Become a semi-expert in wood drying in one minute

Drying wood is like cooking; if the heat is too low, the ingredients will not be cooked and bacteria will not be killed; if the heat is too high, the wood will become stale and burnt, losing all its flavor.

The following is a brief introduction to wood drying in a question-and-answer format:

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

Since the definition of moisture content is: water content / absolutely dry wood content * 100%, it is possible for the content to exceed 100%.

B) What is wood drying?

In flowing air with a certain humidity and temperature, the moisture in the wood is allowed to vaporize and evaporate, leaving the wood.

C) Why is drying necessary?

-- To prevent wood from swelling and shrinking due to moisture, which can cause cracking and deformation (a moisture content of less than 12% can prevent most cracking and deformation, so most furniture, door and window materials require a moisture content of 8-12%)

-- Increase strength

-- Improve electrical and thermal conductivity

-- Anti-mildew and anti-corrosion (wood with a moisture content of less than 20% can prevent most fungal infections, so most building-grade materials require a moisture content of less than 19%)

-- Reduce weight

D) What are the different drying methods?

-- Natural air drying -- takes a relatively long time; generally, it takes about a year for a 3-5 cm thick board to dry naturally.

-- Simple fire pits or stoves with fans: Inadequate airflow circulation leads to uneven drying, increasing the risk of damage or even fire.

-- Steam boiler heating -- Conventional drying

-- Dual circulation of airflow from the external hot blast stove into the kiln - low heat energy consumption, high energy loss, and uneven temperature and humidity field.

-- Dehumidification and heating of air inside the kiln

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

E) What is the structure of the drying kiln? (See diagram, no explanation needed)

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

1. Warping deformation

Causes: The spacing between the dividers is too large, their sizes are inconsistent, they are not stacked neatly, and the drying standard is "too rigid" (i.e., the temperature is too high and the humidity is too low).

Solution: Adjust the material and stacking of the partition strips; use low temperature and low humidity in the early stages of drying, and high temperature and high humidity in the later stages; add weights to the timber stacks.

2. Surface cracking

Causes: The drying standard was too hard (same as above); wood stress was not relieved; uneven temperature at different points in the drying kiln; excessive steaming for initial cracking of air-dried wood.

Solution: Adjust drying baseline; adjust temperature and humidity to relieve stress in the mid-to-late stages; preheat already cracked materials and control steam spraying during the process.

3. Internal cracks

Causes: Excessive stress on the wood; rapid temperature adjustment.

Solution: Slow down the heating process; use intermediate steam spraying; adopt the overall principle of "low temperature/high humidity/slow heating".

4. Port cracking

Causes: The port is too far from the spacer; the gaps between the wood are too large, resulting in excessive airflow at the port; the drying standard is too hard.

Solution: Adjust according to the above reasons; pre-cut the port or apply an impermeable material.

5. Uneven moisture content

Causes: Airflow velocity too low; uneven heat dissipation from heating element.

Solution: Add a circulating fan; adjust the equipment or process to address the above causes.

6. Blue discoloration or mold growth appears.

Causes: The preheating temperature was too low, the humidity was too high, and the time was too long; the airflow was too slow.

Solution: Adjustments should be made based on the above reasons.

7. The color is dark or black.

Cause: High-temperature drying was used when the wood had a high moisture content; substances in the wood became acidic due to the damp heat.

Solution: Adjust the temperature and humidity process.

G) Explain a few drying terms in the simplest terms possible.

1. Moisture content equilibrium point (EMC):

The ideal moisture content for a 1-inch board, where no moisture can enter or leave the wood under certain air conditions, varies by region. For example, it's 11.4% in Beijing, 14.4% in Qingdao, 16% in Shanghai, and 13% in Vancouver. If the drying moisture content exceeds the equilibrium moisture content of the region, problems may occur with the wood products during use.

2. Moisture absorption delay

The process of wood absorbing moisture after it is completely dry (generally, it will absorb 2%-5% more moisture).

3. Free water vs. water that is absorbed

The former flows freely within the cells and only affects the weight, storage, and flammability of wood; the latter binds to the cell wall and microcapillaries, affecting the physical properties of wood such as expansion and contraction.

4. Wet-bulb temperature vs. dry-bulb temperature

The former uses a mercury bulb wrapped in a damp cloth, while the latter does not. The drier the air, the more heat the damp cloth carries away, resulting in a greater temperature difference between the two. Conversely, the wetter the air, the greater its capacity to hold water vapor, and the faster the moisture in the damp wood evaporates.

5. Moisture absorption limit (fiber saturation point)

The moisture content is the point at which the wood begins to evaporate after the free water has evaporated. After this point, the wood begins to shrink in both the tangential and radial directions during drying, and the difference in shrinkage rates can cause warping and deformation of the boards.

About the Author

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