Real-time product moisture content monitoring in batch dryer using psychrometric and airflow measurements

• Psychrometric and airflow measurements are used to monitor a batch dryer.
• Algorithm retrieves sensor signals sampled by DAQ and perform calculations.
• Several sources of error were found to affect the method accuracy.
• Wet bulb temperature measurements are particularly important for accuracy.
• Sensor characteristics and location must be adequately selected.

The possibility of a continuous monitoring of moisture content in batch dryers using air humidity was studied in a laboratory-scale test stand. Psychrometers formed by two K-type thermocouples, one of them kept moist, were installed at the system inlet and outlet to measure the increase in the humidity of the drying air. A grid measurement of air velocity at the inlet duct was used to estimate the airflow from a single measurement point. The sensors were connected to a data logger and this to a computer. An algorithm written in MATLAB allowed the retrieval of measured data from the data logger at specified time intervals and the calculation of the mass of water removed. Numerous drying trials have shown that this method can be suitable for drying monitoring. Errors in the calculation of final moisture content of less than 0.03 (dry basis) could be obtained. The difficulty in measuring wet-bulb temperature with sufficient accuracy over long periods was seen as the main obstacle for a reliable use of the method. Attention must be paid to different possible sources of error, which can be minimized by adequate sensor selection and location.