关 键 词：

教育专区 外文翻译 精品文档 外文翻译

资源描述：

Asimplifiedtwinscrewco-rotatingfoodextruder:design,fabricationandtestingAbstractAsimplifiedco-rotatingtwinscrewfoodextruderwasdesigned,fabricatedandtestedinEngland,followedbyextensivetestinginSriLanka.Itwasbuiltasamodeltomeetthespeci®cproductand®nancialconstraintsoflessdevelopedcountriesandwasexpectedtobeusedinthosecountriestowidentheproductioncapabilitiesofextrudedfoods.Themachinehadanestimateddeliveryof10kg/handwasmademainlywithmildsteel.Twotypesofscrewweremade,onewithaconstantpitchof14mmandtheotherwithvaryingpitchinsegmentsof14,12and10mm.Themachinewaspoweredbya2.2kWelectricmotorwithelectronicspeedcontrols.Themachinealsohadelectricalheatingwithatemperaturecontrollerandapressuresensingdevice.Thecostoffabricationofthemachinewasestimatedat£2000withmostofthepartsbuiltinafairlysimpleworkshop.Amixtureofriceanddriedbananawassuccessfullyextrudedasapotentialsnackfoodandonthebasisofmaximumexpansionthebestresultswasobtainedfromabarreltemperatureof120°C,screwspeed125rpm,feedmoisture15%andwithadieori®cesizeof3mm.Whenthealternativecompressionscrewwastestedverysimilarresultswereachievedwithnosigni®cantimprovementinproductexpansion.Ó1999ElsevierScienceLtd.Allrightsreserved.Keywords:Twinscrewextruder；Design；Lowcost；Snackfood；Continuouscooker；Localconstruction；CerealmixturesNomenclatureADiediameter(mm)BChannelwidth(mm)CScrewcircumference(mm)dScrewcorediameterDOuterdiameterofscrews(mm)HFlightdepth(mm)MMoisturecontent(%wetbasis)nNumberoffightturnsNSpeedangular(rev/min)pPitch(mm)QDeliveryrate(mm3/min)STotalhelicallengthofscrews(mm)tTemperature(C)TResidencetime(min)Overlapangleofscrewfights(degrees)Calendergap(mm)Sideclearance(mm)Productdensity(g/mm3)Helixangle(degrees)1.IntroductionExtrusioncookingisfindingeverincreasingapplicationsinthefoodprocessindustry.Apartfromprovidingameansofmanufacturingnewproducts,ithassuccessfullyrevolutionisedmanyconventionalmanufacturingprocesses(Harlow,1985,Frame,1994).Today,extruderscomeinawidevarietyofsizes,shapesandmethodofoperation.Therearethreetypesoffoodextruderfoundinindustry:hydraulicram,rollerandscrewtypeextruders(Frame,1994).Thescrewextrudersareverydifferenttotheothertwohavingspecialfeaturessuchascontinuousprocessingandmixingability.Singleandtwinscrewtypesarebothwidelyusedinthefoodprocessindustry.Unfortunately,mostofthefoodextrudersavailableinthemarketareeithersocostlythatlessdevelopedcountriescannota.ordtobuythemexceptbysomeformofassistanceoroutsideinvestmentorelsearenotappropriateforthewidevarietyofmaterialsthatneedtobeprocessed.Asaresultthegrowthofextrusiontechnologyoffoodintothesecountrieshasbeenhindereddespiteitsmanyadvantages.Inparticularthereappearsnottobeasuitabletwinscrewextruderavailableatsuffcientlylowcostforuseindevelopingcountries.Thehighcostofthesemachinesislargelyduetothesophisticatedconstructionalfeatureswithmanycontrolsthatarenotessentialformanyapplications.Theinitialpurchasecostisthereforeamajorbarriertothewiderintroductionofextrusiontodevelopingcountrieswheremanyrawmaterialsareavailabletobeprocessedintonutritiousandpalatablefooditems.AttemptshavebeenmadetoutiliselowpressureandsimpleextrudersforbabyweaningfoodssomeofwhichhavebeenverysuccessfulwithprogramsthroughorganizationssuchasFeedtheMillions(SantaMonica,California,90406,USA)andThriposha(USDA/USAID).Theneedhaslongbeenrecognisedforsimple,yetversatilemachinesataffordablepriceswhicharecapableofexploitingthenaturalfoodresources(Sahagun,1977),JansenandHarper(1980),HarperandJansen(1985).Thesecouldincorporatealltheusualcerealssuchasrice,millet,maizeandsorghumwitharichvarietyofnutritionalandtastyadditivessuchasfruits,vegetablesandspices.Theobjectiveofthisstudythereforewastoeliminateorsimplifymanyoftheunnecessaryfeaturesofthecurrentlyavailablemachinessothatfabricationcouldbedoneinanunsophisticatedworkshop,whileensuringtheversatilitytopermittheproductionofacceptableextrudedfoodsespeciallyusingblendsoffruitsandcereals.ThislattertechniquewasdevelopedbyGamlath(1995)usingasinglescrewextrudertoproducearangeofextrudatesthatsheadjudgedtoshowgreatpromiseasasnackfoodorbreakfastcerealinSriLankainparticularbutalsoinmanydevelopingcountriesandalsoasahighvalueexport.Theseproductswerelargelybasedonmixturesoffruitandcerealssuchasmango,bananaandtomatowithmaize,riceandwheat.Attractivelytexturedandflavoureditemswereproducedwhichcouldeasilybefurtherflavouredwithspices,sugarandsalt.Manydevelopingcountriesusehomebasedbusinessestoproducesnacksandready-to-eatmealsandtheseproductwellintothiswellestablishedculture.Machinesmadeinthedevelopingcountry,however,needtobelowcostandrelativelysimple,easytomaintainandoperate.Inthisstudytheparticularfeaturesrequiredweregoodtransportcharacteristicsforstickymaterialsandgoodmixingabilitywhichareneededfortheblendedcropsselected.Shortresidencetimesandlowtomediumpressureswereacceptable.2.DesignapproachThedesignoftheextruderinthisstudywasmostlybasedontheresultsofextrusionstudiesdoneusingasinglescrewlaboratoryscaleextruders(Brabender,Warrington,Cheshire,UK)aspartofthesameprogramme,andsupportedbyengineeringprinciplesandconstraintsapplicabletoadevelopingcountry.Inthedesignprocess,thelevelofpressurelikelytobedevelopedwithintheextruderbarrelbecomesthemostimportantfactorasitdeterminesthesizesofalltheimportantcomponentsoftheextrudersuchasbearings,shaftdiametersandbarreldimensions.Asthesemachinesoftenoperateunderelevatedtemperatures,pressuretendstovaryunpredictablybecauseofcomplexrheologicalpropertiesoffooddoughasrelatedtovaryingtemperaturealongthebarrel.Therefore,actualmeasurementsofextruderprocessvariables,weremadeonthechosenrangeoffoodmaterialsonthelaboratorysinglescrewextruder.Thisapproachwasreckonedtobemoreappropriateratherthanatheoreticalone,thatcannotbegeneralisedespeciallyformaterialsofunknownrheology.Hence,approximatevaluesforsomeofthemostimportantparameterssuchasdiesizes,pressures,temperaturesetc.werequantified.Thesevalueswerethentestedagainintheexperimentsdescribedlaterforthetwinscrewsystem.2.1.SinglevstwinscrewsManycomparisonsbetweentwinandsinglescrewextrudershavebeenmade(Harper,1992)(Hauck,1985)(Hauck&BenGera,1987)(VanZuilichem,Stolp&Janssen,1984)andmanyofthefeaturesdiscussedarerelevanttothecurrentapplication.Singlescrewmachinesmeetmostofthecriteriafordevelopingcountriesbutunfortunatelycertainofthetechnicalrequirementscanonlybemetbytwinscrewssuchasversatility,adequate¯owoffeedintothescrewsand,particularlyinthiscase,¯owofthematerialdownthebarrelwithoutspinningwithinit.ThisisacommonproblemwiththeratherstickyorhighmoisturematerialsthatwereenvisagedwiththeincorporationoffruitsandvegetablesandothertropicalcropsandflavoursandexperiencedbyGamlath(1995).Theforwardmotionofthefooddoughreliesentirelyonthefrictionbetweenthematerialandtheinteriorsurfacesofbothscrewandbarrel.Thisisparticularlysoinasinglescrewbutlesssoinatwinscrew.Singlescrewextrudersaregenerallycheaperthanthetwinscrewtypeowingtotheirsimplerconstruction,buttheyaremorelikelytoblockthantwinscrewextruders.Thedilemmafacingthedesigner,therefore,isthatalthoughtheoperationalproblemscanbeavoidedbyusingtwinscrewstheirsophisticatedconstructionalfeatureswouldhavetobesimplifiedsignificantlytoachievethelowcostobjective.Intwinscrewextrudersthescrewscanbemadeaseitherco-rotatingorcounterrotating,withdifferingamountsofintermesh,pitchandclearances,allofwhichaffecttheprocessingcharacteristicsofthemachine(Martelli,1983).Sincetheflightofonescrewengagesthechanneloftheotherthetwinscrewspreventmaterialfromstickingtothescrewsandrotatingwiththescrew,henceencouragingforwardmotionofdoughwithreducedslip(Jansen,1978).Thisalsoenhancesthemixingofmaterialsinthechannelsofthescrews.Thecounterrotatingtypeoftwinscrewextrudercangivebettermaterial¯owcharacteristicspushingthematerialpositivelyforwardtowardsthediebutreducingthedegreeofmixingofmaterialswithintheextruder.Anotherunfortunatefeatureisthatseparatingforcesbetweenthescrewscancauseseverewearofthesidewalls.Ontheotherhand,co-rotating,twinscrewextrudersgiveadditionaladvantagessuchaspressurebalanceintransverseplanesofthescrewsandawipingactionofthefightsduetotheopposinglinearvelocitiesatthefightsofintermeshingregion.Therefore,itwasdecidedthatatwinscrewco-rotatingextruderwouldprovidealltheversatilityandcharacteristicsrequiredforprocessingthefoodmaterialsenvisagedinadevelopingcountry.Theonlyproblemthenwastodesignasatisfactorymachinethatwouldalsomeetthecostcriteria.2.2.ScrewsThecentralpartofanextruderisthescrew.Thescrewsrotateinatightlyfittingbarrelandconveythematerialfromfeedendtodieend,mixingandcom-pressingtheingredients.Thecapacityoftheextruderismainlydeterminedbythescrewouterandrootdiameters,pitchandfightwidth(Fig.1).Therefore,thescrewoveralldimensionshavetobefirstdeterminedaccordingtothedesiredoutputtoinitiatethedesignprocess.Oneotherimportantfactoristheresidencetime,asitdeterminesthedegreeofcookingtakingplaceintheextruder.ExploratorytrialsandtheworkofGamlath(1995)indicatedthataresidencetimeofabout16swouldbeadequateatthemeanspeed.Itis,however,influencedbymanyofthedesignparametersincludingthelength,diameterandpitchofthescrewsalthoughitcanbecontrolledlargelybytheoperationalscrewspeedoftheextruderandtheselecteddiesize.Therheologyofthematerialalsohasamajorinfluenceonthethroughputsoresidencetimecannotbeaccuratelypredicted.Inthisstudyseveraldimensionsofthescrewsweredeterminedinordertosuittheexpectedoutputandresidencetime.ThegeometryandequationsrelatingmanyofthesefactorsaregiveninFig.1andEqs.(1)±(3)below.Themaximumoutputwasnotthe®rstprioritybutsimilarsizedextrudersandsinglescrewtrialsindicatedatypicaldeliveryrateofupto10kg/h.Theresidencetimewasdesignedtobeshortasthenovelproductsplannedwerenotexpectedtorelyonlongprocessingtimes.TheshortresidencetimewasparticularlysoughtinordertomaintainthevitaminCcontentofperishableproductswhichcouldbeeasilydestroyedifcookedtoolong(Seiler,1984).Theinfluenceofscrewpitchissuchthatlowvalueswithsmallhelixanglesleadtolongerresidencetimesbecauseoftheincreasednumberofcyclesthatthematerialhastogothrough.Ontheotherhand,largepitcheswouldreducethedegreeofmixinganduniformityofheatdistributionduetotheincreaseinvolumeofmaterialcontainedinthescrewchannels.

内容简介：

-