Flow problems can be encountered with very wet and dirty grain as the grain may clog. In one design of the mixing type of continuous-flow dryer, as also shown in Figure 5.14, a baffle system facilitates the mixing of grain and avoids the development of moisture gradients across the drying bed. Higher air temperatures, 60-70 C, can therefore be used without damaging the grain. In this version the drying section consists of a vertical compartment across which rows of air channels are installed. One end of each channel is open and the other closed.
Alternative rows are open to the plenum chamber and intervening rows to the exhaust section.
Alternate rows are also offset such that the channel tops divide the moving stream of grain as it descends providing considerable mixing. As can be appreciated from Figure 5.14 many of these dryers are large and complex structures and are usually designed and constructed by specialist firms. When large volumes of wet grain are to be dried in a single site these are the types to be considered first. Drying at this rate sets up moisture gradients within the individual grains. After each pass the grain is held in a tempering bin where the moisture within the kernel equalises as moisture diffuses from the interior of each kernel to the surface.