Air drying is the drying of timber by exposing it to the air. The technique of air drying consists mainly of making a stack of sawn timber (with the layers of boards separated by stickers) on raised foundations, in a clean, cool, dry and shady place. Rate of drying largely depends on climatic conditions, and on the air movement (exposure to the wind). For successful air drying, a continuous and uniform flow of air throughout the pile of the timber needs to be arranged (Desch and Dinwoodie, 1996).
The rate of loss of moisture can be controlled by coating the planks with any substance that is relatively impermeable to moisture; ordinary mineral oil is usually quite effective. Coating the ends of logs with oil or thick paint improves their quality upon drying. Wrapping planks or logs in materials which will allow some movement of moisture, generally works very well provided the wood is first treated against fungal infection by coating in petrol/gasoline or oil. Mineral oil will generally not soak in more than 1–2 mm below the surface and is easily removed by planing when the timber is suitably dry.
• Benefits: It can be less expensive to use this drying method (there are still costs associated with storing the wood, and with the slower process of getting the wood to market), and air drying often produces a higher quality, more easily workable wood than with kiln drying.
• Drawbacks: Depending on the climate, it takes several months to a number of years to air-dry the wood.
The process of artificial or ‘oven’ drying consists basically of introducing heat. This may be directly, using natural gas and/or electricity or indirectly, through steam-heated heat exchangers. Solar energy is also an option. In the process, deliberate control of temperature, relative humidity and air circulation creates variable conditions to achieve specific drying profiles. To achieve this, the timber is stacked in chambers, which are fitted with equipment to control atmospheric temperature, relative humidity and circulation rate (Walker et al.’, 1993; Desch and Dinwoodie, 1996).
Chamber drying provides a means of overcoming the limitations imposed by erratic weather conditions. With kiln drying, as is the case with air drying, unsaturated air is used as the drying medium. Almost all commercial timbers of the world are dried in industrial kilns. A comparison of air drying, conventional kiln and solar drying is given below:
1. Timber can be dried to any desired low-moisture content by conventional or solar kiln drying, but in air drying, moisture contents of less than 18% are difficult to attain for most locations.
2. The drying times are considerably less in conventional kiln drying than in solar kiln drying, followed by air drying.
• This means that if capital outlay is involved, this capital sits for a longer time when air drying is used. On the other hand, installing, operating and maintaining an industrial kiln is expensive.
• In addition, wood that is being air dried takes up space, which could also cost money.
3. In air drying, there is little control over the drying conditions, so drying rates cannot be controlled.
4. The temperatures employed in kiln drying typically kill all the fungi and insects in the wood if a maximum dry-bulb temperature of above 60 °C is used for the drying schedule. This is not guaranteed in air drying.
5. If air drying is done improperly (exposed to the sun), the rate of drying may be overly rapid in the dry summer months, causing cracking and splitting, and too slow during the cold winter months.
Significant advantages of conventional kiln drying include higher throughput and better control of the final moisture content. Conventional kilns and solar drying both enable wood to be dried to any moisture content regardless of weather conditions. For most large-scale drying operations solar and conventional kiln drying are more efficient than air drying.
Compartment-type kilns are most commonly used in timber companies. A compartment kiln is filled with a static batch of timber through which air is circulated. In these types of kiln, the timber remains stationary. The drying conditions are successively varied according to the type of timber being dried. This drying method is well suited to the needs of timber companies, which have to dry timbers of varied species and thickness, including refractory hardwoods that are more liable than other species to check and split.
The main elements of chamber drying are:
• Construction materials
The chambers are generally built of brick masonry, or hollow cement-concrete slabs. Sheet metal or prefabricated aluminium in a double-walled construction with sandwiched thermal insulation, such as glass wool or polyurethane foams, are materials that are also used in some modern timber ovens. However, brick masonry chambers, with lime and (mortar) plaster on the inside and painted with impermeable coatings, are used widely and have been found to be satisfactory for many applications.
Heating is usually carried out by steam heat exchangers and pipes of various configurations (e.g. plain, or finned (transverse or longitudinal) tubes) or by large flue pipes through which hot gases from a wood burning furnace are passed. Only occasionally is electricity or gas employed for heating.
Humidification is commonly accomplished by introducing live steam into the kiln through a steam spray pipe. In order to limit and control the humidity of the air when large quantities of moisture are being rapidly evaporated from the timber, there is normally a provision for ventilation of the chamber in all types of kilns.
• Air circulation
Air circulation is the means for carrying the heat to and the moisture away from all parts of a load. Forced circulation kilns are most common, where the air is circulated by means of fans or blowers, which may be installed outside the kiln chamber (external fan kiln) or inside it (internal fan kiln).
Throughout the process, it is necessary to keep close control of the moisture content using a moisture meter system in order to reduce over-drying and allow operators to know when to pull the charge. Preferably, this in-kiln moisture meter will have an auto-shutoff feature.
The most widely used method to saturate wood with impregnation resins is by a vacuum treatment process. This process uses a sealable vessel to contain the wooden samples while they are in treatment. After the samples have been oven-dried and placed into the vessel, a vacuum is pulled up to a certain psi depending on the procedure. Then the resin of choice is introduced into the vessel by a backfilling process and remains in a vacuum pulled state for the amount of time dictated by the procedure. Once the samples have been under vacuum for a sufficient amount of time for the resin to enter the wood the vacuum is released, and the samples remain in the resin solution to allow diffusion to happen. Diffusion is not controlled by the vacuum stage of the process, it is purely controlled by time. If the resin meets all of the required specifications, it will diffuse into the cell wall and the impregnation process will be complete
Thermally modified wood, is wood that has been modified by a controlled pyrolysis process of wood being heated (> 180 °C) in absence of oxygen inducing some chemical changes to the chemical structures of cell wall components (lignin, cellulose and hemicellulose) in the wood in order to increase its durability. Low oxygen content prevents the wood from burning at these high
temperatures. Several different technologies are introduced using different media including nitrogen gas, steam and hot oil.
The name is derived from the mechanical process involved. A vacuum is initially used to extract air from the timber. Once the air has been extracted the preservative in introduced and absorbed into the surface.
Following this a second vacuum is employed to remove the excess preservative from the surface. Hence the name Vac-Vac.
This method of preservation is suited to applications where the wood is continually exposed to moisture, for example the portion of a post within the ground.