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.