In deep bed drying all the grains in the dryer are not fully exposed to the same condition of drying air. The condition of drying air at any point in the grain mass changes with time and at any times it also changes with the depth of the grain bed. Over and above the rate of air flow per unit mass of grain is small compared to the thin layer drying of grain. All on-farm static bed batch dryers are designed on deep bed drying principle. The condition of drying in deep bed is shown in the following Figure 29.7.