VACUUM DRYING OF OAK WOOD

Vacuum drying, i.e drying under absolute gas pressure of about 10(4) P a, is an efficient means of reducing the process period and of produci ng good quality wood. We will examine here continuous vacuum drying wh ere the plank surfaces are kept at a constant temperature, which remai ns above the boiling point, and moisture flowing to the surface is ext racted from the kiln. We have carried out an experimental study of oak drying under such conditions, The drying rate and moisture content pr ofile of the sample (40 mm thick) are recorded during the whole drying period. A model of continuous drying is established from general cons ervation equations with the main approximation that the air is rapidly extracted. The two constitutive equations of the model which describe temperature and water content fields are of a diffusive type and coup led through coefficients. The adequate boundary equation is not a conv ective one, but expresses a hygroscopic equilibrium between the vapour in the chamber and the wood surface. The mass diffusive coefficient c an be adjusted to the drying rates through capillary pressure and boun d water diffusion functions. The wood heterogeneity (seasonal growth) is the main factor of discrepancy in these functions. The simulated dr ying rates correspond with the experimental ones.