An intermittent conventional drying schedule for 30 mm-thick Eucalyptus obliqua lumbers

  Objective  The Eucalyptus obliqua wood from Australia has extremely low permeability and is extremely prone to crack and shrink in the conventional drying process, and it is difficult to ensure the quality. In this study, the intermittent treatment was introduced into the conventional kiln drying process, and an intermittent conventional drying schedule was established, which shortened the drying time of E. obliqua lumbers and improved the drying quality as well.

  Method  The drying characteristics and possible drying defects were obtained by the 100 ℃-test method proposed by Tarazawa. Combined with Keylwerth’s chart method, the preliminary drying schedule for 30 mm thick E. obliqua lumbers was proposed. Intermittent treatments were applied based on the extent of collapse observed on the sample surface at the early stage of drying. The final drying quality of lumber samples was evaluated according to GB/T 6491−2012 “Drying Quality of Lumbers” from the aspect of moisture content gradient, degree of bending, checking and collapse.

  Result  The average basic density of E. obliqua samples in this study was (558 ± 21) kg/m³. Based on the defect grades from the flatsawn and quartersawn 100 ℃-test samples, the final defect grade of E. obliqua wood was evaluated as “Grade-4” for the initial checking, “Grade-5” for the internal checking and “Grade-5” for the cross-section deformation. The initial temperature of the conventional drying schedule for 30 mm thick E. obliqua lumber was set at 46 ℃ with the wet-bulb depression of 1.5 ℃, and the final maximum temperature was set at 67 ℃. During the drying process, severe collapse was observed when the average moisture content of the specimens was 37.7%, 34.4% and 24.4%, and three batches of intermittent treatments were followed with the treatment time of 16, 8 and 8 h, respectively. The specimens were dried from an average initial moisture content of 61.0% to a final one of 10.8% in a total of 20 d, with an average drying rate of 0.10%/h. The moisture content deviation in the thickness direction of the specimen was 0.70%. Most drying defects such as spring, bow, twisting, surface and internal checks of the specimens met the first-class standard, but cupping and collapse of the specimen were more serious, only meeting Grade-3 standard.

  Conclusion  The 100 ℃-test shows that the cross-section deformation caused by collapse is the most serious drying defect of E. obliqua woods, and the collapse extent of the quartersawn samples is greater than that of the flatsawn samples. Adding intermittent treatments to the preliminary conventional drying schedule based on the 100 ℃-test could effectively alleviate and prevent further collapse development effectively. A small amount of collapse still occurs on the 30 mm-thick E. obliqua specimens dried in this study, therefore not meeting the Grade-2 drying quality in GB/T 6491−2012, which is usually required by most enterprises in China. However, the drying time was reduced approximately 90% compared with that of the traditional combined air and conventional kiln drying method, and the drying quality was improved. The intermittent drying schedule recommended in this study can provide insights into the actual drying of E. obliqua woods in the industry.