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Drying in a matter of hours

Compression kiln technology from TeknoComp could eliminate the need for air seasoning timber ­altogether in a number of applications, and bring drying time down to a matter of hours in most cases. The method is particularly well-suited to timber such as eucalyptus, oak, and beech, which are not easy to kiln dry normally.
TeknoComp's COWOPRESS® compression kiln technology is suited to both large- and small-section timber, and guarantees much faster drying times and wood with considerably less defects.

TeknoComp's patent-pending COWOPRESS® compression kiln technology combines very short drying times with high-quality output. These are both areas where conventional methods often leave a lot to be desired, as not only are their drying times long, but cracking, twisting, and warping are also problems.

The speed of TeknoComp's COWOPRESS® kilns is based on a three-stage, computer-controlled, high-temperature process, in which the temperature in the drier can rise close to 150 °C at times. Drying times depend on the type and thickness of wood being processed, and range from a few hours to a couple of days. This compares very favourably to conventional compartment kilns, which require between two weeks and a month to achieve the same moisture content.

50-mm pine board used in furniture-making, for example, can be dried to less than 8% moisture content in 24 hours, while a conventional kiln would need around two weeks to do the same job. Throughput times for 22-mm pine board (8-10% moisture content) are less than 12 hours.

The process

Timber to be dried for 24 hours, for example, stays in each chamber for eight hours. During the first stage, the wood is subjected to mechanical compression to remove its free water content. Preheating is also initiated, using waste heat from the following stages.

The wood remains compressed during the second stage of the process, when the temperature is increased to as much as 150 °C. This vaporises the moisture in the timber rapidly, but in a controlled way, as the wood is held firm, preventing it from cracking or twisting.

The drying process is computer-controlled to ensure a maximally even result, and when the required moisture content has been achieved, the timber is moved to the third and final chamber.

Drying continues here as the wood cools, thanks to the thermal energy that has been transferred to the wood itself and the compression press, and the wood gradually reaches its final moisture content level.

When the next batch emerges from the main drying chamber, the wood in the third stage is discharged as ready-to-go product.

Dealing with the difficult ones

In addition to its speed and capability of producing end-product with significantly less defects, TeknoComp's COWOPRESS® process also results in wood with stronger internal structure and integrity. This prevents cracking at a later stage and also enables heart-cut timber to be dried without fracturing, which is virtually impossible using other methods.

Timber coming out of the process is of very high quality, and keeps its dimensions very well subsequently. This is particularly true in the case of wide boards, and enables machining tolerances to be much finer, thereby saving on valuable raw material costs.

The TeknoComp drying process is also ideal for timber that is difficult or virtually impossible to kiln dry using conventional techniques. Young-growth eucalyptus, for example, has been difficult to dry for construction use, and very difficult to dry to furniture-making standards up until now, which has restricted its use to being a raw material for pulping in the main.

Tests over the last couple of years with FEA Ltd. in Tasmania in Australia have confirmed that the TeknoComp process solves this problem, and the company has now ordered a compression kiln line that will enable it to enhance the added value of its output significantly. The energy efficiency of the COWOPRESS®-driven process was particularly attractive to FEA, as drying eucalyptus with the process requires virtually no external energy, thanks to energy released from the wood itself.

Benefits all the way down the line

The stable properties synonymous with compression kiln-dried timber are also a significant benefit to end-users, as wood does not absorb moisture or deform when it comes into contact with moisture in the same way as normal kiln-dried stock.

Floorboards produced from compression kiln-dried wood, for example, can be relied on to stay straight and not develop unsightly gaps as a result of uneven drying. Joinery made from compression kiln-dried timber, such as windows and doors, also retain their dimensions, and open and close year-round, despite seasonal differences in ambient humidity.

Tests have shown that a compression kiln-dried section will retain its 28-mm thickness unchanged in the equivalent of an average year of outside exposure, while a normal kiln-dried section will swell to 29.8 mm. Immersing 8% moisture-content pine board in water for 24 hours results in the moisture level of compression kiln-dried timber rising to 20%, while that of equivalent conventionally dried wood rises to 34%.

The wood on the left has been dried using a COWOPRESS®, while that on the right reflects the defects typical of conventional kiln drying.

The better integrity of compression kiln-dried wood also makes it easier to machine, and results in better surfaces that are easier to treat and absorb less lacquer or other finishes. Compression kiln-dried timber is also more resistant to fire.

Hardness and other properties can also be modified using the COWOPRESS® system. The hardness of Finnish pine, for example, can be enhanced by 10% using TeknoComp's CIUWO® method, and by 40% using the COWO® method. The hardness of spruce and birch can also be improved significantly as well.

> Tapio Kuokkanen
(Published in High Technology Finland )