Effect of kiln drying schedule on the quality of South African grown Eucalyptus grandis poles

In specific cases, kiln drying prior to preservative treatment is the prescribed method of utility pole seasoning in South Africa. The slow drying nature of Eucalyptus grandis poles makes rapid kiln drying potentially detrimental due to development of defects such as excessive surface checking, honeycomb and collapse. To control these defects while maintaining the fastest drying rate possible, the effect of drying schedule on the drying quality of E. grandis poles was investigated. Full length pole drying was simulated in thirty 2.1 m long butt sections of poles of which the top ends were end sealed. Three industrially used schedules that differed in dry and wet bulb temperature and duration of drying (T db, T wb and time) were chosen: 80°C/59°C/8 days, 80°C/69°C/10 days and 70°C/59°C/12 days. Surface check length of each pole was measured after drying. Cross-sections cut at the theoretical ground line (TGL), 1.5 m from the butt end, were used to measure the moisture content (MC) gradient between the shell and core of poles. Digital image analysis of cross-sections of discs cut at TGL was used to measure the area of honeycomb and disc cross-section. Collapse was assessed qualitatively. With all three schedules, MC's in the pole cores at the end of drying were still very high. Some poles were above 25% in the outer 25 mm of the treatable zone. The 70°C/59°C/12 days schedule generated the highest surface check total length. A reduction in the wet bulb depression of the 80°C dry bulb schedules from 21 to 11°C, accompanied by a simultaneous increase in drying time, did not reduce honeycomb or collapse. With a wet bulb depression of 11°C, a slight reduction in honeycombing was observed when T db was reduced from 80 to 70°C. From this preliminary study it can be concluded that at 80°C T db, poles should be dried longer than 8 days and that T db's lower than 80°C should be considered to minimise potentially serious drying defects.