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.
4.1. 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:
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.