(Editor’s Note: In his regular column, Alan Froome continues his discussion of sawmill ideas and methods and how they evolved, along with a review of some recent developments in technology. This series of columns is intended to be a generic overview of different methods used in the sawmill industry in response to readers’ requests.)
Following the previous articles discussing the processes used to produce rough lumber, boards, cants, etc., we
now come to one of the final stages in
the lumber manufacturing process, namely drying. We also will introduce the terminology used in the lumber drying process.
Of course, not every company dries its lumber production. Some sell their products green and rough sawn only. As we said in part one of this series, there are "different courses for different horses." The basic nature of the industry — and what makes it so interesting – includes different and often unique methods used in different regions.
What Is Wood?
To understand how wood dries, first we need to know something about the structure of wood itself — and that water is an integral part of it.
Wood is made up of bundles of tubular cells that run vertically up the axis of the tree. These cells are held together by lignin and convey sap and nutrients from the roots to the leaves. The walls of the cells are made of cellulose, formed in a spiral fashion, which, in combination with the cell’s tubular form, gives the wood its mechanical strength.
The water content of freshly cut green wood is in two forms: water that binds itself to the cellulose in the cell walls and ‘free’ water that is contained in the cell cavities. This ‘free’ water can be removed fairly easily by drying, but the water bound to the cellulose normally cannot be removed. Wood fiber is considered ‘saturated’ when the water content is around 25%. Sapwood contains more water than heartwood and generally takes longer to dry.
From an historical standpoint, lumber drying has been going for hundreds if not thousands of years. The ancient Romans knew that drying reduced the weight of large construction timbers used for bridges and other projects while at the same time increasing strength and stiffness. Later generations of carpenters traditionally seasoned their timber by stacking it and letting it air dry for months before using it. This was done perhaps more to prevent warping and twisting after sawing — another benefit of drying.
Other benefits of high temperature kiln drying are that it kills fungi (such as causes blue stain on Ponderosa pine and other species) and insects, which often is a requirement in export markets.
As each wood species has different properties and dries at different speeds, it is best to sort lumber by species or moisture content prior to kiln drying.
Drying Methods
Since the Romans lit fires in caves to dry timbers, many drying methods have been tried, all of which can apply to hardwood and softwood species.
Some methods used today include:
• air drying (stacked outdoors)
• steam, gas or electric heated kiln, forced air
• front loading package kiln (ideal for smaller loads), forced air
• through track kiln (used for larger loads), forced air
Air drying stacked lumber outdoors can work well in drier regions, in summer at least. It is the lowest cost method but also the slowest. The moisture content can be checked from time to time using a hand-held detector.
Some hardwood mills prefer to partly air dry the lumber prior to kiln drying. For example a mill in Virginia regularly air dries oak and poplar down to 20% moisture content, then kiln dries it to the final 6% or 8%. This is done mostly to reduce energy costs.
The final moisture content varies according to species and the requirements of the end user. For example, Southern Yellow Pine typically is dried to 12%-15% moisture content while 19% often is adequate for western Douglas fir intended for general construction use.
Kiln Construction
To speed up the drying process, compared to stacking it outdoors in order to air dry, some kind of closed chamber with a heat source was devised. Modern kilns can be sized to suit any load capacity, from very small quantities of lumber to 120,000 board feet or more.
Kiln construction varies from one supplier to another. Insulation in the walls and roof is obviously important to keep the heat inside, and the construction materials can be important, too. For example, tannic acid — produced when kiln drying oak — can corrode many types of structures. To prevent corrosion, some kiln manufacturers use a special grade of aluminum for both the structure and cladding. Other species, like western hemlock and Douglas fir, also produce acids; however, the pitch they also emit while drying has the effect, to some extent, of leaving a protective coating inside the kiln, so the acid is a less critical issue.
Through-track kilns use wheeled carts on rail tracks to hold and move stacks
of lumber; they are pushed into the kiln. After drying is complete, the carts
are pushed out the far end of the building, making way for the next ‘charge’
of lumber.
To facilitate the flow of heated air around each board, the lumber is stacked in layers separated by spacers called stickers. They can be used several times and should have a constant thickness. Each stack of lumber is sized to suit forklift loading.
Forced air flow can be low speed or high speed. High speed air flow can dry the lumber faster, but a lot of care must be taken to prevent over-drying. Modern computerized kiln controls have made higher air flows possible. Either way, heated air is blown by electric fans
over the stacked lumber, usually from the entrance or loading end of the
kiln, and the resulting moisture in the form of steam is exhausted at the rear or out the roof. Another wrinkle is a
dehumidification kiln; heated air is
circulated over the lumber like a conventional kiln, but the resulting hot, moist air then passes over a cold refrigeration coil, and the moisture in the air is condensed into liquid. The more sophisticated kilns use variable speed electric fans, which adjust the airflow automatically through the course of the drying cycle. This technology costs more
but over time saves energy and improves efficiency.
Kiln heat sources vary considerably, and kiln makers go to a lot of trouble in order to achieve even heating inside the kiln. Many large softwood mills burn their bark and wood waste in co-generation furnaces, which produce steam and electricity. The steam is piped into an array of tubes or coils located along the kiln walls and ceiling to provide heat. One kiln maker has even devised a swing-out bank of center coils; when in place, the coils are located in the center of the chamber, between the stacks of lumber. Other mills burn natural gas or propane as available in their area, and the heated air is blown directly through the kiln. Dehumidification kilns use electric heat and a heat pump system. In addition, heat exchanger systems are available to recover heat from the exhaust air and recycle it; some kiln suppliers say these systems can reduce heating costs by 15%.
Control of the entire drying cycle or ‘schedule’ is best done by computer. All the well-known kiln manufacturers have developed software to control the drying cycle, with full color graphics on a monitor to clearly illustrate what is going on inside the kiln. A basic desktop computer running a Windows program controls the heat gradient, air flow, roof vent opening and other functions over the seven days or whatever length of time is required to achieve the desired moisture content. Sensors at various locations inside the kiln monitor the temperature, moisture and other factors, relaying the data to the computer on a constant basis. The kiln operator can monitor the progress on the graphic display in the control room or from a remote location (even at home).
Microwave technology has also been tried to dry lumber. This has only been used so far on a test basis for relatively small quantities of lumber, however if natural gas and propane prices continue to rise, we may hear more about this in future.
Lumber Degrade
Lumber degrade occurs when a board or other lumber product does not meet the grade or quality standard required, and it must be reduced to a lower grade. Of course, this means a lower value as well. Degrade losses can occur at every stage in the sawmill process, including drying, whichever of the above methods is used.
The amount of degrade loss in the drying process is the measure of its efficiency, and it tends to be more critical with the wider widths of lumber, so more care needs to be taken with them. For example, tests have shown that the amount of degrade loss from drying 2×12 Douglas fir increases sharply if the moisture content is reduced below 16% (which is why it is often left at 19%). The same tests showed that 2×4 Douglas fir dried to 13% had the same degrade losses as 2×12 at 16%.
To reduce degrade losses, many mills have found it more efficient to do a moisture test on each stack of lumber. Then the lumber is presorted so the kiln charge is comprised of lumber with more or less the same moisture content. The result is that the charge dries more evenly with less degrade.
