continuous flow dryers - extension publish · pdf filecounter-flow dryers in a counter-flow...

Cross Flow Dryers Oneofthemorepopularcontinuousflowdryersisthecross-flowdryer.Theyareavailableinhorizontaldesigns(Figure1)andverticalortowerdesigns(Figure2).Inthistypeofdryer,airflowisperpendiculartograinflow.Heatedairblowsacrossthegrainasitflowsbygravitydownan8-to18-inch-widecolumn.qualityandvalue. Theenergyefficiencyisafunctionoftheplenumairtemperatureandtheairflowrate,withthehighestefficiencyoccurringathighplenumtemperaturesandlowairflowrates(Figure3).Inpractice,energyefficiencyneedstobebalancedwithgrainquality.Thelowertheairflowrate,thehigherthegrainmoisturecontentdifferencebetweenthegrainkernelsontheinsideversustheoutsideofthecolumnandthelargerthetemperaturegradient.Manymanufacturershavedevelopedmethodstomixthegrainasitflowsdownthecolumntolessenthemoisturevariationatthedryerdischarge.Controllerscanbeusedthatsensekernelmoistureandregulateunloadingaugersasgrainmeetsmoisturesetpoints.Theefficiencywillvarydependingonairflowrate,plenumtemperaturesandheatrecoveryoptions.Therangeistypicallybetweenabout2000Btuperpoundofwaterremovedwithenergyefficiencyfeaturesinplacetoabout3000Btuperpoundofwaterremovedforawell-maintaineddryerwithnoenergyconservationmeasures.

Continuous Flow Dryers

ENCON2.3December, 2012

Fact Sheet

Improving Energy Efficiency in Grain Drying

Figure 1: Horizontal cross-flow dryer. Source: Scott Sanford

Figure 2: Tower cross-flow dryer. Source: Scott Sanford

Figure 3: Cross-Flow Dryer Efficiency. Source: University of Nebraska

Counter-flow Dryers Inacounter-flowdryerthegrainflowandtheairflowmoveinoppositedirections.Typicallythisisanin-bincontinuousflowdryer,whichconsistsofaroundbinwithafullperforatedfloorandasweepauger.Heatedairispushedupthroughthegrainandthesweepaugerremovesgrainwhenthecontrolsystemindicatesthegrainhasreachedthedesiredmoisture.Hotgrainismovedtoacoolingbinwhenitisdischargedfromthedryer(Figure4).

Oneadvantageofthisdryertypeisthatwetgraincanbepiledontopofthedryinggrain,eliminatingtheneedforaseparatewetholdingbin.However,increasinggraindepthreducesdryingrateandfanefficiency.Fineparticlescanhinderairflowthroughtheperforatedfloor,soitisrecommendedthatthegrainbescreenedwhenfillingthedryer.Itisusuallynecessarytoshutdownthedryerevery3-4daystoremovefinesthathaveaccumulatedonthefloortomaintainproperairflow. In-bincontinuous-flowdryersareamongthemoreefficientcontinuous-flowtypedryers-about2000Btuperpoundofwaterremoved-whichisabout20to30percentmoreefficientthanatraditionalcross-flowdryer.Thistypeofdryercanberetrofittedintoanexistingbin,andcanbeusedtostoregrainattheendofthedryingseason.Thelastbatchofgrainwouldbedriedwiththedryeroperatinginarecirculationmode(Figure5).Theenergyefficiencyofthisdryercanbeincreasedby10-15percentusingdryerationprocessforcoolingthegraininsteadofin-bincooling.

Mixed-flow Dryers Mixed-flowdryersarecolumndryersthathaveairflowinbothcounterandconcurrentdirections(Figure6).Thesedryersmayhavemultiplezones(Figure7)and,becausethegrainisnotcontinuouslyexposedtohightemperatures,theycanusehigherairtemperaturesthancross-flowdryerswithoutdamagingthegrain.Thesedryersareusuallyself-cleaningandhaveenergyefficienciessimilartoacounter-flowdryer,using20to30percentlessenergythanatraditionalcross-flowdryerwithoutheatrecovery.Onemajoradvantageofthisdryeroveronewithascreencolumn(likeacross-flowdryer)istheabilitytoaccommodateawidevarietyofdifferentgrainsfromsmallrapeseedtocorn.ThistypeofdryerispopularoutsideoftheU.S.buthasnotbeenacceptedasreadilyintheU.S.corngrowingregions,possiblydue

toslightlyhighercapitalcostandlackofunderstandingoftheadvantage.

Cooling Options Coolinggraininadryerreducesdryingcapacityandrapidcoolinginthedryerwillcausekernelstostresscrack,whichmayleadtolowervaluegrain(morecrackedkernels).Usingtheentiredryerfordrying(full-heatdryer)andtransferringthegrainhottoabinforcoolingataslowerratewillsaveenergyandimprovegrainquality.Graincanbetransferredtoacoolingbinat1to3percentmoistureabovethedesiredstoragemoistureleveldependingonthecoolingmethodused. Forin-storagecooling(Figure8),thegrainistransferredtostorageat1to1.5percentabovethedesiredfinalstoragemoisture,whichcansaveabout15percentindryingcostsandincreasethedryercapacitybyabout33percent.Fordryeration(Figure9),thegrain

Figure 4: In-bin continuous-flow dryer and cooling/storage bin. Source: Scott Sanford

Figure 5: In-bin continuous-flow dryer with internal recirculation / discharge auger. Source: Shivvers

istransferredtoabinat2to3percentabovethedesiredstoragemoisture,allowedtosteepfor4to12hoursandthencooledwithambientair.Thisprocesscansaveabout25percentinenergycosts,reducekernelstresscracksandincreasedryercapacitybyupto70percent.Condensationonthebinwallsduringdryerationisextensive,soitisimperativethatthecornbemovedtoanotherbinforstoragesothewetgrainismixedwithdrygrain.Condensationmayalsobeaproblemwithin-storagecooling,particularlywhenoutdoortemperaturesarecool.Thismayleadtostorageproblemsandcornadheringtothebinwall.Partialcoolinginthedryerreducesthepotentialforcondensationinthebin.Seesomeoftheresourcesbelowformoreinformationontheseprocesses.

Energy Efficiency Options Somenewdryershaveenergy-savingfeaturessuchasgrainturners,graindiverters,steepingsections,multiplemeteringrolls,heatrecoverycapabilitiesandcomputerizedcontrols. Grainturnersmovesomeofthegrainfromtheinsideofthecolumnneartheplenumtotheoutsideandgrainfromtheoutsidetotheinside(Figure10).Theturneraidsinreducingthemoisturevariationofthegrainexitingthedryer.Withoutaturner,thegrainneartheplenumoftengetsover-driedwhilegrainontheoutsideofthecolumnisunder-dried.Agraindiverterworkssimilarly,byallowingthegrainontheinsideofthecolumntomovefasterthroughthedryerthanthegrainontheoutersideofthecolumn(Figure11).Somedryersusemultiplemeteringrollsatthebaseofthegraincolumntoremovethegrainontheinsideofthecolumnneartheplenumfaster,providingresultssimilartodivertersinreducingoverdrying. Asteepingsectionisanareawherehotgrainisallowedtopassdownthegraincolumnforaperiodoftimewithouthotairbeingforcethroughthegrain.

Figure 7: Three zone mixed-flow dryer. Source: NECO

Figure 6: Mixed-flow dryer airflow pattern Source: Grain Handler

Figure 9: Dryeration. Source: Scott Sanford

Figure 8: In-store cooling. Source: Scott Sanford

Thisallowstimeforthemoistureinthegrainkernelstoequalizebeforedryingiscontinued.Steepingsectionsaretypicallyofferedasanoptionontowerdryers.Heatrecoveryoncontinuousflowcolumndryerscansave10to30percentinenergycosts.Itistypicallyonlydonewithdryersrunninginaheat-coolmode.Heatisrecoveredastheintakeairfortheheatingsectionisdrawnthroughthegraininthecoolingsection.Thiscoolsthegrainwhilepreheatingtheairenteringtheheatingchamber.Aircanalsoberecycledfromthelowerheatsectiontoreclaimadditionalheat.Recyclingairreducesdryercapacityslightlydependingonthemoisturecontentoftherecycledair.Towerdryersandconventionalcross-flowdryerscanhaveheatrecoverycapabilitiesbuiltintothedryerdesign. Computerizedcontrolsystemswithtemperatureandgrainmoisturesensorscanpreventover-dryingandreduceenergyuse.Thecontrollerregulatestheplenumtemperatureandcontrolsthemeteringrollsorunloadingaugersbasedonthegrainmoisture.Whenthecontrollersensesthegrainhasreachedthesetpointforkernelmoisture,themeteringrollsoraugersareactivatedtoremovethegrain.Insomecasesthespeedofunloadingcanberegulatedwithvariablespeeddrivesasthedetectedgrainmoisturechanges.Ifthegrainexitingthedryerhasamoisturecontenthigherorlowerthanthesetpoint,thespeedoftheunloadingaugercanbeadjustedautomaticallyasneededtorun slowerorfaster,respectively.

Figure 10: Grain turner - top view. Source: K. Hellevang

Figure 11: Grain diverter varies grain flow rate across the drying column. Source: K. Hellevang

References

K.J.Hellevang.1983.CropDryerationandIn-storageCooling,AE-808,NorthDakotaStateUniversityExtension.Availableat:http://www.ag.ndsu.edu/extension-aben/documents/ae808.pdfverified12/2012

H.A.Cloud,Morey,R.V.1998.DryerationandIn-storageCoolingforCornDrying,UniversityofMinnesotaExtension.Availableat:http://www.extension.umn.edu/distribution/cropsystems/DC7356.htmlverified12/2012

B.A.Mckenzie,Foster,G.H.1980.Dryeration&bincoolingSystemsforGrain,AE-107,PurdueUniversityExtension.Availableat:http://www.extension.purdue.edu/extmedia/AE/AE-107.htmlverified12/2012

S.Shouse,Hanna,M.,Petersen,D.2010.ManagingHigh-temperatureGrainDryersforEnergyEfficiency,IowaStateUniversityExtension,PM2089f.Availableat: http://www.extension.iastate.edu/Publications/PM2089F.pdfverified12/2012

USDA/NIFA National Facilitation Project The Role of a 21st Century Engaged University

Thispublicationisavailableonlineat:http://blogs.extension.org/encon1/files/2012/12/FS_FlowDryers.pdf

On-linetrainingprogramsonbioenergyandsustainability,on-farmenergyconservationandefficiency,andanaerobicdigestionareavailableonlineat:http://fyi.uwex.edu/biotrainingcenter