Benefits of Seasoning

·         By seasoning timber, "Movement" in the component of the finished products, relative to the dimensions at the time of fabrication, is kept to a minimum. Thus seasoning is the first step towards realizing maximum attainable dimensional stability from any timber during use.

·         Considerable losses through cracking, splitting, warping etc, which are liable to occur while using green timber under inevitable exposure to surrounding uncontrolled climatic conditions, are avoided by carrying out proper seasoning under controlled drying conditions prior to use. This is of great importance in timber utilization in India where the climatic conditions vary considerably at different times during the year.

·         Seasoning protects timber against primary decay, fungal stain and attack by certain kinds of insects. The organisms, which cause decay and stain, generally can not thrive in timber with moisture content below 20 percent. Seasoned wood is less susceptible to decay than a green wood in indoor use.

·         Seasoned timber is lighter and hence the transport and handling are easy and the costs are reduced.

·         Seasoned timber is stronger than the green timber in most of the strength properties.

·         Finishing, coloring/painting & gluing is resulted better in seasoned & processed timber than in green wood.

·         The electrical and thermal insulation properties of wood are improved by seasoning.

·         Seasoning enables substantial long-term economy in utilization by minimizing replacement, besides rationalizing the utilization of our timber resources.

Compartmental Kiln

Wood Seasoning 

the process of inducing evaporation of the moisture contained in wood; a type of hydrothermic treatment of wood.
The purpose of seasoning is to reduce the moisture content in the wood to a level appropriate to the use of the articles made from the wood. This prevents changes in the dimensions and shape of the articles, eliminates wood rot, increases the strength of the wood, reduces the weight of the articles, strengthens glued joints, and improves the quality of finishing. Wood is seasoned in various forms: lumber, peeled or planed veneers, crushed wood particles, and semifinished articles.
The simplest method of seasoning lumber is open-air seasoning, in which the lumber is stacked in the open air or under canopies for a period of two or three weeks to several months. The principal industrial method, kiln drying, uses kilns fed with hot air, a mixture of air and fuel gases, or superheated steam. Kilns with steam are the most commonly used type. Continuously operating kilns are used primarily for large-scale seasoning of lumber prior to shipping; moisture content is reduced to 18–22 percent. Batch kilns are used to season wood to a moisture content of 7–10 percent.
The reduction of moisture content causes irregular drying of the wood, and internal stresses arise that may cause cracks to form. In order to prevent this, the relative humidity of the drying agent is lowered and the temperature is raised in the course of the seasoning process. Conditions for kiln drying lumber are standardized in the USSR. Depending on the desired intensity of the process and the later use of the wood, the following regimes are used: mild (temperature at the start of seasoning is 40°–50°C), normal (60°–80°C), accelerated (80°–100°C), and high-temperature (over 100°C). The length of the process varies from 15–25 days (mild regimes for hard woods) to 20–30 hr (high-temperature regimes for soft woods). If the wood is to be precision machined, it is steam-treated after seasoning to remove internal stresses. Seasoning in a high-frequency electric field is also used (seeDIELECTRIC HEATING).
Continuously operating roller driers are ordinarily used for peeled and planed veneer. The sheets of veneer are surrounded by hot air (110°–130°C) or fuel gas (150°–250°C) and are moved through the drier on roller conveyors. The process lasts two to 12 minutes. Presses in which the plates are periodically opened and closed may also be used to season veneer. The temperature of the plates is 130°–170°C, and the process lasts up to two minutes. A promising method for the future is the processing of peeled veneer in a continuous ribbon, rather than in sheets, in driers with forced ventilation through nozzles and metal belt or roller-chain conveyors.
Crushed wood particles for chipboard are seasoned primarily in gas drum driers at temperatures up to 500°C. Pneumatic units in which the particles are seasoned while suspended in a gas stream are also used. For packing chips and small semifinished articles, such as matchsticks, belt driers are used in which heated air is circulated around a layer of material on a mesh belt.

Progressive Kiln

The advantage of this system, although much larger, has a continuous flow of seasoned timber coming off line.

A number of commercial processes for seasoning of timber are available, the most common of which is kiln-drying. Kiln seasoning accelerates the process of seasoning by using external energy to drive the moisture out. The timber is stacked in much the same way as it is for air drying, and is placed inside a chamber in which the conditions can be varied to give best seasoning results. Air is circulated around the charge (stacked timber) and the temperature and humidity can be varied to give optimum drying. Each species has different cell characteristics and therefore requires different drying schedules. Typically the timber may be in the kiln for a period of between two days to one week.

Generally, it is not feasible to kiln-dry structural timber in thicknesses greater than 45 mm, although there are limited amounts of 70 mm thick kiln-dried softwood members in the market place. All untreated structural pine and some commercial hardwoods are seasoned, mostly using kilns that are often heated by sawmill by-products or gas.

Plenty of research is continually being done to determine kiln drying schedules for different species. An optimum schedule will remove the moisture as quickly as possible, with the minimum quantity of introduced energy, and giving minimum damage to the dried timber.
After kiln seasoning, there is often some damage to cells near the surface of the wood. (All of the moisture passes through those cells.) They have in fact collapsed, but can easily be ‘pumped back up’ in a reconditioning chamber. This chamber introduces steam for a period and puts some moisture back into the outer cells and removes the effect of seasoning collapse.

Hard woods have closed cell structure, so take more time in being seasoned and dried.

Conventional kiln-drying for hardwoods uses temperatures well below the boiling point of water. This avoids damage to the wood fibres which, if the seasoning is not properly controlled, can experience loss of strength. In seasoning hardwoods, the process is slow and can be economically undertaken by first of all air-drying or partially seasoned for a period which can be anywhere from 3 to 9 months, depending upon the thickness of material, the ambient temperature and humidity and the species. The partial seasoning process can be accelerated for some hardwoods by using fan-forced pre-drying in a controlled (but not kiln) environment, to reduce the lead time for kiln-drying to 20 to 30 days.

Once the sawn hardwood material reaches fibre saturation point or slightly below (at a moisture content of about 20% to 25%), it is then placed in kilns usually for up to 10 - 14 days (depending upon the thickness of the sawn timber) in order to bring the moisture content down to between 10% and 15%. This drying process must be strictly controlled and monitored in order to avoid drying degrade.

The amount and duration of air, heat and humidity again depends on species, size, quantity, etc.

Artificial (kiln) seasoning

Kiln drying of lumber is perhaps the most effective and economical method available. Drying rates in a kiln can be carefully controlled and defect losses reduced to a minimum. Length of drying time is also greatly reduced and is predictable so that dry lumber inventories can often be reduced. Where staining is a problem, kiln drying is often the only reasonable method that can be used unless chemical dips are employed.

Kilns are usually divided into two classes:

1.    Progressive

2.    Compartment

Both methods rely on the controlled environment to dry out the timber and require the following factors:

§  Forced air circulation by using large fans, blowers, etc.

§  Heat of some form provided by piped steam.

§  Humidity control provided by steam jets.

Amount and Duration of Air, Heat and Humidity depends upon:

1.    Species

2.    Size

3.    Quantity

1. Progressive Seasoning:

In the progressive kiln, timber enters at one end and moves progressively through the kiln much as a car moves through a tunnel. Temperature and humidity differentials are maintained throughout the length of the kiln so that the lumber charge is progressively dried as it moves from one end to the other. Progressive kilns may be further subdivided into natural draft kilns in which heated air is allowed to rise through the material by natural convection, and forced draft kilns in which fans are employed to force the air through the wood. A progressive kiln has the stack on trolleys that ‘progressively’ travel through chambers that change the conditions as it travels through the varying atmospheres.

How Seasoning is Done?

The method and technology we use is "KILN", which happens to be the most energy intensive amongst wood processing operations claiming an estimated 50-70 percent of the total energy consumed in manufacturing wood products. The timber is stacked in the chamber. Then with Air blower heat exchanges in chamber at pressure. Timber is kept in chamber for 6 to 12 days as per its fiber property. These removes all possible unwanted moisture through fibers.

The seasoning Kiln is a chamber equipped with arrangements for heating and dehumidifying the drying air to desired conditions of temperature and relative humidity and its circulation over the surfaces of timber stacked inside. Fans are used for faster circulation of air.

Why Seasoning Required?

Green Wood in growing trees contains considerable quantities of water/ moisture. Most of this moisture has to be removed in order to obtain satisfactory performance from the wood in use. Freshly felled wood contains moisture roughly 100 percent based on the oven dry weight of wood. A well seasoned piece of wood should on other hand contain only 10 to 12 percent moisture. An important object of seasoning is to dry the timber to the equilibrium moisture content before use, so that gross dimensional changes through shrinkage, which inevitably occurs in green wood as it tends to attain equilibrium with the surrounding atmosphere, are eliminated.

Some of the Advantages of Wood Seasoning Plant

·         Zero risk of fungal decay.

·         Reduction in weight.

·         Increase in Strength Properties.

·         Improved capacity to hold nail and screw.

·         High gluing capacity.

·         Reduction in moisture content by 8 - 15%.

·         Improved electrical and thermal insulation properties.

Air Seasoning