Progressive Kiln

The advantage of this system, although much larger, has a continuous flow of seasoned timber coming off line.

A number of commercial processes for seasoning of timber are available, the most common of which is kiln-drying. Kiln seasoning accelerates the process of seasoning by using external energy to drive the moisture out. The timber is stacked in much the same way as it is for air drying, and is placed inside a chamber in which the conditions can be varied to give best seasoning results. Air is circulated around the charge (stacked timber) and the temperature and humidity can be varied to give optimum drying. Each species has different cell characteristics and therefore requires different drying schedules. Typically the timber may be in the kiln for a period of between two days to one week.

Generally, it is not feasible to kiln-dry structural timber in thicknesses greater than 45 mm, although there are limited amounts of 70 mm thick kiln-dried softwood members in the market place. All untreated structural pine and some commercial hardwoods are seasoned, mostly using kilns that are often heated by sawmill by-products or gas.

Plenty of research is continually being done to determine kiln drying schedules for different species. An optimum schedule will remove the moisture as quickly as possible, with the minimum quantity of introduced energy, and giving minimum damage to the dried timber.
After kiln seasoning, there is often some damage to cells near the surface of the wood. (All of the moisture passes through those cells.) They have in fact collapsed, but can easily be ‘pumped back up’ in a reconditioning chamber. This chamber introduces steam for a period and puts some moisture back into the outer cells and removes the effect of seasoning collapse.

Hard woods have closed cell structure, so take more time in being seasoned and dried.

Conventional kiln-drying for hardwoods uses temperatures well below the boiling point of water. This avoids damage to the wood fibres which, if the seasoning is not properly controlled, can experience loss of strength. In seasoning hardwoods, the process is slow and can be economically undertaken by first of all air-drying or partially seasoned for a period which can be anywhere from 3 to 9 months, depending upon the thickness of material, the ambient temperature and humidity and the species. The partial seasoning process can be accelerated for some hardwoods by using fan-forced pre-drying in a controlled (but not kiln) environment, to reduce the lead time for kiln-drying to 20 to 30 days.

Once the sawn hardwood material reaches fibre saturation point or slightly below (at a moisture content of about 20% to 25%), it is then placed in kilns usually for up to 10 - 14 days (depending upon the thickness of the sawn timber) in order to bring the moisture content down to between 10% and 15%. This drying process must be strictly controlled and monitored in order to avoid drying degrade.

The amount and duration of air, heat and humidity again depends on species, size, quantity, etc.