Efficiency of radiofrequency-vacuum (RF/V) technology for mixed-species drying of wood disks with inherent defects

Abstract

Freshly cut 30-mm-thick wood disks of Scots pine, Caucasian elm, narrow-leafed ash, wild cherry, black walnut, and European alder with various natural grade defects were dried together in a radiofrequency-vacuum (RF/V) dryer. A mild four-step drying schedule at the maximum temperature of 45 °C and power density in the range of 1-2 kWm⁻³ with an “on-off” control of RF radiation at the final step was used. The moisture content of the disks at the end of drying ranged from 10.6% to 13.4%. An almost uniform radial moisture gradient developed in the dried disks. The six wood species, however, exhibited variations in checking. The RF/V drying schedule used here was able to completely inhibit near-bark radial checking in all disks but pine and elm, whereas it failed to efficiently prevent V-cracking in pine and ash with a high mean tangential to radial shrinkage anisotropy (S_T/S_R). A strong positive correlation was also found between S_T/S_R and the proportion of V-cracked disks. The occurrence of ring failure within the reaction wood-containing disks of walnut, pine, and elm was almost completely prevented by the drying schedule, but a significant correlation was observed between the pith eccentricity and V-crack length in the compression wood-containing disks of pine. All other natural defects, i.e., scar in cherry, bark pocket, double or triple pithing in walnut and elm, sound knots in pine, and roundness deviation, exhibited no statistically significant effect (p > 0.05) on check formation.

Keywords:

• Wood disk
• radiofrequency-vacuum drying
• inherent defects
• check formation

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was financially supported by College of Agriculture and Natural Resources at University of Tehran. This study was carried out in Turkey during a research stay of the first author at Düzce University. We would like to thank Recep Sivrikaya Company in Düzce for allowing us to use the industrial RF/V kiln.