The wet material is placed in the hopper and flows by gravity between the perforated retaining walls and is discharged at the bottom by a continuously operating metering valve. Heated air is forced across the column at right angles to the direction of grain motion. The column may be inclined to simplify construction or fitted with baffles to stir the mass as it progresses through the drier. Many designs are used, but the principle of operation in all cases is that of Fig. 11.9.