Automatic endpoint determination for batch tea dryers

A laboratory batch fluid-bed dryer was developed for handling small samples of tea for experimental batch manufacture, and this dryer required a means of stopping drying when the process was complete. A control system was dev ised which requires only the initial weight of the sample to be entered int o the controller. The system then takes full control, using only an inlet a nd exhaust temperature measurement. A simulation model was used to explore the operating region of the dryer, and how the various disturbances affecte d the drying time. A relationship was found between time to a transition te mperature and time to completion which enabled a simple algorithm to be dev eloped, suitable for implementation on a microcontroller. The method was fo und to be substantially independent of variables such as sample moisture co ntent and ambient air conditions, as well as inlet temperature, over the ra nge normally experienced. The algorithm was tested in practice and found to give adequate control, substantially better than the manual system used pr eviously. (C) 2001 Silsoe Research Institute.