Drying of sapwood, heartwood and mixed sapwood and heartwood boards of Pinus radiata

Abstract

boards in stacks of all sapwood, all heartwood and mixed sapwood and heartwood, using a tunnel dryer. Drying temperatures were 90/60 °C and airflow was reversed every three hours, with an average air velocity of 5.0 m/s. For stacks of all sapwood and all heartwood boards, the influence of board thickness and position of thicker and thinner boards in the stack was also examined. Boards with thicknesses of 37 and 45 mm were placed within a stack of 40 mm boards, and individual drying curves during drying were compared. Due to the higher initial moisture content of sapwood, the drying time for the all sapwood stack was almost twice as long as that for the all heartwood stack. In the mixed stack, the initial drying rate for sapwood was faster and that for heartwood slower than in segregated stacks, so that the difference in the drying time between sapwood and heartwood was less. To dry to an overall average moisture content of 10%, the all heartwood stack needed 20 hours, the sapwood stack 35 hours and the mixed sapwood/heartwood stack (55% sapwood) 30 hours. The drying rate for individual boards in the sapwood and heartwood stacks was related to thickness, but the differences were negligible once the overall moisture content fell below 10%.