Forest Products Notes on...

Advantages and Disadvantages of Air Seasoning of Timber Compared to Kiln Drying

• Kilns can dry to lower moisture contents than can air seasoning – in fact we must use kilns in countries having cool, moist weather conditions (such as U.K.) for most indoor (heated) applications. In the UK the lowest moisture content achievable by air seasoning alone is about 18%.

• Kilns are faster. “Rush” orders can be met. Regular supplies can be ensured regardless of the season of the year.

Air Seasoning:

(green to 20%)

Oak       240 – 360 days

Beech   150 – 200 days

Spruces  45 – 90 days     

Then Kiln Seasoning:

(20-6%)

Oak         6-16 days

Beech     5-8 days

Spruces  3-5 days

Compared to only Kiln Drying

(green to 6%):

Oak        20-30 days

Beech    12-15 days

Spruces   4-7 days

Table comparing the time taken (in days) to dry 1 inch (2.54 cm) stock. Note, it is usual to air season to 20% and then kiln dry down to 6%.

• Air seasoning is at the mercy of the elements. Correctly controlled the drying conditions produced and maintained in a kiln reduces losses of timber due to degradation (i.e. drying “degrade” (defects) is reduced). BUT mistakes can be made very rapidly when kilning and can be very expensive!

• Kilns can provide sterilization of insect or fungal infected timber. The more rapid drying also reduces the propensity to bio-degrade

• The temperature used in kilns sets or hardens gums and resins which might otherwise cause processing or finishing problems.

However:

• Kilns are very expensive (high initial investment). They also have high energy costs. Consequently they must be fully utilized – running 24 hours a day, 7 days a week.

However, air seasoning also has it’s costs:

• Capital is tied up in the necessary large stocks of slowly drying timber.

• The large stocks held must be insured.

• The large yard will incur a purchase or rental cost.

• A lot of expensive stickers are required.

Also:

• Desirable (or undesirable) colour changes may occur using kilns. It has been claimed steaming improves the colour of e.g. beech (as well as sterilising, reducing degrade, giving easier machining and better stability).

Systems used to Control Timber Drying Kilns

Entirely manual. Here an operator periodically checks the moisture content of the wood and changes to the next schedule stage as necessary. A good, reliable operator is vital!

Semi-automatic. Here the change in drying environment is made at pre-set times (there is no automatic measurement of the moisture content of the wood).

Automatically self correcting systems. Here the moisture content of the wood is measured electrically and the equipment automatically selects the next stage when required.

There is increasing computerisation in industry, and (fortunately) increasing importance being given to quality control.

A line of modern timber drying kilns: warm, wet air is being exhausted from the kilns

A stack of timber being moved into a kiln. Note that the stack fits tightly into the kiln to avoid the possibility of circulating air “short circuiting”. Also note the moveable trackway by which means the stacks of timber can be moved from a storage area into the kilns.

The NEXT PAGE of these notes describes a rather novel, “low tech”, but potentially very efficient way of drying timber by Using a Dehumidifier to Dry Wood...

Learn about Kiln Schedules and the basics of Kiln Drying of Timber ...