Natural air-drying of wood can take a very long time, while kiln-drying takes far less time in comparison
Freshly felled timber contains a large quantity of moisture, most of which has to be removed in order to obtain satisfactory performance from the wood and to avoid unequal shrinkage during the equilibration process. While no one argues against the need to dry wood, there is a debate about the preferred method: kiln-drying or air-drying. Air-drying is the drying of timber by exposing it to the air. The rate of drying largely depends on climatic conditions, and on the air movement (exposure to the wind). Kiln-drying is an industrial unit operation used to accelerate the drying of wood, in an enclosed space where air speed, temperature and humidity are controlled.
Both methods bring their own advantages and challenges for importers and sawmills. For manufacturers, they hold appeal for various reasons. Leading regional importers of timber confirm the benefits -and the demand - of kiln-dried timber. "Kiln-dried lumber is of higher quality because - after drying - it will be graded again and any physical deformities or drying defects will be removed before shipment," says Danube Group's Founder and Chairman, Rizwan Sajan. "Of course, most markets prefer kiln-dried wood," states Mohamad Maulana, CEO, Al Nibras General Trading Company. "The main reason behind this is that kiln-drying lowers the moisture level significantly and gives you very stable timber." Kiln-dried timber can be machined to a finish superior to air-dried timber and therefore problems such as raised grain are virtually eliminated. Kiln-dried timber can usually be installed immediately upon delivery to site, which means that you don't need to have expensive timber lying around acclimatizing. Such timber is also suitable for finishing immediately with a paint or stain, which means that the wood can be protected without delay. Sajan tells us that the demand for kiln-dried wood exceeds the demand for air-dried wood in the markets Danube deals with. United Agencies' General Manager Venu Gopal verifies that, in 2011, kiln-dried timber moved faster than air-dried timber, due to its superior properties.
The benefit of kiln-drying timber is that it is dried in a controlled environment, has rigorous testing, and is extremely quick - giving a higher quality end product. Bugs and insects are also killed during the drying process. Therefore it can be more cost effective and is less likely to have distortion, staining or drying stresses (i.e. warping or bowing). Because it offers benefits with respect to ensuring consistent quality and preventing lumber degrade, kiln-dried lumber is pretty much a standard in the global lumber industry nowadays. For large sawmills, an important benefit of kiln-drying is the short lead time required to move the lumber, which reduces inventory costs and saves space in the yard. However, the cost of installing and maintaining kilns may often be prohibitive unless throughput of timber is high.
The obvious advantage of air-drying is its low capital cost in comparison to kiln-drying procedures. Kiln operators often find that it is more economical to air-dry timber to 25 to 30 percent moisture content if the material is likely to take more than 5 weeks to kiln from green, due to its size or the drying characteristics of the timber. However, it is important to remember that air drying can only be used to reduce the moisture content of timber to around 14 to 20 percent. Therefore, for many applications, kiln-drying - which reduces the moisture content down much further - is a necessary final step in the drying process.
"Compared to air-drying, kiln-drying results in lower and more uniform moisture levels," adds Sajan. "For example, with kiln-drying it is possible to go down to moisture content of 8 percent which is not at all possible in air drying. Compared to air-drying, the drying time is quicker with kiln-drying so the timber will be ready for use earlier than air dried timber and there is no dependency on weather. Also, while kiln-drying, all possible insects will be killed and we will get insect-free timber." Modern kilns allow tight control of air-flow, temperature and humidity so that all the timber dries uniformly and to the precise moisture content required.
This results in a more uniform product and the problems often associated with poorly air-dried timber are avoided.
B M Ashraf, Managing Director of Mohiudeen Wood LLC concurs with his peers but also explains why air-dried timber is still selling and continues to be in stock. "The kind of timber required depends on the end use. If the use demands a very stable timber, then the timber has to be kiln-dried. Generally, when engineers have a fixed budget to work with and the end use allows a little flexibility; air-dried timber can be specified after careful thought." The main disadvantage of air-drying is that all pieces of a given bundle of lumber will not necessarily dry to the same moisture content. Boards on the inside of the stack may have completely different moisture content to those on the outside. In other words, when air-drying timber you don't have the same control that kiln-drying offers.
Another disadvantage is that air-dried timber usually needs acclimatizing when delivered to site to prevent further shrinkage once installed. Furthermore, air-dried wood may not be sufficiently dry for a coating to be applied whereas kiln-dried wood can be coated with a wood finish immediately. There are dangers in air-drying timber as well. If allowed to dry out too quickly, casehardening or honeycombing may ruin the structure of the wood.
"Wherever the final destination of sawn timber (lumber) in temperate species, it is always advisable that the wood is kiln-dried before shipment," explains Rod Wiles, AHEC Director for Africa, Middle East, India and Oceania. "The process is a fine art and those who produce the wood in North America or Europe, for example, have perfected it over many, many years. It is a genuine value-addition to the product and should ensure that what is delivered to the customer is higher yielding, pest and fungus free, ready to use and dimensionally stable. In effect, when you buy kiln-dried lumber, you have more chance of knowing what you are going to get and there will be much fewer unwanted surprises."
Some of the benefits associated with kiln-drying:
• Kiln-drying lumber allows for uniform drying in a carefully controlled environment
• Careful drying in a controlled environment minimizes checking (cracking), warping and bowing, creating a stable product, which is ready for shipment and further processing
• Grading after kiln-drying ensures delivery of a quality-controlled product, which minimizes wastage
• The heat and steam used in kiln-drying serve to eliminate pests and disease in the green lumber
• Finishes can be applied to kiln-dried lumber without the need for further drying
• Once the moisture content is sufficiently reduced through the process of kiln-drying, the wood will remain dimensionally stable and allow for a longer lasting performance product.
© Timber Design & Technology 2012
