AUTOMATIC TIMER CONTROL FOR A BATCH-IN-BIN DRYER

R. B. Brown1, L. Otten1, and J. E. Brubaker2

'School ofEngineering, University ofGuelph, Guelph, Ontario NIG 2W1; and 2Ontario Ministry ofAgriculture andFood, Energy Branch, Toronto, Ontario.

Received 24 April 1986, accepted 11 February 1987

Brown, R. B., L. Otten, andJ. E. Brubaker. 1987. Automatic timer control fora batch-in-bin dryer. Can. AgricEng. 29: 179-182.

Acomputer-based control system for abatch-in-bin grain dryer was developed and tested with afarm-scale grain dryerin 1985. Nine drying tests were conducted with wet corn harvested at about 30% initial moisture content (wet basis). Thecontrol system utilized an inexpensive personal computer (Radio Shack TRS-80 Model 100) and an A/D interface unitdesigned tocommunicate with the computer through the RS232 port. The control algorithm was based upon simulation ofthe drying process, with known initial conditions and parameters. Control to within 0.3 percentage points ofthe target finalmoisture content of the corn dried was realized in three of the tests.

INTRODUCTION

Batch-in-bin dryers are commonly usedtodry cornon farmswherethe annualcropdoes not exceed about 500 t. This systemrequires a relatively low initial investment,is flexible and with careful managementcan produce high-quality grain. In addition, energyefficiency is potentially excellentespecially with exhaust heat recovery(Otten 1985).

A typical drying bin is about 7000 mmindiameter andhasa fully perforated drying floor. A fan and burner unit deliversheated drying air, usually at 60-100°C, toan underfloor plenum. This drying airpicks up moisture as it passes through a1000- to 2000-mm depth of wet corn in thebin, and isexhaustedthrough roof vents. Alevel fill of wet grain is dried to an acceptable moisture content and then cooled inthe bin and unloaded to storage. The timerequired for one cycle varies from about 12to 20 h depending upon initial moisturecontent, grain depth and drying temperature.

The deep bed of grain in the dryer creates two problems with the system. First, asteep gradient of moisture content existsfrom the bottom to the topof the grainbedat the end of drying. Although grain ismixed as the dryer is unloading, the difference in moisture content among individual kernels may be as great as 20percentage points. Various schemes to stiror mix the grain as it dries have beenintroduced to eliminate the gradient, anddryer manufacturers supply either augerstirring machines or grain recirculatingdevices.

The second operational problem is oneofcontrol. It is very difficult to monitor themoisture content of grain in the dryer inorder to determine when to shut off theburner. The axial moisture gradients mentioned earlier and also radial fluctuations

resulting from uneven stirring or pocketsof fines make it necessaryto obtain a largenumber of samples from the grain bulk.These samples must be blended and cooledbefore a moisture tester can be used tomeasure average moisture content. Theprocedure must be repeated until the batchis finished. In practice, dryer operatorsdonot test the grain but rely upon experienceto time the batch. This approach is satisfactory when variations in grain moisture,depth and ambient conditions are slight.More typically, the operator acts in such away that the corn is always overdried by2-3 percentage points to be on the safeside.

Apart from a reduction in grain quality,deliberate overdrying costs at least $5.00per tonne of grain marketed at presentprices. The cost is due to additional fuelcosts for drying and to weight loss fromunnecessary moisture removal.

A control system to regulate final average moisture content for a batch-in-bindryer is desirable. A closed-loopfeedbackcontrol system for moisture content is notpossible because average moisture contentis not readily measured. An alternativeapproach is to use a predictor, or a calculated estimate of the moisture content, tocontrol the dryer. The objective of thisstudy was to develop and field-test such acontrol system.

SIMULATION MODELSThree approaches to the mathematical

modelling of deep-bed corn drying wereinvestigated. The logarithmicmodel (Sab-bah et al. 1979; Barre et al. 1971) is thesimplest and fastest to use. It is suited tonear-equilibrium drying since it neglectssensibleheating of grain by the drying air.Moisture and temperature gradients in thegrain bed are assumed to be logarithmicfunctions of timeanddimensionless depth

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units. Since batch-in-bin drying uses airtemperatures above 60°C, equilibrium isattained for only a small fraction of thebed, and the method is inappropriate.

A set of four partial differential equations can be derived to define the changesin grain moisture and temperature and airspecific humidity and temperature for abed of grain (Meiering et al. 1977;Bakker-Arkema et al. 1974). These may be solvednumerically, but very small increments oftime and depth are necessary for solutionstability. For a deep bed and a high initialmoisture content simulation time canbecome excessive.

A similar approach is to perform heatand mass balances around a thin section ofthe bed, and then treat the bed as a stack ofadjacent layers (Thompson et al. 1968).The thin-layer drying rate is calculatedfrom an appropriate empirical equation.The resulting system of equations can besolved quickly if several simplifyingassumptions are made. For example, kernel temperature is assumed to be in equilibrium with the drying air. It was alsoassumed that although condensationoccurs in the upper grain layers, rewettingdoes not occur due to the relatively shortdrying time. This assumption greatlyreduced the computation time for the firsthalfof the dryingperiodwhengrainconditionsabove thedrying front areessentiallyconstant.

The model used wasdevelopedfromtheconcepts presented by Thompson (1970).A time increment of 300 s (5 min) and alayer thickness of 50 mm (2 inches) wasused for simulation. Psychrometric relationships given by Wilhelm (1976) andpublished by the American Society ofAgricultural Engineers (1983, D271.2)wereused to calculatemoist air properties.The Zeroin search technique reported byBakker-Arkema et al. (1974) was used

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with the equation of the wet bulb line todetermine the wet bulb temperature whencondensation occurred in the grain bed.

The specific heat of corn was determined by the relationship published byOtten and Samaan (1980):

Cp = 1.178 + 0.0627 Mw -

8.7 x 10-4MW2 (1)

Equilibrium moisture content of cornwas calculated using the Chung-Pfostequation for desorption and the coefficients developed by Gustafson and Hall(1974):

ln[-/?7rln(RH)] = ln(A) - BMC (2)Dry matter bulk density of corn was

assumed to be 620 kg/m3. This value,along with airflow rate and incrementalbed depth, was used to calculate a drygrain-to-air mass ratio (Rk) which wasnecessary for a heat balance. The ratio wasused to convert specific heat of grain to anequivalent value expressed on the basis ofair mass rather than grain mass (i.e., kJ/kgdry air).

Latent heat of vaporization for water incorn was determined from an expressiondevelopedby Thompson et al. (1968) andconverted to SI units

L = (2502.22 - 2.3867) x

(1 + 4.35 exp (28.25M)) (3)

The drying temperature for each grainlayer was calculated by performing a sensible heat balance of the air and grainbefore and after thermal equilibrium. Thiswas an intermediate estimate only since itdoes not account for moisture evaporation.The solution for the estimated equilibriumtemperature is:

(1.006 + 1.775 Ho) + Cp/?kG„T = • (4)

(1.006 + 1.775 //„) + Cp/?k

The relative humidity of the drying air atthe new equilibrium temperature was thencalculated. If the air was saturated at thattemperature then nodrying waspossibleinthat layer. For an unsaturatedair condition,the water removal for drying for the specific time increment was calculated fromthe following thin-layer equation developed by Thompson et al. (1968) for atemperature range of 60-150°C and anaverage drying air velocity of 0.10 m/s.

t = a \n(MR) + b[\n(MR)]2 (5)A new drying curve is required for each

layer as the drying air temperatureincreases. Therefore, an equivalent dryingtime for the new curve must be calculatedto account for the drying that occurred atthe lower temperature. This equivalenttime is then incremented by the time step

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for simulation and a new moisture ratio is

calculated to determine the final moisture

content after that time step. The waterremoved is found from the change inmoisture content, and the absolute humidity of the drying air is increased by thesame amount.

The final drying air temperature wasthen calculated from a heat balance for the

air, grain and water as:

exchange would be required. The modelapproach would also require a measurement of ambient temperature at the initiation of cooling since this temperaturewould almost certainly differ from themeasurement taken at the start of dryingsome 12-20 h earlier. Due to these difficulties it was decided that rather than com

plicate the control system, a constantcooling interval would be used. The expe-

(1.006 + 1.775 Ho + CpRk)Tc - A//(2501 + L- 4.1868 Te)

1.006 + 1.775//, + Cp/?k(6)

The relative humidity of the air afterdrying was checked for feasibility of thefinal air state conditions. If the relativehumidity was greater than 100% a condensation subroutine was called. A new

final temperature and absolutehumidity ofthe exhaust air at saturation was foundusing the Zeroin algorithm. The differencein water removed between the two airstates was assigned to the corn by adjustingthe moisture content.

Several additions were necessary tocomplete the drying control program. Theairflow rate-pressure drop relationship ofHaque et al. (1982) was used to calculatethe airflow rate from plenum static pressure and grain depth. A grain stirring subroutine was developed assuming completemixing and instantaneous stirring at theend of each stirring interval. The graintemperature and moisture content of eachlevel in the bed were replaced by the overall average value for the entire bed at theend of each stirring cycle.

The dryer used for field trials had beenused in the previous year for an investigation of exhaust air heat recovery. Since theheat exchanger was also used in thesetrials, the control program had to accommodate any effects it had on the system.During the drying stage, the only significant effects would be a slight reduction inairflow rate and lower fuel consumptionfor a given drying temperature. Theseeffects would be accounted for in theparameters determined as inputs to thecontrol program.

The effects during the cooling stagecould not be compensated for easily.Ideally, time for cooling and the amount ofmoisture removal during cooling would becalculated by the simulation model. Thiswould require knowledge or an assumption of inlet air temperature to the dryer.Without the heat exchanger, this wouldsimply be ambient temperature plus asmall temperature increase due to the fan.However, with the exchanger in place,either real time measurements of inlet tem

perature or a very good model for heat

rience of the previous year indicated that2-4 h were required to cool the grain,depending upon grain depth, finalmoisture content and drying temperature.The operator would be free to change thisparameter, and often did, depending uponthe aeration capabilities of the final storagebin.

Data from the previous tests were compared with simulated drying runs for thesame conditions. The mean differencebetween simulated and actual drying timewas 1.5 h, or about 7% of the actual dryingtime. Since some moisture removal occurswith cooling, drying must stop and coolingcommencebeforethe average moisturecontent is at the target level. It was initiallyassumed that 1.5 percentage points ofmoisture were removed in cooling, butsubsequent field tests in 1985 demonstrated that the actual reduction was about3%. In view of inaccuracies in the sourcedata and assumptions a mean error of 7%was considered acceptable for the simulation model.

DRYER CONTROL SYSTEMThe simulation model was the basis for a

very simplecontrolprogramto operate thedryer. The control steps were as follows:

1. Input grain and air parameters —moisture content, depth, target moisturecontent, grain temperature, plenum pressure, ambient temperature and humidity,drying temperature and duration ofcooling.

2. Read start-up time and energize gassolenoid and drying fan relay.

3. Simulate drying for input conditionsand store required drying time.

4. Monitor clock until elapsed real timeequals required time and then close gassolenoid to shut off burner.

5. Monitor clock until cooling time haselapsed, then open fan relay to stop airflow.

The computer selected for the trials wasa Radio Shack TRS-80 Model 100. Advantagesof thismachine were: RS-232-C portfor I/O, addressablesystemclock withcal-

CANADIAN AGRICULTURAL ENGINEERING, VOL. 29, NO. 2, SUMMER 1987

TABLE 1. SUMMARY OF CONDITIONS FOR 1985 CONTROL SYSTEM TESTS

Initial Wet BedAmbient conditions

Drying Grain Duration Final

moisture weight depth Temp. RH temperature temperature of cooling moistureTest (% WB) (tonnes) (m) (°C) (%) (°C) (°C) (h) (% WB)

1 29.9 28.1 1.0 6 80 90 10 2.0 11.92 31.4 29.0 0.9 16 85 no 22 2.5 12.23 27.3 37.5 1.3 11 85 108 11 2.5 12.64 28.0 37.2 1.3 16 50 103 12 3.5 15.25 30.4 35.8 1.2 10 45 109 4 3.5 15.86 31.5 33.2 1.2 8 55 88 5 3.5 11.97 28.9 32.8 1.6 6 60 107 5 3.5 16.48 27.5 43.5 1.5 8 80 108 6 3.5 15.39 27.0 40.0 1.3 5 85 92 4 3.0 16.5

endar, battery back-up power pack in caseof power fluctuations, small size, built indisplay for data input prompts, and lowcost.

A controller and interface unit was built

to set the gas solenoid valve and dryer fanrelay. Two solid state relays were addressed by setting the two bottom bits of acontrol byte and writing the result to thecontroller via the RS-232-C port.

FIELD TESTING

The dryer used for the field tests waslocated near Ayr, Ontario. It was 7300 mmin diameter and 6100 mm to the eave with afull perforated drying floor. A 660-mmdiameter, 7.5-kW axial fan and a naturalgas burner provided heated drying air.Grain was distributed by a centrifugalspreader as it was loaded into the bin, and asix-auger stirring machine traversed thebin once every 4 h during drying.

An exhaustair heat recovery system wasinstalled on the dryer utilizing a crossflowflat plate heat exchanger which was testedthe previous year (Otten 1985).

The first dryer test commenced 16Oct.,and nine tests were conducted between thatdateand 9 Nov. A poor fielddrydown ratecoupled with persistent rainy weathermade for sporadic testing. The conditionsfor the tests are summarized in Table I.

The grain stirrer drive reduction ratiowaschanged prior to test 4 since the operator felt that better mixing and more uniform drying would result from a 2.5-hstirring cycle. The stirrer was not starteduntil 6 h of drying time had elapsed, inorder to establish a uniform drying zonebefore stirring activity disrupted the airflow pattern in the bed. Three tests (3, 6and 9) were conducted without stirring.The heat exchanger was disconnected fortwo tests to evaluate its effect on dryerperformance.

RESULTS

Moisture removal during cooling wasgreater than the 1.5 percentage pointsanticipated, consequently the corn of the

first three tests was severely overdried(Table 1). It was impossible to determinethe average moisture content at the timethe burner was shut off because there were

radially distributed zones of wet and drycorn created by the stirring auger paths.Therefore the third test was unstirred, andthe average moisture content was 17.3%wet basis compared to a simulated value of17.0%, a difference of less than 2%. Aftercooling the average moisture content was12.6% indicating that more than 4 percentage points of moisture were removed withcooling.

The control program was altered assuming a conservative value of 3 percentagepoints moisture removal with cooling. Thesubsequent two tests yielded a finalmoisture content within 0.3 percentagepoints of the target level of 15.5% wetbasis. Test 6 indicated that the moistureremoval during cooling was the same withor without the heat exchanger. The seventhand ninth tests both produced corn wetterthan desired. In both cases there was adiscrepancy of about 1 percentage pointbetween the moisture tester readings at thefarm and the oven-dry moisture determination in the laboratory for initial grainconditions. The tester results were used tocontrol the dryer, and these had been ingood agreement with the oven-dry methodin the earlier tests. Test 8 produced goodresults, and the moisture tester determination of initial moisture content was againclose to the oven-dry method.

The grain stirring augers did not do anadequatejob of mixingthe grain, althoughthere was some reduction of moisturegradient in the dried grain. The delayed stirring scheme (Test 4 and subsequent tests)improved the situation, but radial zones ofwet grain were encountered between adjacent auger paths. The solution to this problem would be more augers, or variable-path stirring augers. The overall energyefficiencyofdrying and theaccuracyof thesimulation model did not appear to beaffected by stirring strategy.

Specific energy consumption averaged

CANADIAN AGRICULTURAL ENGINEERING, VOL. 29, NO. 2, SUMMER 1987

2.45 MJ/kg of water removed for the dryerwith the heat exchanger. Without the heatexchanger, specific energy consumptionwas 2.98 MJ/kg water removed, anincrease of almost 22%.

DISCUSSION

Once the amount of drying that occurredwith cooling was known, the control system worked very well. A target moistureset 3 percentage points above the desiredfinal value (i.e., 18.5% for 15.5% finalaverage) gave good results. For other dryers this parameter may vary, but it wouldbe determined as part of the tuning procedure. Final moisture content of cool

grain was controlled to within ±0.3 percentage points for three of the test runs. Ifthe initial moisture content of tests 7 and 9

had been more accurately determined, it isprobable that the final moisture contentswould have been within the same range. Itis doubtful that greater accuracy could bemaintained consistently because commercial moisture testers have an error range ofthe same magnitude.

Level filling and uniform mixing by thestirring device was assumed in the modeland should be realized in the actual system. An implicit assumption in the controlscheme is that the ambient absolute

humidity is constant throughout drying.Although ambient temperature and relativehumidity can vary significantly in any 10-to 20-h period, it is unlikely that thehumidity ratio will deviate greatly over thesame period. This drying method is quiteinsensitive to ambient conditions in anyevent due to the large temperature increasewith the burner.

The drying temperatures used in thetests were higher than normal for deep-bedbatch drying because the dryer was operated for maximum throughput. For a temperature range of 60-70°C moisturegradients within the bed would be reducedand a more uniform product moisturecould be maintained.

CONCLUSIONSThe main conclusions drawn from the

study are as follows:1. The simulation model allowed an

accurate prediction of the time required todry corn to a specific average moisturecontent.

2. With accurate determination of grainand system operating conditions, the calculated drying time can be used to controla batch-in-bin dryer and regulate the finalgrain moisture.

3. Moisture removal during the coolingstage averaged 3 percentage points for theparticular combinations of temperatureand airflow rates in this study.

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4. The small personal computer andinterface controller comprised an inexpensive and effective control unit.

ACKNOWLEDGMENTSThe research project was jointly funded by

the Ontario Ministry of Agriculture and Foodand the Ontario Ministry of Energy. We aregrateful to Robert and Jeff Hall and to BruceFord for their cooperation and assistance duringthe drying tests.

REFERENCESAMERICAN SOCIETY OF AGRI

CULTURAL ENGINEERS. 1983. Agricultural Engineers Yearbook. ASAE, St.Joseph, Mich.

BAKKER-ARKEMA, F W., L. E. LEREW,5. F DeBOER, and M. G. ROTH. 1974.Grain dryer simulation. Mich. State Univ.Agric. Exp. Sta., Res. Rep. 224.

BARRE, J. H., C. R. BAUGHMAN, andM. Y. HAMDY. 1971. Application of thelogarithmic model to cross-flow deep-bedgrain drying. Trans. ASAE (Am. Soc.Agric. Eng.) 14: 1061-1064.

GUSTAFSON, R. J. and G. E. HALL. 1974.Equilibrium moisture content of shelled cornfrom 50 to 155°F Trans. ASAE (Am. Soc.Agric. Eng.) 17: 120-124.

HAQUE, E., Y. N. AHMED, and C. W.DEYOE. 1982. Static pressure drop in a fixedbed of grain as affected by grain moisturecontent. Trans ASAE (Am. Soc. Agric.Eng.) 24: 1095-1098.

MEIERING, A. G., T. B. DAYNARD, R.BROWN, and L. OTTEN. 1977. Drier performance and energy use in corn drying.Can. Agric. Eng. 19: 49-54.

OTTEN, L. 1985. Exhaust heat recovery in abatch-in-bin dryer. Report to OMAF by

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Wellington Engineering Ltd., Guelph,Ontario, 31 January.

OTTEN, L. and G. SAMAAN. 1980. Deter

mination of the specific heat of agriculturalmaterials: Part II experimental results. Can.Agric. Eng. 22(1): 25-27.

SABBAH, M. A., H. M. KEENER, and G. E.MEYER. 1979. Simulation of solar drying ofshelled corn using the logarithmic model.Trans. ASAE (Am. Soc. Agric. Eng.) 22:637-643.

THOMPSON, T. L. 1970. Simulation foroptimal grain dryer design. Trans. ASAE(Am. Soc. Agric. Eng.) 13: 844-848.

THOMPSON, T. L., R. M. PEART, andG. H.FOSTER. 1968. Mathematical simulation of

corn drying — a new model. Trans. ASAE(Am. Soc. Agric. Eng.) 11: 582-586.

WILHELM, L. R. 1976. Numerical calculationof psychrometric properties in SI units.Trans. ASAE (Am. Soc. Agric. Eng.) 19:318-325.

LIST OF SYMBOLS

G0 =

Ho =

Hf =

A// =

L =

M =

Mc =

Mo =

Mw =

MR =

R =

a, b, coefficients in thin layer drying RH

equation R,a = -1.706 + 0.008787

b = 148.6 exp(-0.05947). T

A,B,C,D,E,F, constants used in TcChung-Pfost equation for equilibrium moisture content (devel TToped in British EngineeringUnits). To

A = exp[(C + DTT)(RTr)].B = (E + FTr)(RTr). TrC = 1.544 x 10-2.D = -1.383 x 105. t

E = 3.211 x 10"3.F = 2.069 x 10"5.

specific heat of grain (KJ/kg K).initial grain temperature in grainlayer (°C).initial absolute humidity of air forgrain layer (kg water/kg dry air).final absolute humidity of air forgrain layer (kg water/kg dry air).H f — //o.latent heat of vaporization for corn(kJ/kg water).grain moisture content,dry basis.equilibrium moisture content,decimal dry basis.initial moisture content, decimaldry basis.grain moisture content, percentwet basis.

M - Mc

decimal

Mo ~ Mc'

gas constant 1.987 Btu/lb mole°R.

relative humidity of air, decimal,dry grain to air mass ratio (kg drygrain/kg air).air or grain temperature (°C).intermediate equilibrium temperature (°C)final equilibrium temperature forgrain layer (°C)initial air temperature for grainlayer (°C)grain temperature used in Chung-Pfost equation (°R)time (h)

CANADIAN AGRICULTURAL ENGINEERING, VOL. 29, NO. 2, SUMMER 1987