Enhancing the exergetic performance of a pilot-scale convective dryer by exhaust air recirculation

Abstract

In this study, an air recirculating pilot-scale convective dryer operating at various exhaust air recycle fractions was exergetically investigated in detail. Two drying air temperatures (55–70 °C), two air volume flow rates (360–450 m3/h), and six exhaust air recycle fractions (0–100%) were considered for drying of poplar wood chips. The effects of drying variables were studied on the exergetic efficiencies of drying system and drying chamber. The total exergy of air exhausting from drying chamber was also fractionated into thermophysical and wet exergies for further evaluating the effect of recycle fraction. The universal exergetic efficiency of drying chamber ranged from 41.84% to 98.07%, while the average overall functional exergetic efficiency of drying system varied from 1.32% to 4.01%. Exhaust air recirculation profoundly improved the overall functional exergetic efficiency of drying system as a decision-making parameter up to over two times. Although the recycle fraction of 100% showed the highest improvement in the overall functional exergetic efficiency of drying system, the drying time drastically increased at this condition as expected. Overall, a compromise should be made between drying time and exergetic improvement in order to select a proper recycle fraction for recovering exergy from outflow air.