Advances in wood drying research and development

Figures & data

Figure 1. Wood drying kiln (a) and schematic end view of the kiln inside configuration (b).

Figure 2. Moisture content distribution among boards toward the end of drying.^[1]

Figure 3. Moisture content gradients along board thickness for sapwood (a) and heartwood (b) at different drying times.^[3]

Table 1. Common defects in wood drying.^[4–6]

Drying defects	Illustrations	Consequences
Distortion: Crook (spring), Twist, Bow and Cup		If the distortion is over a certain limit, the dry wood needs either to be planed to make surface flat, which induces material loss, or to be thrown away as waste.
Checking (cracking): Surface checking, Internal checking (honeycomb), End checking		This type of drying defect negatively affects the wood integrity and reduces strength. However, the surface checking is more severe for appearance and outdoor applications.
Internal stresses		The defect can induce distortion when the dried timber is cut or it is left for some time without constraints.
Collapse		Collapse manifests as beyond-normal shrinkage in the earlywood zone of annual rings, resulting in depression bands on surfaces and within-ring internal checks.

Figure 4. Various possible modes of water transport in different types of wood adapted from Chen et al.^[15] Penvern et al.^[16] and Cocusse et al.^[17]

Figure 5. Wood shrinkage as a function of moisture content in tangential direction (a) and longitudinal direction (b), adapted from Pang and Herritsch.^[25]

Figure 6. Stress-free shrinkage in tangential direction (solid lines) and in radial direction (dotted lines) as a function of moisture content and temperature, adapted from Lazarescu et al.^[35]

Figure 7. Drawing adapted from Hawley^[58] from early wood-water-relationships explaining the phenomenon of capillary pressure inside wood cells during drying.

Table 2. Equivalent pit radius and maximum capillary pressure for selected softwood species^[66] in comparison with the transverse tensile strength in green condition^[67].

	Heartwood		Sapwood
	Pit radius	Capillary pressure	Pit radius	Capillary pressure	Tensile strength perpendicular to grain
Species	R (µm)	ΔP (kPa)	R (µm)	ΔP (kPa)	(kPa)
Northern white cedar	0.028	5290	0.24	619	1700
Redwood	0.050	2909	0.12	1212	1800
Incense cedar	0.014	10391	0.10	1455	1900
Douglas-fir	0.035	4156	0.20	727	2100
Eastern red cedar	0.015	9698	0.13	1119	2300
Eastern larch	0.017	8816	0.55	265	2300*

* Tensile strength was only available for Western Larch.

Figure 8. The top two rows are, from left to right, microscope images of air dried, kiln dried and oven-dried (103 °C) Eucalyptus nitens samples adapted from Dickson.^[71] Colors in the bottom two rows show deformation visualization in which color change from green to magenta represents deformation levels, and white represents full cell collapse.

Figure 9. Cross sectional images of Eucalyptus nitens taken using a medical CT-scanner at different stages of drying. The grayscale from black to white is a measure of density, meaning that brighter zones represent higher MC.

Figure 10. Example of MC profiles simulated with the percolation model.^[75] Drying proceeds from top-left to bottom-right. Blue areas represent free water, red areas represent dried cells, and black histograms represent average MC profiles.

Table 3. Calculated RH, EMC and capillary pressure in equilibrium at 20 °C, with corresponding water potential, equivalent capillary R, and wet-bulb temperature.

Dry bulb (°C)	Wet bulb (°C)	RH (%)	EMC (%)	ΔP (kPa)	Chemical potential µ (J/mol)	Capillary radius (µm)
20.0	20.0	100.0	28.9	0	0.0	∞
20.0	19.8	98.2	26.9	−2435	−44.0	0.060
20.0	19.6	96.4	25.2	−4895	−88.5	0.030
20.0	19.4	94.7	23.7	−7383	−133.4	0.020
20.0	19.2	92.9	22.4	−9899	−178.9	0.015
20.0	19.0	91.2	21.3	−12443	−224.9	0.012

Figure 11. CT-scanner cross section image of 25 mm thick radiata pine sample dried at 90/60 °C for 5 h.^[81] The CT-scanner image was treated digitally to show MC > FSP (blue) and MC < FSP (red) in a scale from 200% to 0%.

Figure 12. Schematic diagram of closed-loop heat recovery system, adapted from Aziz et al.^[119]

Figure 13. Example of a two-stage progressive kiln commonly used one decade ago in the lumber drying industry.^[31]

Figure 14. Example of progressive kiln offered commercially by a local kiln manufacturer in New Zealand (www.windsorengineering.co.nz/drying/kilns/).

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