Energy conservation by recirculation of drying air

An air recirculation system was used to reduce energy cost for batch food dehydration. The specific objectives of this study were: to test the effect of different air recirculation rates on energy consumption and drying time; and to identify the drying schedule that could maximize the energy savings without decreasing drying rate. In the study, apples and peaches were used as test materials. Fixed recirculation rates of 0, 70, 80, and 90% were applied to both apple and peach drying. Variable-rate recirculation was applied to apple material only. For both apple and peach drying tests, very significant differences in energy use were observed between no recirculation and the 70, 80, and 90% recirculation rates. Recirculation rates of 70, 80, and 90% were not significantly different in energy consumption. The 70% or higher recirculation rate produced an energy savings over no recirculation of approximately 50% for drying apples and 46% for drying peaches. Apple drying took approximately 5 hours, while peach drying took approximately 35 hours to reach the target moisture content (25% w.b.). For all tests of each product, the total drying times were nearly the same for all recirculation rates. The moisture diffusion models were successful in providing good prediction of drying rates for both apples and peaches. The model results also indicated that air recirculation rates did not significantly affect drying rates. Variable recirculation rate drying yielded the same energy consumption and drying time as fixed recirculation rate drying. Based upon these results, a fixed recirculation rate of 80% for apple ring and 70% for peach half drying were suggested.