An experimental determination of the critical diffusion coefficient and critical relative humidity (RH) of drying air when optimizing the drying of three hardwood species (birch, aspen, and black alder)

There are no systematic approaches available in current specialist literature when it comes to the optimization of the process of drying out hardwood species of wood, such as birch, aspen, or black alder, all of which find industrial uses in Estonia. In order to investigate the drying of these three hardwood species, this paper has made use of an experimental and theoretical drying optimization methodology which was developed previously for pine sapwood within the framework of the EIC Grant No. 16200 project. The same methodology was used to determine the critical diffusion coefficient and corresponding critical relative humidity levels (RH) in relation to the surrounding air, with those levels being important from the point of view of drying optimization, and calculating those levels separately for each species of hardwood. For alder it was found that the critical diffusion coefficient is Dcr = 36.57 * (10⁻⁴ mm² s⁻¹), while for aspen the figure was Dcr = 30.71 * (10⁻⁴ mm² s⁻¹), and for birch it was Dcr = 16.35 * (10⁻⁴ mm² s⁻¹). It was found that the time dependencies for the total deformation of different tree species tend to differ considerably, although the same drying regime was used for all of those tree species which were incorporated into the experiments.