Impact of heating speed on permeability of sub-alpine fir

Abstract

Sub-alpine fir (Abies lasiocarpa (Hook)) is characterized by its low permeability and that it is then hard-to-dry. Long drying time for the species causes kiln capacity problems for numbers of spruce-pine-fir (SPF) lumber producers. A high-temperature drying schedule with a rapid rise in dry-bulb temperature to a temperature above boiling point at the beginning is a potential approach to enhance the drying productivity. Determinations in permeability of ten sub-alpine fir boards after slow heating and fast heating were carried out in this study. The results indicated that a rapid rise in temperature increased the permeability of sub-alpine fir. By statistical analysis, the permeability was increased significantly due to the fast heating in seven boards. The permeability in other three boards was also increased, but not significantly. Using a scanning electron microscope (SEM), the impact of heating speed on pit membrane of sub-alpine fir lumber was investigated. A small number of fine fractures were observed on the pit membrane after slow heating, while in case of fast heating, the torus was partially ruptured and a separation occurred in pit border and cell wall. By observing the SEM images, it was suggested that the force of vaporization and/or thermal stresses due to the fast heating were able to open the aspirated pits and/or break the membranes in the wood cells, and therefore increase the permeability, intensify the moisture transportability and improve the dry-ability of sub-alpine fir lumber.