6. High-capacity Drying

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.

Figure 1.29. A mixed-flow grain dryer.

Table 1.21. Specifications of a typical commercial mixed-flow wheat-and-barley dryer

Characteristic	Specification
Overall height	12.74 m
Number of standard modules	8
Cross-sectional area	5.44 m2
Holding capacity	27 ton
Capacity at 4% moisture removal	14.5 ton/h
Drying air temperature	68°C
Airflow	68000 m3/h
Heat consumption	760 x 103 kcal/h

Figure 1.30. Two-stage concurrent-flow dryer with counterflow cooler, tempering section, and recirculation of the cooling air and part of the drying air.

Concurrent-flow Dryers. Figure 1.30 shows aschematic ofa two-stageconcurrent-flow graindryer. Atempering section separates the two adjoining drying stages. The wet grain flows from a garner bin through the two drying sections and the tempering section in the same direction as the drying air. There is no airflow in the tempering section. (The function of the tempering process is to reduce the temperature and the moisture gradients in the kernels before subsequent further drying, and thus improve the grain quality.) In the cooler, the grain and air flow in opposite directions. The depth of the grain bed (or layer ) in a concurrent-flow dryer and the static pressure and inlet-air temperature are substantially larger and higher than in cross-flow and mixed-flow dryers. Table 1.22 contains the specifications of a typical commercial concurrent-flow maize dryer.

Continuous-flow Dryer Controls. The moisture content of wet grain reaching a high-temperature continuous-flow dryer over a 24-hour period can vary greatly. At commercial elevators, it is not unusual to encounter moisture-content differences of 10% to 15% in lots of maize received from different growers. All the grain must be dried to approximately the same average moisture content, however, by properly varying the speed of theunload augerand thusthe residence time of the grain in the dryer.

Manual control of continuous-flow dryers often leads to significant overdrying or underdrying because manual-control decisions in changing the auger speed are based on hourly readings of the inlet and outlet moisture contents of the grain. Automatic controllers receive this information continuously and thus can minimize the overdrying or underdrying of the grain.

For many years, the automatic control of continuous-flow grain dryers was limited to temperature-activated feedback-type controllers that measure the grain or the exhaust-air temperature at one or several locations along the drying column. A temperature-activated controller is inaccurate and inconsistent at moisture content changes exceeding 3%, due to the nonlinearity of the drying process. Therefore, the temperature-activated controllers are slowly being replaced by moisture-activated systems.

Figure 1.31. Schematic of an automatic control system for continuous-flow grain dryers. A/D, analog to digital; D/A, digital to analog.

Figure 1.32. Inlet and outlet moisture contents versus time during a typical test of automatic control of a cross-flow maize dryer with a set point of 15.5% (w.b.).

Figure 1.31 shows the schematic of a moisture-based automatic control system in­stalled on a high-capacity continuous-flow grain dryer. Figure 1.32 illustrates the varia­tion in the outlet moisture content of maize dried in a cross-flow dryer operating under feedforward/feedback control.