Chinese Journal of Chemical Engineering, Vol.12, No.6, 762-766, 2004
External heat transfer in moist air and superheated steam for softwood drying
In kiln drying of softwood timber, external heat and moisture mass transfer coefficients are important in defining boundary temperature and moisture content at the wood surface. In addition, superheated steam drying of wood is a promising technology but this has not been widely accepted commercially, partially due to the lack of understanding of the drying phenomena occurred during drying. In this work, experimental investigation was performed to quantify the heat transfer between wood surface and surrounding moist air or superheated steam. In the experiment, saturated radiata pine sapwood samples were dried using dry-bulb/wet-bulb temperatures of 60degreesC/50degreesC, 90degreesC/60degreesC, 120degreesC/70degreesC, 140degreesC/90degreesC, 160degreesC/90degreesC, 140degreesC/100degreesC and 160degreesC/100degreesC. The last two schedules were for superheated steam drying as the wet-bulb temperature was set at 100degreesC. The circulation velocity over the board surface was controlled at 4.2 m(.)s(-1). Two additional runs (90degreesC/60degreesC) using air velocities of 2.4 m(.)s(-1) and 4.8 m(.)s(-1) were performed to check the effect of the circulation velocity. During drying, sample weight and temperatures at wood surface and different depths were continuously measured. From these measurements, changes in wood temperature and moisture content were calculated and external heat-transfer coefficient was determined for both the moist air and the superheated steam drying.
Keywords:wood drying;external heat transfer coefficient;moist air;moisture content;softwood timber;superheated steam