Moisture content timber drying

Langrish, The Significance of the Gaps between Boards in Determining the Moisture Content Profiles in the Drying of Hardwood Timber, Drying Technol 17(R. 7-8) 1481-1494 (1999). 

The maximum moisture content for a wood can be estimated easily. In a timber having a basic density of 300 kg m the oven-dry cell tissue (density 1500 kg m )... 

If extractives were located in the lumens rather than in the cell walls this timber would shrink by 15% on oven-drying. However, the expected shrinkage for extractive-free wood of this density (530 kg m ) would be 15.9%. The estimated shrinkage on drying to 15% moisture content is 94% (7.5/7.95) of that predicted (7.95%) and the estimated fibre saturation point would be 28%. 

Lumber is not used in the oven-dry state. For many uses the desired in-service moisture eontent falls within the range of 8-15% moisture content. The shrinkage will then be only a half to three-quarters of the oven-dry shrinkage value. Therefore the timber trade is more interested in shrinkage values from the green to 12% moisture content (Table 4.1). These values indicate to the sawmiller the green... 

If wood is conditioned at 20°C and 65% relative humidity the sapwood of most timbers will equilibrate to about 12% moisture content. This corresponds to the typical moisture content of lumber in an unheated building in many temperate regions. There are exceptions. Nothofagus fusca, one of the Southern Beeches, would dry to 9.5% or so under these conditions, so drying to 12% would be inadequate (Harris, 1961). Medium density fibreboard and particleboard also equilibrate at a lower moisture content of about 8-9%. For this reason Hoadley (1979) observed that it is more logical that wood products be dried to a specified relative humidity than to a specified moisture content (Table 4.1). 

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. 

Author Species, size of timber Moisture content (%) Initial Final Relative drying times microwave/ conventional... 

If lumber were taken out of the kiln immediately there would be a risk that the hot wood will heat the cool air around the stack, making the air warmer and much drier. The warm dry air would then lead to further drying and checking at the surface of the boards - eool saturated air at 20°C has a relative humidity of only 12% if heated to 60°C and the moisture content of the wood in equilibrium with that air would be only 2%. For the better grades of timber the heat is turned off and the load cooled under a constant wet-bulb depression of about 5°C until the temperature is within 15-20°C of that outside. Only then can the stacks be removed safely from the kiln. 

Some time ago Kamke and Vanek (1994) compared the performance of a number of within-the-timber drying models, representing mainly diffusion-like and multiple-transport mechanism approaches, for predicting average moisture contents and moisture-content profiles. Four data sets were used, with three sets representing idealised problems. The fourth data set was the experimental results of drying 40 mm boards of Norway spruce, Picea abies, from initial moisture contents of 29-66% at a dry-bulb temperature of 60°C, wet-bulb depressions of 8-25°C, and an air velocity of 6 m s. The required inputs for the models, including physical properties... 

Wood for charcoal production must first be cut, split and dried because wet wood has a low heat value. In the moist tropics it is difficult to hold timber in stack for more than a couple of months without noticeable deterioration, but even in that time the moisture content can drop from 60% towards 30%. Stock holding also ensures continuity of supply. 

CSIRO, Division of Forest Products Technological Paper No. 37 Nijdam 11 (1998) Reducing moisture-content variations in kiln-dried timber. PhD thesis. University of Canterbury, Christchurch, New Zealand Nijdam 11 and Keey RB (1996) Influence of local variations of air velocity and flow direction reversals on the drying of stacked timber boards in a kiln. Transactions of the Institution of Chemical Engineers, 74A 882-92... 

In solving the diffusion equation for moisture variations in wood, some authors have assumed that the diffusion coefficient depends strongly on moisture content [5-9] while others have taken the diffusion coefficient as constant [10-14], It has been reported [15-19] that the diffusion coefficient is influenced by the drying temperature, density and moisture content of timber. The diffusion coefficient of water in cellophane and wood substance was shown by... 

Moisture Content. In a specific circumstance (a treatment problem of a group of oak timbers), the moisture content and the amount of internal sound wood core were used instead of chemical analyses to assess degradation (6). The wood condition has been classified as follows Class I, the most deteriorated wood, contains over 400% water ([weight of water] / [oven-dry weight of wood] X 100) and virtually no core Class II is 185-400% water, with a core present and Class III is less than 185% water, with only the surface degraded. The analyses included moisture contents and the pin test to determine the extent of sound wood. 

Savory (82) reported that soft-rot attack could occur at quite low moisture contents, as well as under waterlogged conditions. He stated that the mi-croftingi can attack wood which is too wet or too dry for Basidiomycete decay. He also reported severe soft rot in beech strips exposed at a constant relative humidity of 90%, corresponding to an equilibrium moisture content of 20-21%. Apparently it is a misconception that soft rot only occurs in very wet wood. Kirk and Cowling (9) state, for example, that only water-soaked timber is attacked by soft rot. In fact, soft rot may occur over a wide range of moisture conditions, from quite dry to fully waterlogged wood. 

Figure 2 Typical strength and wood failure increase as a function of time of a pure PRF honeymoon adhesive system on timber at 12% and 22% moisture content [26-29]. Note that at the unusually high moisture content of 22% the requirements of the standards are passed in less than 24h as the timber starts to dry [26-29]. Test on beech strips according to British Standard BS 1204. 

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