Temperature, timber drying

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... 

Kho PCS, Keey RB and Walker JCF (1989) Effects of minor board irregularities and air flows on the drying rate of softwood timber boards in kilns. Proceedings 2nd lUFRO Wood Drying Symposium, Seattle, Washington, DC, 150-7 Kho PCS (1993) Mass transfer from in-line slabs application to high-temperature kiln drying of softwood timber boards. PhD thesis, University of Canterbury, Christchurch, New Zealand... 

Williams DH and Kininmonth JA (1984) High-temperature kiln drying of radiata pine sawn timber. New Zealand Forest Service, Forest Research Institute Bulletin 73 Williams LH (1990) Potential benefits of diffusible preservatives for wood protection an analysis with emphasis on building timbers. First International Conference on Wood Protection with Diffusible Preservatives. Forest Products Research Society, 29-34 Williamson TG (ed) (2002) APA Engineered Wood Handbook. McGraw-Hill, New York, NY Williston EM (1981) Small log sawmills profitable product selection, process design and operation. Miller Freeman, San Francisco... 

The inherent heterogeneity of the material leads to variations in the responses of wood to thermal modification. The rate of transfer of heat into the interior of the wood is of paramount importance in order to ensure that there is a constant temperature throughout the sample. The thermal conductivity of dry wood is low and the heating method employed must ensure that the treatment is as even as possible. Heat transfer into the interior may be improved by the use of steam-heating. Heat transfer is a very significant factor in the treatment of timber of larger dimensions. 

The Royal (or Royale) process was originally developed as a method for drying timber, in which the wood is heated in oil under vacuum. The temperatures used are low (60-90 °C) compared to other thermal oil treatments, and although sufficient to lead to some curing of the oil itself, there is no direct modification of the wood as a result of this process. The oil does not penetrate the cell wall. In this process, wet timber is placed in a treatment vessel and oil is then introduced, which is heated to the desired temperature, whilst a vacuum is applied. Water is removed from the timber and the vapour is transported away by the vacuum system. When the wood has reached the desired MC, the oil is removed from the treatment vessel. After this, a vacuum is applied to removed excess oil from the wood. Some dimensional stability is imparted to the timber due to the water repellency of the oil. This treatment is marketed by Osmose as the Royale process. 

The elements that eontrol the drying rate are the relative humidity of the air, the air temperature, and the airflow aeross the timber surfaees. In a kiln the temperature and (paradoxieally) relative humidity are maintained at higher levels than in the open air, while powerful fans eontrol the air velocity. In air-drying the same elements eannot be controlled or manipulated nearly as effectively. 

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. 

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. 

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. 

The compressive strength of the saturated cell walls decreases with increasing temperature, so raising the temperature inereases the likelihood of collapse. Thus collapse is most likely when a waterlogged impermeable timber of low density (and so of low strength) is kiln-dried above 50°C. Collapse is best avoided by drying at low temperatures. 

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. 

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... 

---