A Decade of Improved Lumber Drying Technology

Abstract

In this paper, we comprehensively review the relevant literature published from 2005 to 2016, focused on lumber drying and provide a summary of where we feel future research will focus. Drying is a critical part of most wood products manufacturing process, and the methods used and proper control are key to achieving the appropriate production level, quality, and costs. While a combination of drying methods may be used, most lumber is dried in a kiln at some point in the process. The most common commercial kilns can be classified as conventional, high temperature, and vacuum; however, there continues to be some interest in solar and compression drying. While no new drying technologies have been proposed, work has continued on improving the existing methods. Control of the drying process varies with the type of kiln used, the species being dried and the temperatures used in the process; however, it usually involves some type of measurement of the moisture content of the wood being dried. The development of new methods for controlling the drying process focuses on new ways to measure moisture content or moisture content variation, temperature drop across the load, and drying stresses. While wood quality can be defined differently by its various users, for example, industrial or end users, certain aspects of quality remain constant across these groups, such as minimizing warp, checks, and splits, and discoloration, and maintaining or enhancing mechanical properties. New schedules have been proposed to increase drying rate and improve drying quality. Methods to reduce drying defects and improve its quality have focused mainly on mechanical restraint to prevent warp, better understanding of defect formation, and pre-treatments to speed up the drying process or reduce final moisture content variation. Finally, concerns regarding the environmental impacts of wood drying, most importantly the high energy demands and emissions, have increased in importance as concerns about sustainability and health issues become more mainstream.

Effect of the Drying Method of Pine and Beech Wood on Fracture Toughness and Shear Yield Stress

“…The fracture toughness increases when wood is dried in other than air drying conditions. The air drying [ 1 ] is considered as a reference method that affects the native physical and chemical properties of wood to a small extent [ 2 ]. That leads to the conclusion that industrially dried wood becomes more brittle than wood not exposed to elevated temperatures.…”

“…Drying of wood is, therefore, an important step in the manufacturing process of any wood product [1]. Several methods of drying wood are commonly used in the modern wood industry with air drying, kiln drying and vacuum drying recognized as the most frequent [1][2][3][4]. Alternative methods using super-critical drying, higher temperature schedules, low pressures, assistance of microwaves, among others, were developed to shorten time of this process [5,6].…”

Prediction of Maritime Pine Boards Modulus of Elasticity by Means of Sonic Testing on Green Timber

“…Considering the energy required for the various processes that make up the structural timber supply chain, from the forest to the end user, and the related emissions, it is immediately evident that their optimization is important to keep the use of this material sustainable. Given that drying is the most environment-impactful phase of the entire process [19][20][21][22], due to the high energy required to remove moisture from the wood and the possible emission of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) [23], the advantage that would be obtained in drying only the material of proper quality is evident. Discarding wood with inferior properties prior to drying can result in significant cost savings in the drying process.…”

Engineering properties of wood under different drying methods

“…Drying is a process of removing moisture involving a simultaneous heat and mass transfer under the influence of air temperature, velocity and relative humidity [2]. Drying ensures the attainment of equilibrium moisture content of timber, thereby enhancing shelf life, value addition, volume reduction, workability, strength and quality enhancement [3,4]. In open-air or natural drying method, which takes a lot of drying time, drying is achieved under direct exposure of woods to ambient air, ambient temperature, relative humidity and natural wind.…”