Iranian Journal of Wood and Paper Industries

Iranian Journal of Wood and Paper Industries

Preparation of time-based schedule for drying of walnut slab with different width in vacuum/hot-air kilns

Document Type : Research Paper

Authors
Asghar Sistani 1
Asghar Tarmian 2
Foroogh Dastoorian 3
1 Department of Wood and Paper Science and Technology, Faculty of Natural Resources, College of Agriculture and Natural Resources, University of Tehran, Iran
2 Department of Wood and Paper Science and Technology, Faculty of Natural Resources, College of Agriculture and Natural Resources, University of Tehran, Iran.
3 Department of Wood Engineering and Cellulosic Products, Faculty of Natural Resources, Sari Agricultural Sciences and Natural Resources University, Iran
10.22034/ijwp.2025.2072884.1729
Abstract
Problem definition and objectives: Drying wood is one of the most important stages of its primary processing, which affects the final quality and cost of producing wood products. Drying valuable hardwood slabs such as Persian walnut in a conventional kiln is associated with serious challenges due to their large dimensions, including prolonged drying time, cracking, deformation, and the occurrence of internal stresses. Similar to conventional kilns, for drying wood in a vacuum kiln, an optimal schedule should be developed according to the type of species and its dimensions. In addition, the type of vacuum kiln and its technical specifications, such as the heat transfer system, also affect the optimal schedule. In other words, a separate schedule must be prepared for each type of vacuum kiln. The main objective of this study was to develop a time-based schedule for drying Iranian walnut (Juglans regia L.) slabs in an industrial-scale vacuum/hot air kiln.
Methodology: Persian walnut slabs with a nominal thickness of 7 cm and a length of 180 cm, in two width ranges (below and above 400 mm), were dried. Initial moisture content of the slabs ranged from 60% to 70%. An industrial vacuum/hot air kiln with a nominal volume of 12 cubic meters and a hot oil heating system with intermittent vacuum application was used to dry the slabs. Three time-based schedule were used in 9 to 10 steps with a maximum temperature of 70°C and a maximum vacuum of 0.45 bar. In the first, second and third schedule, the temperature of the first step was 45, 40 and 35°C, and the drying time was 152, 167 and 176 hours, respectively. The vacuum and heating time used for each schedule was also different. After the end of drying, the final moisture content, moisture gradient in thickness and width, residual stress (casehardening), cracking and warp intensity were determined. The experiment was based on a Completely Randomized Design and the significance of the mean data was analyzed with Duncan's multiple range test.
Results: The final moisture content of the slabs in all drying schedules was in the range of 10–12%, which is suitable for the final use of the material in the production of office and household rustic-style furniture. The highest moisture gradient across the thickness and width of the slabs was observed in schedule 1, whereas schedule 3 showed the lowest gradient. Surface and end checking were significantly more severe in schedule 1 (total check length exceeding 1500 mm in wide slabs), while the length was reduced to less than 200 mm in schedule 3. Case-hardening also decreased significantly in schedule 3. Furthermore, deformation defects such as cupping, bowing, springing, and twisting were minimized in schedule 3, leading to a substantial improvement in the final quality of the slabs. In all schedules, wide slabs (over 400 mm) exhibited greater checking and deformation compared to narrow slabs.
Conclusion: Drying schedule strongly determines the final quality of walnut slabs. In the schedule 1, rapid drying of the slabs due to higher temperatures and greater vacuum resulted in severe defects, while schedule 3, with mild drying condition improved moisture uniformity, reduced cracks and deformations, and enhanced overall quality. Therefore, schedule 3 is recommended as the optimal drying schedule for Iranian walnut slabs in vacuum/hot-air kilns.
Keywords
Drying
Persian walnut slab
Case hardening
Deformation
Surface checks
Subjects
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Iranian Journal of Wood and Paper Industries
Volume 17, Issue 1
Spring 2026
Pages 35-51