High-temperature Drying.A number of the in-store drying processes illustrated in Fig. 1.24 can be classified as high-temperature systems. The simplest design is the bin-batch dryer. A relatively shallow (1–1.5 m ) layer of wet grain is dried at a temperature of 45 to 65◦C and an airflow rate of 5 to 10 m3/(m3·min) in 8 to 12 hours depending on the initial moisture content. A major disadvantage of a bin-batch system is the overdrying of the bottom grain layers. Cooling of the grain requires 1 to 2 hours.
In the roof bin-batch dryer, the drying floor is elevated in the bin; the grain depth is limited to 0.5 m. The wet grain is partially dried at a relatively high temperature (60–80◦C) and airflow rate (10–15 m3/(m3·min) before being dumped to the bottom of the bin, where the final drying and cooling of the grain occurs. The roof bin-batch system is a versatile system employed mainly for maize.
The in-bin counterflow drying process can operate as a grain-recirculating system, or as a continuous-flow system. Grain is continuously or intermittently loaded into a drying bin and is intermittently removed in thin layers by a tapered auger located at the bottom of the bin. The partially dried, hot grain is either recirculated to the top of the bin or moved to a second bin for slow in-bin final drying and cooling. The automatic nature of the process and the capability to produce high-quality grain at fairly high capacities have contributed to its commercial success. The airflow rate, bed depth, and drying-air temperature in in-bin counterflow dryers are similar to those in bin-batch systems; however, the in-bin counterflow systems produce dried grain of fairly uniform moisture gradient while bin-batch dryers do not.
Combination drying is a process in which high-temperature in-store or high-capacity drying is followed by low-temperature, low-capacity in-bin drying and cooling (see Fig. 1.24). Combination drying mainly is used for maize and rice; it is also called, a combination of the terms drying and aeration. In a combination-drying system, wet grain is dried from 22% to 28% moisture in a high-temperature dryer to an intermediate moisture content of 18% to 20% and then moved hot to an in-bin dryer. After tempering for 6 to 8 hours, it is slowly final-dried and cooled with ambient air. The main advantages of combination drying are the increased drying capacity and the improved energy efficiency and grain quality.
In-bin Dryer Controls.It may be economically advantageous to operate the fan on an ambient-air/low-temperature drying system, and on a silo-aeration system, intermittently because of favorable or nonfavorable weather conditions. The hardware of the two controller types is similar, but their software differs.
In-bin control systems usually measure the temperature and relative humidity of the ambient air and the temperature of the grain. Some controllers also measure the relative humidity of the air in the interstices of the grain mass. The proper location of sensors in the bin is critical. Control actions are based on the maximum temperature and equilibrium relative humidity of the grain, and thus the sensors should be located where these values are likely to occur, namely in the center of the bin under the loading spout. Multiple sensors increase the chance of detecting a hot spot; the choice of the number of sensors is an economic compromise.
Figure 1.26. Schematics of the four major types of high-temperature grain dryers: cross-flow, counterflow, concurrent-flow, and mixed-flow.
Modern in-bin controllers are equipped with microprocessors that allow the user to change the strategy of the control action. Following is a list of a number of performance criteria for in-store drying (and aeration).
Minimize fan operation (h) Minimize overdrying (%, w.b.) Minimize moisture content range (%, w.b.) Minimize time to finish drying (h) Minimize average dry matter loss (%) Minimize cost of overdrying ($/tonne) Minimize cost of energy usage (kwh/tonne) Minimize net cost ($/tonne)
It is clear that all eight of the criteria cannot be minimized simultaneously. Thus, the manager of a grain depot has to decide which performance criterion should be minimized before a microprocessor is programmed.
Figure 1.27. Moisture and temperature changes during cross-flow drying (a), concurrent-flow drying (b), counterflow drying (c), and mixed-flow drying (d).
Figure 1.28. Conventional cross-flow dryer with forced-air drying and cooling.