Natural and solar drying
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The traditional practice of grain drying is to spread crop on the ground, thus exposing it to the effects of sun, wind and rain. The logic of this is inescapable; the sun supplies an appreciable and inexhaustible source of heat to evaporate moisture from the grain, and the velocity of the wind to remove the evaporated moisture is, in many locations, at least the equivalent of the airflow produced in a mechanical dryer. In tropical countries, for at least several months of the year, the mean level of insolation upon the ground is more than 0.5 kW/m² (measured as a mean over the hours of daylight). The heat available therefore, assuming a 12 hour day, is 21.6 MJ/m², a quantity theoretically sufficient to evaporate 9 kg of water.
Even today, sun drying of grain remains the most common drying method in tropical developing countries. It is first employed when the crop is standing in the field prior to harvest; maize cobs may be left on the standing plant for several weeks after attaining maturity. Although not requiring labour or other inputs field drying may render the grain subject to insect infestation and mould growth, prevent the land being prepared for the next crop and is vulnerable to theft and damage from animals. Drying in the field may also be carried out after harvest with the harvested plants laid in stacks with the grain, maize cobs or panicles raised above the ground and exposed directly to the sun. Data on the drying of paddy in the field has been gathered by Angledette (1962) and Mendoza et al. (1982).
Drying on flat exposed surfaces is the most common way of drying grain after harvesting and threshing. For drying small amounts on the farm grain may be spread on any convenient area of land. Contamination with dirt cannot be easily avoided with this method and cleaner dried grain can be obtained by drying the grain on plastic sheets, preferably black.
Purpose-constructed drying floors are commonly used where there is a need to dry large quantities of grain during the season, e.g. at most rice mills. The floors are usually made of concrete or brick, these materials presenting a relatively smooth and hardwearing surface. Floors should be constructed to withstand the movement of vehicles and sloped or channelled to hasten the runoff of rainwater. The paddy is spread in a thin layer on the floors and raked at intervals, preferably 7-8 times daily, to facilitate even drying. At night the paddy is heaped into rows and covered with sheeting.
Work by Chancellor (1965) and Soetoyo & Soemardi (1979) has demonstrated that paddy can be dried from 24-26% moisture to 14% moisture at depths of 50-100 mm at a rate of 3.3 kg/m².h for stirred paddy and 1.9 kg/m².h for unstirred paddy. The grain can reach temperatures as high as 60°C under clear skies and the rate of drying can be extremely high. Under these circumstances kernel cracking and loss of head rice can be appreciable, particularly if paddy is dried to below 14% moisture. Covering the paddy around midday may be beneficial under particularly hot and sunny conditions. Experiments at IRRI have shown that cracking can be reduced by 25% if paddy is dried in the shade but the benefit from the improved quality is generally more than offset by the longer drying times and hence reduced throughput and increased costs.
In rainy weather, even though drying will be slow, every effort should be made to prevent wet freshly-harvested paddy from over-heating with deterioration in quality by spreading on floors rather than let it remain in heaps and sacks. Under these conditions or when there is great demand for drying space paddy can be dried to 17-18% moisture and then temporarily stored for 15-30 days before final drying.
Compared with paddy, cob maize can remain at relatively high moisture contents, in excess of 20% with natural ventilation for considerably longer periods, from one to three months. The maize crib in its many forms acts as both a dryer and a storage structure. The rate and uniformity of drying are controlled by the relative humidity of the air and the ease with which air can pass through the bed of cobs. The degree of movement of air through the loaded crib is largely attributable to the width of the crib; research in West Africa has shown that crib widths should not exceed 0.6 m (Anon 1980). Guide-lines on crib design, construction and operation have been prepared by Bodholt (1985). Further information on crib design is available in Chapter 6.
An improved technology in utilizing solar energy for drying grain is the use of solar dryers where the air is heated in a solar collector and then passed through beds of grain. There are two basic types of solar dryer appropriate for use with grain: natural convection dryers where the air flow is induced by thermal gradients; and forced convection dryers wherein air is forced through a solar collector and the grain bed by a fan (Brenndorfer et al. 1985).
Natural convection dryers are generally of a size appropriate for on-farm use. A design that has undergone considerable development by the Asian Institute of Technology (AIT) in Bangkok, Thailand (Boothumjinda et al. 1983; Exell 1980) is shown in Figure 5.7 (see Figure 5.7. Natural Convection Solar Dryer.). The dryer consists of three components, a solar collector, the drying bin and a solar chimney. For a one tonne capacity dryer the collector is 4.5 m long and 7.0 m wide with the solar absorber base of burnt rice husks or black plastic sheet covered with clear plastic sheet. The drying bin is 1.0 m long and 7.0 m wide with a base of perforated steel or bamboo matting.
The solar chimney provides a column of warm air that increases the thermal draught of air through the dryer. It is made of a bamboo frame covered with black plastic sheet. In Thailand paddy was dried from 20% moisture to 13% moisture in 1-2 days and the rice quality was appreciably greater than that from sun dried paddy. A disadvantage of the dryer is its high structural profile which poses stability problems in windy conditions, and the need to replace the plastic sheet every 1-2 years. A smaller (100 kg capacity) and simpler version of this type of dryer has also been developed (Exell & Kornsakoo 1978; Oosthuizen & Sheriff 1988) as shown in Figure 5.8 (see Figure 5.8. Small scale Solar Paddy Dryer.).
The forced convection solar dryer can be considered as a conventional mechanical drying system in which air is forced through a bed of grain but the air is heated by a flat plate solar collector rather than by more conventional means. Several types of flat plate collector are shown in Figure 5.9 (see Figure 5.9. Flat Plate Collectors.).
The performance of a flat plate collector can be quantified by calculation of the collection efficiency; the ratio of the heat gathered by the collector to the insolation incident on its surface. The collection efficiency is a function of the air velocity through the collector, the geometry of the air duct, the absorptivity of the absorption surface, and the transmissivity of the cover(s).
Considerable work has been undertaken in developing low-cost and efficient solar collectors for crop drying applications (Brenndorfer et al. 1985; Davidson 1980). The simplest type of collector is the bare plate (Figure 5.9) which consists simply of an air duct the uppermost surface of which acts as the absorber plate. The covered plate collector in its many forms utilises a translucent cover above the absorber plate; four versions of this type are also shown in Figure 5.9. Compared with the bare plate collector higher collection efficiencies are obtainable with covered plate collectors but at the expense of increased complexity and cost.
The optimum design suitable for use at farms and mills in developing countries is probably the bare plate collector which is capable of operating at a collection efficiency of 40-50% with an airflow of 0.10 kg/s.m². With typical insolation levels in many tropical countries of the order of 0.5 kW/m² such collectors are capable of providing mean daily elevations in air temperature of 5-10°C with heat outputs of 0.20-0.25 kW/m² of collector area. Covered plate collectors, operating at efficiencies of 60-70%, are capable of providing air temperature elevations of 10-30°C but at a lower airflow.
A major advantage of the bare plate collector is that it can be easily incorporated into the roof of a dryer or storage building. Corrugated iron is a popular and inexpensive roofing material in many areas and when painted black forms an excellent solar absorber. A false ceiling can be fixed to the roof joists so forming a shallow duct running the length of the building and easily connected to a fan via ducting at one end of the building. The heat available from the collector is weather dependent and consideration should therefore be given as to whether solar energy should be the sole source for heating the air or a supplement to more conventional heating systems.
Research and performance studies on forced convection solar dryers have been reported by Bose (1978), Muthuveerappan et al. (1978) and Soponronnarit et al. (1986). Damardjati et al. (1991) have described the performance in Indonesia of a 10 tonne/day paddy dryer (Figure 5.10. Forced Convection Solar Paddy Dryer.) that incorporated a 225 m2 roof-type collector together with a moisture extraction unit (MEW, see below). Heat output from the collector averaged 60 kW over daylight hours and that from the MEU 35 kW with a mean daily elevation in air temperature of 7-9°C.
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