In a sawmill process, wood drying is a critical stage that can determine the end-product quality. When wood dries, it can suffer from bending, cracking, hardening or discoloration, and if not optimised, the drying stage can result in damaged products. Between 10-12 % of each kiln load can end up as reject wood due to distortion. At this stage, when the wood has already been cut and trimmed into nearly finished products, loss of material is especially costly. This can also cause drying to be a bottleneck in production.

Meanwhile, drying is also a major energy consumer, using on average over 85 % of the heat energy and up to 40 % of electricity consumed by a sawmill.

Therefore, drying needs to be done in precisely appropriate conditions, where temperature, moisture content and air circulation are optimised. Continuous kilns with automated control systems offer a potential solution.

Methods to minimise quality problems during drying include:

• Logs can be pre-sorted based on their spiral angle, and material with a higher risk of twisting during drying can be directed to uses with less strict limits on distortion.
• Logs can also be pre-sorted based on their moisture content and density, to form more uniform drying batches to optimise drying conditions.
• Pressure framing is important for successful drying; mechanical frames are pressed into the top-most timber layers to keep them from deteriorating.

Continuous kilns allow higher drying capacity compared to a batch kiln. In continuous kilns, the timber is transported through a drying channel, and the air is circulated around it with fans, transporting heat and moisture. The kilns can be divided into zones with different climates, which helps to maintain more accurate target moisture content and temperature during the drying process. The air can also be blown in different directions at different speeds in each zone.

The energy efficiency of drying kilns can be improved with variable frequency drives that allow the slowing airflow towards the end of the drying process. Heat recovery systems can also be fitted to the kilns.

Benefits

• Optimising drying conditions and minimising drying faults significantly improves profitability.
• With energy efficiency measures, drying energy costs can be cut - e.g. Variable frequency drive can bring up to 60 % energy savings in drying. Heat recovery can reduce energy consumption by up to 20–30%.