Wood drying process principle

1. The microstructure of the wood

Wood is made of a number of cells that make up of the formation. Each cell has cell nucleus ( wall). Cell wall, which is also made of Micro fiber ,and there are space among the Micro fiber. Between the cells and inside the cells of space is water. Cell nucleus is made of organic compounds, cellulose is the most important of organic compounds.

2. Wood moisture content

2.1 The lumber cells gap and the space between cells with water is called free water. The fiber cell formed microscopic capillary system.

3. Free water evaporation reduced is wood quality, not its specifications. Lose the free water can lead to physical dimension of be dry wood membrane changed.

4.
4.1. Wood saturation----- the wood neither absorb nor evaporate water.
4.2. Fiber saturation point depends on wood and temperature in the 22% to 35% range for change.

5. Wood has water absorption performance.When the moisture of wood higher than the environment, water has evaporated from wood and wood will dry up; otherwise, when the moisture of wood lower than the environment,wood will start to absorb the moisture.

6. Only when the wood and the environment under same water content(Moisture equilibrium), the wood neither absorb nor evaporate water.

Wood and the local climate conditions should be consistent(humidity, temperature,etc.),only then the wood won't have a crack.Wood humidity should be 2.5% lower than the average humidity.

7. Measure moisture content

7.1.Wood moisture content is connected with the absolutely dry wood as follows: Wood moisture content= The quality of the wet wood-The quality of the absolutely dry wood/ The quality of the absolutely dry wood×100%
7.2. The quality of the absolutely dry wood is the wood (water-free) quality.

Measuring the quality of the absolutely dry wood method:

a.Claim:

1) Cut down each side of drying samples for 300mm
2) Specimen thickness 10-12mm
3) Wood sample should be clears, free from putrefaction and other defect, And should be polished.

b.Accurate to 0.01gweight Settings

c. Method : timber should be drying at temperature 103±2℃ in the kiln, measure the wood quality every 2 hours.If in four hours period qualitative change doesn't happen, then the wood is thought to be absolutely dry.

7.3 The initial water content: the moisture content before drying wood.
7.4Final moisture content: Humidity level after drying wood, it is unchanged in any case.
7.5. Equilibrium moisture content

①The air relative humidity is 30-80% in a standard humidity. The equilibrium moisture content calculating equation as follows:  Equilibrium moisture content =relative air humidity/5
②Equilibrium moisture content: the wood neither absorb nor evaporate water
There are data, grade, wood thickness and the initial water content since drier.

•  Absolute humidity and relative humidity (RH)

Moisture absorption depending on the temperature
Absolute humidity(P) : One cubic of water vapor content in the air(g/m3)

P=Vg/V(Vg — water vapor content(g/m3);V — Wet air volume(g/m3))

• Maximum Humidity(Pm):(under the same temperature and atmosphere),one cubic of the greatest water vapor content in the air in determining temperature(kg/m3).

P м = The greatest possibility of water vapor content in the air/ Wet air volume
• Relative humidity (Y):Absolute humidity(P)/ Maximum Humidity(Pm) × 100% Dry and wet bulb thermometer is used for testing- air humidity. Dry and wet bulb thermometer working principle is based on determining the readings of the dry and wet bulb thermometer (psychrometric difference ),the difference data depends on the surrounding air humidity.

The difference wet/dry bulb temperature(℃)

Dry bulb temperature
( ℃ )

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

40

100

94

88

82

76

71

66

61

57

53

48

44

40

36

32

29

44

100

94

89

83

78

73

68

63

59

55

50

47

43

40

36

33

50

100

95

90

84

79

75

70

66

62

58

54

50

47

44

41

37

54

100

95

90

84

80

76

72

68

64

60

56

52

49

46

43

39

60

100

95

90

86

81

77

73

69

65

61

58

55

52

49

46

43

64

100

95

91

86

82

78

74

70

67

63

60

57

54

51

48

45

70

100

96

91

87

83

79

76

72

68

64

61

58

55

52

50

47

74

100

96

92

87

84

80

76

72

69

65

63

60

56

53

51

48

80

100

96

92

88

84

80

77

73

70

66

64

61

58

55

53

50

84

100

96

92

88

84

80

77

74

71

68

65

62

59

56

54

51

90

100

97

93

89

85

81

79

75

72

69

66

63

61

58

56

53

94

100

97

93

89

86

82

79

76

73

70

67

65

62

60

57

54

100

100

97

94

90

87

83

80

77

74

71

68

66

63

61

59

56

104

100

 

94

91

88

84

81

78

75

72

69

67

64

62

60

57

110

100

                   

69

67

65

63

61

58

114

100

                      

65

63

61

58

120

100