Application of the Response Surface Methodology for Designing Oscillation Drying of Beech Timber

Abstract

Oscillation drying is one of the possible approaches for reducing drying time and limiting drying defects of hardwood timber. This study aimed to design oscillation-drying schedules for beech (Fagus sylvatica L.) timber using the response surface methodology (RSM) and to develop an empirical model describing relationships between drying time, drying rate, moisture content gradient after drying, and the parameters of oscillation drying, i.e., dry-bulb temperature increase, equilibrium moisture content (EMC) decrease, and the duration of phase #1 in the drying schedule. The design employed 8 unique drying schedules for which early stage of drying was studied. The Gompertz model was used to describe the change in moisture flux as a function of moisture content, with estimated parameters of the model used to determine relations between the maximum flux at the initial moisture content, the maximum rate of flux change, and the critical moisture content for the maximum rate of flux change for each oscillation-drying schedule. Analysis of variance (ANOVA) revealed that the decrease in EMC was the only factor significantly influencing oscillation drying. For the most intense oscillation-drying schedule, maximum moisture flow was ca. 75% higher compared with the control drying schedule. Drying processes that accounted for a decrease of EMC from 15% to 12% were characterized by significantly shorter drying time (by 35.8%), 52.6% higher drying intensity, and ca. two times larger moisture content gradient. These results confirm theoretical findings relating the increase of oscillation-drying intensity with the difference in wet-bulb temperature between phases of the drying processes.