Mechanical dryers

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Batch-in-Bin Dryers

The small capacity version of the batch-in-bin dryer, otherwise known as the flat-bed dryer, has been developed for farm- or village-level use. Its capacity is of the order of 1-3 tonnes/day with drying times of 6-12 hours.

As represented in Figure 5.11 (see Figure 5. 11. Flat Bed Dryer.) the flat-bed dryer is simple to construct using easily available and inexpensive materials and easy to operate with unskilled labour. The walls of the drying bin can be constructed of wood, brick or metal. The floor of the drying chamber is preferably made from fine wire mesh, suitably supported, or perforated metal. If these are not available then sacking spread over a coarser but stronger wire mesh can be used. To facilitate an even airflow through the bed the length of the drying chamber should be 2-3 times the width. The height of the plenum chamber is of the order of 0.3 m. Unloading ports can be fitted at intervals in the walls of the drying chamber.

In order to prevent excessive moisture gradients through the bed, the depth of grain in the bin is relatively shallow, 0.4-0.7 m. and the air velocity is usually of the order of 0.08-0.15 m/s for maize and 0.15-0.25 m/s for paddy. The temperature of the air is selected according to the desired safe storage moisture content of the grain. For the drying of paddy in tropical areas an air temperature of 40-45C is usually used, with a heater capable of raising the air temperature by 10-15C. With such bed depths and air velocities the pressure drop over the bed is relatively low, 250-500 Pa, and therefore simple and inexpensive axial-flow fans can be used. Typically power requirements are 1.5-2.5 kW per tonne of grain for a belt-driven fan powered by a petrol or diesel engine.

Operation of flat-bed dryers invariably results in a moisture gradient between the lower layers and the higher layers of the bed (Soemangat et al. 1973). This problem can be reduced by careful selection of drying temperature and airflow conditions but, even so, gradients of 34% moisture are to be expected. Turning of the grain in flat-bed dryers at intervals can alleviate the problem but this extends the drying time and requires additional labour.

The flat-bed dryer is easily loaded from sacks by hand. However, unloading the dried grain into sacks can be time- and labour-consuming; placement of the drying bin on a tilting frame has been investigated (Wimberly 1983) but this incurs additional costs.

Dryers of this type have been developed in many countries and designs are available from the University of the Philippines at Los Baos (UPLB), Los Baos, Philippines and the International Rice Research Institute (IRRI), Manila, Philippines.

IRRI have also developed a vertical batch-in-bin dryer which operates more efficiently than the flat-bed dryer. It differs from the latter in that the airflows horizontally through the bed on either side of the plenum chamber and exhausts through slatted sides. The bin is easily unloaded by removing the slats. Details are available from IRRI.

Both direct and indirect heaters can be used with the flat-bed dryer (see below). Solar air heating (see above) can also be an option. The waste heat from the engine used to power the fan can be used (Esmay & Hall 1973; Soemangat e' al. 1973; Teter 1987). Heating of the air by 5-10C using waste engine heat is possible but the engine exhaust gases should not be drawn through the grain; the exhaust gases should be ducted outside the housing around the engine. A development of this principle is the moisture extraction unit (MEW) in which the fan is directly driven by the engine and the air is drawn over and around the engine block and exhaust pipe.

Large capacity batch-in-bin (or in-store) dryers can be used in cooler dryer areas. The advantage of this technique is that the bin is used for both drying and storage with savings in both capital and operating costs. With heated air at a temperature of 40-45 C, bed depths of 2-3 m can be used with air velocities through the bed not exceeding 0.08 m/s. Since drying times to achieve reductions in moisture content of 5-10% can be of the order of 20-40 days, this method should not be used in humid areas with grain of moisture contents greater than 18% because of the risks of sprouting and mould growth in the upper layers of the bed.

Large batch-in-bin dryers are usually round or rectangular and range in capacity from ten to several hundred tonnes. With large bins, air distribution ducts at the base of the bin are used rather than a plenum chamber. The ducts can be semi-circular, rectangular or triangular as shown in Figure 5.12 (see Figure 5.12. Air ducts for large Batch-in-Bin Dryer.). To ensure good air distribution through the bed, ducts should be spaced from each other at a distance of half the depth of grain and one quarter the depth from the end and side walls. Air velocity through the ducts should not exceed 5 m/s because of pressure drop factors. More than one fan can be used to provide the airflow required. Detailed information on duct design and airflow distribution is presented by Brooker et al. (1974).

In-bin layered drying with ambient air can be performed with confidence in locations where the relative humidity of the air is less than about 70%. An initial layer of grain, 0.6-0.9 m deep, is loaded into a storage bin, 5-10 m deep, and further layers are added as drying proceeds. Over-drying of the grain is minimized because of the low air temperature. In the USA an airflow of 0.025-0.06 m3/s per tonne was used to dry paddy from 20% moisture to 16% moisture within 14 days when ambient temperature ranged from 18-24C (Houston 1972). Careful and skilled management is required to ensure that each layer is dried before the succeeding layer is loaded into the bin.

View showing three different forms of Air Duct: Rectangular, Triangular, and SemiCircular. Dimensions are in relationship to grain depth D.

Some success has been reported in Indonesia for the drying of paddy from 18% moisture to 13% moisture (Gracey 1978; Renwick & Zubaidy 1983). The latter also demonstrated that field-wet paddy (24% moisture) could be dried safely in bulk to 18 % moisture by continuous aeration (24 in/day) with ambient air, regardless of the humidity of the air. Subsequent drying to 14% moisture or less was accomplished by drying at times of lesser humidity and with the addition of waste engine heat.

There can be a need to dry grain in sacks in certain instances: for example, at central drying facilities where farmers wish to retain access to their own grain. Stacks of sacks are laid over air distribution ducts with no need for a conventional drying bin. Drying proceeds in much the same manner as for bin drying. After use, the air distribution ducts can be dismantled easily to allow use of the building for other purposes.


Re-circulating Batch Dryers

This type of dryer avoids the problems of moisture gradients experienced with bin dryers by re-circulating the grain during drying. One version of a re-circulating batch dryer is shown in Figure 5.13 (see Figure 5.13. Re-circulating Batch Dryer.). The dryer is a self-contained unit with an annular drying chamber, 500 mm thick, around a central plenum chamber, a fan and heater, and a central auger for transporting the grain from the bottom to the top. When drying is complete the grain is discharged from the top. Most dryers of this type are portable and can be moved relatively easily from farm to farm.

Air temperatures of 60-80C are employed with air flowrates of 0.9-1.6 m3/s per tonne of grain, twice that used in flat-bed dryers (Wimberly 1983). However, since the grain is only exposed to the flow of hot air for relatively short times within each cycle, too rapid drying rates are avoided and moisture distribution within individual grains is equalised during the period the grain remains in the non-drying sections at the top and bottom of the dryer. Control of the drying rate can be effected by adjusting the auger speed to regulate the flow of grain through the dryer.

Another version of a re-circulating batch dryer is rectangular with drying chambers on either side of the heater, fan and plenum chamber. Under each drying chamber are horizontal screw conveyors that collect the grain and return it to a screw auger at one end that lifts the grain to a holding section at the top. A screw conveyor in the holding section distributes the grain evenly along each drying chamber.

The capital cost of re-circulating batch dryers is considerably greater than batch-in-bin dryers (Table 5.5. Dryer Specifications, Estimated Performance, and Cost for drying Freshly Harvested Field Paddy (Raw Paddy) from 20% to 14% Moisture) because of their greater complexity and incorporation of handling and conveying devices. However, throughput is greater due to the shorter drying times and the quality of the dried grain is likely to be higher. Re-circulating batch dryers require specialist skills for construction and trained operators for successful operation and therefore are not generally suitable for operation by small-scale farmers or enterprises.


Continuous-flow Dryers

Continuous-flow dryers can be considered as an extension of re-circulating batch dryers. However, rather than the grain re-circulating from bottom to top, as in the latter, the grain is removed from the bottom, in some systems, cooled, and then conveyed to tempering or storage bins. In their simplest form continuous-flow dryers have a garner (or holding) bin on top of a tall drying compartment. With some dryers a cooling section is employed below the drying compartment in which ambient air is blown through the grain. At the bottom of the dryer is the flow control section that regulates both the circulation of grain through the dryer and its discharge.

There are three categories of continuous-flow dryers based on the way in which grain is exposed to the drying air:

Probably the most commonly used continuous-flow dryer is the crossflow columnar dryer, which can be classified as non-mixing and mixing types.

In one version of a non-mixing dryer (Figure 5.14. Continuous Flow Dryers.), drying takes place between two parallel screens, 150-250 mm apart on either side of the plenum chamber. The air escapes from the dryer through louvres on either side of the dryer. The flow rate of grain through the dryer is controlled by a regulator gate at the base of the drying column. Since the grain flows plug-like through the drying section the layer of grain closer to the plenum chamber is dried by hotter and drier air than is the grain on the outside. However, mixing is effected to a fair degree when the grain is discharged and conveyed to tempering and storage bins. Air temperatures of 45-55C and airflows of 2-4 m3/s per tonne of grain are used. Flow problems can be encountered with very wet and dirty grain as the grain may clog. Teter (1987) notes that if very wet paddy is to be dried then the grain should be cleaned and also pre-dried to at least 22% moisture before a non-mixing dryer can be used.

In one design of the mixing type of continuous-flow dryer, as also shown in Figure 5.14, a baffle system facilitates the mixing of grain and avoids the development of moisture gradients across the drying bed. Higher air temperatures, 60-70C, can therefore be used without damaging the grain. Unless screens are fitted on the outside of the drying section lower airflows, 1-1.5 m3/s per tonne of grain, have to be used to avoid grain being blown out of the dryer.

Another design of this type is the LSU (Louisiana State University) dryer (Figure 5.15. Louisiana State University (LSU) Continuous Flow Dryer.). In this version the drying section consists of a vertical compartment across which rows of air channels are installed. One end of each channel is open and the other closed. Alternative rows are open to the plenum chamber and intervening rows to the exhaust section. Alternate rows are also offset such that the channel tops divide the moving stream of grain as it descends providing considerable mixing.

Further information on continuous-flow dryers has been presented by Bakker-Arkema et al. (1982), Fontana et al. (1982) and Houston (1972). As can be appreciated from Figure 5.14 many of these dryers are large and complex structures and are usually designed and constructed by specialist firms.

Compared with batch-in-bin dryers and re-circulating batch dryers, continuous-flow dryers offer the largest drying capacity. When large volumes of wet grain are to be dried in a single site these are the types to be considered first. They are most commonly used in a multi-pass drying operation as shown in Figure 5.16 (see Figure 5.16. Large drying system using Continuous-flow Dryer, Conveying Equipment, and Tempering Bins.). Investment costs are high (Table 5.5) but because of the large throughputs operating costs per tonne can be lower than the larger batch-in-bin dryers and re-circulating dryers.

In a multi-pass drying system, continuous-flow dryers are used in association with tempering bins. During each pass through the dryer the grain is dried for 15-30 minutes with a reduction in moisture content of 1-3%. Drying at this rate sets up moisture gradients within the individual grains. After each pass the grain is held in a tempering bin where the moisture within the kernel equalises as moisture diffuses from the interior of each kernel to the surface. The combination of rapid drying and tempering is repeated until the desired moisture content is attained. Using this procedure the actual residence time of the grain within the continuous-flow dryer is of the order of 2-3 hours to effect a 10% reduction in moisture. Selection of the number of passes is a compromise between the dryer efficiency, ie fewer passes, and grain quality, ie longer drying time. Tempering periods are usually 424 hours in duration. The tempering bins may be aerated with ambient air to cool the grain with some slight moisture removal.

It is vital that the operation of drying with tempering is carefully planned and managed to ensure maximum throughput and efficiency. This usually means that the plant is operated 24 hours a day with two or more batches of grain being dried at a time. Well trained management and staff are essential.

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