Control of fluid bed tea dryers: controller design and tuning

Variations in moisture content of dried tea are considerable, hence the moisture control of tea dryers needs to be improved. An experimentally validated simulation model of a continuous fluid bed tea dryer was used to design a control system. Feedback from moisture sensing was found to give good results, but a predictor removes some of the effects of lag in the system. As an alternative to the high-cost moisture measurement, the use of exhaust temperature sensing in the feedback is evaluated. Direct feed back of air outlet temperature at a position two-thirds distance along the dryer does not provide effective control, but with use of inferential control good results are obtained. Normal disturbances in feed rate of 5%, which caused an increase in discharge moisture (wet basis) from 3 to 4.5% without control, were restricted to an increase to 3.4% moisture with direct feedback of exhaust temperature. This control error is eliminated by the use of an inferential estimator, which enables control of the discharge-moisture by using only a temperature measurement.