In order to determine the duration of drying of sawn timber with regard to various factors, the method developed at the Central Scientific Research Institute for Mechanical Wood Processing is recommended.

The method predicts coefficients from tables 1–8. They take into account the type of wood, the thickness, width and length of the material, the quality of its drying, the construction of the dryer, the initial and final humidity.

Coefficients according to the type and dimensions of wood

Table 1 — Coefficient according to the type of wood

Table 1: Coefficient according to wood type
Type of wood Spruce, fir Pine Birch Beech Larch Oak
Coefficient 0 4 26 41 46 71

Table 2 — Coefficient according to material thickness

Table 2: Coefficient according to material thickness
Thickness, mm 13 16 19 22 25 30 35 40 45 50 55 60
Coefficient 0 9 16 23 28 36 43 49 54 59 65 70
Thickness, mm 70 80 90 100 110 120 130 140 150 180 220 280
Coefficient 80 89 97 104 110 116 121 126 130 142 155 171

Table 3 — Coefficient according to material width

Table 3: Coefficient according to material width The ratio of the width of the material to its thickness W:D to 1,5 2 3 5 10 Coefficient 0 5 9 14 18

Table 4 — Coefficient according to the length of the material

Table 4: Coefficient according to material length Length, m 0,3 – 0,4 0,5 – 0,6 0,7 – 0,8 0,9 – 1,0 via 1,0 Coefficient 0 6 11 14 15

Coefficients according to drying quality and dryer

Table 5 — Coefficient according to material drying quality

Table 5: Coefficient according to material drying quality Drying Category Drying of normal lumber (II category) Quality drying of lumber for furniture, making wagons.. (I category) High-quality drying for making carts, oak rainbows.. Coefficient 0 8 20

Table 6 — Coefficient according to the design of the dryer

Table 6: Coefficient according to dryer design Essence type SRC With countercurrent circulation With natural or weak driving circulation Channels with natural circulation Coefficient 0 4 8 15

Conditional material was taken into account when calculating the coefficient. Conditional material means pine boards with a thickness of 50 mm, a width of 150 mm and a length of over 1 m, with an initial humidity of 60% and a final humidity of 12%. This material, placed on mats 25 mm thick, should be dried in dryers with natural air circulation for 6 days.

Using the coefficients given in the tables 1–8, it is possible to determine the normative drying time of sawn timber.

An example of a calculation

It is necessary to determine the normative duration of drying of pine boards with thickness 40 mm, width 20 mm (ratio of width to thickness 200:40 = 5) and length over 1,0 m. The boards are intended for the production of door leaf frames, i.e. the drying quality is category I, and they are dried in dryers with accelerated reverse air circulation from initial humidity 60% to final humidity 9%.

The corresponding coefficients give:

K = 4 + 49 + 14 + 15 + 8 + 0 + 82 = 172

Coefficients according to humidity and stages of drying

Table 7 — Coefficient according to the initial and final humidity of sawn timber

Table 7: Coefficient according to the initial and final humidity of sawn timber
Initial humidity, % Final humidity, %
6 7 8 9 10 11 12 13 14 15 18 20 22 25
20 62 56 50 45 38 32 25 18 9 0 / / / /
25 70 65 60 55 50 45 41 35 30 25 6 / / /
30 75 70 66 62 58 54 50 46 42 38 25 15 30 /
35 79 75 71 67 64 60 57 54 50 47 38 29 21 7
40 82 78 75 71 68 65 62 59 56 53 44 38 32 21
45 85 81 77 75 72 69 66 63 61 58 50 45 40 31
50 87 83 80 77 75 72 69 67 65 63 55 51 46 38
55 89 85 82 80 77 75 72 70 68 65 59 55 50 44
60 90 87 84 82 79 77 75 72 70 68 62 58 55 48
65 92 89 86 84 81 79 77 75 73 71 65 61 58 52
70 93 90 88 85 83 81 78 76 75 73 67 64 61 55
75 94 92 89 87 85 82 80 78 77 75 69 66 63 58
80 95 93 90 88 86 84 82 80 78 77 71 68 65 61
85 96 94 91 89 87 85 83 82 80 78 73 70 67 63
90 97 95 92 90 88 86 85 83 81 79 75 72 69 65
95 98 96 93 91 89 87 86 84 82 81 76 73 71 67
100 99 97 94 92 90 88 87 85 83 82 77 75 73 69
110 101 98 96 94 92 90 89 87 85 84 80 77 75 74
120 102 99 97 95 94 92 90 89 87 86 82 79 77 76
140 104 102 100 98 96 95 93 92 90 89 85 83 81 78

Table 8 — Coefficient for recalculating the drying mode in time according to the decrease in humidity

Table 8: Coefficient for recalculation of the drying mode in time after humidity decrease Decrease in material humidity, % 60-50 50-40 40-35 35-30 30-25 Drying stage coefficient 0,11 0,14 0,08 0,10 0,11 Decrease in material humidity, % 25-20 20-15 15-12 12-10 10-8 Drying stage coefficient 0,14 0,17 0,15 0,11 0,14

Related topics: softwood and hardwood lumber, wood drying, wood drying temperature and humidity, wooden window and door production and request for proposal.