4. Sun Drying
In many tropical and sub-tropical regions, sun drying remains the preferred method of grain drying, mostly for economic reasons. Traditional sun drying has changed little over the centuries. The grain is spread on mats or paved ground in layers of 5- to 15-cm thickness and is exposed to the ambient conditions. The grain is stirred intermittently, usually is covered at night, and dries adequately in 2 to 4 days.
The fixed costs of sun drying are low (except if a special drying floor has to be constructed). However, sun drying is an unreliable process because it is weather-dependent. Also, the solar radiation changes with the season and the time of day, and the flux density is low.
Notwithstanding its disadvantages, it is possible to produce dried grain of superior quality if sun drying is practiced competently. Of particular importance are the proper selection of the maximum layer thickness and the initial moisture content of the grain, and the recognition that during certain periods of the year (i.e., the wet season) adequate sun drying of grain is not feasible.
Notwithstanding the widespread use of the sun drying of grains in the developing world, few controlled scientific experiments have been conducted. Therefore, no general recommendations valid for every region can be made for the sun drying of grains.
Physics of Sun Drying. The sun drying of grain is affected by the solar radiation, the ambient air temperature, the ambient relative humidity, the wind velocity, the soil temperature, the grain-layer thickness, and the grain type. Simulation of the process [9] has resulted in a better understanding of sun drying, in particular of the effects of initial moisture content, layer thickness, and season on the drying process (see Figs. 1.21–1.23). The specific information in the three figures pertains to Jakarta (Indonesia), but the trends are valid for other locations.
Figure 1.21. Average moisture content during sun drying of rice at different initial moisture contents.
Figure 1.22. Rice-grain moisture content during sun drying
with grain-bed depths of 5, 8, 10, and 15 cm.
Figure 1.23. Moisture content of rice during sun drying in July and March.
5. In-store Drying
In-store (i.e., in-bin) grain drying commonly is practiced on the farm and usually employs ambient air or slightly heated air as the drying medium. The major in-store drying methods are illustrated in Fig. 1.24 [1]. The objective of in-store drying is to decrease the moisture content of the grain at a rate that will prevent product deterioration. Both the minimum drying rate and the maximum final moisture content are locality-dependent; that is, in hot and humid regions the drying rate of wet grain has to be higher, and the final moisture content lower, than in cold and dry areas.
Figure 1.24. On-farm in-bin and non-bin grain-drying systems.
Ambient/Low-temperature Drying. Drying a bin of grain with ambient air is possible if the initial moisture content of the grain is not excessive, the average daily relative humidity of the ambient air is not too high, and the airflow rate is sufficient. The recommended maximum initial moisture contents of maize and wheat, and the minimum airflow rates, for ambient-air drying under midwestern U.S. conditions are listed in Table 1.19. In the tropics, the initial moistures are lower and the required airflow rates are higher. The reader should obtain the exact values for a locality from the regional extension service.
In ambient-air layer drying, a bin is filled in stages; a wet layer is added only after the lower grain layers have been partially dried. In stir drying, one or more vertical stirring augers slowly move the dried grain from the bottom of the bin to the top, and the top grain layers to the bottom of the bin. Layer drying and stir drying require lower airflow rates and permit higher initial moisture contents than ambient-air drying.
In periods of zero drying potential during rain or high humidity, the relative humidity of the air can be decreased to the desired level (i.e., 60%–70%) by increasing the air temperature by 2 to 3◦C. This process of in-bin drying frequently is called low-temperature drying. The airflow rate and maximum initial moisture content of the grain are similar for ambient and low-temperature drying. Stirrers are frequently employed in an in-store/low-temperature drying system.
A basic in-bin ambient-air/low-temperature grain dryer is shown in Fig. 1.25.
Figure 1.25. In-bin grain-drying system.