenrangeoffoodmaterialsonthelaboratorysinglescrewextruder.Thisapproachwasreckonedtobemoreappropriateratherthanatheoreticalone,thatcannotbegeneralisedespeciallyformaterialsofunknownrheology.Hence,approximatevaluesforsomeofthemostimportantparameterssuchasdiesizes,pressures,temperaturesetc.werequantified.Thesevalueswerethentestedagainintheexperimentsdescribedlaterforthetwinscrewsystem.2.1.SinglevstwinscrewsManycomparisonsbetweentwinandsinglescrewextrudershavebeenmadeHarper,1992Hauck,1985HauckBenGera,1987VanZuilichem,StolpJanssen,1984andmanyofthefeaturesdiscussedarerelevanttothecurrentapplication.Singlescrewmachinesmeetmostofthecriteriafordevelopingcountriesbutunfortunatelycertainofthetechnicalrequirementscanonlybemetbytwinscrewssuchasversatility,adequate¯owoffeedintothescrewsand,particularlyinthiscase,¯owofthematerialdownthebarrelwithoutspinningwithinit.ThisisacommonproblemwiththeratherstickyorhighmoisturematerialsthatwereenvisagedwiththeincorporationoffruitsandvegetablesandothertropicalcropsandflavoursandexperiencedbyGamlath1995.Theforwardmotionofthefooddoughreliesentirelyonthefrictionbetweenthematerialandtheinteriorsurfacesofbothscrewandbarrel.Thisisparticularlysoinasinglescrewbutlesssoinatwinscrew.Singlescrewextrudersaregenerallycheaperthanthetwinscrewtypeowingtotheirsimplerconstruction,buttheyaremorelikelytoblockthantwinscrewextruders.Thedilemmafacingthedesigner,therefore,isthatalthoughtheoperationalproblemscanbeavoidedbyusingtwinscrewstheirsophisticatedconstructionalfeatureswouldhavetobesimplifiedsignificantlytoachievethelowcostobjective.Intwinscrewextrudersthescrewscanbemadeaseithercorotatingorcounterrotating,withdifferingamountsofintermesh,pitchandclearances,allofwhichaffecttheprocessingcharacteristicsofthemachineMartelli,1983.Sincetheflightofonescrewengagesthechanneloftheotherthetwinscrewspreventmaterialfromstickingtothescrewsandrotatingwiththescrew,henceencouragingforwardmotionofdoughwithreducedslipJansen,1978.Thisalsoenhancesthemixingofmaterialsinthechannelsofthescrews.Thecounterrotatingtypeoftwinscrewextrudercangivebettermaterial¯owcharacteristicspushingthematerialpositivelyforwardtowardsthediebutreducingthedegreeofmixingofmaterialswithintheextruder.Anotherunfortunatefeatureisthatseparatingforcesbetweenthescrewscancauseseverewearofthesidewalls.Ontheotherhand,corotating,twinscrewextrudersgiveadditionaladvantagessuchaspressurebalanceintransverseplanesofthescrewsandawipingactionofthefightsduetotheopposinglinearvelocitiesatthefightsofintermeshingregion.Therefore,itwasdecidedthatatwinscrewcorotatingextruderwouldprovidealltheversatilityandcharacteristicsrequiredforprocessingthefoodmaterialsenvisagedinadevelopingcountry.Theonlyproblemthenwastodesignasatisfactorymachinethatwouldalsomeetthecostcriteria.2.2.ScrewsThecentralpartofanextruderisthescrew.Thescrewsrotateinatightlyfittingbarrelandconveythematerialfromfeedendtodieend,mixingandcompressingtheingredients.Thecapacityoftheextruderismainlydeterminedbythescrewouterandrootdiameters,pitchandfightwidthFig.1.Therefore,thescrewoveralldimensionshavetobefirstdeterminedaccordingtothedesiredoutputtoinitiatethedesignprocess.Oneotherimportantfactoristheresidencetime,asitdeterminesthedegreeofcookingtakingplaceintheextruder.ExploratorytrialsandtheworkofGamlath1995indicatedthataresidencetimeofabout16swouldbeadequateatthemeanspeed.Itis,however,influencedbymanyofthedesignparametersincludingthelength,diameterandpitchofthescrewsalthoughitcanbecontrolledlargelybytheoperationalscrewspeedoftheextruderandtheselecteddiesize.Therheologyofthematerialalsohasamajorinfluenceonthethroughputsoresidencetimecannotbeaccuratelypredicted.Inthisstudyseveraldimensionsofthescrewsweredeterminedinordertosuittheexpectedoutputandresidencetime.ThegeometryandequationsrelatingmanyofthesefactorsaregiveninFig.1andEqs.1±3below.Themaximumoutputwasnotthe®rstprioritybutsimilarsizedextrudersandsinglescrewtrialsindicatedatypicaldeliveryrateofupto10kg/h.Theresidencetimewasdesignedtobeshortasthenovelproductsplannedwerenotexpectedtorelyonlongprocessingtimes.TheshortresidencetimewasparticularlysoughtinordertomaintainthevitaminCcontentofperishableproductswhichcouldbeeasilydestroyedifcookedtoolongSeiler,1984.Theinfluenceofscrewpitchissuchthatlowvalueswithsmallhelixanglesleadtolongerresidencetimesbecauseoftheincreasednumberofcyclesthatthematerialhastogothrough.Ontheotherhand,largepitcheswouldreducethedegreeofmixinganduniformityofheatdistributionduetotheincreaseinvolumeofmaterialcontainedinthescrewchannels.

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