An introduction to wood drying terminology and processes

The drying process is one of the most time- and energy- consuming processes in all of wood processing. Many structural properties of wood limit the ability for water to move throughout the material. Stress build up and the possibility of drying defects also retards the maximum drying rate, as rapid drying increases these defects substantially. The variability of wood properties between species and even within species further complicates the drying process. The optimal drying rate and temperatures are very specific and complex, and in some ways even an art. However, there have been many studies conducted by many agencies including the USDA that have helped quantify this art so that wood drying can be done well by anyone with the proper equipment. In order to understand the process of wood drying, it is first crucial to grasp at least basic knowledge of the structural features of wood, wood-moisture relations, how wood dries, shrinkage, and drying stresses.Wood is classified into two general categories: hardwood and softwood. The first includes maple, oak, and birch and has broad leaves while the latter includes woods such as pine and spruce with needle-like leaves. The categories or classes do not necessarily correspond to actual hardness properties of the wood, but are used primarily for classification of species. Regardless of class, all wood is made up of a few different layers and types of growth. Starting from the outside of the tree outer bark is composed of dead and dry tissue and purposed primarily for protection. Inside the outer bark is inner bark made up of living tissue and inside of the inner bark is a cambium layer which forms bark. Inside of the cambium layer both sapwood and heartwood are contained. These two parts of the tree are used for lumber and each have their own properties. Sapwood is usually higher in moisture content than heartwood, but at the same time is more permeable to water movement.In the drying process, it is important to know the moisture content of the wood in order that drying can be carried out with the minimal probability of defects. This relation is called the moisture content and can be calculated on the following formula:

Employing this formula allows the operator to accurately prescribe a drying plan based on other charts found in the literature. Water is held in wood as either bound water contained in cells or free water. The first must diffuse out of the cells before it can evaporate while the other can move out of the wood much more quickly. An important definition is the fiber saturation point at which is the moisture content level at which water is bound in the fibers, but no free water remains. The equilibrium moisture point is the point at which the relative humidity in the wood is the same as the ambient humidity; in other words, the partial pressure of the wood and atmosphere are equal.Now that jargon has been defined and explained, the process of wood drying can now be explained. Wood dries from the outside in. Capilarry action causes free water to move from the cell walls to places of lower moisture content. Differences in partial pressures cause water to diffuse through cells from areas of high partial pressure to lower partial pressure. Water evaporates from the cell walls to the outside air. Differences in moisture content cause bound water to move cell to cell until it reaches the surface of the wood where it is evaporated into the atmosphere. These three processes happen continuously until the wood is completely dry.In general, as wood dries, it shrinks in different ways based on the geometry of the cut. In details too lengthy to discuss in this paper, it can be estimated how much a board will shrink after drying. This shrinking causes drying stresses to develop and thus must be monitored during the drying process. In general, too rapid of drying causes these stresses to increase to critical levels at which cracks form, and too slow of drying can lead to mold formation. As discussed previously, many charts have been developed for which drying times and temperatures can be referenced with relative accuracy for many types of wood. These are what sawmill operators use when selecting a plan. They are found in USDA publications.

Dereks Documents:

Chapter 1 Properties of Wood Related to DryingUSDA Agricultural Handbook AH-188: Dry Kiln Operators ManualSimpson2001http://www.fpl.fs.fed.us/products/publications/specific_pub.php?posting_id=17263&header_id=p

This source is a handbook for kiln drying published by the USDA Forest Products Laboratory. It is a chapter from a comprehensive manual on wood drying. The first chapter focuses on general principles of wood drying and introduces important equations. These principles include types of wood, structural features of wood, wood-moisture relations, how wood dries physically, and wood shrinkage as well as other concepts. It is a great place to start learning jargon in forestry and wood drying.

USDA Forest Products Laboratory General Technical Report FPL-GTR-121William T. Simpson, C.A. Hart2000http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr121.pdf

Simpson and Hart have published a large database of estimated drying times for an extensive list of lumber types and moisture contents. Though, this data was acquired through previously published data and computer simulations, the authors suggest it could be used as a starting point for drying time estimates. They caution the reader that variability can occur based on location of drying and type. Data published is exclusive for air drying, though it could serve as a benchmark for comparison for our acquired data later on.

Dry Kiln Schedules for Commercial Woods Temperate and TropicalFPL-GTR-57R. Sidney Boonehttp://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr57.pdf

A schedule for dry-kiln drying for over 500 types of wood. This could be referenced when choosing drying temperatures and times for different wood types. It also includes corrections for different cut directions. Correction on drying time formulas are also included for wood chemical treatments. This, along with most other wood drying process sources is published by the USDA Forest Products Lab. Therefore, it is very reliable.