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Beyond batch kilns: alternative solutions for drying your lumber

Qing Q

Qing Qiu digital analyst at Algorex

In prior blog articles, we have discussed the motivations behind lumber drying and explained conventional approaches. Batch kilns – the time-tested solution – remain integral in turning moist wood into dry lumber. There are ongoing endeavors that are directed towards advancing alternative technologies in recent years with the aim of achieving faster drying time, improved quality, as well as enhanced sustainability. In this blog, we are excited to delve into three additional state-of-the-art drying solutions, discovering the opportunities these modern technologies may bring to the world of lumber processing.

Continuous Drying Kiln (CDK)
One of the groundbreaking advancements in lumber drying is continuous drying kilns (CDKs), also known as continuous flow kilns or progressive kilns. While the concept of continuous drying has been existing for many years, its widespread commercialization initially took root in Europe and, in the past decades, has made great inroads into the North American market. In contrast to batch kilns that follow a cyclical operation, CDKs are specifically designed for continuous operation. If we draw a metaphor, envisioning a batch kiln as an oven, a CDK could be likened to a conveyor toaster. Continuous operation offers the advantages of higher throughput, streamlined processes, reduced handling time, as well as better boiler utilization thanks to the homogeneous demand for energy.

The North American counter-flow CDK typically consists of three sections, with only the central section being heated. The other two sections are strategically designed to pre-heat the lumber upon entry and equalize the lumber at the exit, respectively. These CDKs are usually equipped with dual tracks, where lumber in separate tracks is loaded from opposite ends. This approach leverages the moisture content of green lumber to condition the dry lumber as it exits from the other track, helping foster a more uniformed moisture content distribution. Simultaneously, this configuration is also known for enhancing energy utilization, since the hot and dry lumber exiting from one track could heat up the green and moist pieces entering the other track. Given that lumber drying is a highly energy-intensive process, initiatives to reduce energy consumption in this process can significantly contribute to lowering the carbon footprint.

Vacuum Drying
Diverging from using high temperatures and air circulation that are employed by batch kilns and CDKs, vacuum drying involves creating a vacuum within the drying chamber, enabling wood to undergo the drying process at a pressure significantly below atmospheric pressure. As the pressure decreases, the boiling point of water also decreases, facilitating the removal of water at lower temperatures compared to traditional drying methods. The capability to dry at lower temperature could reduce the risks of having drying defects. In the literature, benefits of vacuum drying also include significantly reduced drying time, particularly for hardwoods and large sections. It also enhances color preservation, boasts higher energy efficiency, provides better control over volatile organic compound emissions, and allows for the effective drying of exceptionally large cross-sections [1]. Despite its existence since the early 1900s, vacuum drying has not achieved mainstream status as a wood-drying method. Its utilization remains confined to high-value species, large cross sections, and specific niche applications where the unique advantages of this method outweigh its limitations.

Radio Frequency (RF) Drying
Another drying technology that is known for its speed is radio frequency (RF) drying. In brief, RF drying utilizes high-frequency electromagnetic waves to excite the water molecules in wood. As the molecules rapidly and constantly move, heat is generated. Different from convective approaches that heat up wood from the surface to the core, RF drying guarantees consistent heating throughout the entire lumber thickness. This ensures a more uniformed moisture content distribution across the thickness, mitigating the risks of wet pockets and other drying defects like checks and warp, and thus making faster drying feasible. Because of the high expenditure on energy and capital investment, RF technology is often combined with other drying approaches [2].

It is truly thrilling to observe the continuous evolution of innovative technologies enhancing lumber drying solutions. Here at Algorex, we are servicing CDK through our KMS 360 service. Simultaneously, we eagerly anticipate integrating our digital solutions with these cutting-edge technologies, empowering you to swiftly adapt to new technologies and efficiently control your kilns.

[1] Espinoza, O., & Bond, B. (2016). Vacuum drying of wood—state of the art. Current Forestry Reports, 2, 223-235.
[2] Lavoie, V. (2016). Séchage hydride conventionnel et par haute fréquence en continu du bois d’épinette noire destiné aux produits à valeur ajoutée (Master Thesis, Université Laval).

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