Drying Grains

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    17 HEATED AIR DRYING

    17.1 INTRODUCTION

    Heated air drying of grain is any system that is dependent on the addition of heat to the

    ambient air to produce the necessary drying. The performance of heated air driers ismeasured in terms of capacity, efficiency, and product quality (Foster 1973). The design ofa drier is usually a compromise among several factors. For example drying capacity anduniformity of drying in cross-flow driers can be increased by increasing the airflow rate butefficiency will decrease because the air passes through the grain too fast to pick up themaximum amount of moisture it could from the grain. Airflow rates for drying with heated airare 250 to 600 (L/s)/m3 for in-bin driers and 600 to 2500 (L/s)/m3 for portable driers (Foster1973).

    Air drying of grain is inherently an inefficient process because only a portion of the heat addedto the air can be used to evaporate moisture. A portion of the heat is retained as sensibleheat in the air because the air comes into moisture equilibrium or becomes saturated beforethe air cools to its initial temperature. Efficiency increases as added heat is increased, asthe temperature of the ambient air increases, or as the relative humidity of the ambient airdecreases. Maximum temperature of the drying airis normally controlled in a drier, however,

    it is the maximumgrain temperature that affects grain quality. Depending on the type of drier

    the grain temperature can be much less than the air temperature because of evaporativecooling.

    17.2 BATCH IN-BIN DRIERS (Friesen 1982)

    17.2.1 Description

    The simplest batch in-bin driers consist of bins with fans, air heaters, and perforated floors(Fig. 17.1). A grain spreader, sweep auger, and under-floor unloading auger can be addedto reduce the physical labour of grain handling. The bin floor must be completely perforatedand the top grain surface must be level to have near-uniform airflow through the grain and todry it uniformly.

    Fig. 17.1 Typical batch dryer bin (Friesen 1982)

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    To obtain uniform airflow through the floor, the under-floor plenum and air ducts from the fanto the plenum must be designed carefully (i.e. to provide a constant static pressure throughoutthe plenum). The surface area of the perforated flooring through which the air is passing intothe grain can be quite large. The floor area of 5.8-m (19-ft) diameter bins is 26 m2 while thefloor area of 9.1-m (30-ft) bins is 65 m2.

    Condensation may collect on the underside of the roof or on bin walls above the grain duringdrying in cold weather and at night. If these metal surfaces are not insulated, they can coolbelow the dew point temperature of the air exhausting from the grain. Where water drips ontothe grain wet spots and spoilage may develop. On some bins, a continuous opening can beprovided around the bin eaves to allow water condensed on the underside of the metal roofto run down the underside of the roof to the outside and then drip down on the outside. Toprovide unrestricted exits for the moist air, the total area of the exhaust openings should beat least 3% of the floor area of the bin (Friesen 1982). Condensation can be reduced by

    insulating the bin roof so that its underside stays near the temperature of the exhaust air.

    Small seeded crops, e.g. canola, may pass through or block the openings in perforated floors

    designed for cereal grains. To prevent this, a shallow layer of larger-seeded grain, e.g. wheator barley, can be spread over the floor before adding the small-seeded crop. When removingthe batch the sweep auger is set so that it runs over the top of the thin layer of other grain. Ifsome mixing does occur, the two grains of different sizes can be separated easily.

    Sweep augers do not clean the drying floor completely. To prevent contamination ofsuccessive crops to be dried a seed grower usually has to clean the floor manually. Sweepaugers break kernels and these broken kernels and other fine materials accumulate towardsthe centre of the bin between the sweep auger and the perforated flooring. Periodic removalof this layer is necessary to prevent excessive resistance to airflow. Fines can also fallthrough the perforated flooring into the plenum chamber under the floor. If it is too difficult to

    clean under the floor regularly, insects and mites can grow unmolested. This infestationbecomes a reservoir from which every new bin filling can become infested.

    17.2.2 Drier management

    The fan and heater are usually started after the first grain is spread on the floor. To reduce theamount of dust and debris that falls through the perforated flooring and to reduce plugging ofthe floor openings with particles the fan can be switched on as the first grain is loaded into thebin. Additional grain can be added as it is harvested. The grain nearest the floor will dry firstand the drying front will then move up through the grain while the fan is operated. When the

    drying front has moved through the bulk the heater is turned off and unheated ambient air isblown through the grain until the cooling front passes through the bulk. The grain can be leftin the bin when it is dry and cool or it can be moved to another bin for storage.

    If several batches are to be dried in the bin each year, the cycle time for each batch includes:time to move wet grain into the bin, drying time, cooling time, time to empty the bin, and a fewadditional hours for unexpected delays (e.g. repairs, errors in management, unexpected highmoisture contents, operator's sleep). A common design is for a cycle period of 24 h. Sizesof bin, fan, heater, and grain handling system are determined by the daily harvest rate.

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    If more than one batch is to be dried each day, a surge bin to hold the damp-grain as it isharvested may be needed. Another approach is to position the heater and fan so that theheated air can be easily switched between two drying bins. One bin can be emptied andrefilled while the other is being dried. This eliminates the down time during grain handling andthereby increases total drying capacity for a given fan and heater size.

    As a manager gains experience with a drier and grain conditions, the manager can probablyturn off the heat to the drier and begin cooling the grain before the drying front is completelythrough the batch. The heat stored in the grain is then used to dry the last amount of grain nearthe top of the batch and over drying can be reduced.

    17.2.3 Uniform airflow

    The top surface of the grain must be kept level to have uniform airflow. A grain spreader atthe centre inlet of a cylindrical bin can level the grain to some extent. The need for manuallevelling with a shovel depends on the design, adjustment, and proper operation of the

    spreader. Spreaders consolidate the grain, increasing airflow resistance but also increasingthe total mass of grain that can be stored in the bin (Sec. 7.5.1, 7.7.4). Foreign material in thegrain must also be evenly spread throughout the bulk to have uniform airflow (Sec. 7.6). Themovement of fines towards the centre by the sweep auger also reduces airflow uniformity. Thelocation of the under-floor unloading auger and supports for the perforated flooring can alsoaffect airflow uniformity.

    Uneven drying can be caused by incomplete mixing of the air streams after they pass throughthe heater. Only part of the air has direct contact with the heated surface or flame. Air doesnot readily mix in the short distance from the heater to the plenum chamber under the bin floor.Changes in the design of the airflow paths may be necessary to obtain the desired uniformity

    of drying across the whole area of the bin.

    17.2.4 Sophisticated management

    A major difficulty of in-bin, heated-air drying is over drying the grain. Under weather conditionsduring the fall harvest in western Canada heat added to the air will reduce its relative humiditybelow that in equilibrium with the maximum moisture content allowed for dry grain (Sec. 7.1).

    Ambient air in September could be at 15C and 90% relative humidity that is in equilibriumwith wheat at about 20% moisture content. If this air is heated to 25C its relative humidity isreduced to about 50%. This air will overdry wheat to 11% moisture content. If the maximum

    allowable moisture content is 14.5% without a sales penalty then the saleable mass has beenreduced by 4%. If the air is heated to 35C its relative humidity drops to 25%, wheat is driedto about 8%, and the value of the grain is reduced by 7%.

    A manager may decide to continue drying until the top layer is dry because of a concern thatundried grain may deteriorate during storage. Also other workers with little experience caneasily carry out such management decisions. Unseen costs of reduced mass of grain for sale,reduced drying capacity, and increased energy consumption can be high.

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    Overdrying can be reduced by using a humidistat to switch off the heater or the fan and heaterwhen the relative humidity drops below a set point. Turning off the fan, however, may leavesome grain hot and moist providing ideal conditions for mould growth. Humidistats tend tobe unreliable and can be affected by dust, although sling psychrometers can be usedfrequently to check the humidistat.

    To reduce overdrying an experienced operator can stop the heated air drying when theaverage moisture content of the batch is equal to or less than the target moisture content. Thedrying front, however, will be only partially through the batch. Grain farthest from the air inlet,usually the top layers of the bulk, could spoil quickly because it is still at a high moi