High-temperature dryers are employed for the drying of grains if high drying capacities are required. These dryers are unable to produce grains of the same high quality as low-temperature in-bin drying systems. However, in many cases a slight decrease in grain quality is acceptable to the end-user of the grain.
The three major high-temperature dryer types are cross-flow dryers, mixed-flow dryers, and concurrent-flow or counterflow dryers. A schematic of each type is illustrated in Fig. 1.26, and the moisture content and temperature distribution of the grain in each is shown in Fig. 1.27. The air and grain move in perpendicular directions in crossflow dryers, in the same direction in concurrent-flow dryers, in opposite direction in counterflow coolers, and in a combination of cross-flow, concurrent-flow, and counterflow directions in mixed-flow dryers. In theory, the variation in moisture content and temperature, and thus in grain quality, in a sample of dried grain is substantial in cross-flow dryers, less in mixed-flow dryers, and almost nonexistent in concurrent-flow or counterflow dryers.
High-temperature dryers contain a cooling section in which the hot grain after moving through the drying section is reduced in temperature to within 3 to 5◦C of the ambient temperature. Grain remains in the cooling section of a dryer about half as long as in the drying section.
Cross-flow Dryers.Figure 1.28 illustrates a cross-flow grain dryer. The wet grain flows by gravity from a wet holding bin through screened grain columns surrounding the plenum. A heater-fan assembly is located within the drying section of the heated air plenum and forces the hot air through the grain to the ambient in a direction perpendicular to the flow of the grain. In the cooling section of the dryer, ambient air is drawn by cross-flow through the grain into the heater-fan assembly.
Table 1.20 contains the specifications of a typical commercial cross-flow maize dryer.
Mixed-flow Dryers.Figure 1.29 is a schematic of a mixed-flow grain dryer. The wet grain flows from a garner bin over alternate horizontal rows of hot inlet-air ducts and cold outlet-air ducts. The spacing between the airducts determines the grain-layer depth through which the air is forced. Air from the inlet-air ducts flows upwards and downwards to the surrounding outlet-air ducts, in a combination of cross-flow, concurrent-flow, and counterflow with respect to the grain. The bottom series of inlet-air and outlet-air ducts in a mixed-flow dryer serves as the cooling section. Table 1.21 contains the specifications of a typical commercial mixed-flow maize dryer.