AIR SEASONING
270
DAA
Architectural Association Hooke Park Beaminster Dorset DT8 3PH
KILN SEASONING
DAA
Timber Shelter Research Program Design & Make
PASSIVE AIR SEASONING SHELTERS
Architectural Association Hooke Park Beaminster Dorset DT8 3PH
Timber Shelter Research Program Design & Make
KILN SEASONING Kiln drying is effected in a closed chamber, providing maximum control of air circulation, humidity and temperature. In consequence, drying can be regulated so that shrinkage occurs with the minimum of degrade, and lower moisture contents can be reached than are possible with air seasoning. Some of the advantages of kiln-drying to be secured over air-drying in addition to reducing the shipping weight and lessening quantity of stock are the following:
Figure 6: In 1771 the Commissioners of the Admiralty Board had visited Chatham and were shocked to see howships constructed from poorly seasoned timber had rotted. Thereafter plans were made for the construction of timber seasoning sheds to a standard design at the various Royal Dockyards. They could hold up to 3 years supply of timber at each yard. There are two rows of these at Chatham consisting of a total of 75 bays.
1. Less material lost 2. Speed: quicker method than air seasoning 3. Consistency of output 4. Dependable supply at any time of year 5. Prevention of sap stain and mould and fixation of gums and resins 6. Reduction of hygroscopicity KEY KILN DESIGN REQUIREMENTS There are many different types of kilns being used around the world with no clear leading design solution, however all kilns require the following: -Control of humidity at all times. -Ample air circulation at all points. -Uniform and proper temperatures.
Figure 6 Figure 7: Air Seasoning in Scandinavia. Stacks piled high are roofed. Transport to seasoning yard is by boat. Source: Trada slides
TYPES OF MECHANICAL KILNS There are two main methods used in artificial seasoning of timber. Both methods rely on a controlled environment to dry out timber. They are: - compartamental kilns - progressive kilns Figure 11: Industrial scale progressive kiln. Material moves through a series of controlled sub-chambers . Figure 7
Air Seasoning - Wood can be properly air seasoned without the use of mechanized kilns down to 20% moisture content (MC) depending on the species and the local environment. MC 20 is adequate for firewood however timber used for construction requires a lower moisture content, as low as 6-8% for indoor jointery and furniture. The most effective seasoning is without doubt that obtained by the uniform, slow drying which takes place in properly constructed piles outdoors, under exposure to the winds and the sun and under cover from the rain and snow. What is suggested for Hooke Park is an air season, kiln finish strategy.
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Figure 13: Example of a compartment kiln. Compartment kilns are only a viable option on an industrical scale and therefore inappropriate for Hooke Parks’ seasoned timber demand. The basic concept is that the timber moves through a series of kilns programmed for each specific step of the process. The usual flow goes from a very humid chamber to a dry chamber, allowing for a continuous flow or output of seasoned wood. The apartment method can be arranged so that it will not require any more kiln space or
Figure 8: Air Seasoning in the round under covered shelter. Figure9 9 &10: Invisible Studios designed timber seasoning shelter for firewood. Thin battens are used horizontally to draw air across the shelter.
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Figure 12: Compartment kiln. Material is placed in the chamber which is then sealed for a number of days, the length of the entire seasoning process.
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