PREHEATING AND CONTINUOUS AND INTERMITTENT DRYING IN BOARDS OF EUCALYPTUS-REGNANS F MUELL .2. CHANGES IN SHRINKAGE AND MOISTURE-CONTENT DURING DRYING

Matched, green. flat-sawn board-sections of Eucalyptus regans were preheated in water at temperatures of 50 to 90 degrees C and dried in a 12''r EMC conditioning room on a continuous or intermittent basis. For all temperatures. including controls. shrinkage in width (tangential shrinkage) was higher for intermittently dried material but shrinkage in thickness (radial shrinkage) was higher for continuous drying, volumetric shrinkage was little different between the two schedules. It was argued that higher tangential shrinkage in intermittent drying was caused by lower permanent set in the shell because of lower drying stresses, and that higher permanent set in continuous drying limited tangential shrinkage and resulted in increased radial shrinkage. Overall drying rate (L(r)) was lower for intermittent drying than for continuous drying, even when periods if interruption in the drying process were omitted from consideration. L: was also comparatively lower for continuous drying at higher preheating temperatures. During continuous drying, sequential differences from controls in shrinkage were shown to be highly negatively related to differences from controls in moisture content (Delta M); Delta M was negatively related to board-end checking in continuous drying and negatively related to surface checking in intermittent drying, thus suggesting a role for checking in modulating change in moisture content during drying.