Become a half-expert in wood drying in one minute

Drying wood is like cooking. If the heat is not enough, the food will not be cooked and the bacteria will not be killed. If the heat is too much, the food will become old and stale and have no taste.

The following is a brief introduction to wood drying in the form of self-questioning and self-answering:

A) What is the approximate moisture content of freshly sawn lumber?

Since the moisture content is defined as: water content / absolutely dry wood content * 100%, there may be cases where the value exceeds 100%.

B) What is wood drying?

In the flowing air with a certain humidity and temperature, the moisture in the wood is vaporized and evaporated and leaves the wood.

C) Why dry?

-- Prevent wood from swelling and shrinking due to moisture and drying, which can cause cracking and deformation (a wood moisture content of less than 12% can prevent most cracking and deformation, so most furniture, doors and windows require a moisture content of 8-12%)

-- Increase strength

-- Improve electrical and thermal conductivity

-- Anti-mildew and anti-corrosion (a wood moisture content of less than 20% can prevent most fungi from invading, so most construction-grade materials require a moisture content below 19%)

-- Reduce weight

D) How many drying methods are there?

-- Natural air drying - It takes a long time. Generally, it takes about 1 year for 3-5 cm boards to dry naturally.

-- Simple fire pit or stove with fan: Improper air circulation, uneven drying, and easy to burn or even cause fire

-- Steam boiler heating - conventional drying

-- Double circulation of air flow from hot blast furnace outside the kiln into the kiln - low heat energy, high consumption, uneven temperature and humidity field

-- Dehumidification and heating of air in kiln

-- Others: Solar drying/Vacuum drying/High frequency drying/Microwave drying

E) What is the structure of a drying kiln? (See picture, no explanation required)

F) What are the causes and solutions to common problems with board drying?

1. Warping

Reason: The distance between the strips is too large, the sizes are different, the stacking is not neat, and the drying standard is "too hard" (that is, the temperature is too high and the humidity is too low)

Solution: Adjust the spacer material and stacking; set the drying standard to low temperature and low humidity in the early stage and high temperature and high humidity in the later stage; add weights on the wood stack

2. Surface cracking

Causes: Drying base too hard (same as above); Wood stress not eliminated; Uneven temperature at different points in the drying kiln; Too frequent steaming of the initial cracks in air-dried wood

Solution: Adjust the drying benchmark; adjust the temperature and humidity in the middle and late stages to eliminate stress; preheat the cracked material in the early stages and spray steam during the process

3. Internal cracks

Cause: Excessive wood stress; temperature adjustment too quickly

Solution: Slow down the heating process; spray steam in the middle; generally adopt the principle of "low temperature/high humidity/slow heating"

4. Port cracking

Causes: The port is too far from the spacer; the gap between the wood is too large, resulting in too high wind speed at the port; the drying base is too hard

Solution: Adjust the above reasons; pre-cut the port or apply impermeable material

5. Uneven moisture content

Cause: The air flow speed is too low; the heat dissipation of the heating tube is uneven

Solution: Add a circulating fan; adjust the equipment or process according to the above reasons

6. Blue or mildew occurs

Cause: The temperature in the low-temperature preheating state is too low, the humidity is too high, the time is too long; the air flow is too slow

Solution: Make adjustments based on the above reasons

7. Dark and black color

Cause: High temperature drying is used when the wood has a high moisture content; the substances in the wood are acidified by moisture and heat.

Solution: Adjust the temperature and humidity process

G) Explain a few drying terms in simplest terms?

1. Equilibrium moisture content (EMC):

The ideal moisture content of a 1-inch board under certain atmospheric conditions, where no moisture enters or exits, varies from region to region, such as 11.4% in Beijing, 14.4% in Qingdao, 16% in Shanghai, and 13% in Vancouver. If the drying moisture exceeds the equilibrium moisture content in the region, problems may arise during use of the wood product.

2. Hygroscopic hysteresis

The process of moisture absorption after the wood is completely dry (usually 2%-5% of water will be returned)

3. Free Water vs. Absorbed Water

The former flows freely in the cells and only affects the weight, storage and combustion properties of the wood; the latter is bound to the cell walls and microcapillaries, affecting the expansion and contraction and other physical properties of the wood;

4. Wet Bulb Temperature vs. Dry Bulb Temperature

The former has a mercury ball wrapped in a wet cloth, while the latter does not. The drier the air, the greater the heat the wet cloth takes away. The greater the difference between the two temperatures, the wetter the air, the stronger the air's ability to hold water vapor, and the faster the moisture in the wet wood evaporates.

5. Moisture absorption limit (fiber saturation point)

The moisture content at which the free water evaporates and the absorbed water begins to evaporate. After this point, the wood begins to shrink in the tangential and radial directions during drying, and the difference in shrinkage may cause the board to warp.

About the Author

Mr. Peter Zhao has been engaged in international trade between Asia, America and Europe for over 20 years. He has extensive professional theoretical knowledge and market experience in international timber trade. He is a recognized expert in the timber field and has a deep understanding of timber sizing and grading, processing and use, industry status and development trends.