Portable batch and continuous flow are two verypopular high-speed grain drying techniques. Both dryertypes are similar in appearance and operation andrequire wet grain storage ahead of the dryer. The basicprinciple in both dryer types is to force high quantities ofair (50{25 cu fUbu) through relatively thin grain columns(12-24 inches) to obtain relatively high drying rates. Theportable batch units usually dry a fixed amount of grain,cool it and then unload to storage whereas the continu-ous flow units will meter cool-dry grain at a steady-adjust-able flow rate from the drying chamber. While the dryingunits are movable or portable as the name implies, a per-manent centralized location is generally desirable.

Department of Agricultural Engineering AEN-61

Universityof Kentucky. Collegeof Agriculture. CooperativeExtensionService

Equipment Requirements

Portable Batch

Figure 1 is a schematic drawing of a typical columnbatch dryer. The dryers are self-contained units with fansizes generally ranging from 10 to 20 hp, drying tempera-tures of 160" to ZOO"F and heater capacities of 2 to 5 mil-lion BTU/hr. Larger units than the one shown may havemultiple fans and drying chambers which allow the grainto be dried in stages. Most portable batch units come witha complete set of controls to automatically load, dry cooland unload the grain which eliminates the need for anymanualoperation of the dryer and allows extended dryingperiods after nightfall. Many portable batch units have anoptionalcooling cycle which can be eliminated when dry-eration is practiced. This option will increase the capacityof the dryer.

Continuous FlowFigure 2 is a schematic drawing of a typical continu-

ous flow dryer showing the configuration of the heatingand cooling chambers. The basic ditference between thistype dryer and that of a portable batch unit is that thegrain continually flows through this dryer and is regulatedby the grain meters shown in Figure 2. Fan sizes for con-tinuous flow dryers are comparable to those of the porta-ble batch but the drying temperatures are higher(180o-230"D and the heater sizes are generally larger;ranging from &12 million BTU/hr depending on the indi-vidualdryer. Most continuous flow dryers are completelyautomated and do not require constant supervision.

Handling

While the name "portable" implies that these dryingunits are movable, they will function better in a central-

Portable Batch-Gontinuous Flow DryingMethod and ManagementT. C. Bridges, O. J. Loewerand G. M. White

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UNLOADING AUGER

Figurc 1. - Schematic drawing ot a typical column batch dryer.

WET GRAINSUPPLY

GRAIN SLIDE

HEATED AIRCHAMBER

DRYING COLUMNSWITH PERFORATEDWALLS

CONVEYOR FORREMOVING DRIEDGRAIN

FiguE 2. - Schematic drawing of a continuous flow dryer with aheat rccirculation feature.

ized location with permanent handling equipment for thewet grain. The central handling unit may be either abucket elevator or transport augeri with both requiring aminimum handling capacity of 1500 bu/hr. In systemswhere transport augers are used, a second auger unitshould be available to unload the dry grain into storage. Insystems using bucket elevators, a smaller dry leg may beinstalled just to handle dry grain. An important consider-ation for grain systems requiring high speed dryers is notto limit the drying capacity of these units with insufficienthandling capability for movement of wet and dry grain.

Wet HoldingBoth the portable-batch and continuous flow dryers

require wet grain storage preceding the dryer itself. Thisstorage may be a ground-level tank that feeds the dryerby means of a portable auger, or an elevated wet grain binwhich provides faster gravity loading of the dryer. lf theelevated tank is chosen, care should be taken to properlydesign the support structure to accommodate the heavystresses that occur when the tank is filled with wet grain.A rule of thumb is that minimum wet storage requirementfor these systems is generally 1llzto 2 times the batchsize of the dryer or vehicle size, whichever is larger. Agood estimate of the maximum size tank may beobtained by using the following relationship:

Tank capacity = Daily harvest - Drying time xEtfective drying rate

where: Tank capacity = the size of the wet grain tank inbushels.Daily harvest = the amount of grain harvestedeach day in bushels.Drying time = hours of dryer operation while har-vest is still continuing.

Effective drying rate = the rate of the dryer in bu/hr, including the time for loading and unloadingplus that of the cooling cycle, if any.

Remember that in most cases the dryer will not bestarted until after the tirst or second load reaches thegrain facility.

For example suppose that the producer is harvesting2200 bu in a 10 hr harvest period. The batch size of thedryer is 150 bu, dried in approximately 40 minutes; thecooling cycle requires 15 minutes and unloading involvesanother 5 minutes for a total cycle time of t hour; then theetfective drying rate forthis dryer is 150 bu/hr. Because ofvehicle travel and loading time, the dryer does not begindrying until 1.5 hours after harvest has begun. Then:

fbnk capacity = 22OO - 8.5 x 150 = 925 bu

Surge BinAnother convenient item of equipment for handling

the dry grain in these systems is the addition of a surgebin. This item of equipment is generally used in systemscontaining bucket elevators and allows the dryer tounload dry grain when ready, thus facilitating the dryingprocess. In addition, the surge bin unloading auger orbucket elevator is required to operate only when thesurge bin is unloading rather than for the entire dryeroperation time. The surge bin should have a minimumcapacity equal to the batch size of the dryer although itmay be much larger if desired. Surge bins are generallyequipped with sensing devices that allow automatic load-ing of the dry grain to storage.

Advantages and DisadvantagesThe main advantage of a portable batch or continu-

ous flow drying unit is drying speed. These units aredesigned to give the necessary drying capacity for grainharvesting situations that require fast harvest rates andlarge harvest volumes. Most dryer units of either type arecompletely automated, reducing labor requirements forloading and unloading, and are available in many differ-ent sizes to accommodate a wide range of drying needs.The drying units are generally small enough so that theymay be moved if necessary but generally will operate bet-ter in a centralized location. Their portability allows for rel-atively easy replacement if they wear out or if expandedcapacity is needed.

The main disadvantage of these drying units is theirreduced drying efficiency in terms of energy when com-pared to other drying methods. This is the tradeoff thatgenerally occurs with increased drying speed. The porta-ble batch-continuous flow units are generally moreexpensive and require more associated handling equipment (wet holding, surge bins, dry legs, etc.) than doother drying methods. Grain must be handled at least

twice in systems using these dryers, but this is less of aproblem than before with advances in automatic grainhandling.

Physical PerformancePortable batch and continuous flow dryers are very

similar in appearance and performance. Portable batchdryers are classified as stationary bed dryers. The grainmass does not move during the drying process and thedrying air is blown across the grain column. These dryersoperate with drying temperatures of 160" to 200oF andairflow rates of 50 to 100 cfm/bu. Their drying capacitiesfor typical farm units will normally range from 100 to 400bu for 10 points of moisture removal. Continuous flow dry-ers may be categorized by 3 types:

r crossflow - where the drying air is blown acrossthe grain column the same as in a portable batch;

. counterflow - where the drying air and the grainmove in opposite directions;

. concurrent flow - where the drying air and grainmove in the same direction.Continuous flow farm units typically operate with dryingtemperatures of 1800 lo 220oF, airflow rates of 75 to 125cfm/bushel and at drying capacities of 125 to 600 bush-els/hr for 10 points of moisture removal. Most continuousflow farm drying units used are of the crossflow type, sothis publication will not provide further discussion of theother kinds.

An important consideration in the use of high speedgrain dryers is the temperature at which the dryer is oper-ated. The safe maximum temperature of the heated airfor drying grain depends on the following factors: (1) thefinal use made of the grain, (2) the moisture content of thegrain and (3) the type of grain.

Recommended safe maximum drying temperaturesfor shelled corn are presented in Table 1.

Table 1. - Maximum SafeDrying Temperatures for Shelled Corn,Categorized by Use.

Grain Use Drying Temperature, oF

Presented in Figure 3 are simulated performancecurves for a crossflow dryer drying shelled corn in the typical operating ranges of 1600 to 240oF for drying temper-ature, 50 to 125 cfm/bu for airflow rates and 10 points ofmoisture removal (25-150/o wb). The top graph (Figure 3)shows heat energy use of the dryer (solid lines) and mois-ture spread (dashed lines) across the grain column forthe operating ranges. Note that the drying energy will

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decrease and moisture spread across the grain columnwill increase for a given airflow rate as drying temperatureincreases. Converselythe drying energy will increase asthe airflow rate is increased for a given drying tempera-ture. This says that the hotter you can run the dryer for agiven airflow rate, the more efficient it will be. The limitingfactor here is the maximum safe grain temperature.

The bottom chart (Figure 3) presents the averagegrain temperature (solid lines) and drying rate (bu/hr/sq ftof dryer surface area - dashed lines) for the sameranges of temperature and airflow rate previously indi-cated. Note that both increase with drying temperaturefor constant airflow rates. For example, if the desiredaverage grain temperature is limited to 180oF, the bottomchart (Figure 3) indicates that the drying temperaturecould range as high as 22OoF depending on the airflowrate of the dryer. The expected moisture spread and dry-ing energy then for 2200 is about 6.30/o and 2300 BTUper pound of water removed, respectively. The tradeoff isthat though the drying temperature can be raised todecrease the drying energy, the grain quality may bereduced significantly if temperatures are too high. Thegrain temperature should be monitored closely duringdrying and if the grain is brought out of the dryer "hot," itshould not be cooled rapidly.

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An easy way to measure the grain temperature is toplace a grain sample in a largemouth thermos bottlecontaining a thermometer that reads up to 200oF. Afterfilling the thermos, read the temperature as soon as thereading stabilizes. Leave the hot grain in the thermosuntil the next sample is to be taken. This reduces the heatdrawn from the new sample by keeping the containerwarm.

Econom ic ConsiderationsPortable drying units are expensive and may require

a large capital outlay second only to that of the combine.Generally they are used where it is necessary to dry largevolumes of grain rapidly. These systems can be com-pletely automated and will require wet storage ahead ofthe drye4 which is also an added expense. When select-ing or replacing these units the following factors shouldbe considered.

Drying GapacityA dryer should be selected that is capable of drying

the daily harvest rate. The hours of dryer operation (gen-erally around 1&18 hours) is an important factor in arriv-ing at the necessary dryer capacity. However these unitsmay be sefected and designed to operaletor 24 hours perday. This reduces the necessary dryer capacity andexpense, but leaves few options if reserve capacity isneeded to overcome breakdown, delays, etc. Futureexpansion of crop size or harvest rate must also be con-sidered when selecting the appropriate dryer capacity.

Drying EfficiencyAs LP gas and other fossilfuels increase in price, the

etficiency of the dryer becomes even more important.The primary feature of portable dryers is drying speed,but generally the producer pays for this speed withreduced drying efficiency compared to other types of dry-ers. The more etficient the drye4 the less the cost of thisspeed. The continuous flow dryer shown in Figure 2 usesa heat recovery system. The drying air is pulled inthrough the cooling section and while cooling the grain,recovers some of the heat used in drying. This air thenmoves on into the drying sec.tion where this recoveredheat is reused. Depending upon dryer design and operat-ing circumstances, energy requirements of a crossflowdryer may be reduced by as much as 350/o by using aheat recovery system.

DryerationDryeration is a popular technique that increases

both dryer efficiency and capacity. In this method the

grain is removed "hot" 1120o-140oD from the dryer at amoisture content 1-3 points higher than the desired finalvalue. This "hot" grain is transferred to a separate hold-ing bin and held there from zt-10 hours. lt is then cooledslowly taking out the excess moisture before being trans-ferred intostorage. Thetempering and slowcoolingof thegrain reduces stress cracks and improves overall quality.

Dryeration benefits dryer capacity and efficiency in 3ways. First, less moisture needs to be removed in thedrye6 second, higher drying temperatures may be used,and third, the cooling cycle of the dryer is eliminated.Removing the cooling cycle reduces total batch time for aportable batch dryer and provides more drying area in acontinuous flow dryer. For an existing system, dryerationis a viable alternative to purchasing a new unit. Depend-ing upon the individual situation, significant gains in dry-ing capacity and etficiency may be made for a minimalcost compared to that of a new dryer.

SummaryPortable batch-continuous flow drying is a good dry-

ing method for on-farm systems where drying speed isessential. Both dryer types require wet grain storageahead of the dryer and each may be completely automated so that only little or moderate supervision is neces-sary. When selecting a dryer of either type, the unitshould be chosen so that the drying capacity is equal tothat of the daily harvest rate. These units are relativelyexpensive, but with proper selection and managementthey provide a viable high speed drying alternative whererapid harvest rates are necessary.

RefercncesBrooker, D. 8., F. W. Bakker-Arkema and C. W. Hall. 1974.

Drying Cereal Grains. The AVI Publishing Company,Westport, CT.265p.

McKenzie, Bruce A. 1966. Selecting a grain dryingmethod. Extension Bulletin AE-62 CooperativeExtension Service, Purdue University, West Lafay-ette,lN.

McKenzie, Bruce A. and George H. Foster. 1980. Dryera-tion and bin cooling systems for grain. ExtensionBulletin AE-102 Cooperative Extension Service,Purdue University, West Lafayette, lN.

Pierce, R. O. and T. L. Thompson. 1981. Energy use andperformance related to crossflow dryer design.Transactions of the ASAE, 24(1):216220.

Thecollegeo|AgticulturisanEqualoppo|tunityoranizetionwithrspecttoeduca|ionandomp|oymon|endauthoizationtoptovidersedrh,educationise|vicasonly|oindividual8andinstitutions|hat|unctionwTi t lev l lo t thec iv i lR igh tAc to |1964 'T i t le lxo t theEducOllica, College ol Agriculturc, Univercity ot Kentucky, R@m S-105, Agriculturcl Scienca Building-North, Lexington, Kentucky 10546.

lssusd in turth€rance ot Cooporalive Extonsion work, Acts ol May 8 and June 30, 1914, in cooperation with ths U.S. D€partment of Agricultur€. Charlss E. Barnhari, Oirector olCoopsrative Extension Seruice, Universily ot Kentucky Coll6ge of Agriculture, Lexington. and Ksntucky Stato University, Franklort.

lssued 7-86, 5M