s see Chapter Cooling options see page onwa B4-SH7A Bevel-helical speed reducer B4

Flender/Flender Gear Units/Bevel-helical speed reducer B4

slow process that requires manual supervision and control time, which is demanding and expensive.Another reason is that dryers exhibit relatively complicated dynamics, as result, an auto-matic controller often effects more accurate control than the manual alternative. 5.1 Dryer Description In the following, continuous cross-ow dryer described in detail in Nybrant (2, and Farkas and Nybrant ( is considered for the control study. schematic view of such adryer is given in Fig. 9. The wet grain is normally kept in container on the top of the dryer. While it continuously ows through the dryer, it rst passes the drying section, where it is dried by preheated air of typically 4 C6C. In the cooling section the grain is cooled by ambient air before it leaves the dryer. The speed of the discharge motor determines thegrain ow through the dryer and thereby the average amount of moisture removed fromthe grain. 8 Farkas Fig. 9 Schematic view of continuous cross-ow dryer. 5.2 Dryer Control Strategy control objective for continuous cross-ow dryer can be formulated to determine the discharge rate so that the outlet moisture content is as close as possible to referencevalue, regardless of variations in inlet moisture content and grain and dryer characteristics. According to the control objective, an obvious control strategy would be to use feedback from the outlet moisture content and feedforward from the inlet moisture content, which is the most dominant measurable disturbance. Such feedback-feedforward control- lers are described, for example, by Toftdal and Olesen (, Nybrant (2,, and Moreiraand Bakker-Arkema (. In physically based model for dryer control, the grain mois-ture distribution can then be calculated and used for control purposes, as discussed byFarkas (. Since it is expensive to measure moisture content accurately, an alternative, espe- cially for smaller dryers, is to measure and control the exhaust air temperature. This tem- perature is thermody