MOISTURE MOVEMENT IN SOFTWOOD TIMBER AT ELEVATED TEMPERATURES

The anatomy of a common plantation-grown softwood in New Zealand, Pinna radiata, is reviewed and the differences between the drying behaviour of softwoods and hardwoods are discussed. These differences have significant effects on the paths for moisture movement, and this review of the timber physiology has been used as the basis for a model of the moisture movement at elevated temperatures which distinguishes between the drying behaviour of sapwood and heartwood. It is postulated for the drying of heartwood that liquid flow does not occur within a board due to the aspirated state of the pits, so an evaporative plane at the boiling point of water sweeps through the timber until the centre of the board is reached. In the case of the much wetter sapwood where the pits are not initially aspirated, we assume that liquid flow initially maintains an evaporative plane just below the surface until the moisture is no longer funicular. Thereafter, drying proceeds in a similar way to heartwood. Independent experimental measurements of the temperature profiles at the centre and surface of heartwood and sapwood bkards (100 × 50 mm) have been compared with model predictions, and the close agreement gives some confidence in the model. The model predicts that the drying process for this timber can be divided into three periods, marked by transitions in the gradients of the drying rates when plotted against the average moisture contents. This behaviour has been observed experimentally for Pinus radiata, giving us further confidence that our drying model includes the important physical processes for moisture movement in this timber.