# Drying Behaviour of Western Hemlock with Schedules Developed for Norway Spruce and Scots Pine
###### Abstract
Wet pockets manifest during drying as localised regions within the timber with significantly higher MC, often above FSP; meanwhile, other regions are well below it. This could be one of the causes for the drying of hemlock to be considerably more time-consuming than spruce and pine. In practice, wet pockets are assumed to be the reason for increased drying time, higher energy consumption, and non-uniform final MC of the timber, which may cause quality-reducing defects in drying. [PERSON] et al. [8] found extractive subtraction by dissolving in chemical composition to be more effective in achieving uniformed final MC than raising the temperature. [PERSON] et al. [9] confirmed [PERSON]'s results and observed a reduced presence of wet pockets in young western hemlock trees and proposed sorting the timber prior to drying according to MC, preferably determined by using electrical resistance MC meters. Modern tools such as machine learning and stochastic models are applied prior to drying to sort western hemlock timbers based on the initial MC with the goal of achieving homogeneity in the final MC [10; 11; 12]. Implementing post-drying equalisation of the MC may be effective in preventing non-uniformity in the MC levels by decreasing dry bulb temperature, which leads to reducing over-drying, while it prolongs drying time [13]. [PERSON] et al. [14] found an inverse correlation between basic density and heartwood percentage of hemlock on drying rate, while initial MC was positively correlated. [PERSON] et al. [15] also reported pit aspiration causing a reduction in diffusion coefficient as a possible reason for the slow drying rate. X-ray computed tomography (X-ray CT) is one of the most accurate non-destructive methods utilised in industry for measuring the inner properties of logs. X-ray CT detects the density of a material, which can be used to evaluate density distribution, and on a lab-scale, can also be used to determine the MC distribution in the entire timber volume. Different methods for determining MC precisely from CT data have been investigated [16]. Analysing CT data would also lead to enhancing our comprehension of the effect of important parameters in drying behaviour, such as temperature and density, by improving the numerical modelling of moisture gradients and flow speed, especially in transition, diffusion, and conditioning regimes [17]. The objective of this study was to determine the causes of the well-known longer drying time and drying behaviour (wet pockets) for western hemlock compared to Norway spruce and Scots pine using X-ray computed tomography (CT), a technology that allows for internal imaging of the wood structure due to density variations, combined with timber drying in a custom-made experimental drying kiln. Developing the knowledge of the dominant wood-drying regime by utilising CT image processing can lead to providing an optimised drying schedule that minimises energy consumption. The drying behaviour of heartwood and sapwood of the three species was studied for pine and spruce industrial drying schedules. Furthermore, a comparative assessment was conducted to determine if high green MC and wet pockets of western hemlock were visible and if they affected the drying rate and regimes.