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 . 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.