WOOD RESEARCH Volume 71, Number 2, 2026
YUQI SUN, XUANJIANG LIU, JIANING LI, TONGTONG LI, and HONGHAI LIU

Comparative Study on Characteristics of Vacuum and Conventional Kiln Drying of Non-Pith Heartwood of Rubberwood

This study compared the drying characteristics of non-heartwood rubberwood in vacuum drying (VD) and conventional kiln drying (CKD). The results show that overall, VD had a 21.6% higher drying rate than CKD. Especially below the fibre saturation point (FSP), VD’s rate was 1.56 times that of CKD because the low pressure in VD reduces the bound- water diffusion resistance. An ANOVA test shows that VD had significantly lower uniformity of moisture content (MC) in the longitudinal direction and higher uniformity of MC in the radial direction, especially when the MC drops below the FSP. VD also had significantly lower shrinkage ratios, colour change, and residual stress than CKD (p < 0.05). The greatest differences in shrinkage occurred at 20% MC. Overall, the colour change in VD was 30% lower, and the maximum decrease in the residual stress index in VD was 17.9% at 40% MC. These advantages of VD are likely related to its more uniform internal moisture gradient and low-oxygen environment.

WOOD RESEARCH Volume 61, Number 3, 2016
Zhengbin He, Yu Zhang, Shu Qiu, Zijian Zhao, and Songlin Yi

Water distribution during ultrasound-assisted vacuum drying of wood

Wood ultrasound drying is an innovation method, it can reduce overall drying time, increase the mass transfer rate, and increase the effective water diffusivity. In this paper, poplar was taken as experimental material. The drying process was carried out under the conditions that the drying medium temperature is 60°C, the absolute pressure is 0.02 MPa, the ultrasound power is 100 W and the ultrasound frequency is 20 kHz. The moisture content distribution and water diffusion coefficient were studied, and the model among wood moisture content variation, drying time and water diffusion coefficient was established. Results indicated that the moisture gradient increases along with the increase of drying time during the drying process, free water and bound water are dried simultaneously when wood moisture content is above the fiber saturation point; Wood moisture decreased linearly when moisture is above the fiber saturation point, while the descending rate decreases when the moisture content is below the fiber saturation point. The water diffusion coefficient decreases along with the increase of drying time and increases exponentially along with the increase of moisture content. The moisture diffusion coefficient is 2.89×10-4 at the beginning stage, it is 3.02×10-6 when the moisture content is at the fiber saturation point, and it is 2.27×10-7 cm2.s-1 when the moisture content is 10 %; The equation between the water diffusion coefficient and the moisture content was established and it could be used to predict the water diffusion coefficient during ultrasound-assisted vacuum drying.