# Drying Behaviour of Western Hemlock with Schedules Developed for Norway Spruce and Scots Pine
## 3 Results and Discussions
### MC Distribution  
Figure 5 shows the pixel-wise MC calculated from the density-based CT images for three drying runs for each 5 h of drying up to 30 h, and as explained, the last drying run only included spruce sapwood. In all drying runs, pine and spruce reached the desired target MC and capillary, transition, and diffusion regimes occurred. The moisture reduction starts from the sawn-timber surfaces, creating a drier \"shell\". Liquid water migrates from the inner regions of the specimens toward their surface (the capillary phase). The water undergoes rapid evaporation on the specimen's surface, faster than new water is added to this region, creating the \"shell\". The dominant heat-transfer regime is convection through air, which is needed to transport the evaporated water away from the specimen. As drying continued and liquid free water reduced, the diffusion phase with conduction-dominated heat transfer was governed, which, because of the high initial MC of hemlock sapwood, after 30 h liquid water observed in the images, proved that it was still in the capillary phase. Figure 5a shows that once the surface has dried out in both pine and hemlock specimens, the evaporation front recedes deeper towards the centre of the timber, resulting in the formation of a dry layer on the surface and a still moist core. This recession is faster for pine compared with hemlock except for the first 5 h of the drying process, so it can be inferred that two different drying behaviours occurred in hemlock compared to pine. When the moisture content moved from above FSP to below FSP for pine, the drying process was slowed down. This can be observed in pine specimens for around 10 h and 15 h, which showed free water discontinuity in the pine and implies a transition to diffusion known as irreducible saturation, where shrinkage begins on the outer region of the timber. However, the hemlock specimen remained at the capillary regime. As can be seen, it might be said that the MC distribution and changes in hemlock sapwood are uniform during the drying, and there is no abnormal local high MC, which can be inferred as the absence of wet pockets. At the same time, a high initial MC impacts on prolonging the drying time. Figure 5b indicated MC loss of spruce and hemlock heartwood, where spruce reaches the desired target MC while hemlock has abnormally high MC regions (wet pockets), which makes it have high MC resistance to drying in some regions, even when other regions are below FSP or target MC, as shown in 20 h of drying. This specimen faces overdrying and high local MC issues, which enhance drying stress and defects. Permeability is a critical factor in wood drying, especially during the initial stages when water migrates to the surface of the wood through capillary flow. As the drying process progresses, the permeability diminishes due to pit aspiration [23]. Apart from the inherent higher initial MC and wet pockets, lower permeability in hemlock sapwood and heartwood compared with spruce in all directions could be another reason for having prolonged drying time [24; 25]. The behaviour of the evaporation front is not like the first run, and it can be said that in all drying times, it recedes slower into hemlock heartwood compared to spruce heartwood. Figure 5c shows that the MC change for the spruce sapwood specimen started from high green MC, which achieved the desired target MC uniformly with the proposed drying schedule. Compared with drying spruce heartwood specimens, it can be seen that free \begin{table}
\begin{tabular}{c c c} \hline
**Drying Run** & **Specimens** & **Error (\%)** \\ \hline \multirow{2}{*}{I} & Pine sapwood & 2.03 \\ \cline{2-3}  & Hemlock sapwood & 0.24 \\ \hline \multirow{2}{*}{II} & Spruce heartwood & 4.53 \\ \cline{2-3}  & Hemlock heartwood & 1.51 \\ \hline III & Spruce sapwood & 2.71 \\ \hline \end{tabular}
\end{table} 
Table 2: The errors of the MC were obtained from the CT-processed images as a percentage of the gravimetrically measured MC.