Commercial Grain Dryers

7Commercial Grain Dryers

G. S. VIJAYA RAGHAVANMcGill University, Sainte-Anne-de-Bellevue, Quebec, Canada

1 INTRODUCTIONFor centuries, cereals have been the primary source of food for the human population.Maize, rice, and wheat together account for over 80% of the world cereal production of2 billion metric tons (MT) in 1997. Other important cereal productions include barley,rye, oats, millet, and sorghum. Although the cultivated land base for cereals droppedslightly from 716 to 701 million ha1, from 1980 to 1997, FAO statistics show that produc-tivity increased by over 30% to 2.97 MT ha1 during this period (1). Although this is aclear indication of technological improvements in production, crop production energy hasbeen estimated to be but 2025% of the total used (2). Postharvest energy consumptionis primarily in processing (including drying), transportation, and cooking. This breaksdown to roughly 15, 5, and 60% of the total, respectively, depending on the commodityand in which region it is produced. In the developing nations, very little commercial energyis used for drying because the natural-drying potential of the climate is high and usuallysufcient to bring moisture content to safe storage levels. However, about 34% of theworlds cereal crop is produced in nations where articial drying of certain species is anecessity, and commercial energy inputs for reducing moisture in crops, such as graincorn; can be as high as 10% of the energy input (3). Thus, substantial energy inputs areused in drying cereal crops from typical harvest moisture contents of 2030% to safelevels for storage (1013%).

About 126 kg of water must be removed per MT of coarse grain harvested for usein the long term, and the corresponding energy requirements range from 0.38 to over 0.63GJ/MT of grain, depending on the type of commercial dryer used, the required moisturereduction, and the operating conditions. One can estimate that the over 1 billion L ofdiesel fuel are used each year for this operation. Although the energy and investment costs

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Copyright 2003 by Marcel Dekker, Inc.

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of grain drying are signicant, they are offset by several advantages. First, the crop maybe harvested earlier and closer to its ideal moisture content, thus reducing losses in theeld owing to inclement weather, pathogenic microorganisms, birds, and wildlife. Thisalso permits autumn eld operations to be performed while the soil is relatively dry, reduc-ing damage to the physical structure of the soil, and in some regions, may permit a secondcrop. Last, proper drying and aeration of tough or damp grain reduces or eliminates spoil-age problems in storage caused of hot spots and insect infestations. However, when hightemperature-drying systems are used, a fraction of the load becomes overdried, and lossesin marketable weight and quality are incurred.

Among convective-drying technologies, there are literally hundreds of variants; yetonly certain types of dryer are presently used in the grain industry. Not all grain dryersare suited to a given geographic area or farm. The choice of system depends on the annualvolume produced, the marketing pattern, the type of farm, the material to be dried, andthe capacity and nature of existing facilities. Presently, most commercially available dryersare based on heated air (convection dryers); however, there has been substantial researchaimed at adapting other technologies to grain drying. Two general areas are involved: (a)improvement of heated air dryers to improve energy efciency or to eliminate overdrying(e.g., uidized and spouted beds concepts), and (b) alternative modes of heat or masstransfer. Among the alternative modes are conduction drying, conduction drying usingparticulate media, infrared drying, microwave drying, and heat pump drying. Althoughimproving fuel efciency is one of the main motivations for continued research, severalother advantages may be sought. Among them are faster drying, better quality of driedgrain, and incorporation of renewable energies; however, advances in one direction oftenrequire a trade-off somewhere else. For example, faster drying is often associated withpoorer quality; improved fuel efciency requires trade-offs in drying time or system capac-ity. This chapter is intended to provide an introduction of the various types of grain dryerspresently available on the market so that the reader may understand how a given dryeris selected for a given farming operation.

2 CROP CONDITIONINGThe term crop conditioning refers to moisture-reduction methods that are implemented be-fore heated-air drying or, in some circumstances, replace heated-air drying altogether. Cropconditioning includes aeration, natural air drying, in-storage drying with supplemental heat,and multistage drying. Aeration, natural air drying, and natural air drying with supplementalheat are in a category referred to as low-temperature drying, which was the norm beforethe 1940s. Since then, the enormous increases in per hectare productivity and in productionlevels of individual farms have led to the implementation of high temperature dryers, fuelledby cheap fossil fuels, to streamline operations at the end of the cropping season. Withincreases in fuel prices and concern about the use of nonrenewable energies, there is arenewed tendency to implement low-temperature drying techniques when possible.

2.1 AerationAeration consists essentially of moving small amounts of unheated air through a pile ofgrain to equalize the grain temperature and prevent moisture migration in bins exposedto signicant changes in ambient temperature. Aeration is also used to cool grain afterdrying, to keep damp grain cool until it can be dried, to remove storage odors, or to


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distribute fumigants in the grain mass. This operation is usually carried out in a storagebin that is equipped with a fan, duct system, and perforated oor, along with exhaust ventsto provide escape for moist air. Whether or not the ventilating air is blown upward orsucked down through the grain is largely a matter of choice. Upward ventilation is morecommonly used, although there are advantages and disadvantages to each of these meth-ods. An important advantage of using upward ventilation is that it allows storage tempera-tures to be measured easily because the warmest air is always at the top of the pile. Therecommended airow rate for normal aeration of shelled corn, soybeans, and small grainsat 125 Pa (0.5 in H2O) is 5 m3h1m3 of grain (0.1 cfm bu1) (4). Aeration of damp grainat 500750 Pa (23 in H2O) requires higher ow rates of the order of 50 m3h1m3 ofgrain (1 cfm bu1) (5).

It is important that the aeration fan should not be run when the relative humidityof the ambient air is too high. For example, during fall and winter, the operator shouldselect days when the average relative humidity is less than or equal to 70% (6), and theair temperature is more than 1.1C (30F). Furthermore, bins of 40 m3 (1000 bu) or lessgenerally do not require aeration if the grain is loaded dry.

2.2 Natural Air DryingNatural air drying requires higher airow rates than those used for aeration, but is con-ducted in the same type of setup, as just described. Airow rates are typically 150250m3h1m3 (35 cfm/bu) and 250500 m3h1m3 (510 cfm/bu) for bed depths of 1.21.8 m (46 ft), respectively (4). These gures apply to small grains, peas, and beans, andshelled and ear corn (6).

2.3 In-Storage Drying with Supplemental HeatDrying can be accomplished in the storage bin by ventilating with air heated to 412C(722F) above ambient through a duct system or through one centrally placed cylinder,as with batch drying. This method is adequate for bins having capacities of up to 100 ton(7). This method usually requires continuous operation of the ventilation system over aperiod of 13 weeks, but has the advantage of permitting the handling of huge amountsin one ll.

In-storage drying may also be carried out on a bar oor provided with a fan capableof providing airow rates of 80165 m3h1ton1 of grain and an appropriate system ofoor and lateral ducts. The advantages of this method are low-cost and simplicity.

2.4 Multistage DryingThe term multistage drying refers to any process that uses high-temperature drying incombination with aeration or natural air drying. An outline of two such processes, dryera-tion and combination drying, follows.

2.4.1 DryerationDryeration is a two-stage process by which grain is dried in a heated air dryer to withinabout 2% of its dry moisture content and then moved to an aerating bin where it isleft to steep without ventilation for about 10 h (5). This allows time for moisture to migrateto the surface of the kernel. The grain is then aerated for about 12 h at airow rates onthe order of 2550 m3h1m3 of grain (0.51 cfm/bu).


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The advantages of this process are

1. Higher drying temperatures can be used during the removal of most of the mois-ture, as the residence time is shorter, which improves the speed of the initialdrying.

2. The drying system capacity can increase by up to 60% because the initial dryingis faster, and there is no residence time for cooling.

3. The last few moisture percentage points, which are the most difcult to removeefciently, require only energy for the blower because the heat already con-tained within the grain is sufcient to drive the moisture to the surface, resultingin fuel savings of 20% or more.

4. The grain quality is improved by slower cooling, which results in fewer heatstress cracks.

If the air is blown up through the grain, there is often considerable condensationon the roof and walls of the bin. Therefore, the grain must be moved to another bin forstorage. The amount of condensation on the roof can be reduced by pulling the air downthrough the grain or by cooling the grain immediately after it comes out of the dryer.

2.4.2 Combination DryingCombination drying is an extension of the dryeration process, and it is primarily used forgrain with very high harvest moisture25% (8). This method involves reducing the mois-ture content to 1923% in a high-temperature dryer, then moving the grain to a bin dryerin which drying is completed using natural air or supplemental heat. The use of hightemperatures when the grain is relatively moist reduces the risk of stress cracking. Withthis method the output of the high-temperature dryer is increased to two or three timesthat obtained when it is used for complete drying. In addition, energy requirements maybe reduced by as much as 50%. Airows for the bin-drying portion of the process arebetween 45 and 90 m3h1m3 of grain (0.9 and 1.8 cfm/bu). Combination drying is anexcellent compromise between fuel efciency and processing time.

The choice between dryeration and combination drying depends on the amount ofgrain to be dried, its typical initial moisture content at harvest, and the cost of energy andcapital investment involved. If small amounts of grain at relatively low-moisture contentsare to be dried, the purchase of equipment for combination drying would not be warranted.Combination drying is more suited to high moisture contents and large volumes of grain.In all cases, for bins 100 m3 or larger, aeration ducts large enough for airows of at least36 m3h1m3 of grain (0.7 cfm/bu) should be provided. Because fully perforated bin oors

new, large storage bins.

3 ARTIFICIALLY HEATED AIR DRYINGThe drying temperature is an important consideration in any heated air process, for energyefciency, fuel costs, processing time, and output product quality depend to a large extenton this factor. Suggested ranges for drying temperatures vary, depending on the intendedend-use of the grain. A few recommendations for natural- and heated-air drying and themaximum drying temperatures to be used on grain for seed, commercial use, and animal

vary according to the drying temperature and type of dryer used. Finally, signicant energyfeed are listed in Table 1. Drying time and airow rate are also important. However, these

allow the greatest number of options, they should be considered for installation on all


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Dryers

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Table 1 Recommendations for Drying Grain with Natural Air and Heated Air

Ear ShelledParameter corn corn Wheat Oats Barley Sorghum Soy Rice Peanuts

Max. moisture content of crop at harvesting for satisfactory drying:With natural air (%)a 30 25 20 20 20 20 20 25 4550With heated air (%)a 35 35 25 25 25 25 25 25 4550

Max. moisture content of crop for safe storage in a tight structure (%)b 13 13 13 13 13 12 11 12 13(12%)a (12%)c

Max. relative humidity for natural air drying to safe storage level (%) 60 60 60 60 60 60 65 60 75Max. temp. of heated air when crop is:

Used for seed (C) 43 43 43 43 41 43 43 43 32Sold for commercial use (C)a 54 54 60 60 41 60 49 43 32Animal feed (C)a 82 82 82 82 82 82

a Moisture contents on wet basis: (a) higher temperatures than those listed may be used when the corn is dried under carefully controlled conditions; temperature of the kernelsdoes not exceed 54C at any time: (b) if there is any possibility that the crop may be sold, use the lower temperature as listed for commercial use.b If the products are to be stored for long periods, the moisture content should be 12% lower than shown in this tabulation.c Seed.


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reductions may be obtained by matching the nal moisture content to the required storagetime and the anticipated storage temperature. For example, if a storage temperature of 15Cis maintained, corn can be safely stored for nearly 2 months at 18% moisture, whereas itcan be stored for only about 10 days at 24% moisture. At these moisture contents, corncan be stored for up to 9 months and 1 month, respectively, at a storage temperature of4C (9).

The two major types of heated air grain dryers are bin dryers and portable dryers.Bin dryers are available in batch, recirculating, and continuous categories, whereas porta-ble dryers are commonly available in nonrecirculating and recirculating types.

3.1 Bin DryersBin dryers are usually operated at lower airow rates than other types and are generallymore energy-efcient, although slower than most other types of dryer. The rule of thumbin the selection of bin dryer size and capacity is that the grain harvested in a day shouldbe dried to safe storage level within 24 h to permit loading of the next days harvest.

3.1.1 Batch DryersBatch-in-bin systems are the least expensive for drying grain. The main componentsare a bin with a perforated oor, a grain spreader, a fan and heater unit, a sweep auger,

grain is put in and continues to operate as long as is required to lower the average grainmoisture content to the desired level. The drying rate depends on grain depth, temperatureof the heated air, and airow rate. As a general rule, an airow of 450 m3h1m3 of grain(9 cfm/bu) leads to efcient drying. This is the airow on the exhaust side of the grainbed and is a function of the initial ow rate, the grain bed depth, and the grains packingcharacteristics. The pressure drop across the grain bed is usually measured with a manome-ter and used to determine the required fan capacity from charts usually supplied by thefan manufacturer.

Fig. 1 Typical batch bin dryer.

and an underoor unloading auger (Fig. 1). The heater fan starts when the rst load of



For a given grain depth, the drying rate can be increased by raising the air tempera-

The grain must be cooled after drying and before storage. This is done by using the dryerfan to blow cool air over the bed, or by transferring the grain to an aerated storage bin.

differential between the drier grain near the perforated oor and the damper grain nearthe top of the grain column.

Some bin dryers have overhead, perforated, cone-shaped drying oors supportedabout 1 m below the roof (Fig. 2). A heater fan unit is installed below the perforated oorand blows warm air up through the grain.

When one batch of dry grain is dropped to a perforated oor at the bottom of thebin where it is cooled by an aeration fan, the next batch is loaded and dried on the dryeroor above. Cool, dry grain is transferred to another storage bin by an underoor auger.The advantage of this system is that drying can continue while the grain is being cooledand transferred.

Vertical stirring augers may be installed in bin dryers to promote more uniformdrying and permit a higher airow rate, thus increasing the drying rate for a given crop.Although the use of stirring augers may result in slightly lower fuel efciencies, the largerpossible batch size, the increased drying rate, and the reduction in quality losses fromoverdrying at the bottom outweigh this disadvantage.

3.1.2 Recirculating DryersIn recirculating dryers, grain is constantly mixed during heating. One example of a recircu-

auger situated in the center of the dryer. The auger picks up the grain and delivers it tothe top of the grain bin. The result is a more uniformly dried crop than that obtained usingnonrecirculating types.

Fig. 2 A bin dryer with overhead drying oor.

for the crop being dried can be determined from the initial moisture content (see Table 1).ture, but this increases the chance of overdrying near the oor. An adequate air temperature

Alternate heating and cooling cycles are sometimes invoked to reduce the moisture

lating dryer is shown in Fig. 3. A slanted oor causes the grain to move toward a vertical


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Fig. 3 Recirculating dryer.

The dryer shown in Fig. 4 is used as a recirculating batch or continuous-ow dryer.When being used as a recirculator, an under grain sweep-auger moves grain to thecenter of the perforated bin oor where it is picked up by a vertical auger and deliveredto a grain spreader. When the dryer is operated as a continuous-ow dryer, the graintraveling up the vertical auger is transferred to an aeration bin by an inclined auger.

3.1.3 Continuous-Flow DryersOf the many types of continuous-ow dryers, the cross-ow dryer is the most commonly

The grain is loaded at the top and passed down both sides of the hot and cold

by a thermostat near the outside of the grain column. As fan capacity is decreased orcolumn width increased, more efcient use of heat results; however, the moisture differen-tial between grain on the inside and outside layers increases.

Fig. 4 Recirculating bin dryer.

plenums before entering the unloading augers. Grain ow rate is controlled manually or

used for grain drying (Fig. 5).



Fig. 5 Cross-ow dryer.

Some continuous-ow dryers use three fans and three plenums, each with individualtemperature controls. These may be run with two heating sections and one cooling section,or with three heating sections. In the latter case the grain must be cooled in an aeratedbin (see Figs. 6a, 6b).

Farm Fans (Indianapolis, IN) has a series of dryers of this type that they term contin-uous multistage dryers, ranging in capacity from about 5 to 27 ton/h (2651220 bu/h)based on drying and cooling corn from 25 to 15% moisture.

Several companies recycle drying or cooling air. Two common techniques of ac-complishing this are shown in Fig. 6. Some manufacturers use the system shown in Fig.6a. Here, ambient temperature air is drawn through the grain in the cooling section andthen passed through the fan heater unit of the midsection. This system results in moreenergy saving than the system shown in Fig. 6b because air from the rst heating section

Fig. 6 Heat recovery systems: (a) reverse cooling; (b) one-way airow.


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is recycled. Its disadvantage is that chaff and ne material may be drawn into the midsec-tion hot air plenum, necessitating frequent cleaning.

Most continuous-ow dryers are of the stationary type, although some of the smaller-sized dryers are portable. For example, Gilmore and Tatge Manufacturing Company, Inc.(Clay Center, KS) make a concentric cylinder-type portable dryer that handles 7.8 ton/h(350 bu/h) based on moisture removal from 20.5 to 15.5%. Grain column width on manyof these dryers is 0.30 m as compared to the 0.45 m found on the GT-Tox-o-Wik recirculat-ing batch dryers. Note that the moisture differential across the grain column is loweredas its width is decreased. A thinner column, therefore, means that, for a given averagemoisture content, the inner layer is overdried less. Thus, using a continuous-ow dryermight be of some benet when drying heat-sensitive small grains, such as wheat, oats,and barley.

Another type of continuous-ow dryer is the parallel-ow dryer in which the grainmoves in the same direction as the hot airow. This results in more uniform drying andreduces the danger of heat damage. Furthermore, because no screens are used in parallel-ow dryers, small seed crops can be dried without leakage.

Continuous-ow dryers are not well suited for the drying of small quantities ofdifferent types of grain because start-up and emptying of these dryers is inefcient. Accu-rate moisture control is difcult to achieve until a uniform ow is established. Continuous-ow dryers are best in situations in which large quantities of grain must be dried withoutfrequent changes from one type to another.

3.2 Rotary DryersRotary dryers are now employed in the grain industry to a limited extent. The majorcomponents of the system are a long, inclined cylindrical shell, a fan and heater unit,loading and unloading augers, and a variable-speed drive. The operating principle of thistype of dryer is to repeatedly lift the grain using a set of ights along the perimeter ofthe cylindrical shell and drop them into a stream of heated air. Most rotary grain dryersare of the concurrent-ow type in which grain and heated air are introduced at one endof the shell and dried grain and moist air exit at the other end. The grain is moved throughthe entire length of the shell by cascading a certain distance along the periphery of theinclined shell, such that in each fall, the grain is moved closer to the exit.

A typical commercial rotary dryer has a shell diameter of 12 m, a length of 1530 m, and a slope 24 from the horizontal. The shell rotates at 48 rpm, and the dryingair temperature is 121288C (10). Rotary dryers are increasing in popularity for theparboiling of rice because they are particularly suited to drying high moisture particlesthat tend to stick together and cannot be suitably dried in bin or column dryers. Thespecic energy consumption of the system cannot be fairly compared with that of otherdryers, for the parboiled rice is at a much higher moisture content than those materialsdried in other types of units.

3.3 Portable DryersPortable dryers generally appeal to the farmer who has grain bins in various locations, orwho does custom drying off the farm. Portable dryers may be used without a proper grain-handling system to ll an immediate need in an emergency situation; however, they arenormally not used when drying is benecial, but not necessary, owing to the inconvenience



of setup and dismantling of the system. The two types of portable batch dryers are nonrecir-culating and recirculating.

3.3.1 Nonrecirculating DryersMost nonrecirculating dryers have a fully enclosed concentric cylinder conguration. Theyare loaded from the top, and drying is accomplished by blowing hot air radially througha column of grain. Although the inside grain layer (the layer near the hot air plenum)tends to overdry while the outside layer remains underdried as occurs in the batch-in-bindryer, the damp and dry grain are mixed as the grain is removed from the dryer so thata satisfactory product results.

Other types of portable nonrecirculating batch dryers exist, such as wagon or truckbox dryers. These use a heater fan unit, similar to that used for bin drying, that is connectedto smaller air ducts suspended at midheight of the box or located on its oor. If suspendedair ducts are used, exhaust ducts on the oor are a necessity.

These types of automatic dryers are equipped with thermostats or timers to controlheating and unloading cycles and can be completely automated if a mechanized grain-handling system is used.

3.3.2 Recirculating DryersPortable recirculating batch dryers are essentially the same as nonrecirculating modelsexcept that they have a central auger that picks up grain near the bottom of the columnand deposits it at the top (Fig. 7). A complete recirculation of grain occurs roughly every15 min. Most common dryers of this type come in sizes ranging from 10 to 18.5 m3 (300525 bu) bin capacity. These dryers are often used by medium-sized of farms in eastern

Fig. 7 Typical portable dryer.


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Table 2 Recommended Drying System Based on the Annual FarmProduction at Harvest

Annual production (ton) Type of drying system2260 Natural air drying60445 Natural air drying with supplemental heat4451556 Batch-in-bin dryersAbove 1556 Portable and continuous ow dryers

North America that cannot afford a more expensive continuous-ow model. The dryersmay be used for virtually any crop, if the maximum safe drying temperature is not ex-ceeded. However, their disadvantage is that constant augering can cause damage to certainseeds, such as beans, peas, and malting barley, especially when they are nearly dry.

3.4 Dryer SelectionSelection of a continuous-ow, batch, or batch-in-bin dryer depends largely on the amountof grain to be dried and the facilities a farmer already has available when the dryer ispurchased. For example, a farmer who already has a good-sized storage bin and only asmall volume of grain to store would likely use an in-storage dryer, rather than purchasea portable dryer and wet grain holding bin. In-storage dyers usually are suitable onlyfor farms harvesting less than 160 ha.

The recommendations concerning the type of system to be used can be made basedon the annual production of a given farm, as illustrated in Table 2. Although the capacityrange presented here is for corn, it can be extended to other grains and cereals. The recom-mendations are based on harvest moisture conditions in the central United States region.The crops most often dried in Canada and the United States by articial means are corn(maize) and beans. Wheat, oats, and barley are harvested in the dry season and usuallycome off the eld at a low enough moisture content for safe storage. If need be, the grainmay be dried with natural air on sunny, warm days.

Most of the dryers in Canada are found in Quebec and Ontario (11), many of thesebeing portable batch types. Larger, continuous-ow models may be found at cooperativesacross the country or on the larger farms in southwestern Ontario where farmers are grow-ing 320360 ha of their own crop.

4 ARTIFICIAL DRYING IN DEVELOPING COUNTRIESWhere humidity is too high to allow grain to be adequately dried by natural means, it isnecessary to supply heat to the drying crop. The most popular forms of articial dryingmay be categorized according to the depth of grain being dried. These are (a) deep-layerdrying, (b) in-sack drying, and (c) shallow-layer drying.

Deep-layer dryers consist of silo bins (rectangular warehouses) tted with ductingor false oors through which air is forced. Depths of up to 3.5 m of grain may be driedat one time (12).

An in-sack dryer is made of a platform that contains holes just large enough to holdjute sacks full of grain. Heated air is blown up through the holes (and grain) by a heaterand fan unit. The platform may be constructed from locally available material. A typical



oil-red unit that handles 25 ton of grain is equipped with a fan that delivers 9700 m3h1of air heated to 14C above ambient temperature and consumes about 4.5 L h1 of oil(11). For 2-ton loading, the moisture removal rate is about 1%/h.

Shallow-layer dryers are those consisting of trays, cascades, or columns in whicha thin layer of grain is exposed to hot air. In these dryers, the hot airstream is at the highestsafe temperature and the amount of drying is determined by the length of time the grainis allowed to remain in the dryer, either as a stationary batch or as a slow-moving stream.Because the layer of grain being dried is thin (less than about 0.20 m), no signicantmoisture gradient develops through the grain (11). This means that the drying temperatureis limited only by the possibility of heat damage to the grain.

Another simple, but effective, type of articial dryer utilizes a locally built platformdryer in which the products of combustion of local fuel are not allowed to pass throughthe grain. The heated air passes through the produce by means of natural air movementor convection currents. One such dryer built at Mokwa, Nigeria, uses a pit (which becamethe hot air plenum) covered by the drying oor, the rebox being located outside theplenum chamber.

Yet another type of dryer is the horizontal dryer, which contains a number of cham-bers, each being divided by horizontal, equidistant, screen-bottomed trays placed on hori-zontal pivots. Damp grain is placed on the top tray in a layer 0.16 to 0.18-m deep and istipped to the next set of trays after an initial drying period. As this type of dryer is normallyoperated as a batch dryer, it is an advantage to have two cooling chambers per unit sothat one batch may be loaded into the dryer while the other is being removed from themachine. A typical setup of this type would include a double drying chamber, a cleaningunit, and augers or elevating units for tilting the dryer and elevating the grain to storage.

5 SOLAR ENERGY IN DRYINGAn alternative that is being encouraged in hot, dry countries of Asia and Africa is solardrying. Solar heat is trapped with a solar collector constructed from an aluminum sheetpainted black. The collector may be xed to the drying bin in such a way that an air spaceexists between it and the bin wall. Energy absorbed by the collector heats the ventilatingair by a few degrees as it is forced through the air space. In North America, these typesof dryers have been known to operate satisfactorily with grain moisture contents up to25%, even on cloudy days (7). The reason for this is that solar energy is about half visiblelight and half infrared rays, the latter being able to penetrate clouds. On rainy days andnights supplemental heat may be supplied electrically.

In countries where harvesting time occurs at the beginning of the dry season, themost popular method of drying is exposure to the sun. Crops are often left to dry in theeld before harvesting. In some countries various crops are dried on scaffolds or invertedlatticework cones. Another method is to lay paddy, maize, cobs, and other crops on heapsof stubble and then to cover them with stubble. At the village level, probably the mostcommon practice is to spread the harvested threshed or shelled crop on the ground or ona specially prepared area (e.g., matting, sacking, mud/cow dung mixture, or concrete)exposed to the sun.

In humid countries, initial crop drying may take place as just outlined; however,further drying is accomplished by placing the crop in a ventilating storage area. A moreeffective type of drying than sun drying is shallow layer drying. This form of drying maybe achieved by spreading the produce in a layer on the ground or on wire bottom trays


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that are supported above the ground. Cribs may also be constructed for drying maize onthe cob or unthreshed legumes and cereals. These are usually oriented so that the longaxis is facing the prevailing wind. They often have roofs or wide overhangs to protectthe drying crop from rain.

In most warm countries, a commercial dryer is too expensive and not essentialenough for a single farmer to consider its purchase. China, India, and countries on thecontinent of Africa are examples of places where solar drying by direct exposure or bya cheaply constructed collector is employed.

Solar energy can be and is used by some North American farmers for low-tempera-ture drying situations with unsophisticated collectors (12).

6 NONCONVENTIONAL METHODSIncreases in the price of fossil fuels have prompted researchers to investigate and developmore energy efcient dryers (1316). One attempt at reducing fuel cost was to pass un-heated air through large beds of absorbent material such as silica gel before passing itthrough the grain. The problem was that the gel itself had to be dried at high temperatures,making the operation expensive.

Other methods related to enhancement of heat-transfer techniques are also beingstudied. Particleparticle heat transfer is one such technique that has led to the design ofmany experimental dryers. Although the heat-transfer rates and efciency have beenhigher than for convection systems, the moisture removal was low owing to restrictionof mass transfer of moisture. If the moisture to be removed is limited, conduction dryersmay be recommended. Hybrid systems consisting of a initial heating by particle-to-particleconduction, followed by heated-air drying have been contemplated. Richard and Raghavanhave dealt with this topic extensively (17). They discuss the theoretical aspects, experimen-tal data, and demonstrate the potential of this method. The main advantage of this typeof dryer is its rapidity. Following this concept, a continuous-ow conduction grainprocessor/dryer was developed in the late 1980s at McGill University, Quebec, Canada;it is shown in Fig. 8 and fully described in a paper and two patents by Pannu and Raghavan

Fig. 8 Particulate medium dryer developed at Macdonald Campus.



Fig. 9 Particulate medium thermal processor, Macdonald Campus.

(1820). It is based on particleparticle heat transfer and was designed to control mixingand heating time and provide ease of separation of the grain and the particulates. As adryer, however, the most serious limitation was resistance to mass transfer owing to rapidsaturation of the air in the conical drum. Some work was performed using zeolites as theparticulate media in an attempt to improve removal of moisture from the air. This improvedthe mass transfer (moisture removal) by 50130%, and although ne zeolite powderstended to stick to the nished product, nutritional studies on ruminants did not indicatedeleterious effects (21,22).

The design was extensively modied in the 1990s (23), leading to the constructionof a 1 ton1h1 thermal processor for grain to efciently accomplish tasks such as roasting,popping, disinfestation, and degermination (Fig. 9). A 10 ton1h1 commercial unit is nowbeing developed.

In regions where hydroelectric power is available, it may be reasonable to adaptmicrowave-heating technology for grain drying applications. One motivation for this ap-proach is that hydroelectric power is considered to be nonpolluting. The main disadvantageis the initial capital cost of equipment. Nevertheless, there may be substantial savings indrying time owing to rapid internal heat generation and pressure-driven expulsion of inter-nal moisture. Some recent work has shown that it is possible to dry grains to seed qualityin a combined microwaveconvective environment (24). As with particulate medium heat-ing, microwaves may be more practical as an initial heating method integrated with heatedair drying.

ACKNOWLEDGMENTSThe author thanks the following companies for providing information on different typesof dryers: Beard Industries, Frankfort, IN; Caldwell Manufacturing Company, Kearney,NE; Farm Fans, Indianapolis, IN; Gilmore and Tatge Manufacturing Company, Inc., ClayCenter, KS; Long Manufacturing N.C. Inc., Tarboro, NC; Martin Steel Corp., Manseld,OH; and Mathews Company, Crystal Lake, IL. The help of Mr. P. Alvo in revising the


182 Raghavan

original version of this chapter which appeared in the Handbook of Industrial Drying.Vol. 1, 1995 (AS Mujumdar, ed.), Marcel Dekker, Inc. is also appreciated.

REFERENCES1. FAO Statistics Database on Internet. 1998.2. Parikh JK, Syed S. Energy use in the post-harvest (PHF) system of developing countries.

Energy Agric 6:325351, 1988.3. Smil V, Nachman P, Long TV II. Technological changes and the energy cost of U.S. grain

corn. Energy Agric 2:177192, 1983.4. Agriculture Canada. Drying and conditioning. In: Agricultural Materials Handling Manual,

Part 3. Ottawa: The Queens Printer, 1962, pp 131.5. Foster GH. Drying cereal grains. In: Storage of Cereal Grains and Their Products. St. Paul,

Min: Am Soc Cereal Chem 1984, pp 79116.6. Brooker DB, BakkerArkema FW, Hall CW. Grain drying systems. In: Drying Cereal Grains.

Westport CT: AVI Publishing, 1974, pp 145184.7. Nash MJ. Cereal grains, legume grains and oil seeds. In: Crop Conservation and Storage, New

York: Pergamon Press, 1978, pp 2779.8. Friesent OH. Heated-Air Grain Dryers, Ottawa Information Services Agriculture Canada Pub-

lication 1700, 1981, pp 825.9. Ritchie JD. Sourcebook for Farm Energy Alternatives. NY: McGraw-Hill, 1983.

10. Brooker D, Bakker-Arkema FW, Hall CW. Drying and Storage of Grains and Oilseeds. West-port, CT: AVI, 1992.

11. Otten L, Brown R, Anderson K. A study of a commercial cross-ow grain dryer. Can AgricEng 22:163170, 1980.

12. Hall DW. Handling and Storage of Food Grains in Tropical and Subtropical Areas. Food andAgriculture Organization of the United Nations, 1970, pp 1198.

13. Meiring A, Daynard TB, Brown R, Otten L. Dryer performance and energy use in corn drying.Can Agric Eng 19:4954, 1977.

14. Mittal S, Otten L. Evaluation of various fan and heater management schemes for low tempera-ture corn drying. Can Agric Eng 23:97100, 1981.

15. Mujumdar AS, Raghavan GSV. Canadian research and development in dryingA survey.In: Drying 84 New York: Hemisphere/McGraw-Hill, 1984.

16. Sturton SL, Bilanski WK, Menzie DR. Drying of cereal grains with the dessicant Bentonite.Can Agric Eng 23:101104, 1981.

17. Richard P, Raghavan GSV. Drying and processing by immersion in a heated particulate me-dium. In: Advances in Drying. Vol. 3. New York: Hemisphere, pp 3970, 1984.

18. Pannu K, Raghavan GSV. A continuous ow particulate medium grain processor. Can AgricEng 29:3943, 1987.

19. Raghavan GSV, Pannu KS. Methode et appareil de se`chage et de traitement a` la chaleur dunmateriau a letat granulaire. Canada Patent 1254381, 1989.

20. Raghavan GSV, Pannu KS. Method and apparatus for drying granular material. US. Patent4597737, July 1, 1986.

21. Raghavan GSV, Alikhan Z, Fanous M, Block E. Enhanced grain drying by conduction heatingusing molecular sieves. Trans ASAE 31:12891294, 1988.

22. Alikhani Z, Raghavan GSV, Block F. Effect of particulate medium drying on nutritive qualityof corn. Can Agric Eng 33:7984, 1990.

23. Sotocinal S. Design fabrication and testing of a particulate medium thermal processor. PhDdissertation McGill University, 1997.

24. Raghavan GSV, Alvo P, Shivhare US. Microwave drying of cereal grain: advantages andlimitations. CAB Postharvest News Inform 4:79N83N, 1993.


Chapter 7: Commercial Grain Dryers1 INTRODUCTION2 CROP CONDITIONING2.1 Aeration2.2 Natural Air Drying2.3 In-Storage Drying with Supplemental Heat2.4 Multistage Drying2.4.1 Dryeration2.4.2 Combination Drying

3 ARTIFICIALLY HEATED AIR DRYING3.1 Bin Dryers3.1.1 Batch Dryers3.1.2 Recirculating Dryers3.1.3 Continuous-Flow Dryers

3.2 Rotary Dryers3.3 Portable Dryers3.3.1 Nonrecirculating Dryers3.3.2 Recirculating Dryers

3.4 Dryer Selection

4 ARTIFICIAL DRYING IN DEVELOPING COUNTRIES5 SOLAR ENERGY IN DRYING6 NONCONVENTIONAL METHODSACKNOWLEDGMENTSREFERENCES

Documents

Commercial Grain Dryers