Lumber drying mills are the industry that turns trees into manufactured wood products. Throughout its transformation process, wood’s inherent moisture content (MC) fluctuates according to the relative humidity (RH) and temperature of the surrounding air.
Wood manufacturing transformation process:
- Head Rig: the primary saw cuts the tree into sawn pieces.
- Edging: removing irregular edges and defects from sawn pieces.
- Trimming: the trimmer squares off the ends of lumber into uniform pieces based on market dimensions.
- Rough Lumber Sorting: pieces are segregated based on dimension and final product production: unseasoned (known as green), or dry.
- Stickering: lumber destined for dry production is stacked with spacers (known as stickers) that allow air to circulate within the stack. (Green product skips this stage).
- Drying: lumber is kiln-dried to facilitate natural moisture content evaporation.
- Planing: smoothing the surface of each lumber piece and making its width and thickness uniform.
- Grading: the process of assessing the characteristics of each lumber piece in order to assign its “grade” (quality).
The success of wood manufacturing rests on the mill’s ability to retain the wood’s quality throughout the manufacturing process. Wood continually loses or gains moisture until the amount it contains is in balance with the surrounding environment. The amount of moisture at that point is called the equilibrium moisture content (EMC) and this depends mainly on the RH and temperature of the surrounding air. Kiln drying usually requires controlling the EMC conditions of the wood by monitoring and controlling the RH and temperature in the kilns. A mill’s primary job is to stabilize and maintain optimum drying conditions for the wood during the kiln drying process.
MC technology is a vital component of modern lumber drying manufacturing. Technicians monitor moisture management systems. In fact, there are numerous electronic manufacturers that produce a range of lumber MC measurement systems for large lumber drying manufacturing operations.
While maintaining and controlling the RH and temperature levels in the kilns, lumber producers continuously measure and monitor the changes in the wood’s MC to eliminate the over-drying or under-drying of the wood. These MC measurements can apply to a whole stack of wood in the kiln, or to an individual spot-checked wood piece. From the moment the log enters into production to its shipment, lumber manufacturers depend upon integrated MC measurement and management systems to achieve quality production on every piece of lumber manufactured by the mill.
The lumber drying process is the origin requiring mills to depend upon proper wood moisture management procedures. However, many consumers and builders mistakenly believe that wood MC concerns end when the wood is shipped out of the mill. In fact, the MC of wood must be constantly measured and controlled as wood will continually lose and gain moisture until it is in balance with the surrounding atmospheric conditions. The conclusion of our series, Wood Production, takes MC control to the installation site.