Pre-sorting for density in drying batches of Norway spruce boards

One of the key processes for product performance, and the most energy consuming process in the production of solid timber, is the drying process, a process heavily influenced by the wood density. Normally no information of density in a timber drying batch is known today, apart from previous experience of the level of density for the timber of that particular mill. In this study the effect of dividing boards into a high and a low density drying batch was investigated. 66 boards of 50 x 150 mm 2 were collected randomly from the green sorting at a Norwegian sawmill. The boards were cut into 1200 mm long pieces and grouped in a high density group, low density group and a mixed density group. The batches were dried in a laboratory kiln with a constant dry bulb temperature of 70 °C and a decreasing wet bulb temperature. The drying schedule was designed with the help of simulation software and was adapted to the average density and average moisture content of each batch. Total drying times for the low, mixed and high density batch was 58 h, 63 h and 70 h respectively. The downgrading due to checks showed no statistically significant difference between the density batches and the final moisture content was statistically equal. Only the level of case hardening was significantly lower for the low density batch. This density separation resulted in a net saving of 4 % in drying time at the test mill.