Asimplifiedtwinscrewco-rotatingfoodextruder:design,fabricationandtestingS.A.M.A.N.S.Senanayakea,B.Clarkeb,*DivisionofAgriculturalandPlantationEngineering,TheOpenUniversityofSriLanka,Nawala,Nugegoda,SriLankaDepartmentofPostharvestTechnology,SchoolofAgriculture,FoodandEnvironment,SilsoeCollage,CranfieldUniversity,Silsoe,BedfordshireMK454DT,UKReceived6July1998；accepted10February1999AbstractAsimplifiedco-rotatingtwinscrewfoodextruderwasdesigned,fabricatedandtestedinEngland,followedbyextensivetestinginSriLanka.Itwasbuiltasamodeltomeetthespecificproductandfinancialconstraintsoflessdevelopedcountriesandwasexpectedtobeusedinthosecountriestowidentheproductioncapabilitiesofextrudedfoods.Themachinehadanestimateddeliveryof10kg/handwasmademainlywithmildsteel.Twotypesofscrewweremade,onewithaconstantpitchof14mmandtheotherwithvaryingpitchinsegmentsof14,12and10mm.Themachinewaspoweredbya2.2kWelectricmotorwithelectronicspeedcontrol.Themachinealsohadelectricalheatingwithatemperaturecontrollerandapressuresensingdevice.Thecostoffabricationofthemachinewasestimatedat2000withmostofthepartsbuiltinafairlysimpleworkshop.Amixtureofriceanddriedbananawassuccessfullyextrudedasapotentialsnackfoodandonthebasisofmaximumexpansionthebestresultswasobtainedfromabarreltemperatureof120C,screwspeed125rpm,feedmoisture15%andwithadieorificesizeof3mm.Whenthealternativecompressionscrewwastestedverysimilarresultswereachievedwithnosignificantimprovementinproductexpansion.1999ElsevierScienceLtd.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()TResidencetime(min)aOverlapangleofscrewfights(degrees)dCalendergap(mm)eSideclearance(mm)qProductdensity(g/mm3)/Helixangle(degrees)*Correspondingauthor.Fax:+01525-863277；e-mail:b.clarkecran-eld.ac.uk0260-8774/99/$seefrontmatter1999ElsevierScienceLtd.Allrightsreserved.PII:S0260-8774(99)0004971.IntroductionExtrusioncookingisfindingeverincreasingapplicationsinthefoodprocessindustry.Apartfromprovidingameansofmanufacturingnewproducts,ithassuccessfullyrevolutionismanyconventionalmanufacturingprocesses(Harlow,1985,Frame,1994).Today,extruderscomeinawidevarietyofsizes,shapesandmethodofoperation.Therearethreetypesoffoodextruderfoundinindustry:hydraulicram,rollerandscrewtypeextruders(Frame,1994).Thescrewextrudersareverydifferenttotheothertwohavingspecialfeaturessuchascontinuousprocessingandmixingability.Singleandtwinscrewtypesarebothwidelyusedinthefoodprocessindustry.Unfortunately,mostofthefoodextrudersavailableinthemarketareeithersocostlythatlessdevelopedcountriescannotaffordtobuythemexceptbysomeformofassistanceoroutsideinvestmentorelsearenotappropriateforthewidevarietyofmaterialsthatneedtobeprocessed.Asaresultthegrowthofextrusiontechnologyoffoodintothesecountrieshasbeenhindereddespiteitsmanyadvantages.Fig.2.Plandrawingofthetwinscrewextruderwithdrivesystem.1-Vbeltpulley,2-gearbox,3-foodseal,4-angeclampbolt,5-dieplate,6-die,7-twosegmentsoftheextruderchamber,8-extruderscrew.weremadesothattheycouldbeexternallyscrewedtothedieplate.2.5.DrivesystemThemachinewasdrivenbyanelectricmotorof2.2kWusingatwinbeltdrivebetweenthemotorandagearboxshowninFig.2.Thespeedreductionintheboxwas2.08whileanelectronicspeedcontrollerwasusedtocontrolthespeedcontinuouslyovertherangerequired.Fig.3.Frontportionofbarrelshowingprovisionforheaters,temperatureandpressuresensors.1-slotsforheaters,2-endflanges,3-sideflangestobarrel,4-holeforpressuresensor,5-twinholestoformthebarrel.2.MotorpowerIntwinscrewextrudersthemotorpowerisutilizedmainlytocompressandshearthefooddoughthatsqueezesthroughvariousgapsintheintermeshingscrewsandthegapbetweenthescrewsandthebarrel.Whendealingwithawiderangeoffoodsunderdifferentprocessconditionstheshearresistancecanvarywidelybecauseofchangesintherheologicalbehaviourwhichwouldpreventaccurateestimateofthemotorpower.OwingtotheunknowncharacterthereforeofthenovelmaterialsamotorpowerwasselectedbasedonthatusedforsimilarmaterialsinsimilarsizedextruderswithasafetymarginandfromexploratorytrialsintheBrabenderextruder.RossenandMiller(1973)givearangeofspecificenergyconsumptionfiguresfordifferentextruderswhichrangedfrom0.02to0.10kWh/kg.At10kg/hthroughputthisgaveamaximumpowerrequirementof1kWwhiletheBrabendertrialstendedtoindicateapowerrequirementofabouthalfofthisvalue.The2.2kW,3phaseACmotorusedwasamplycapableofsupplyingthispowerplusallotherdrivefrictionlosses.3.GearboxIntheco-rotatingextruderthetwoscrewshaftsaredrivenatthesamespeedinthesamedirection.Themainproblemisthattheyareveryclosetogether.Thegearboxwasdesignedtodrivetwopinions,coupledtotheshaftsbyshearpins,byusingagearwheelofmorethandoublethewidthofthepinions.InthiswaythetwopinionscouldtsidebysidedrivensimultaneouslyandmaximisetheirdiameterspaceasshowninFig.2.Lubricatedphosphorbronzethrustbearingswereusedtoresisttheaxialloadgeneratedbythematerialalongtheshaft.2.6.HeatingandtemperaturecontrolHeatingofthebarreltogivenecessarythermalinputforcookingthefoodwasdonebytwosetsofcartridgeheatershavingcapacitiesof800and1200W.TheheaterswerepositionedinthegroovesmadeonthetopandbottomofthebarreltowardsthedieendasshowninFig.3.Asingletemperaturecontrollerwassetuptogetherwithathermocoupletosensethetemperatureinsidethebarrelveryclosetodieplate.Owingtotheshortnessofthebarrelonlyonethermocouplewasconsiderednecessary.Inanearlydesignheaterswerealsousedneartothefeedhopperbutwerenotusedastheytendedtocauseprematuregelatinizationofthestarchandblockageofthefeed.4.PressuresensorPressuremeasurementsarenotsoimportantinthecommercialproductionprocessesasitcannotbedirectlycontrolledtomonitortheproductcharacteristics.Neitherwassuchadeviceneededasasafetymeasureasthiswascoveredbyanoverloadcutoutontheelectricalsupply.However,inexperimentalworkthemeasurementofpressureisusefultoascertaintherelationshipbetweenthepressureandtheothercontrollableparameterssuchasdiesize,temperature,moisturecontentandspeed.Inthisstudy,adevicewasbuiltusingstraingaugesmountedonasmallcantileverbeaminordertomeasurethepressureinsidetheextruderbarrel(Fig.4).Afourarmstraingaugebridgewasfixedatthepointofmaximumbendingmoment.Thepressurewastappedfromasmallholemadeinthedieendofthebarrelinwhichaplunger,sealedbyanO-ring,actuatedthecantileverbeamtotransmitthepressureforce.Thestraininthebeamwasdetectedasavoltagedifference.Thi