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Benefits of Kiln Dried Timber production compared to Air Seasoning Timber

 

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

• Kilns dry timber faster than air seasoning does - so “Rush” orders can be met, and regular supplies can be ensured regardless of the season of the year.

It is often the case that timber is first air seasoned (down to the minimum that can be obtained in this way) and then, if required kiln dried:-

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!

• Another benefit of kiln drying timber is that 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.

But with regard to kiln drying of timber:

• Desirable (or undesirable) colour changes may occur using kilns. However it has been claimed steaming improves the colour of e.g. beech (as well as the other benefits of kiln drying timber such as sterilizing, reducing drying degrade, giving easier machining and better stability).

To realise the full benefits of kiln drying timber it is vital that the kilns are carefully controlled otherwise expensive financial losses can be incurred and the timber might be rendered valueless.

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.
A stack of timber being moved into a kiln.

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.

Find out How Kilns are Used to Dry Timber and how to avoid common, costly drying defects such as Case Hardening and Splits and Warping

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Contents

Reasons to Dry Timber: An Introduction to Timber Drying

 

Timber Drying - Fundamentals Concepts and Definitions

 

Factors controlling the Drying of Wood

 

The Structures of Softwoods and Hardwoods and their effect on Wood Drying

 

An Introduction to the Air Seasoning of Timber

 

Layout of a Timber Drying Yard

 

Design of Stacks in the Timber Drying Yard

 

Kiln Drying of Timber

 

Types of Kiln Drying Equipment

 

Benefits of Kiln Dried Timber production compared to Air Seasoning Timber

 

Using a Dehumidifier to Dry Wood

 

High Temperature Timber Drying

 

Solar Kilns for Drying Timber

 

Drying Defects in Sawn Timber

 

Case Hardening of Timber

 

Avoiding Case Hardening by Monitoring the Drying of Timber. Also Collapse & Staining of Timber