Microcomputer-based control of exhaust air recirculation in a fruit dehydrator

A microcomputer-based control system was developed to monitor and control the exhaust air recirculation in a batch-type fruit dehydrator. The purpose of this system was to increase the recirculation of the exhaust air gradually as dehydration progressed. The system was able to maintain the humidity ratio of the drying air at different specified levels at different stages of drying. Type T thermocouples were used as temperature sensors to measure the dry and wet bulb temperatures of the drying air. The humidity ratio of the drying air was determined by the dry and wet bulb temperatures. Control of the humidity ratio of the drying air was accomplished by adjustment of the exhaust air recirculation. A stepping motor and valve mechanism were used to adjust the recirculation ratio. A microcomputer was used to control the motor through the I/O of a multifunction board and a stepping motor driver. The number of steps and direction for the motor were determined by the difference between the measured humidity ratio and the set point. The effects of proportional coefficients and sampling periods on the humidity control were investigated. The control system was able to maintain the humidity ratio of the drying air at an accuracy of ±0.001 kg water/kg dry air.

The system was tested under actual drying conditions to evaluate its performance. The dehydration started with an initial recirculation rate (70%) and was programmed to maintain the humidity ratio of the drying air at different specified levels as dehydration progressed. After the drying sample weight dropped to 70% of its original weight, the system adjusted to maintain the humidity ratio of the drying air at the set point of 0.025 kg water/kg dry air. Approximately five minutes were needed for the system to stabilizeat the set point after the adjustment. The recirculation of the exhaust air was increased gradually as dehydration progressed. The energy savings with this system was approximately 31% over the dehydration without air recirculation.