departmentofagr icu l tura lengineer ing AEN-56

layer Drying ofStored GrainT.C. Bridges, O.J. Loewer and C.M. White

university of kentucky . college of agriculture . cooperative extension serviceagriculture . home economics. 4h . development

In-storage layer drying is a process whereby thegrain is dr ied in layers in the storage structure withthe ent ire grain depth ul t imately being dr ied in place.The process starts when an ini t ia l grain layer is placedin the dry ing b in and a dry ing f ron t i s es tab l i shed andbeg ins to move th rough the gra in . Add i t iona l layersof wet grain are added periodical ly so that a depth ofwet grain always precedes the drying front. Theamount o f g ra in tha t can be p laced in any one layer i sl imited to the amount that can be dr ied before exces-sive mold growth or af latoxin develops in the top ofthat layer. This drying technique is used in grain sys-tems where relat ively slow harvest rates are accept-able and harvest volumes are low to moderate.

EquipmentThe necessary equipment in a layer drying bin

should include a ful l perforated drying f loor, a fanand heater unit with a transi t ion, a grain spreader, asweep auger and an un ioad ing auger (F igure 1) . Fanand heater requirements wi l l vary with di f ferentdrying si tuat ions but typical fan sizes range f rom 5 to10 hp wh i le heater capac i t ies genera l l y w i l l vary f rom300 to 800,000 Btu per hour. l t is essent ial to havesuff ic ient fan capacity to dry the successive grainlayers as the bin becomes f i l led to prevent excessivemold growth. Bin eaves are general ly no more than 16feet high since most fans do not del iver suff ic ient air-f low for drying depths above this level. Ask yourcounty Ex tens ion agent o r equ ipment dea le r aboutappropriate fans for your part icular s i tuat ion.

Cenera l l y , the b in un load ing auger w i l l be a 5 -inch tube type and i ts capacity should be designed tomatch that of the central handl ing unit , whether i t bea transport auger or bucket elevator. The capacity forhand l ing wet g ra in in a layer d ry ing un i t i s no t c r i t i ca ldue to the slowness of the drying process; the onlynecessity is that there should always be wet grain

ahead of the drying front. However, the centralhand l ing un i t shou ld p rov ide su f f i c ien t hand l ingcapacity to load out a semi-trai ler load in no morethan 2 hours . Th is means a min imum hand l ing capac-i ty of 400 to 500 bushels per hour for layer-dryingsystems.

Figure 1.-Typical drying bin equipment for layer drying.

Advantages and DisadvantagesLayer drying offers some advantages when com-

pared to other drying methods. The system requiresa low heat input, making i t one of the most energy-eff ic ient drying methods in terms of the amount ofheat required to move moisture from the grain. Thegrain remains in place in the storage structure afterbe ing dr ied , thus requ i r ing a min imum of hand l ingand labor. By combining the dryer and storage unit

Kenlucky Room 5.105 Agi lcul lutal Sctence Butdng Notth Lextngtan Kentuc^r 1A516

Servi . : . Unrversr ly ol Kentucky Col lege ol Agncul lure Lexrnglo. and Kentucky Slale Unrversrty Frankior l

f-l l*""0Iffi

" Perforated

into one structure, layer drying presents an economi-cal drying alternative for those producers with lowharvest rates and harvest volumes.

There are also several disadvantages that areinherent with a layer drying system. The speed of thesystem is slow when compared to most other types ofdrying systems, requir ing greater system manage-ment. The slow speed of the system may restrictharvest rates and normal ly el iminates the chance ofusing the same bin and associated drying equipmentmore than once during the drying season. This mayl imit the operator to one drying experience per binper year, which would require very careful manage-ment during operat ion. A layer drying system haslittle reserve capacity to overcome any mistakeswhich may result in the loss of an ent ire bin of grain.

? 5

5

t

f -zoF-lrlFzo(,ut 15E3F2o-- l of

G,gJ?r r 5

40 50 55 60R E L A T I V E H U M I D I T Y , 7 o

Physical PerformanceCharacteristically, layer

systems where slow harvestper day) and low harvest

drying is used in grainrates (200 to 500 bushelsvolumes (up to 15,000

bushels per year) occur. The drying air temperatureshould be l imited to no more than a 20oF r ise aboveambient condit ions and usual ly the added heat iscontrol led by a humidistat which senses the humidi tyunder the plenum chamber. Ceneral ly, the controlfevel for the humidistat will range from 50"/" to 60"/"relat ive humidi ty (rh)with 55% being a typicalsett ing.

Figure 2 shows equi l ibr ium moisture contents forshel led corn at var ious temperatures and humidit ies.I t can be seen from the f igure that at relat ive humidi-t ies below 55"/" , the grain may be dr ied to undesirablylow moisture contents and that there wi l l be a largepotent ial for overdrying in the bottom layers of grain.Care should be taken to l imit overdrying as much aspossible since no premiums wi l l be paid for graindried below the base moisture content. Maintaininga humidistat sett ing of Sl" /"should keep overdrying toa minimum. The addit ion of st i rr ing devices wi l l a lsohelp reduce moisture spread and increase dryingcapacity; however, these gains may be offset by theaddit ional cost of the st i rr ing equipment.

Figure 2.-Equillbrium moisture conlents for shelled corn for various drying temperatures and humidities.

Warmer temperatures a'nd higher humidi t iesduring the harvest season pose problems for grainproducers in Kentucky and other areas of the south-east that their counterparts in northern cl imates donot face. ln addit ion to the normal problems of grainspoi lage due to mold growth, the producer may alsobe faced with the potent ial for af latoxin develop-ment. Whi le taking these restraints into accountwhen determining a layer drying schedule, the pro-ducer must also consider the fol lowing parameters:the air f low of the drying fan, the outside temperatureand humid i ty , the humid is ta t se t t ing , the in i t ia l mo is -ture content of the wet grain and the desired f inalmoisture content when dry.

To a id g ra in p roducers in de termin ing the f i l l i ngschedule for their individual layer drying systems, acomputer model (Br idges et al . ,1982) has been devel-oped. The model uses specif ic drying fan and bininformation as wel las the projected drying condit ionsduring harvest. Output f rom the model provides theproducer with a tentat ive f i l l ing schedule for a spe-ci f ic set of input condit ions whi le considering the po-tent ial for mold growth and af latoxin development.

Table 1 presents f i l l ing schedules for shel led corndeveloped by the computer model for an 18-footdiameter drying bin with 16-foot eaves, a 5 hp dryingfan and f ive ini t ia l grain moisture contents. Eachschedule in Table 1 was determined for average out-side air condit ions of 60"F and 65% rh (which aretypical of Kentucky during the harvest season), ahumidistat control value of 55% and a f inal storagemoisture ol 13"/" wb. The various schedules in Table 1provide such management information as the num-ber of f i l l ings or layers required to f i l l the bin for aspecif ied harvest moisture content, the volume andapproximate drying t ime for each layer and the totalt ime requ i red to d ry and f i l l the b in . As each schedu leis o r ien ted toward an ind iv idua l s i tua t ion , i t shou ldprovide the producer with an excel lent guide as towhen the dry ing f ron t w i l l pass th rough an ind iv idua llayer and how often one can expect to add a newlayer. The various schedules can also be used todetermine new layer volumes as the grain moisturedecreases during the harvest per iod.

For the example s i tua t ion in Tab le 1 , as the in i t ia lmoisture content is reduced trom25"/"to19"/" wb, theto ta l number o f layers o r f i l l i ngs per schedu le de-creases from 11 to 5 whi le the total drying t ime alsodecreased f rom 23.5 to 15.3 days. For Schedules 1 and2 (Table 1) at the higher ini t ia l moistures, there is apotent ial for af latoxin development so the individuallayer sizes were l imited to prevent i ts occurrence. Asthe ini t ia l grain moisture decreased to 2ly",af latoxin

was no longer a crucial factor and so mold growthbecame the control l ing factor in determining thedrying schedule. This is a less restr ict ing condit ion asshown by the relat ively larger layer volumes anddry ing t imes in Schedu les 3 and 4 . When the in i t ia lgrain moisture is suff ic ient ly low enough to preventspoi lage as in Schedule 5, the ent ire grain bin may bef i l led at one t ime. This may not be advantageous(unless i t is sui ted to the harvest ing schedule) asshown by the 2-day increase in drying t ime over thatof Schedule 4.

The example si tuat io 'n Table 2 is also for shel ledcorn and the same dry ing cond i t ions as those in Tab le1 except for cooler and less hu mid outside condit ions.In this example the average outside air condit ions are55oF and 55"/o rh, typi fy ing a late fal l harvest season inKentucky. The humidistat control value was also 55%for th is example , thus a l low ing on ly a min imum tem-perature r ise of 1.5oF over the fan. The schedules inTable 2 indicate that when layer drying in cooler tem-peratures and reduced humidit ies, the size of thelayers can be increased signi f icant ly over those inTable 1 since af latoxin condit ions are not presentand mold growth is minimal. The trade-off in thiss i tua t ion is tha t by no t add ing much heat ( to min i -mize overdrying), the drying t imes are much greaterthan fo r the cond i t ions in Tab le 1 . So wh i le coo le r ,dr ier c l imates general ly al low more grain to be placedin the individual layers, the drying process may beextended due to less drying potent ial .

The drying schedules (Tables 1 and 2) developedby the computer mode l a re the max imum f i l l i ng ;a testhat wi l l prevent grain spoi lage for the si tuat ions pre-sented. The layer volumes and depths are based onundr ied gra in and w i l l be somewhat less a f te r d ry ingdue to shr inkage. Care shou ld be taken in us ing theschedu les in Tab les 1 and 2 fo r s i tua t ions o ther thanthose shown. Each dry ing s i tua t ion is un ique due todif ferent air condit ions, grain moisture contents anddrying fans, and may require di f ferent drying sched-ules. However, for drying si tuat ions using condit ionsthe same as those in Tables 1 and 2 with di f ferent fansand b in d iameters , one cou ld expec t s im i la r layerdepths provided the fan wi l l del iver approximatelythe same a i r f low a t the f i l l ed b in depth (16 fee t ) as theexample 5 hp fan . The dry ing t imes fo r the ind iv idua llayers may deviate somewhat from those shown in thetables so i t would be necessary to maintain a closewatch over the drying front. The example fan wasprojected to del iver 1.24 ctm/bu at the f i l led bindepth. Drying schedules for condit ions other thanthose shown may be obtained through your localcounty Extension agent.

Table l.-Layer drying schedule for corn at various initial moisture contentsand average outside air conditions of 60oF and 65% relative humidity.t

Layer Layer Total layer Total Approx. Drying Total DryingNo. Depth Depth Volume Volume Time per Layer Time

(ft) (ft) (bu) (bu) (days) (days)

Schedule 1. Initial Moisture =25"/owb

3.52.52.02.01 .51 .51 .51 .5

4.53.03.02.52.01 .0

1234567I9

1011

12345

67I

1234)6

3.02.02.01 .51 .51 .51 .01 .01 .01 .00.5

5.03.53.02.52.0

15.0

3.05.07.08.5

10.01 1 . 5'12.5

13.514.515.515.0

2.72.22.62.22.32.61 .81 .92.02.11 . 1

2.74.97.59.7

12.014.616.418.320.322.423.5

6111018142517302035234025442748295232563257

Schedule 2. lnitial Moisture = 23% wb713

1222163120382343264829533257

Schedule 3. f nitial Moistute = 21"/o wb

916152721 38264730543257

Schedule 4. lnitial Moisture = 19"/" wb

61140740730530530s207204204204101

2.95.57.9

10.512.815.217.820.5

2.92.62.42.72.22.42.62.7

713509407407305305305305

916511511509407203

3.56.08.0

10.011 .513.014.516.0

1234)

1

4.57,5

10.513.015 .016.0

3.53.03.53.43.01 . 6

3.43.13.23.02.6

17.3

3.56.5

10.013.415.418.0

3.46.59.7

12.715.3

17.3

5.08.5

1 1 . 514.016.0

10187't3511509405

10181731234228513257

15.0

Schedufe 5. tnitial Moisture = 17"/o wb

3257

lFrom computer simulation model using the f ollowing drying conditions: 18Joot drying bin with 16Joot eaves, 5-hpdrying fan, a humidistat control value of 557", and a final grain moisture of 13% wb.

ln summary, management of layer drying in-volves keeping track of the drying front and adjust ingthe f i l l ing rates accordingly to keep potent ial spoi lagedevelopment at a minimum. The posit ion of the dry-ing front can be determined at any t ime by probingthe grain mass from above and locat ing the point

where the grain temperature begins to increasesignif icant ly. Many t imes the drying front may bedetected by feel ing the temperature difference alongthe bin wal l . Management in layer drying is verycr i t ical because the operator wi l l have only onelearning experience per year.

Table 2.-Layer drying schedule for corn at various initial moisture contentsand average outside air conditions of 55oF and 55% relative humidity.r

layer LayerNo. Depth

(ft)

TotalDepth

(ft)

LayerVolume

(bu)

Total Approx. Drying TotalDryingVolume Time per Layer Time

(bu) (days) (days)

Schedufe 1. Initial Moisture = 25Y" wb

18.333,3

18.315.0

10.06.0

10.0 203615.0 1221

20363257

Schedufe 2. Initial Moisture =237" wb25.65 .4

29.1

13.52.5

16.0

13 .515.0

2748509

27483257

25.631.0

29.116.0

Schedule 3. Initial Moisture = 21Y" wb

3257 3257

lFrom computer simulation model using the f ollowing drying conditions: 7\-foot drying bin with 16-foot eaves, 5-hpdrying fan, a humidistat contol value of 55"/o, and a final grain moisture of 13o/" wb.

Economic Considerationslncreases in harvest volume are general ly ac-

companied by an increase in the harvest rate neces-sary to properly store the grain. One method bywhich the drying capacity of a layer drying system maybe increased is to enlarge the fan size or add mult iplefan units per storage structure. This addit ional horse-power may prove to be expensive and the increaseddrying capacity may not be signi f icant when related tothe cost. Also, dupl icat ing drying fans and associateddrying equipment may be more cost prohibi t ive thanselect ing another drying method. This would suggestthat layer drying is usual ly physical ly and economi-cal ly undesirable when higher drying rates areneeded.

SummaryLayer drying is a good drying technique for smal l

harvest rates and harvest volumes. The method issimple, requires l i t t le labor input but yet necessitates

superior management ski l ls. The system has l i t t lereserve capacity to overcome any mistakes and is noteasi ly adaptable when expansion is required. Whenlayer drying is used with sound equipment andcorrect management pract ices, though, the resultscan be excel lent. This is evidenced by the many layerdrying systems in use today.

References CitedBridges, T.C., C.M. White, LJ. Ross and O.J. Loewer.

1982. A computer aid for management of on-farmlayer drying systems. Transact ions of the ASAE2 5 ( 3 ) : 8 1 1 - 8 1 5 .

Brooker, D.8., F.W. Bakker-Arkema and C.W. Hal l .1974. Drying Cereal Crains. The AVI Publ ishingCompany, Westport, CT. 255 p.

McKenzie, Bruce A. 1956. Select ing a grain dryingmethod. Extension Bul let in AE-67, Cooperat iveExtension Service, Purdue Universi ty, West Lafay-ette, lN.

lssued 10-84,5M