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Asimplifiedtwinscrewcorotatingfoodextruderdesign,fabricationandtestingS.A.M.A.N.S.Senanayakea,B.Clarkeb,DivisionofAgriculturalandPlantationEngineering,TheOpenUniversityofSriLanka,Nawala,Nugegoda,SriLankaDepartmentofPostharvestTechnology,SchoolofAgriculture,FoodandEnvironment,SilsoeCollage,CranfieldUniversity,Silsoe,BedfordshireMK454DT,UKReceived6July1998accepted10February1999AbstractAsimplifiedcorotatingtwinscrewfoodextruderwasdesigned,fabricatedandtestedinEngland,followedbyextensivetestinginSriLanka.Itwasbuiltasamodeltomeetthespecificproductandfinancialconstraintsoflessdevelopedcountriesandwasexpectedtobeusedinthosecountriestowidentheproductioncapabilitiesofextrudedfoods.Themachinehadanestimateddeliveryof10kg/handwasmademainlywithmildsteel.Twotypesofscrewweremade,onewithaconstantpitchof14mmandtheotherwithvaryingpitchinsegmentsof14,12and10mm.Themachinewaspoweredbya2.2kWelectricmotorwithelectronicspeedcontrol.Themachinealsohadelectricalheatingwithatemperaturecontrollerandapressuresensingdevice.Thecostoffabricationofthemachinewasestimatedat£2000withmostofthepartsbuiltinafairlysimpleworkshop.Amixtureofriceanddriedbananawassuccessfullyextrudedasapotentialsnackfoodandonthebasisofmaximumexpansionthebestresultswasobtainedfromabarreltemperatureof120°C,screwspeed125rpm,feedmoisture15andwithadieorificesizeof3mm.Whenthealternativecompressionscrewwastestedverysimilarresultswereachievedwithnosignificantimprovementinproductexpansion.©1999ElsevierScienceLtd.Allrightsreserved.KeywordsTwinscrewextruderDesignLowcostSnackfoodContinuouscookerLocalconstructionCerealmixturesNomenclatureaDiediametermmBChannelwidthmmCScrewcircumferencemmdScrewcorediameterDOuterdiameterofscrewsmmHFlightdepthmmMMoisturecontentwetbasisnNumberoffightturnsNSpeedangularrev/minpPitchmmQDeliveryratemm3/minSTotalhelicallengthofscrewsmmtTemperature℃TResidencetimeminaOverlapangleofscrewfightsdegreesdCalendergapmmeSideclearancemmqProductdensityg/mm3/HelixangledegreesCorrespondingauthor.Fax01525863277emailb.clarkecran®eld.ac.uk02608774/99/±seefrontmatter©1999ElsevierScienceLtd.Allrightsreserved.PIIS026087749900049–71.IntroductionExtrusioncookingisfindingeverincreasingapplicationsinthefoodprocessindustry.Apartfromprovidingameansofmanufacturingnewproducts,ithassuccessfullyrevolutionismanyconventionalmanufacturingprocessesHarlow,1985,Frame,1994.Today,extruderscomeinawidevarietyofsizes,shapesandmethodofoperation.Therearethreetypesoffoodextruderfoundinindustryhydraulicram,rollerandscrewtypeextrudersFrame,1994.Thescrewextrudersareverydifferenttotheothertwohavingspecialfeaturessuchascontinuousprocessingandmixingability.Singleandtwinscrewtypesarebothwidelyusedinthefoodprocessindustry.Unfortunately,mostofthefoodextrudersavailableinthemarketareeithersocostlythatlessdevelopedcountriescannotaffordtobuythemexceptbysomeformofassistanceoroutsideinvestmentorelsearenotappropriateforthewidevarietyofmaterialsthatneedtobeprocessed.Asaresultthegrowthofextrusiontechnologyoffoodintothesecountrieshasbeenhindereddespiteitsmanyadvantages.Fig.2.Plandrawingofthetwinscrewextruderwithdrivesystem.1Vbeltpulley,2gearbox,3foodseal,4¯angeclampbolt,5dieplate,6die,7twosegmentsoftheextruderchamber,8extruderscrew.weremadesothattheycouldbeexternallyscrewedtothedieplate.2.5.DrivesystemThemachinewasdrivenbyanelectricmotorof2.2kWusingatwinbeltdrivebetweenthemotorandagearboxshowninFig.2.Thespeedreductionintheboxwas2.08whileanelectronicspeedcontrollerwasusedtocontrolthespeedcontinuouslyovertherangerequired.Fig.3.Frontportionofbarrelshowingprovisionforheaters,temperatureandpressuresensors.1slotsforheaters,2endflanges,3sideflangestobarrel,4holeforpressuresensor,5twinholestoformthebarrel.2.MotorpowerIntwinscrewextrudersthemotorpowerisutilizedmainlytocompressandshearthefooddoughthatsqueezesthroughvariousgapsintheintermeshingscrewsandthegapbetweenthescrewsandthebarrel.Whendealingwithawiderangeoffoodsunderdifferentprocessconditionstheshearresistancecanvarywidelybecauseofchangesintherheologicalbehaviourwhichwouldpreventaccurateestimateofthemotorpower.OwingtotheunknowncharacterthereforeofthenovelmaterialsamotorpowerwasselectedbasedonthatusedforsimilarmaterialsinsimilarsizedextruderswithasafetymarginandfromexploratorytrialsintheBrabenderextruder.RossenandMiller1973givearangeofspecificenergyconsumptionfiguresfordifferentextruderswhichrangedfrom0.02to0.10kWh/kg.At10kg/hthroughputthisgaveamaximumpowerrequirementof1kWwhiletheBrabendertrialstendedtoindicateapowerrequirementofabouthalfofthisvalue.The2.2kW,3phaseACmotorusedwasamplycapableofsupplyingthispowerplusallotherdrivefrictionlosses.3.GearboxInthecorotatingextruderthetwoscrewshaftsaredrivenatthesamespeedinthesamedirection.Themainproblemisthattheyareveryclosetogether.Thegearboxwasdesignedtodrivetwopinions,coupledtotheshaftsbyshearpins,byusingagearwheelofmorethandoublethewidthofthepinions.Inthiswaythetwopinionscould®tsidebysidedrivensimultaneouslyandmaximisetheirdiameterspaceasshowninFig.2.Lubricatedphosphorbronzethrustbearingswereusedtoresisttheaxialloadgeneratedbythematerialalongtheshaft.2.6.HeatingandtemperaturecontrolHeatingofthebarreltogivenecessarythermalinputforcookingthefoodwasdonebytwosetsofcartridgeheatershavingcapacitiesof800and1200W.TheheaterswerepositionedinthegroovesmadeonthetopandbottomofthebarreltowardsthedieendasshowninFig.3.Asingletemperaturecontrollerwassetuptogetherwithathermocoupletosensethetemperatureinsidethebarrelveryclosetodieplate.Owingtotheshortnessofthebarrelonlyonethermocouplewasconsiderednecessary.Inanearlydesignheaterswerealsousedneartothefeedhopperbutwerenotusedastheytendedtocauseprematuregelatinizationofthestarchandblockageofthefeed.4.PressuresensorPressuremeasurementsarenotsoimportantinthecommercialproductionprocessesasitcannotbedirectlycontrolledtomonitortheproductcharacteristics.Neitherwassuchadeviceneededasasafetymeasureasthiswascoveredbyanoverloadcutoutontheelectricalsupply.However,inexperimentalworkthemeasurementofpressureisusefultoascertaintherelationshipbetweenthepressureandtheothercontrollableparameterssuchasdiesize,temperature,moisturecontentandspeed.Inthisstudy,adevicewasbuiltusingstraingaugesmountedonasmallcantileverbeaminordertomeasurethepressureinsidetheextruderbarrelFig.4.Afourarmstraingaugebridgewasfixedatthepointofmaximumbendingmoment.Thepressurewastappedfromasmallholemadeinthedieendofthebarrelinwhichaplunger,sealedbyanOring,actuatedthecantileverbeamtotransmitthepressureforce.Thestraininthebeamwasdetectedasavoltagedifference.Thisfeaturecouldhavebeenusedasanautomaticsafetycutoutbutreliancewasplacedinsteadonbeltslipintheinitialdrivestageandthemotoritselfhadanoverheatingcutout.Fig.4.Positionofpressureandtemperaturesensorsontheextruderbarrel.1locationofstraingaugesonthepressuresensor,2cantileversupporttoplunger,3temperaturesensor.5.TestingandevaluationArangeofriceandbananamixtureswereselectedasbeingbothnovelyethavinghighpotentialasprocessedfoodsinSriLanka.Thesematerialsarecheapandcommoncropsinmostdevelopingcountriesandrepresentanopportunitytoproduceanattractive,nutritiousandtastysnackfood.Thiswouldprovidelabour,utilisationofexcessperishablefruitsinseasonandameansofstoringthemforatleastoneyearinappropriatepackages.Themainproductqualities
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