Drying timber schedules

The LCA use in drying timber is a relatively new area and no study is available in the literature. In this section, the use of LCA has been demonstrated using an example of radiata pine drying. The drying schedules, drying time, and energy footprint data have been used from a published study (Ananias et al., 2012). These data have further been judged, analyzed, and validated independently for use in this LCA study. 

According to Bannister et al. (1999), timbers that are physically slow to dry, tend to warp, split, or collapse during drying. These timbers are classified under hard-to-dry woods. Many of these timbers have high commercial value and so there is a need to establish a drying schedule that is able to produce consistent high quality results. Examples of hard-to-dry timbers in Australia include Red Beech (Notofa-gus fused). Hard Beech (Notofagus truncatd), and many Eucalyptus species (Bannister et al., 1999). 

Consider the surface temperature of lumber during the course of a kiln schedule in which the dry-bulb and wet-bulb temperatures, and so the wet-bulb depression, are held constant. The initial rate of evaporation is independent of the air temperature (dry-bulb temperature) but is proportional to the wet-bulb depression. This is because evaporation is sustained by the rate of heat transfer, and the rate of heat transfer from the warm air to the moist wood surface is proportional to the temperature difference between the air and the wood surface which is at the wet-bulb temperature. Thus, provided die wet-bulb depression is the same, say AT = 5°C, the rate of evaporation from a wet timber surface is essentially the same whether the kiln air temperature is 40, 70 or 100°C. 

Even with a permeable wood diffusion assumes increasing importance as the average moisture content approaches the irreducible moisture content indeed, in every part of the board where the moisture eontent approaehes this value drying is diffusion controlled. Permeable and impermeable timbers of similar densities should dry from fibre saturation at about the same rate. The behaviour of mixed heart/sapwood boards is eomplieated sinee, at first, there is both an evaporative interface near the sapwood surfaee and one in the interior at the zonal boundary between heart and sapwood. For a board with only a slither of heartwood along one face, mass flow can only move to the sapwood faee so in effeet the board appears to be twice the width than it aetually is. Pang et al. (1994) predieted that such a 50 mm thick board would dry from green to 6% moisture eontent using a 140°C/90°C schedule in 14 hours, compared to 10 hours for sapwood and 11 hours for heartwood. 

Pines such as P. radiata are permeable and ean be dried quiekly (Table 8.4a). However their pits aspirate and if subsequently pressure impregnated with an aqueous preservative such as copper-chrome-arsenate using the full eell proeess they eannot be redried nearly as rapidly. If sueh resaturated, preservative treated timber were to be dried using the schedule in Table 8.4a steep moisture gradients would develop leading to severe checking. A milder schedule is required. Treated pine takes approximately twiee as long to dry. 

Traditional schedules result in a drying rate that decreases with time, only partly countered by increases in the dry-bulb temperature and the wet-bulb depression as the schedule proceeds. Modem automatic process control means that the kiln schedule can be adjusted continuously so ensuring a more constant rate of heat transfer and evaporation. Also this avoids any shock that an abrapt change in the schedule imposes on the timber. 

Some success has been reported with hardwoods, but only for those species that are not particularly difficult to dry using conventional schedules. Overall drying time can be halved but at the expense of additional degrade such as honeycombing and collapse. Some recalcitrant timbers may be dried successfully but only if they have been pre-dried to the fibre saturation point. Other hardwoods cannot be high-temperature dried at all. 

Species sueh as oak and beech check quite readily, and to avoid this problem the humidity is kept high early in the kiln schedule. Also the temperature is kept low in order to maintain the timber s strength. Only as the lumber dries and becomes stronger can the humidity be lowered and the temperature raised to provide more rapid drying eonditions. Surfaee ehecks forming early in the kiln schedule may close up later when the surfaee fibres go into compression and the core into tension, although the failure plane remains in the tissue. Such checks can be revealed subsequently as hairline streaks if the wood is stained. 

Journal of the American Chemical Society, 40 1361-403 Langrish TAG, Keey RB, Kho PCS and Walker JCF (1993) Time-dependent flow in arrays of timber boards flow visuahsation, mass-transfer coefficients and numerical simulation. Chemical Engineering Science, 48 12) 2211-23 Langrish TAG, Brooke AS, Davis CL, Musch HL and Barton GW (1997) An improved drying schedule for Australian ironbark timber optimisation and experimental validation. Drying Technology, 25(1) 47-70... 

Whenever it is important to avoid significant color development in the wood on drying, low kiln tanperatures have to be used. Even though in New Zealand where accelerated conventional schedules (with dry/wet-bulb tanperatures of 90°C/60°C) are used to dry appearance-grade timbers, commercially, kiln temperatures not greater than 50°C are employed to get very pale wood (Keey, 2003). 

Langrish, T.A.G., Booker, A.S., Davis, C.L., Muson, H.E., and Barton, G.W., 1997. An improved drying schedule for Australian ironbark timber Optimisation and experimental verification. Drying Technol., 15 47-70. 

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