The objective of this project was to provide the information required by Canadian lumber producers to evaluate the superheated steam/vacuum (SS/V) drying process by comparison with conventional methods as applied to 4/4-inch red oak with respect to drying time, final product quality, efficiency and costs. The study was divided into two parts. In the first part, three loads of red oak were dried in Forintek’s eastern SS/V kiln to identify a suitable schedule with respect to time and quality. In the second part, comparative tests were conducted between the SS/V kiln and a conventional kiln of similar capacity. The tests involved measurements of drying time as well as wood quality before and after the drying operation, including distortion, drying checks and splits, final moisture content (MC) gradient, shrinkage, and residual stress (prong test). Energy consumption was also measured for the SS/V and conventional kilns. Finally, drying costs were evaluated for the two systems in an industrial scenario.
We successfully developed a schedule to efficiently dry 4/4-inch red oak through the SS/V process. With this schedule, 4/4-inch red oak can be dried in approximately a third of the time required for a similar load in the conventional kiln. Our results from parallel measurements indicate that the final quality of the wood dried with the SS/V process is quite comparable to that obtained with conventional drying. In addition, quality results proved achievable without a conditioning phase at the end of the cycle. Parallel drying tests conducted on similar loads of 4/4 red oak indicated that total energy consumption was 15% lower with the SS/V kiln than with the conventional kiln. A comparison of drying costs for the two systems based on annual production of 10 million fbm suggested that capital and operating costs might be slightly lower with the SS/V kiln.