Stacking, timber

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... 

If the timber is prone to degrade due to too rapid drying the thickness of the stickers can be redueed, to say 15 mm, to reduce the airflow through the stack. Stacks can be moved eloser together and stored at the leeward side of the yard. To further inhibit airflow, staeks ean be eovered with horticultural shade-cloth (50% open mesh), but the boards will need to have had an anti-sapstain treatment. 

Moist warm air from the stacks is drawn through the evaporator coils and is cooled until the absolute humidity of the air corresponds to the absolute humidity at saturation (Table 8.1), the dew point. On further cooling moisture condenses on the cold evaporator coils. This water is collected and drained from the kiln. The cold air then passes through the warm condenser coils where it is reheated. The dry warm air then passes through the timber stacks again. 

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. 

Cupping. Flat-sawn boards cup (Figure 8.14). The outcome of cupping is such that the growth rings straighten out a little. Only boards at the top of timber stacks can cup in this way as the others are held down flat by the weight of the timber above - and may crack instead. 

Figure 9.6. (a) Timber about to be immersed in a boron dip tank. Concrete drip storage area to the right, (b) Block stacked and covered timber is held for 4-8 weeks to allow salts to diffuse into the core. 

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. 

Nijdam 11 and Keey RB (2002) New timber-kiln designs for promoting uniform airflows within a timber stack. Chemical Engineering Research and Design, 80 1) 739-44 Nishiyama Y, Chanzy H and Langan P (2002) Crystal structure and hydrogen-bonding system in cellulose lb from synchrotron x-ray and neutron fiber diffraction. Journal of the... 

Traditionally, the variation of evaporative rates across the stack has been counteracted by the installation of bidirectional fans and by periodically reversing the airflow direction through the stack. This policy has minimal effect on the drying rates in the center of the stack, but reduces the variation in behavior between the two end zones. If only moisture content variations are considered, many reversals are not needed to achieve this equalization (Pang et al., 1995 Nijdam and Keey, 1996 Wagner et al., 1996). However, if stress development in the surface layer with the likelihood of checking is taken into account, then the flow reversals for a timber such as Pinus sylvestris should be less than 2 h apart (Salin and Ohman, 1998). A period of 4 h is a common industrial practice for permeable softwoods such as P. radiata. 

Riley and Holmes, 2001). The first prototype tested used a waveguide, 40 x 20 x 1.2 mm, with angled slots in the long face, connected by a high-temperature coaxial cable to the oscillator and placed in a stack so that the slots were in contact with the lower face of a board. A later development enabled the possibility of the stack s moisture distribution to be determined. The experiments have demonstrated the potential for the manufacture of a rugged industrial system. However, unless the timber is very uniform in properties, this technique is uncertain as the relationship between the microwave signal and the moisture content becomes fuzzy because of factors such as variations in wood density among the boards. 

Nijdam, J.J. and Keey, R.B., 2000. The influence of kiln geometry on flow maldistribution across timber stacks in kilns. Drying Technol., 18 1865-1877. 

Salin, J.-G., 2001. Global modelling of Idln drying Taking local variations in the timber stack into consideration, in Proceedings of the Seventh International lUFRO Wood Drying Conference, Tsukuba, Japan, pp. 34—39. 

Salin, J.-G. and Ohman, G., 1998. Calculation of drying behaviour in different parts of a timber stack, in Proceedings of the 11th International Drying Symposium (IDS 98), Vol. B, KaUddiki, Greece, pp. 1603-1610. 

Where materials are not designed for external environments all stacked units should be protected from the elements by waterproof and opaque coverings. The bottom layer should be placed on packings, such as clean untreated softwood timber, in order to keep the products clean and dry and prevent scratching to finishes. 

Timber storage points, stack of harvesting residues, preparation for transport by trailer lorries Pictures RaimoAlen... 

Load displacement occurs because recognised methods of load restraint are not used. The full height of any load should be supported by a load rest (Figure 27.6). Careful use of backward tilt will prevent the load falling forward. Bonded stacking and load ties of paper, cardboard, thin timber... 

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