1HEATED-AIR GRAIN DRYERSPUBLICATION 1700

2A FEDERAL / PROVINCIAL PUBLICATION

CANADA / MANITOBA

HEATED-AIR GRAIN DRYERS

O.H. FRIESEN, Agricultural Engineer,Manitoba Department of Agriculture,Winnipeg, Manitoba.

This publication was originally produced by theManitoba Department of Agriculture. Under theprovisions of the Federal-Provincial RegionalCooperative Publishing program, AgricultureCanada has agreed to print this publication.

PUBLICATION 1700, available fromInformation Services, Agriculture Canada, Ottawa K1A 0C7

Minister of Supply and Services Canada 1980Cat. No. A531700/1980E ISBN: 0-662-10865-5Printed 1980 10M8:80

Aussi disponsible en franais

3CONTENTS

Advantages of grain drying .................................................................. 4

Grain moisture relationships ............................................................... 5Maximum storage times for damp grain ...................................... 5Maximum storage moisture contents ............................................ 5Moisture migration .......................................................................... 5Moisture testers ................................................................................. 5Moisture rebound ............................................................................ 7

Weather effects on drying ..................................................................... 7Relative humidity ............................................................................ 7Wind and sun ................................................................................... 7Ambient air temperatures ............................................................... 8

Maximum drying temperatures .......................................................... 8Temperature sensing ........................................................................ 8Testing for heat damage ................................................................... 9

Grain-handling systems for dryers ..................................................... 9

Dryer types ............................................................................................ 10Bin dryers ........................................................................................ 11

Batch-in-bin process ................................................................. 11Overhead drying floors ........................................................... 13Stirring augers ........................................................................... 13Recirculating and continuous-flow devices ....................... 13

Portable dryers ................................................................................ 14Batch dryers ............................................................................... 14

Non-recirculating type ...................................................... 15Recirculating type .............................................................. 16

Continuous-flow dryers .......................................................... 16

Multistage drying ................................................................................ 18Dryeration ....................................................................................... 18Combination drying ...................................................................... 19

Aeration ................................................................................................ 19

Drying rates and efficiencies .............................................................. 19Dryer capacities .............................................................................. 20Dryer efficiencies ............................................................................ 21

Drying costs .......................................................................................... 21

Fires in dryers ....................................................................................... 22

Appendix ............................................................................................... 23

Acknowledgments ............................................................................... 26

4HEATED-AIR GRAIN DRYERS5% loss in price and a 5% loss in weight, for a total loss of10%. Harvesting before, instead of after, a rainy spellcan therefore result in a considerable saving.Overdrying of crops in the field, which leads to shat-tering and crop loss, can also be prevented by earlierharvesting. Straight combining of some crops, such assunflowers and corn, is a necessity and if these cropsare harvested damp, field losses are greatly reduce.A dryer in the system may also make it feasible tostraight combine other crops. Replacing a windrowerwith a dryer is therefore an alternative that shouldbe considered.

Eliminate Spoilage in StorageWhen tough or dampgrain is harvested and not dried, long-term storage fre-quently results in grain spoilage. Moisture migrationwithin grain bins can make slightly tough or even drygrain unsuitable for storage. Spoilage problems causedby hot spots and insect infestations can be reduced oreliminated by proper drying and aeration in storage.

The initial cost of a grain dryer is one of the majordeterrents to grain drying. Where only small amountsof grain are to be dried each year, the initial cost of thedryer should be a primary consideration (see DryingCosts). Where large amounts of grain are dried, fuel effi-ciency becomes a more important factor.

The labor and inconvenience of drying are also de-terrents on many farms where no centralized grain-stor-age facility exists. To obtain maximum benefits from agrain dryer, it is necessary to set up awell-organized sys-tem for grain handling and storage.

Because of climatic variations, not all areas have thesame need for grain dryers. Figure 1 shows prairie areasof equal net evaporation for themonth of September. Thisis based on long-term data obtained from the Atmos-pheric Environment Service, Environment Canada. Val-ues for individual years may vary slightly from thelong-termaverages. Thephysical features of an area, suchas lakes, hills and woods, may also cause notable varia-tions from the general pattern. In general, however, theneed for grain drying on the prairies increases from thesouthwest to the northeast. In the driest areas, a dryermay be needed only 1 year in 10 for wheat, oats and bar-ley, while in the northernmost areas it could be neededas often as 9 out of 10 years. If grain corn is grown forcommercial sale, a dryer is essential in all areas. Sun-flowers, rapeseed, and several other crops should alsobe harvested damp every year in order to minimize fieldand harvesting losses.

1 Tough = MC values between those given in Table 1 and 17%for cereal grains or 13% for oilseeds

2 Damp = MC above 17% for grains and above 13% for oilseeds

This publication discusses the principles and prob-lems of heated-air grain drying, and the charac-teristicsof common types of grain dryers. It does not give spe-cific operating instructions for different makes andmod-els of dryers. Operating instructions and informationon specific control and safety features are contained inthe operators manual. Read the manual carefully, andmake sure the dryer meets all required federal and pro-vincial safety standards. The dryer should have eitherCanadian Standards Association (CSA) or Canadian GasAssociation (CGA) approval, or a special acceptance fromyour provincial department of labor.

ADVANTAGES OF GRAIN DRYING

Grain drying has become more common across theCanadian prairies. Instead of drying only during verywet harvest seasons, many farmers now use a dryer aspart of their normal grain-harvesting system. There area number of advantages to doing this:

Longer Harvest SeasonExtra hours of harvesting inthe morning and evening of each day and several extraharvest days each year are possible when a grain dryeris used. The number of available harvesting hours is in-creased substantially in most areas, which could reducethe overall investment in machinery. A smaller combineplus a dryer could be used instead of buying a secondcombine or trading up to a large one. For a farmer rely-ing on custom harvesting, a grain dryer can be helpfulin getting his harvesting done early. In any case, everycombine can be used to harvest more grain if a dryer isincluded in the harvesting system.

Earlier Harvesting Earlier harvesting is possiblewhen a grain dryer is used. Wheat, oats and barley canbe threshed at 20%moisture content (MC) and thendriedwithout loss of quality, grade or germination. Whencompared with harvesting at 14%, there may be a differ-ence of only 1 or 2 days in mid-August, but by mid-Sep-tember it may be 4 days ormore. If awet spell occurs, thedifferences could bemuch greater. Early harvestingmayallow a farmer to harvest nearer to the maximum drymoisture content, which results in the highest weight ofgrain for sale. Harvesting early also allows a farmer todo a better job of weed control through timely chemicalapplication and tillage practices.

Reduced Field Losses Weather damage and lossescaused by wildlife can be reduced by harvesting at thetough1 or damp2 stages. A loss of one grade maymean a

5MOISTUREMIGRATION

Grain that is placed in storage at or near the maxi-mum safe moisture content may develop localizedhigh-moisture zones because ofmoisturemigration. Thisis caused by changes in outdoor air temperatures whichset up convection air currents in the bin (Figure 5). Thismay lead to grain spoilage near the top center of the binin winter, or near the bottom center in summer. Mois-ture migration is more severe if the grain is hot whenbinned, and if the grain is stored in large bins. An aera-tion system can prevent these problems by maintaininga uniform temperature and moisture content through-out the bin. Any bin to be used for cooling grain or anybin of 100m3 or larger should be equipped with an aera-tion system (see Aeration).

MOISTURETESTERS

There are various types of moisture testers on themarket, with widely varying degrees of accuracy andrepeatability. Some of them are reasonably accurate onlynear the dry range and become progressively more in-accurate as themoisture content increases. Some are quiteaccurate for one type of grain and very inaccurate forothers. A few are very accurate for most grains, and for awide range of moisture contents. Detailed informationon various testers is available from the PrairieAgricultural Machinery Institute, Humboldt, Sask., andthe Canadian Grain Commission, Winnipeg, Man.

GRAIN MOISTURE RELATIONSHIPS

MAXIMUMSTORAGE TIMES FORDAMPGRAIN

The length of time that damp grain can be storedbefore drying depends on the grain temperature andmoisture content. Figures 2, 3 and 4 give estimated al-lowable times for various crops and conditions. It shouldbe noted that when heating starts, spoilage due to moldgrowth and insect infestation proceeds rapidly. There-fore, the temperature of damp grain must be monitoredclosely to ensure safe storage, even when air tempera-tures are well below OC.

MAXIMUMSTORAGEMOISTURECONTENTS

Maximummoisture contents for safe storage over thefirst winter are shown in Table 1.

Grain put into a bin on a hot day or grain contain-ing greenmaterial could heat even at the moisture levelsgiven in Table 1. Longer-term storage or processing ofspecial crops may require lower moisture contents thanthose shown.

Figure 1. Long-term average net evaporation (mm) during September.

6Figure 2. Effect of temperature and moisture content onallowable storage time of wheat, oats and barley.

Figure 4. Effect of temperature and moisture contenton allowable storage time of corn.

Figure 5. Moisture migration.

Figure 3. Effect of temperature and moisturecontent on allowable storage time of continuously

ventilated rapeseed.

7MOISTURE REBOUND

When grain is dried rapidly (for example, a 6% dropin 2 hours or less) in a heated-air dryer, it is common toexperience a moisture rebound. Grain coming out of thedryer may test dry, but after a few days on storage it maytest 1% or even 1.5% higher. This is caused by the differ-ence in moisture content within the grain kernels andthe type of tester used. The outer part of the kernel driesfaster and gives a lower moisture reading immediatelyafter drying. When the moisture content becomes uni-form, the testerwill give a somewhat higher reading thanit did earlier. Problems of this nature are usually solvedby initially overdrying the grain by about 1%. Extremecare should be taken to not overdry the grain in total.The lower the moisture content, the more susceptible isthe grain to heat damage. Also, overdrying requiresmoretime and fuel, and results in an unnecessary loss in grainweight. Overdrying by 2%means a loss of 2.5% inweightand sale value.

When testing hot grain from a dryer, the sampleshould be placed immediately in a plastic bag to preventmoisture loss, and it should be allowed to cool to ambi-ent temperature before testing.

WEATHER EFFECTS ON DRYING

RELATIVEHUMIDITY

Natural drying of grain occurs when the relativehumidity (RH) of the air is below the equilibrium

moisture content of the grain. When the relative humid-ity is above the equilibrium level, the grain takes onmois-ture. The approximate equilibrium moisture contentsfor cereal grains and oilseeds are given in Table 2.

If unheated air is used for drying, the RH of the airhas a substantial effect on the drying rate. However,when air is heated the RH drops rapidly, and if the air isheated 30C or more, the initial RH can be practicallyignored. Air at 15C and 100% RH, when heated to 65C,will have only 7% RH. Even if 15C air is heated to only45C, the RH will not be over 20%. Only when fairlyhigh initial air temperatures (over 20C) are combinedwith high RH and low drying temperatures (below 40C)does the RH act as a noticeable deterrent to drying.

WIND AND SUN

To understand the effects of wind and sun on graindrying, it is necessary to understand the operation of thedryer controls. With most continuous-flow dryers andbatch dryers with automatic shutoffs, the drying time isregulated by a thermostat located near the outside of thegrain column. When the grain temperature reaches thepreset value, the unloading mechanism is started, or in abatch dryer the heat is shut off.

Unshielded thermostats located on the outside of adryer can be affected by wind or sunshine. A cold windblowing against the thermostat will delay its activationand, therefore, can cause overdrying at a setting that waspreviously satisfactory. In a two-columndryer, with eachcolumn controlled by its own thermostat, equal settingswill result in one grain column moving faster than the

TABLE 1. MAXIMUMSTORAGEMOISTURECONTENTS

Wheat 14.5% Rapeseed 8.5%Barley 14.8 Corn 15.5Oats 14.0 Peas 16.0Rye 14.0 Sunflowers 9.5Flax 10.5 Mustard 11.0Buckwheat 16.0 Canary seed 12.0

TABLE 2. EQUILIBRIUMMOISTURECONTENTOFCEREALGRAINSANDOILSEEDS

Relative Cereal grains Oilseedshumidity of air

at 25C at 10C at 25 C at 10C

58% 12% 13% 7.5% 8.5%65 13 14 8.4 9.471 14 15 9.3 10.376 15 16 10.2 11.280 16 17 11.2 12.283 17 18 12.2 13.285 18 19 13.2 14.2

8other if a wind is blowing against one of them. This isoften interpreted to mean that the windward side is dry-ing much slower. However, this is not the case since thepressure created by the wind is insignificant comparedwith the pressure inside a dryer (dryers frequently op-erate at a static pressure of 750 - 1000 Pawhereas a 30 km/h wind creates a pressure of only 50 Pa). The column onthe windward side can become seriously overdried as aresult of the winds effect on the thermostat, and graindamage is more likely. This can be prevented by (1)shielding the thermostat from thewind; (2) running bothdischarge augers from the leeward thermostat, or (3)mechanically operating the windward discharge augerat a speed equal to the leeward discharge auger.

The wind itself has little or no detrimental effect onthe operation of a dryer and it is not necessary to shieldthe dryer from the wind. Shielding may, in fact, be det-rimental because it may cause moist air to be drawn backinto the dryer, and may also increase operator discom-fort from swirling moist air and debris. Completely sat-isfactory drying has been obtained with many dryersoperating without shielding in winds up to 50 km/h.

The effect of sunshine striking a thermostat will bethe opposite to that caused by the wind. The thermostaton the sunny side will allow a higher grain-flow ratethan will the one on the shady side. As wind and sunconditions change, unshielded thermostats may have tobe readjusted to prevent underdrying or overdrying.However, the effects of wind and sun on the actual per-formance of the dryer are insignificant.

AMBIENTAIRTEMPERATURES

Differences in ambient air temperatures have littleeffect on drying rates if the temperature of the drying airis kept constant. However, the ambient air temperaturedoes have a very significant effect on fuel consumption.Table 3 shows the relative fuel consumption in maintain-ing a drying temperature of 65C.

Early fall drying is much less expensive than winterdrying and allows a farmer to take advantage of more ofthe benefits of grain drying. When ambient air tempera-tures are high, the initial grain temperatures are also highand this further reduces fuel costs and drying times.Some dryers may have insufficient burner capacityto produce high plenum temperatures at very low out-door temperatures.

MAXIMUM DRYING TEMPERATURES

The rate of grain drying is increased as the tempera-ture of the drying air is raised. However, the grain willbe damaged if the temperature is too high. Damage ismore likely to occur when the grain is dry or nearly dry.Damp grain remains relatively cool due to theevaporative cooling that occurs when moisture is re-leased. As the grain becomes drier, the moisture releaseslows down and the grain temperature increases. By the

time the grain is dry, it is possible that some of it willhave reached the same temperature as the drying air. Toprevent grain damage, it is important that the maximumair temperature does not exceed themaximum allowabletemperature of the grain being handled. The maximumdrying temperatures for various cereal grains andoilseeds are given in Table 4.

The temperatures given in Table 4 are conditionalon drying to not more than 1% below the safe storagemoisture content (Table 1), and on the removal of notmore than 6%moisture in one pass through a high-speeddryer. With dryers where the grain is exposed to heatfor long periods (such as in non-recirculating bin dry-ers), it is advisable to use temperatures 5 - 10C lower thanthose listed for commercial use. This is particularly im-portant when drying rapeseed since the oil quality isaffected by long exposure to high temperatures.

Air temperatures higher than those shown in Table4 are safe under some conditions. If the grain is verydamp, air temperatures 20C above those indicated canbe used safely during the early drying stages sinceevaporative cooling keeps grain temperatures low. Asthe grain approaches the tough stage it is importantthat the temperatures be reduced to those shown inTable 4, or even lower. This can be done either manuallyor by the use of timers in any batch dryer. Continuous-flow dryers with the crossflow design (a crossflow dryeris one in which the airflow is perpendicular to the grainflow) and a single heating section should not use tem-peratures above those given in Table 4. Other typesof continuous-flow dryers may be able to use slightlyhigher temperatures, but the dried grain should bechecked for damage.

TEMPERATURESENSING

A common problem in grain drying is inaccuratetemperature sensing. Thermometers may become dam-age, or sometimes they are improperly located and dontindicate the highest temperature in the air plenum. Thisoften results in damages grain, even though the tempera-ture reading is within the city limits. The best insuranceagainst this problem is to use extra thermometers in thehot-air plenumand/or check the temperature of the grainnearest the plenum with a temperature probe. Varioustypes of temperature indicators are available, althoughgas-filled thermometers aremost commonly used in dry-ers. These may be obtained from dealers selling dryersor from thermometer suppliers in the larger cities.

If grain temperatures are being measured it is veryimportant to know the exact location of the sensing bulbin the grain column. Typical temperature and moisturevariations in a stationary batch or continuous-flow dryerare shown in the followingdiagram. A temperature-sens-ing bulb could give a reading of anywhere between 35and 65C depending on its location in the grain column.

The temperature and moisture differentials inrecirculating batch dryers are normally less than in

9stationary batch or non-mixing continuous-flow dryers.Lower air speeds (such as obtainedwith rapeseed or flax)increase the differentials, and higher air speeds reducethem. Since the grain-temperature indicators on dryersmay be located at various places across the grain column,these temperature readings should not be taken as indi-cators of safe drying temperatures.

Commission. Two samples (minimumof 500 g each), onetaken before and one after drying, should be sent to theGrainResearch Laboratory, CanadianGrainCommission,Winnipeg, Man. The samples should be representativeof the grain going in and coming out of the dryer. If thegrain quality is unchanged, a higher temperature can betried and two more samples sent in for testing. Severalcompanies that purchase oilseeds, corn, andgrain for seedalso conduct tests for heat damage. Check with the pur-chaser at the start of drying to ensure that the dried grainmeets their standards.

GRAIN-HANDLING SYSTEMSFOR DRYERS

A convenient grain-handling system is one of themost important factors in grain drying. If drying grainduring harvest creates bottlenecks, inconvenience andlabor problems, then drying will tend to be used only asan emergency procedure. Many of the potential ben-efits of drying are lost in such a situation. The grain-handling system should be arranged so that there is anabsolute minimum of extra work involved in using thedryer. It should be only a matter of minutes to changeback and forth between drying and not drying. The dry-ing and storage system should be set up in such a waythat the grain coming from the dryer can be transferreddirectly to a cooling bin or storage without having to beloaded into a truck. All large storage bins should haveaeration systems to help ensure safe storage.

Damp grain coming from the field will normally

TABLE 3. RELATIVE FUELCONSUMPTIONSATVARIOUSAMBIENTTEMPERATURES

Ambient air Relative fueltemperature consumption

15C 1.0-7C 1.5-30C 2.0

TABLE 4. MAXIMUMDRYINGTEMPERATURES

Seed or malting Commercial use Feed

Wheat 60 C 65 C 80 - 100 COats 50 60 80 - 100Barley 45 55 80 - 100Rye 45 60 80 - 100Corn 45 60 80 - 100Flax 45 80 80 - 100Rapeseed 45 65 Peas 45 70 80 - 100Mustard 45 45 Sunflowers 45 50 Buckwheat 45 45

TESTING FOR HEAT DAMAGE

Various tests may be conducted to find the highestacceptable drying temperatures for a particular grain anddryer. If the grain is to be used for seed, a germinationtest should be used. In addition, a sample of the grainshould be taken before drying for testing to help deter-mine whether there was any change in the germinationrate during the drying process. Other commercial grains(milling wheat, malting barley and oilseeds) are testedfree of charge for heat damage by the Canadian Grain

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There are several bin arrangements that allow easytransfer of grain from the dryer to storage with a mini-mum of cost and inconvenience. Figure 6 shows onesuch arrangement using augers. Another arrangement,using a bucket elevator, is shown in Figure 7. Both sys-tems can be built up in stages, starting with just a fewbins. Detailed plans for these and other arrangementscan be obtained from your provincial Department ofAgriculture engineer, or from agricultural engineeringconsultants. Equipment details and other planning in-formation can be found in the publication Grain Han-dling and Storage Systems, available from Information

Services, Agriculture Canada, or the information may beobtained from your local engineer.

Continuous-flow dryer and semicircular storage layout.

Figure 6. Grain drying and storage system with augers.

A special problem to be considered in planning is thepossibility of fire when drying certain crops (particularlysunflowers). A convenient water supply and avoidanceof combustiblematerials near the dryer can keep this haz-ard to a minimum.

DRYER TYPES

There are three major types of dryers: the non-recirculating batch type where the grain is loaded as abatch and remains stationary in the dryer throughoutthe drying period; the recirculating batch type wherethe grain is loaded as a batch and is constantly mixedwhile drying; and the continuous-flow type where thegrain is loaded and unloaded continuously or intermit-tently. The three types are available for stationary binsand as portable dryers. The drying characteristics of thethree types vary according to the grain depth and therate of airflow. The individual characteristics, opera-tional procedures and problems of each type are dis-cussed in the following sections.

Figure 7. Grain drying and storage system usingbucket elevator.

need to be placed in a surge bin to allow the truck to un-load quickly and return to the combine. The damp grainis then fed as required into the dryer from the surge bin.The bin should be large enough to hold about half a daysharvesting volume. This is adequate for a system inwhichthe dryer dries at half the harvesting rate and operatestwice asmany hours per day as the combine. Dependingon the drying procedure and dryer capacity selected, adamp-grain surge bin may not be required if a bin dryeris used.

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BIN DRYERS

Bin dryers are available in many sizes and capaci-ties, and can be operated in different ways to obtain vari-ous drying rates. They normally have lower airflow ratesthan other dryer types, resulting in slower drying butlower fuel consumption. Since bin dryers usually con-tain a greater volume of grain than other types, theamount of grain dried in a day can equal thatwith a highspeed dryer. A bin dryer should be sized to dry as muchgrain in 24 hours as will be harvested in a normal day.Manufacturers of bin drying equipment can providedetailed information on drying capacities with variousbins, heaters and auxiliary equipment. An example ofsuch information is given in the Appendix. A bin dryersetup can begin with a simple batch process, and if morecapacity is desired, stirring augers or a continuous-flowdevice can be added later.

operate as long as required to get the average moisturecontent of the grain to the desired level. Additional grainis loaded into the bin as harvesting progresses. By se-lecting an appropriate final depth of grain, based ondryer capacity and grain moisture content, a batch canbe dried in 12, 24 or any other convenient number ofhours. There can be substantial differences in the air-flow resistance of different batches of grain dependingon the filling method used, the size of seeds, and theamount and type of foreign material in the grain. Forthis reason the recommended depths given in the litera-ture should be taken as a guide only. The best depth fora particular batch can be determined with the aid of amanometer or U-tube to measure the static pressureunder the floor (Figure 9). The actual airflow rate canthen be determined by using the charts supplied withthe fan unit. Airflows of about 125 litres per second percubic metre of grain (L/s m3) are recommended for effi-cient drying. Figure 10 shows the effect of grain depthon airflow rate for a typical fan and bin size. Typicalgrain depths for wheat, oats and barley are 1.5 - 2 m atmoisture contents below 20%. Flax and turnip-typerapeseeds use depths of about 0.5 - 1 m. Lower moisturecontents and lower temperatures permit greater depths(which result in lower airflows), while higher moisturecontents and temperatures require the use of shallowerdepths (which result in higher airflows).

Batch-In-Bin Process

The lowest-cost setup uses the batch-in-bin process.A bin with a fully perforated floor, a grain spreader, fanand heater, sweep auger and under-floor unloading au-ger make up the basic equipment needed (Figure 8). Theheater and fan unit is started up as soon as the first loadof damp grain is dumped into the bin, and continues to

Bin dryer.

Figure 8. Typical batch dryer bin.

Figure 9. Static pressure measurement with manometer.

It is essential for the grain to be the same depth overthe entire floor to ensure a uniform airflow and dryingrate. Proper adjustment of the grain spreader shouldminimize or eliminate the need for manual leveling.

The grain nearest the floorwill dry first and the dry-ing front will then move up through the grain. The lo-cation of this frontwill give an indication of the additionaldrying time needed. The use of higher air temperatureswill speed drying somewhat, but will also increaseoverdrying near the floor.

With a bin dryer, the best way to hasten drying is toreduce the depth of grain. This increases the airflow and

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also providesmore-uniformdrying. A common tendencyis to put too much grain in each batch. This reduces thenumber of times that the bin must be unloaded, but seri-ously reduces the overall drying capacity. The unload-ing of the dryer bin and the transfer of the grain to storageshould be able to be carried out as quickly and conven-iently as possible, so that several smaller batches can bedried each day when there is need for more drying ca-pacity. The sweep auger should be at least 150 mm in di-ameter and have a backshield to clear the bin in one pass.Theunderfloor auger on all except fairly small bins shouldbe at least 200 mm in diameter and have an exposedflighting of at least 300mm, and preferably 500mm, at thehopper intake. Adequate power for the augers is also es-sential. If several batches are tobedried inoneday, adamp-grain surge bin may be needed. Another approach is toposition the heater and fan unit to alternately serve twobins. This allows one bin to be filled and emptied whilethe other is being dried, thus eliminating dryer down-time and the possible need of a surge bin.

aerated storage is available the grain can be transferredhot to the storage bin and cooled by an aeration fan. Ineither case, the grain should be cooled to within 5C ofambient air or 2C, whichever is higher, to preventcondensation or moisture migration. Since the graindoes not have a uniform moisture content when it is re-moved from the dryer, and since complete mixing is notensured, it is recommended that grain from a batch-in-bin dryer be placed in aerated storage to level out anymoisture variations.

Drying in cold weather may result in condensationon the underside of the roof or on the walls of bin dry-ers. Troublesome wet spots may be created if this wateris allowed to drip onto the grain. On some bins, a con-tinuous opening can be provided around the bin eavesto allow water to run along the underside of the roofand drip down outside. With any drying bin, the totalexhaust opening should be at least 3% of the floor area toallow easy exit of the moist air. The addition of an ex-haust fan may be helpful in increasing the airflowthrough the bin. The fan must not be placed in any ofthe existing exhaust openings since this could cause adecrease in total airflow rather than an increase. Insu-lating the bin roof is another method of reducing con-densation when drying in cold weather.

Drying small oilseed crops may create a problem onsome perforated floors if the seeds block the openings. Ifthis occurs, a shallow layer of larger-seeded grain, suchas barley or oats, may be placed on the floor and theoilseed crop placed on top of it. The grain acts as an airdiffuser and allows a greater airflow as well as prevent-ing leakage of small seeds through the perforated floor.When removing a batch, the sweep auger is set so that itdoesnt remove the large seeds on the bottom. Shouldsomemixing occur, it is not difficult to separate the smallseeds from the larger ones. The sweep augers in bin dry-ers do not completely clean the drying floor, so a seedgrower would likely find a bin dryer unsuitable becauseof the extra work required for cleaning. The smallamount of mixingwhen changing grain types is not usu-ally a problem in the case of commercial grain sales.Sweep augers tend to accumulate fines near the bincenter, so periodic cleaning of the perforated floor is re-quired to prevent uneven airflow through the floor.

There are a number of variations of the basic batch-in-bin drying procedure. An alternate heating and cool-ing cycle may be used to reduce the moisture differentialbetween the top and bottom of the batch. A cycle time of3 or 4minutes is used, with 75%of the time spent in heat-ing and 25% in cooling. During the cooling part of thecycle the heat from the grain nearest the plenum is car-ried to the cooler grain and this results in a more-uni-form temperature and moisture content throughout thebatch. In a comparison with continuous heat drying,themoisture differential in a 1m depth of grain was onlyabout half as great with cycled drying. Slightly higherair temperatures can be used with this method to com-pensate for the reduced heating time.

Some experience is required to determine when toturn off the heat and begin cooling. Usually the heatcan be turned off when the drying front is about 150 mmfrom the surface. Some moisture will be removed dur-ing the cooling process, so it isnt necessary to dry thebatch completely with hot air. The amount ofmoisture removed during cooling will depend to a largeextent on the decrease in grain temperature. If the grainis cooled 40 - 50C, it could lose between 1 and 2%, whilea smaller decrease would reduce the moisture contentby a smaller amount. Very high ambient air tempera-tures, together with high relative humidities, might re-sult in no moisture being removed, or even a slightincrease in the moisture level if the fan is run too long.

Cooling may be done with the dryer fan, or if

Figure 10. Airflow vs. depth of wheat for a particular fan-binsize combination.

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Overhead Drying Floors

Some bin dryers use an overhead cone-shaped dry-ing floor supported about 1 m below the roof (Figure 11).The heater and fan unit is mounted just below the dry-ing floor. When the grain is dried it is dropped to a per-forated floor below where an aeration fan is used to cool

the grain while the next batch is loaded and dried onthe floor above. Additional batches are dropped on topof the cooled grain until the bin is filled to the level ofthe heater unit. The dried grain is then transferred toanother storage bin. With this system, drying can con-tinue while the grain is being cooled and transferred.Unloading of the drying floor requires only abouta minute.

StirringAugers

Vertical stirring augers can be added to a bin dryerto increase the allowable depth of grain and to providemore-uniform drying (Figure 12). The augers transferdry grain from the bottom up through the grain mass.Stirring also fluffs up the grain which increases the air-flow substantially. This normally permits slightly higherdrying temperatures which, together with the increasedairflow, result in significantly higher drying rates (seeAppendix). Grain depths of 2 - 4 m may be used withwheat, oats or barley depending on the initial moisturecontent. Small oilseeds may use depths of 1 - 2.5 m. Theuse of stirring augers may result in slightly higher fuelconsumption, but this is outweighed by the increaseddrying rate, the reduction is overdrying at the bottom,and the larger batch size.

Figure 11. Bin dryer with overhead drying floor.

Figure 12. Bin dryer with double-auger stirring device.

Recirculating and Continuous-flow Devices

There are a number of other devices that can beadded to bin dryers, either to recirculate the grain or toprovide a more or less continuous flow through the bin.Figure 13 shows a bin dryer equipped as a recirculatingbatch dryer. The perforated floor is sloped towards thecenter so that the grain flows into a central chamberwhere it is picked up by a vertical auger and deliveredto the top of the grain bin. The grain is mixed continu-ously as it is dried, and the drying is more uniform thanin a non-recirculating batch dryer.Bin dryer with overhead drying floor.

14

Figure 14 shows another unit that can be used ei-ther to recirculate the grain or to provide a continuousflow. A sweep auger is used to move the dried grain tothe center of the perforated floor where it is picked upby a vertical auger and carried to an inclined transferauger which takes the grain to a cooling bin. The opera-tion of the sweep auger is controlled by a thermostat.The grain can also be recirculated by augering it up tothe grain recirculator instead of to the transfer auger.When the unit is operated as a continuous-flow dryerthe coolingmust be done elsewhere, so this systemneedsat least one more aerated bin to complete the cycle.Dryeration or combination drying is normally usedwiththis type of system. Excessive grain depths inrecirculating and continuous-flow dryer binswill reducedrying rates substantially. Grain depths should be thesame or only slightly greater than those used in thebatch-in-bin process.

PORTABLE DRYERS

Batch Dryers

There are two basic types of portable batch dryersavailable - the recirculating and non-recirculating types.These dryers are equipped with wheels to make themportable, but once they arrive at the farm it is essentialthat they be placed in a permanent position in a well-planned grain-handling system if drying is to be doneefficiently and conveniently. The initial attraction of aportable dryer is usually to a farmer who has grain binsscattered around in various locations or does customdrying off his farm. While this may fill an immediateneed in an emergency situation, a portable dryer with-out a proper grain-handling system will normally notbe used in situations where drying would be beneficialbut not absolutely essential. Many of the potential ben-efits of a dryer will be lost if it is not convenient to use ata moments notice. The selection of a dryer should notbe made on the basis of portability unless it is to be soldagain immediately after the crisis is past.

Most batch dryers must be operated with a full ornearly full batch of grain. If the top of the air plenum iscovered with only a few centimetres of grain, most ofthe air will escape through the top and most of the heatwill be wasted.

When batch dryers are used to remove only a fewpoints of moisture from grain, their time efficiency isgreatly reduced. Filling, cooling and unloading timesare unchanged but the heating time is reduced. The ac-tual drying time therefore becomes a lower percentageof the total operating time.

Considerable variation in drying rates and fuelefficiencies can be expected with batch dryers on differ-ent types of grain. Since the grain depth remains thesame regardless of seed size, the airflow will be muchlower for small oilseeds than for coarse grains and corn,particularly where an axial flow fan is used. This resultsin a reduced rate of moisture removal for crops such asflax and rapeseed, but also a substantial increase in fuelefficiency. Since it remains in contact with the grainlonger, the air has more time to pick up moisture and isexpelled at a higher RH. Increases of over 25% in fuelefficiency have been observed in batch dryers when dry-ing flax as compared with wheat. A corresponding re-duction in rate of water removal normally accompaniesthe increased fuel efficiency.

When comparing different batch dryers, fan capac-ity and column width are important variables. Higherfan capacity and narrower column widths will producehigher drying rates, but lower fuel efficiencies. Fan typeis also important. A centrifugal fan will normally pro-duce a more-constant airflow with different types ofgrain and will usually be quieter in operation than anaxial flow fan.

Problems with temperature measurement, as dis-cussed previously, are common in batch dryers as well

Figure 13. Recirculating batch bin dryer.

Other drying procedures, such asmultistage drying,are frequently used in bin dryers. Since these procedurescan also be used with any other type of dryer, they arediscussed later (seeMultistage Drying).

Figure 14. Continuous-flow or recirculating dryingbin equipment.

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as in other types. Malfunctions of sensors, and their im-proper location in the hot-air plenum and grain columnscan result in misleading temperature information.Extra thermometers should be installed in the plenumand in the grain column next to the plenum to reducethese dangers.

Samples taken frombatch dryers formoisture checksmust be representative of the grain throughout theentire thickness of the grain column or bed. Carelesssampling can result in seriously overdried or under-dried grain.

Substantial improvements in both drying rate andfuel efficiency can be obtained by removing the hot grainfrom a batch dryer before the grain is fully dried andusing unheated air to complete the process. This is dis-cussed in more detail under Multistage Drying.

Batch dryers have some advantages over other typeswhere relatively small amounts of different grains are tobe dried. Start-up and changing from one type of grainto another is relatively easy compared with a continu-ous-flow dryer, and unloading and cleaning operationsare usually easier than with a bin dryer. However, mostbatch dryers have much higher airflow rates than bindryers and, consequently, use considerably more fuel todo the same amount of drying.

It is essential to have high-capacity loading and un-loading equipment for a batch dryer to keep downtimeto a minimum. An auger size of 150 mm or larger shouldbe used to transfer grain into and out of the dryer. Adamp-grain surge bin is also essential for efficient op-eration of a batch dryer.

Non-Recirculating Type - There are several variationsof non-recirculating batch dryers, butmost are of the fullyenclosed, two-column type (Figure 15). The damp grainis loaded into the dryer from the top until it is filled andthen the hot air is forced through the grain until it isdry. The grain does not move inside the dryer, so theinside layer becomes overdried while the outside layerremains underdried. After the heat cycle, the grain iscooled inside the dryer by shutting off the heat, or it istransferred to an aeration bin for cooling. As the grain isunloaded, the wetter and dryer grain is mixed and if safetemperatures were used and the grain was sufficientlydried and cooled, a satisfactory product results.

There are a number of automatic controls and safetydevices available for these dryers. As the grain dries itshrinks, so with the use of a pressure switch and timerthe dryer can be refilled from the surge bin. The dryermay also be set to operate at a higher temperature for thefirst part of the drying cycle if the grain is very damp,and then at a lower temperature for the latter part. Athermostat or timer may be used to regulate the heatshutoff control. The cooling time may also be control-led by a timer and the unloading augers started auto-matically after cooling. A batch dryer equipped with allof these controls is commonly referred to as an automaticbatch dryer. If all of the controls are set and workingproperly, a batch dryer can fill, dry, cool and unload one

There are also other types of non-recirculating batchdryers such as wagon or truck-box dryers. A heater andfan unit, similar to those used for bin drying, is connectedto a main plenum which is connected to smaller ductsinside the truck box. These ducts may be located on the

Figure 15. Non-recirculating batch dryer.

Non-recirculating batch dryer.

batch after another without manual supervision or con-trol. As grain conditions change, timers and thermostatshave to be reset, and manual correction is needed in thecase of a malfunction. A totally mechanized grain-han-dling system is essential for such a system to operate andget the grain into storage.

16

floor or suspended at mid-height of the box. If they aresuspended, exhaust ducts must be provided at the floorof the box. Capacities vary with the fan and burner ca-pacity, and size of the truck box.

Recirculating Type - Recirculating batch dryers haveone central air plenum surrounded by grain, and oper-ate in the same sequence as non-recirculating batch dry-ers. The main difference is that the grain is constantlyrecirculated throughout the heating and cooling cycles.The dryer is usually circular in shape to accommodatethe vertical auger in the center of the dryer (Figure 16).The auger picks up the grain at the bottom of the dryerand deposits it at the top. It is also used to unload thedryer. A complete recirculation of the grain occurs aboutevery 15 minutes, which is the same as the unloadingtime. The constant mixing results in a more uniformlydried batch of grain than in a non-recirculating batchdryer. However, the constant augering can cause dam-age to certain seeds (including beans, peas and maltingbarley), particularly when they are nearly dry.

Continuous-flow Dryers

There are many types of portable continuous-flowdryers. One of the more common types uses two or fourvertical grain columns, with the air passing through thegrain at right angles to the grain flow (Figure 17). Thegrain is loaded into a hopper on top of the dryer, flowsdown on both sides of the hot-air plenum, then past thecold-air plenum, and is removed by augers. The grain-flow rate is normally controlled by a thermostat locatednear the outside of the grain column. The rate of flowmay also be controlled manually on most dryers. Thegrain is not mixed as it flows downward and, as a result,the grain next to the hot-air plenum becomes overdried,while the outside grain is underdried. Moisture differ-entials across a 300mm grain columnmay be as high as 6 12% when the average moisture content is dry. Thegrain is mixed as it is unloaded and an acceptable prod-uct is produced if safe temperatures were used.

The moisture differential across the grain columndepends on initial and final moisture content, air tem-perature, flow rate, and the column width. In a com-parison of two dryers with 150 and 300 mm graincolumns, using fans of the same type and size, the mois-ture differential was about 3.5% and 6% respectively.There was less resistance in the 150 mm column and the

Recirculating batch dryer.

Figure 16. Recirculating batch dryer.

Drying temperature 20 C higher than those given inTable 4 can be used while the grain is damp, but whenthe grain gets into the tough range the temperaturesshould be reduced to the levels indicated.

Automatic controls, such as those described in thesection on the non-recirculating types of batch dryer, canalso be usedwith recirculating batch dryers to reduce theamount of labor and supervision required.

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Some continuous-flow dryers use three fans andplenums, each with individual temperature controls.The dryers can be run with two heating sections andone cooling, or with heat in all three, in which case thegrain would be cooled in an aerated bin. The top sec-tion is normally run at a higher temperature since thisair is in contact with the wettest grain. The dryer canalso be operated as a continuous batch dryer, where thegrain is dropped one section at a time instead of in acontinuous flow.

Some continuous-flow dryers draw the cooling airthrough the grain and then pass it through the fan heaterunit into the hot-air plenum. This reclaims heat givenup in cooling the grain and reduces the air-grain tem-perature differential as the grain passes from the heatingto cooling sections. Somemoisture is also picked up andsent through the heater unit, but there is still a net gainin fuel efficiency. Chaff and fine material picked up in

the cooling section will accumulate on the inside of thehot-air plenum and, because of this, frequent checkingand cleaning of the plenum may be required. The con-struction of this type of dryer maymake it unsuitable foruse in a dryeration system because it may not be possibleto use the entire dryer for heating.

Another type of continuous-flow dryer is shown inFigure 18. It also has two vertical grain columns, but theairflow is parallel to the grain flow. This provides a uni-form drying rate across the column because the entirewidth is subjected to the same air, and the grain is mixedas it passes through the dryer. Since this design reducesthe danger of heat damage, slightly higher temperaturescan be used than with the two-compartment crossflowdryer. This type of dryer has no screens; consequently,depending on quality of construction, small-seededcrops may be dried without leakage and cleaning maybe easier. The dryer can be operated as a batch type, withboth top and bottom sections receiving either hot or coldair. This feature, which is also available on some othercontinuous-flow dryers, is useful in start-up or whensmall batches of grain are to be dried.

Figure 17. Continuous-flow dryer.

Figure 18. Parallel continuous-flow dryer.

fan outputwas almost 50% greater thanwith the 300mmcolumn. Fuel consumption was also increased by about50%, with very little difference in drying rates. As fancapacity is reduced or width of grain column increased,more efficient use of the heat results, but moisture dif-ferentials are increased.

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It consists of removing hot grain from the dryer at mois-ture levels about 2% above dry and completing the dry-ing and cooling process in a storage binwith airflows of 7 13 L/sm3 (Figure 19). Aeration is begun 6 12 hoursafter the grain is binned. This allows time for moisturetomove from inside of the kernels to the outside for easierremoval. There are a number of advantages to this proc-ess as compared with using a grain dryer only:

Slightly higher drying temperatures can be usedsince the grain isnt completely dried in the dryer.

The need for cooling time in a batch or bin dryer,or for a cooling section in a continuous-flowdryer, is elimi-nated. The use of higher temperatures, coupledwith theelimination of the cooling time and a reduction in the dry-ing time in the dryer, can result in an increase of 50% ormore in dryer output.

Fuel savings of 20% ormore are common. The lastfew points ofmoisture are themost difficult and costly toremove, so by removing them in the bin using the heatcontained in the grain, much less fuel is required for thefinal drying.

Grain quality may be improved since rapid cool-ing of hot grain, which could cause stress cracks, isavoided. There is also less chance of heat damage.

Drying system with continuous-flow dryer and parallel airflow.

Continuous-flow dryers are generally not wellsuited for drying small volumes of grain since startingand emptying them is quite inefficient, and accuratemoisture control is difficult to achieve until a uniformflow is established, usually requiring a couple of hours.They are best suited to drying large volumes of grainwithout frequent changes from one type of grainto another.

Moisture controls on continuous-flowdryers are fre-quently affected by the sun and wind (seeWeather Effectson Drying). Temperature sensors in hot-air plenumsmaybe improperly located or may be inaccurate. Extra sen-sors should be placed in the plenum to get an accuratereading in the hottest part. Temperature sensors installedin the grain column next to the hot-air plenum can alsobe used to help guard against grain damage. Gas-modu-lating valves should allow the dryer to be operated attemperatures as low as 40C.

Airflows in continuous-flow dryers are normallyin the same range as those in batch dryers, and bothtypes of dryer have similar fuel efficiencies. Dryingrates will vary according to seed size, which has anaffect on the rate of airflow. Faster drying rates canbe obtained by using auxiliary cooling, dryeration orcombination drying.

Large continuous-flow units can dry grain quicklywhen only a few points of moisture are being removed,so augers and surge bins must have considerable capac-ity. Augers of at least 150 mm diameter should be used.

Many different automatic controls and safetyshutoffs are available for these dryers, allowing theiroperation without constant supervision. However, be-cause controls need to be readjusted as grain conditionschange, and because malfunctions can occur, frequentchecks of their operation must still be made.

MULTISTAGE DRYING

DRYERATION

Dryeration, or two-stage drying, is a process that usesboth a grain dryer and a high-capacity aeration system.

Low-capacity aeration systems are inadequate fordryeration cooling. Larger fans and duct sizes are re-quired to achieve airflows of 7 13 L/sm3. A fully per-forated floor is essential for the cooling bin since this willeliminate excessive pressure drops and produce a more-uniform airflow through the grain as compared withaeration ducts.

Because a considerable amount of moisture is re-moved during cooling, condensation on the roof andwalls of the bin is often a problem if the air is blown upthrough the grain. Usually thismeans that the grainmustbe transferred from the cooling bin to another storagebin to mix any wet grain with the dry grain. The extragrain transfer and the extra aeration equipment neededare the main disadvantages of this process.

Pulling the air down through the grain will reducethe condensation on the roof and may make it unneces-sary to transfer the grain from the cooling bin. Anotherway to eliminate the transfer is to begin cooling the grainjust as soon as it comes from the dryer. This preventsmost of the condensation by having a continuous air-flow through the bin right from the start of filling. The

Figure 19. Dryeration process

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grain can then be left in the cooling bin for storage. Adisadvantage of thismethod is that all storage binswouldneed to be provided with fully perforated floors to ac-commodate the higher airflow rates. Also, some of thebenefits of the heat soak are reduced since the grainhas less time to attain internal moisture equilibrium.This would mean that slightly less moisture (about 1%)would be removed during the cooling stage. Becauseof this, the grain would need to remain in the dryer alittle longer.

COMBINATION DRYING

Combination drying is an extension of thedryeration process. It is used primarily when grain witha very high moisture content (above 25%) is to be dried.A high-temperature dryer is used to lower the moisturecontent to 19 23%, then the grain is transferred to abin dryer for completion of the drying, either with orwithout supplemental heat. This makes use of the mostefficient operating ranges of both types of dryers. Theoutput of the high-temperature dryer is increased by twoor three times compared with its use for complete dry-ing. Total energy requirements are reduced by about 50%,depending on the amount of moisture removed in eachstage and on the suitability of ambient air conditions fordrying with unheated air. There is also an improvementin grain quality as compared with completing the dry-ing in the high-temperature dryer.

Airflows for the bin drying portion of combinationdrying are in the range of 10 - 25 L/sm3. Fully perfo-rated floorsmust be used. The grainmay be put througha heat soak dryeration process before the final bin dry-ing or itmay go directly from the high-temperature dryerto the bin dryer for immediate cooling and final drying.Bin dryers may use unheated air when the conditionsare right or supplemental energy sources such as elec-tricity, propane, solar heat or crop-residue burners toreduce the relative humidity of the air and increase thedrying rate.

The choice of using dryeration or combination dry-ing will depend on the amount of grain and its initialmoisture content, the cost of energy, and the capital in-vestment required. Where small amounts of grain areinvolved and the moisture content is relatively low, theinvestment in equipment for combination drying wouldnot be warranted. Higher initial moisture contents,larger grain volumes and higher fuel prices makedryeration and combination drying more attractive. Inall cases, however,where bins of 100m3 or larger are built,aeration ducts should be provided, even for grain that isbrought in dry from the field. Building the aeration ductslarge enough for airflows of at least 10 L/sm3 is recom-mended so that the future options are left open to ac-commodate some type of multistage drying. Because itallows the greatest variety of options, a fully perforatedfloor should be seriously considered when a large stor-age bin is being planned. Every grain storage system

should contain at least one and preferably two or threebins with perforated floors.

AERATION

In aeration, unheated air is passed through a grainmass to cool it, remove moisture, or make the tempera-ture more uniform. Aeration normally involves movingsmall amounts of air (about 1 2 L/sm3) through dryor nearly dry grain. This is done to lower and equalizethe grain temperature in the bin and prevent moisturemigration (Figure 5), and to cool warm grain comingfrom the field. Small amounts of air such as this will dovery little drying unless the weather is very dry and thefan is run for a long time. Grain that is put into storagewith varying amounts of moisture can benefit from aera-tion since it will assist in the transfer of moisture fromthe wetter to the drier grain. Higher airflows (7 13L/sm3) are needed to maintain damp grain in conditionwhile it is waiting to be dried. If outdoor temperaturesbecome very warm, even these airflow rates may not en-sure safe storage. Damp grain should be aerated con-tinuously until it is dried or its temperature is reducedto near 0C, especially at night when air temperaturesare normally lower. When using aeration to cool dry,hot grain coming from a dryer, the fan should be oper-ated continuously until the grain is within 10C of theaverage outdoor temperature. At 1 L/sm3, this may re-quire up to a week of operation . Care should be takento avoid overdrying of the grain by excessive aerationduring very dry weather, or to run the fan during longperiods of dampweather since the addition of 0.5%mois-ture can cause structural damage to the bin.

It is important to select ducts, perforated areas andfans of adequate size when setting up any aeration sys-tem. The required information and design assistance isavailable from your local agricultural engineer.

DRYING RATES AND EFFICIENCIES

The design and operation of grain dryers is fre-quently a compromise between drying rate and fuel ef-ficiency. Increasing the airflow will speed drying butwill also increase fuel consumption. Increasing the graindepth or column width will give better fuel efficiencybut slower drying rates. If a fixed columnwidth or depthof grain is used for large and small-seeded crops, fuelefficiency and drying rates will vary greatly. Smallerseeds have a higher resistance to airflow and thereforereduce the fan output. This increases the amount ofwaterthat is absorbed by the air before leaving the dryer. There-fore, a dryer will give higher fuel efficiency with flaxthan it will when wheat or barley is being dried. Re-ducing the fan speed on batch and continuous-flow dry-ers will increase fuel efficiency, but will also reduce thedrying rate.

Several practices contribute to both improved fuelefficiency and faster drying rates. The elimination of

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overdrying will do both as well as reduce the chance ofgrain damage. The drier the grain gets, the more timeand fuel are required to remove moisture.

The effect of initial moisture content on drying ratesand fuel efficiencies is shown in Table 5.

Removing fine weed seeds and broken kernels willalso improve drying rates and reduce the total amount ofdrying required. Running the grain through a precleaneror screened auger before drying can help to speed up thedrying and to reduce variations in drying rates.

Differences in initial grain temperatures have aslight effect on the drying rate and fuel consumption.A difference of 30C in the initial temperature results ina difference of about 10% in the time and fuel require-ments. Changes in temperature occur very slowly ingrain unless air is being blown through it, or heating istaking place.

Outside air temperatures have a very significant ef-fect on fuel consumption. With a 25C temperature rise(e.g., 15C ambient air and 40C dryer temperature) achange of 1C in ambient air temperature changes thefuel consumption by 4%.

With a 50C temperature rise, a 1C change inoutdoor temperature changes the fuel consumption by2%. Drying rates are not affected by the outdoor air tem-perature as long as the dryer and grain temperaturesremain constant.

TABLE 5. EFFECTOF INITIALMOISTURE CONTENT (MC) ONDRYINGRATES ANDFUELEFFICIENCIES

Dryer Grain Initial MC Water removal Fuel usage% kg/h kJ*/kg of water

A Wheat 18.0 480 305016.4 425 3200

B Corn 26.2 530 355022.9 510 370018.7 465 4200

C Corn 26.6 1110 300022.7 850 3400

*kilojoules

TABLE 6. TYPICALWATERREMOVALRATESANDFUELUSAGES

Dryer Water removal Drying rate Fuel usagekg/h t/h kJ/kg of water

Recirculating batch, 13m3 volume 125 2.5 4000Non-recirculating batch, 4.5m3 125 2.5 4500Continuous-flow, 16m3 perforated dryer area 300 6.0 4000Bin, 9.1 m dia overhead drying floor 500 10.0 3000

Passing the cooling air from a continuous-flow dryerthrough the burner and into the heating section resultsin a net improvement in fuel efficiency. The heat that issalvaged from the cooling process more than offsets themoisture gained and can yield fuel savings of 10 - 15%.The fan must have slightly increased capacity to movethe same amount of air through the dryer.

DRYERCAPACITIES

Dryer capacity is best expressed in terms of waterremoval rates, such as kilograms per hour (kg/h). Dryersused on prairie farms have widely varying rates, fromless than 100 kg/h to over 1000 kg/h. Table 6 gives typicalwater removal rates and fuel usages for some of the com-mon farm dryers when drying wheat from 18% to 14%at 15C ambient air temperature.

Water removal rates with wheat, oats and barley areapproximately the same if the same drying temperaturesare used. Water removal from flax and rapeseed is about20%slower at the same temperatures. Sincewheat is com-monly dried at a somewhat lower temperature than flax,oats and barley, the actual drying rates (in kg/h) are nor-mally about equal for wheat and flax, and about 15%higher for oats and barley.

The amount of water that must be removed fromgrain to dry it is given in Table 7.

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TABLE 7. AMOUNT OFWATER REMOVEDWHENDRYINGGRAIN

kg/t removed from grain with an initial MC ofFinalMC 16% 18% 20% 22% 24% 26% 28% 30%

14% 24 49 75 103 132 162 194 22910% 71 98 125 154 184

TABLE 8. FUEL COST FOR REMOVINGWATER (/kg)

Fuel price (/L)Fuel usage(kJ/kg) 10 12 14

2500 1.25 1.50 1.753000 1.50 1.80 2.103500 1.75 2.10 2.454000 2.00 2.40 2.804500 2.25 2.70 3.155000 2.50 3.00 3.505500 2.75 3.30 3.856000 3.00 3.60 4.20

Adryer removing 200 kg/hwould therefore dry about2 t/h of grain from 22% to 14%.

DRYEREFFICIENCIES

Fuel efficiencies aremeasured in terms of the amountof energy required to remove water. The units arekilojoules of energy per kilogram (kJ/kg) of water re-moved. A litre of propane will produce about 20 000 kJof energy with good burner efficiency. A kilowatt hour(kWh) of electricity is equal to 3600 kJ. Typical fuelefficiencies for continuous-flow and batch dryers areapproximately 4000 kJ/kg forwheat, oats, barley and corn.Some dryers with high airflows may require as much as6000 kJ to remove a kilogram of water. Bin dryers nor-mally use about 3000 kJ/kg.

Table 8 gives the fuel costs for removing water fordryers with various fuel efficiencies, and with variouspropane prices.

The cost of drying a tonne of grain, using a dryerwith an efficiency rating of 3500 kJ/kg, is determined inthe following example:

Grain being dried from 22% to 14%Fuel price 12/LDryer fuel usage 3500 kJ/kgFuel cost = 103 kg/t (Table 7) x 2.10/kg(Table 8) = $2.16/t

If the dryer in the above example used 5000 kJ/kg,the fuel cost would be $3.09/t. If 200 tonnes per year aredried, the difference in total fuel cost would be only $186per year. This amount would need to cover any addi-

tional capital costs (if any) for the more efficient dryer. Ifa dryer is to be used for drying large amounts of grain,fuel efficiency would become a more important factor.Amore expensive dryer does not necessarily use less fuel.Some dryers with a low initial price are also the mostfuel-efficient.

DRYING COSTS

Costs of drying grain vary according to the type ofdryer, amount of grain dried annually, and outside airtemperatures. The cost figures in Table 9 are based onthe following conditions:

Average outdoor temperature: 15CDepreciation rate: 8%Interest rate: 12%Repairs and maintenance: 1.5%/100 h of operationPropane: 10/LElectricity: 2.5/kWh5.8 m dia. Drying bin complete (drying rate 2 t/h):$6000

13m3 recirulating batch dryer (drying rate 2 t/h):$10 000

Continuous-flow dryer (drying rate 4.2 t/h):$16 000

All dryers equipped with electric motor-driven fansWheat dried from 20% to 14%The three types of dryers do not all have the same

drying rates and do not all require the same amount ofsupervision or labor. These are also factors to be con-sidered in selecting a dryer, particularly one that will

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TABLE 9. GRAINDRYINGCOSTS

Dollars per tonne

Tonnes per year Bin dryer Recirc. batch Cont.-flow

Fixed Costs

200 $4.30 $6.95 $11.15400 2.15 3.50 5.60600 1.45 2.30 3.751000 0.90 1.40 2.25

Operating costs (excluding labor)

1.80 2.55 2.25

Total costs (excluding labor)

200 6.10 9.50 13.40400 3.95 6.05 7.85600 3.25 4.85 6.001000 2.70 3.95 4.50

see a higher level of use annually. Where large amountsof grain must be dried, a faster drying rate is requiredto minimize field and harvest losses. Where smallamounts of grain are dried, initial purchase cost is theprimary consideration.

Once a dryer is purchased, the fixed costs (interestand depreciation) occurwhether or not the dryer is used.The decision on whether or not to use the dryer in a par-ticular year should, therefore, be based primarily on theoperating cost rather than on the total cost.

Where a substantial increase in grain drying is an-ticipated, consideration should be given to the additionof a dryeration or combination drying process ratherthan to increasing the dryer size. Besides increasing thedrying rate, this would decrease fuel costs and improvegrain quality.

FIRES IN DRYERS

With any crop, fires can occur in dryers if dirt andresidue accumulate in the burner area. However, firesare not common, except when sunflowers are beingdried. Sunflower seeds often have fuzz attached to themwhich is released in the drying process (particularly withrecirculating batch dryers). If this material is drawnthrough the fan and burner, it can ignite and start a firein the dryer. Anything that will reduce the chances ofthe material being drawn through the burner will helpto reduce the risk of fire. This includes cleaning to

remove any light or fine material from the seed beforedrying, and the provision of wind deflectors to preventairborne material from being drawn through the burner.Also, dust and fuzz should not be allowed to accumu-late on the walls and other parts of the dryer.

The chance of fire will also be lessened by ensuringthat the seed is not overdried and that the temperatureof the drying air is not too high. Hot and overdried seedsare more easily ignited by the burning fuzz. High airtemperatures by themselves are not the cause of fireswhen drying sunflowers. Even where drying tempera-tures were below 40C, fires have occurred when fuzzparticles were drawn through the burner and ignited.

In spite of all precautions, it is advisable to remainnear the dryer and to be alert for signs of fire wheneversunflower seeds are being dried, especially near the endof the heating cycle. If a fire occurs, shut off the heat andfan. The fire may snuff itself out in a recirculating dryerif the auger is left running, but it is often necessary touse some water to extinguish it.

A number of fires have also occurred when dryingrapeseed. Precautions similar to those recommended fordrying sunflowers should be followed.

Nearly every fall there are a number of warm, drydays when drying is possible without the use of heat.On such days, putting sunflowers or rapeseed througha dryer without starting the burner can result in verylow cost drying, and eliminate any chances of burner-induced fires.

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APPENDIX

Drying Charts for Bin Drying (Tables courtesy of Westeel Rosco)

FLAX,MUSTARDANDRAPESEED (Final moisture 10%)

Flax Mustard or rapeseedInitial

moisture Depth Volume Time Depth Volume TimeDryer size % m m3 hr m m3 hr

5.8 m DIA BIN 40C 40C

3.75 kWmotor 22 0.45 12 19 0.6 16 17.020 0.6 16 21 0.9 24 21.018 0.9 24 23 1.2 32 26.016 0.9 24 21 1.35 36 25.014 0.9 24 17 1.5 40 22.0

5.4 kWmotor 22 0.6 16 19 0.9 24 19.020 0.6 16 17 1.05 28 20.518 0.9 24 21 1.2 32 21.016 0.9 24 19 1.5 40 25.014 0.9 24 13 1.8 48 23.0

9.3 kWmotor 22 0.9 24 21 1.05 28 21.020 0.9 24 19 1.2 32 20.018 0.9 24 18 1.5 40 23.016 1.2* 32 21 1.8 48 21.014 1.2* 32 17 2.1 56 19.0

5.8 DIA BIN(With twin-screwstirring devices) 55C 45C

3.75 kWmotor 22 0.9 24 23 1.5 40 36.020 0.9 24 23 1.8 48 32.018 0.9 24 22 2.25* 60 30.016 1.2* 32 19 2.25* 60 45.014 1.2* 32 15 2.25* 60 20.0

5.4 kWmotor 22 0.9 24 23 1.5 40 32.020 0.9 24 21 1.8 48 28.018 1.2 32 24 2.4 64 30.016 1.2 32 18 2.7* 72 34.014 1.2 32 14 2.7* 72 32.0

9.3 kWmotor 22 1.35 36 25 1.65 44 31.020 1.35 36 24 1.95 52 30.018 1.5 40 23 2.4 64 26.016 1.8* 48 21 3.0* 80 29.014 1.8* 48 19 3.0* 80 22.0

* Depths shown are maximum recommended because of static pressure limitations of the fan.

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WHEAT, BARLEY AND OATS (Final moisture 14%)

Wheat Barley or oatsInitial

moisture Depth Volume Time Depth Volume TimeDryer size % m m3 hr m m3 hr

-day fill5.8 m DIA BIN 55C 55C

3.75 kWmotor 24 0.6 16 7.5 0.9 24 10.522 0.9 24 9.5 1.2 32 11.520 1.2 32 10.0 1.5 40 11.518 1.5 40 9.5 1.8 48 10.016 1.6 48 8.0 2.1 56 7.0

5.4 kWmotor 24 0.6 16 6.5 0.9 24 9.522 0.9 24 8.5 1.2 32 10.020 1.2 32 8.0 1.5 40 10.018 1.5 40 8.5 2.1 56 9.516 2.1 56 8.0 2.4 64 8.0

9.3 kWmotor 24 0.9 24 9.0 1.2 32 10.022 1.2 32 9.5 1.5 40 9.020 1.5 40 9.0 1.8 48 9.518 1.8 48 9.0 2.1 56 8.516 2.4 64 7.5 2.7 72 7.0

1-day fill5.8 DIA BIN 40C 40C

3.75 kWmotor 24 0.9 24 19 1.2 32 23.022 1.2 32 21 1.5 40 22.020 1.5 40 22 1.8 48 22.018 2.1 56 21 2.7 72 25.016 3.0 80 18 3.6 96 18.0

5.4 kWmotor 24 1.05 28 21 1.2 32 21.022 1.35 36 23 1.5 40 20.020 1.65 44 22 2.1 56 23.018 2.25 60 21 2.7 72 22.016 3.3 80 17 3.9 104 18.0

9.3 kWmotor 24 1.2 32 23 1.35 36 21.022 1.5 40 24 1.65 44 22.020 2.1 56 23 2.4 64 24.018 2.7 72 25 3.0 80 25.016 3.6 96 18 4.2 112 19.0

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WHEAT, BARLEY AND OATS (with twin-screw stirring devices; final moisture 14%)

Wheat Barley or oatsInitial

moisture Depth Volume Time Depth Volume TimeDryer size % m m3 hr m m3 hr

5.8 m DIA BIN 55C 55C

3.75 kWmotor 24 1.65 44 23 1.9 50 2322 1.9 51 22 2.25 60 2220 2.4 64 21 2.8 75 2218 3.1 82 22 3.7 99 2216 3.3 88 15 4.35 116 17

5.4 kWmotor 24 1.8 48 22 2.1 56 2422 2.1 56 21 2.25 60 2220 2.55 68 21 3.05 82 2118 3.3 88 21 4.05 108 2216 3.9 104 16 4.35 116 15

9.3 kWmotor 24 1.95 52 23 2.3 61 2422 2.3 61 22 2.7 72 2220 2.75 74 21 3.3 88 2218 3.6 95 20 4.3 114 2216 4.35 116 17 4.35 116 14

SHELLED CORN (Final moisture 13%)

75C (use60C stirring device)

Initialmoisture Volume Depth Volume Depth

Dryer size % m3 m m3 m

5.8 DIA BIN

5.4 kWmotor 30 35* 1.3 43 1.625 45 1.7 62 2.320 70 2.65 96 3.6

9.3 kWmotor 30 35 1.3 49 1.825 48 1.8 66 2.520 78 2.9 107 4.0

* Based on 21 hours drying time. All other capacities based on 18 hours drying and 2 hours cooling time. At drying temperatures over60C, a stirring device should be used. For more-even drying it may be used at temperatures of 60C or less. Drying rates are based onoutside air of 10C - 65% RH. These rates may vary under other conditions.

The federal and Manitoba Departments of Agriculture have not checked the drying rates indicated and are not responsible for deviationsfrom these figures.

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ACKNOWLEDGMENTS

The author expresses his appreciation to the following for their assistance in preparing this publication:

Lorne Parker, Ste. Agathe, Manitoba

Franklin Voth, Manitou, Manitoba

Grant Henry, Pioneer Grain Co. Ltd., Winnipeg, Manitoba

Alan Roberts, Ag-tech Consultants, Elie, Manitoba

Murray Green, Alberta Department of Agriculture

Paul Barlott, Alberta Department of Agriculture

Bob Brad, Saskatchewan Department of Agriculture

Roy Button, Saskatchewan Department of Agriculture

Tony Protz, Saskatchewan Department of Agriculture

Paul Gebhardt, Saskatchewan Department of Agriculture

Eric Moysey, University of Saskatchewan

Bill Muir, University of Manitoba

Eldon Norum, University of Saskatchewan

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