Wood working25.01.2016

Drying sawn timber in chamber and progressive drying kilns

In Koskisen’s timber yard, imposing piles of freshly cut sawn timber packages sit in the green timber yard awaiting transfer to stick-stacking and drying. In stick-stacking, the bundles are re-stacked in a manner that ensures successful drying.

In the green timber yard, sawn timber bundles await stick-stacking.

In the green timber yard, sawn timber bundles await stick-stacking.

Sawn timber is artificially dried in controlled conditions in the chamber or progressive drying kilns of modern and efficient sawmills.

Stickers ensure successful drying

In stick-stacking, the solid sawn timber bundles are mechanically stacked into piles, one row at a time, with a sticker, i.e. a strip of wood, placed in between each row to ensure good air flow between the drying boards.

The stick-stacked drying loads,

The stick-stacked drying loads are sturdy, like sugarloaves.

“The stick-stacked loads must be sturdy enough to withstand being moved and dried – like sugarloaves. In addition, the right number of stickers must be placed in between each layer to ensure quality,”  Explains processing foreman Esa Hiltunen.

"Two people work in shifts in the stick-stacking plant. The work demands a keen eye and continuous line monitoring to ensure no disturbances occur.”

Tne major condition for successful stick-stacking is well-executed production planning. It can be compared to cleaning: when done well, no one notices it, but when done poorly, everyone sees it. At Koskisen, the matter doesn’t come up much, which means it is done well.

Shipping-dry timber

In drying, surplus moisture is removed from the sawn timber using heat and steam. Freshly cut sawn timber contains some 60 to 80 per cent moisture, which drops to about 10 to 20 per cent after drying. The term ‘shipping-dry timber’ is used to refer to sawn timber with a moisture content of roughly 18 per cent. Exterior cladding boards and structural timber generally contain 16 per cent, interior cladding boards 14 per cent, and floorboards less than 10 per cent moisture.

“Water is dispersed from the timber using heat, and steam is used to prevent the timber from drying too quickly, which could cause cracks,” says Hiltunen.

“Drying times vary according to the timber’s thickness and intended use. Products that are special-dried for applications according to customer specifications require the longest drying times.

Keep an eye in the drying process

Esa Hiltunen and Jyri Tervonen keep an eye on the drying process.

The drying process is guided by so-called drying parameters, which are used to adjust the moisture and temperature of the drying kiln as the process proceeds. The drying parameters are adjusted according to both the type of product and the time of year.

“In the timber yard, the drying kilns run non-stop, with a staff of five working in 3/7 shifts,” explains shift supervisor Jyri Tervonen.

“The work involves monitoring, adjusting the drying parameters and quality control. Our own power plant provides the necessary heat and steam for the drying process,” says Tervonen.

Chamber or progressive drying kilns?

Koskisen’s timber yard has 26 chamber drying kilns and 5 progressive drying kilns; upon completion of the fifth and latest progressive drying kiln investment in March 2016, Koskisen’s drying capacity will exceed 350,000 m3 per year.

In chamber kilns, where timber is loaded in one go, the drying conditions are highly controlled; the timber is fed into and out of the same door and the door is not opened while the drying takes place. The chamber kiln can accommodate roughly one lorry load of sawn timber, and the drying time varies from 2 to 10 days.

“Timber thicker than 50 mm, U/S grade pine and square-edged pine sideboards are always dried gently in chamber kilns to ensure high quality,” stresses Hiltunen.

“It is also possible to dry batches for specific customers in chambers,” he adds.

In the efficient progressive drying kiln, sawn timber moves along conveyor chains through the drying channel, going in one end and out the other. Roughly 6–7 lorry loads of timber can fit in the kiln at the same time, with the drying time ranging from two to five days, depending on the grade.

Check the moisture of dried sawn timber

Jyri Tervonen checks the moisture of dried sawn timber using a hand-held moisture meter.

The moisture levels of the sawn timber are monitored during the drying process and, after drying, the final moisture is measured using a hand-held moisture meter. After the drying process, the moisture level of every piece of sawn timber is checked using the meters on the line of the trimming and grading plant, and if necessary, the pieces containing too much moisture are separated and dried to the correct moisture class.