Classification of Timbers:
(on basis of Seasoning)
On the basis of characteristics of seasoning, woods are divided into three categories:
Those timbers which are most difficult to season and such timbers must be seasoned under highly controlled conditions. They are liable to cracks, splits, and wraps (cupping, bowing, twisting, crooks, etc) during seasoning.
Moderately Refractory Timbers:
These are comparatively easy to season. They are also liable to crack, splits, and wraps.
These are very easy to season and having staining, modulus, and decay.
Methods of Seasoning:
There are three major methods of seasoning:
- Air Seasoning:
- Kiln Seasoning:
- Chemical Seasoning or Salt Seasoning:
There are many other methods but are not commercially important and known as Orthodox Methods.
- Air seasoning involves making of best use of prevailing winds and sun for seasoning while protecting wood from the rain.
- Wind prevents the air to become saturated by absorbing moisture from season wood by regularly circulating.
- Sun lowers the relative humidity of air by raising the temp.
- Three things are very important of air seasoning ie (1) control of climatic factors; (2) control of air circulation, and (3) control of the more rapid movement of moisture along the grain than across it.
- These can be achieved by construction of seasoning sheds, piling or stacking of timber and protection of pieces of wood respectively.
Importance of Seasoning sheds:
When a piece of wood is put in the open air, outermost layers dry up very soon but inner portion is still at higher moisture content. When moisture contents of layers go below fiber saturation point, shrinkage occurs whereas inner portion resist it, thus, a stress is developed in outermost fibers. When this stress exceeds breaking strength of the fiber, cracks occur so the rate of evaporation should be equal to the rate of migration of moisture from one later to the other. This is done by Construction of different types of sheds
There are three different
Types of sheds:
This type of shed is closed from all the sides as well as from the top. Since there is no free circulation of air in this type of shed, the rate of evaporation is very slow. This type of shed is suitable for refractory timbers.
Open on North Sheds:
This type of shed is covered from the top as well as from three sides. The side which is towards the north is open. The rate of evaporation is comparatively more than in closed sheds. This is suitable for seasoning of moderately refractory timbers.
It is only covered from the top but all four sides are open. There is more circulation of air, so the rate of evaporation is high. This is suitable for non-refractory timbers.
Essential for Seasoning Sheds:
- A light and watertight ceiling are necessary to keep off the sun and rain.
- The shelter should be provided at sticks which are exposed to sun and drying winds.
Proper ventilation under the roof and below the piles for admitting fresh air and getting rid of moisture evaporated from wood.
Piling or Stacking of Timber:
The wood is arranged in the seasoning sheds in a specific manner to provide spacing for the movement of air. This is called Stacking or Piling.
The wood should be stacked on a concrete floor. If it is not possible then the place where wood is to be stacked should be cleared of vegetation. This would minimize the chance of attack of insect and fungal. Timber should be stacked (8 inches – 12 inches) above ground level.
Following are different methods of stacking. The choice depends upon seasoning characteristics of a particular spp.
- Open Grip Method:
- Stacks are in layers
- Stacks are separated by stickers or crossers
- Size of crosser is 1 ¼ inch × 1 ¼ inch
Movement of moisture is rapid along the grin then across it. If loss of more moisture from the ends is not checked, serious stresses are set up that result in the end splitting. To minimize end splitting, the following measures should be taken.
- Coat some waterproof substance on the ends ie coal tar, wax, etc
- A strip of wood or metal is nailed at ends of wood.
A kiln is defined as a closed compartment in which conditions of temperature and humidity is always controlled.
Following are the factors which should be controlled in kiln seasoning:
- Circulation of air:
Temp serves two purposes in kiln drying:
- It helps in evaporation of moisture
- It helps to build up high humidity. High humidity can be maintained at high temperate.
Humidity controls the rate of evaporation. At high humidity rate of evaporation will be slow.
- Circulation of air:
Circulation of air prevents the air to become saturated and it is necessary for uniform drying of wood.
For Refractory timbers, seasoning should start at high humidity and low temp to keep evaporation rate low. For non-refractory timbers seasoning may be started at low humidity with high temp.
Types of Kilns:
- Compartment Kilns
- Progressive Kilns
- Furnace Heated Kilns
- Direct Air Heat Kilns
- Steam Heated Kilns
- It consists of the single closed chamber.
- Timber remains in the same position in the kiln throughout drying period.
- Temp and relative humidity of air is constantly changed.
- Air may be circulated cross-wise from top to bottom or vice versa or from end to end.
- In the beginning, temp and relative humidity of air is low but as drying proceeds, temp is increased and humidity is reduced which increases evaporation rate.
- There is flexibility in operation which is desirable when out-turn of the mill is constantly changing both in spp and stock.
- In progressive kilns, timber is admitted at one end and is moved gradually to the other end where it emerges dry.
- Air flows in opposite direction of movement of timber.
- While passing through the piles of timber, air absorbs moisture which increases its relative humidity and decreases temp.
- The severest dry conditions are at exit end mildest conditions are at the leading point.
- After circulation through the length of the kiln, a part of the air is discharged into the atmosphere while remainder returns below the kiln to re-circulate and fresh air is admitted at the suitable opening to compensate discharge of air.
- They require less skill for efficient operation
- The output is continuous as compared to compartment kilns
- The successful operation depends upon the steady supply of timber of the same spp and dimensions because drying conditions cannot be modified when a new load of different spp is added.
- These kilns cannot be regulated with great precision so unsuitable for those timbers which are difficult to season.
Furnace Heated Kilns:
- In this type, wood wastes are burnt and heat produced is directly used to warm air in the kiln.
Steam Heated Kilns:
- A boiler is used to produce steam for conventional steam heated kilns.
CHEMICAL SEASONING OR SALT SEASONING:
When wood is seasoned with the help of chemical, it is termed as Chemical Seasoning.
Principles of Salt Seasoning:
- The principle is that vapor pressures of aqueous solutions of salts ie common salt have lower vapor pressure than pure water.
- By treating those salts on the surface of the wood, the vapor pressure of water contained in surface cells of wood lowered and a vapor pressure gradient is established which tends the moisture to move outward from the center of the wood.
- Salt is of low hygroscopic nature than wood. This salt acts as a barrier b/w wood surface and atmosphere.
- Water is first transferred into the salt and then it is evaporated.
- Simply, salts perform two functions:
- Act as a barrier to avoid quick removal of water.
- Salt also expels out water present in the wood.
Comparison of Air and Kiln Seasoning:
|Air Seasoning||Kiln Seasoning|
1. It is a very simple method
2. It is economical because not capital investment is needed.
3. Better use of air.
4. Air seasoning can be done at a place where timber already stores.
1. It is an uncontrollable process
2. The yield of seasoned wood is not continuous.
3.There are more chances of seasoning defects because conditions are not controllable.
1. It is a controllable process
2. It is a very fast process
3. It supplies seasoned wood in all seasons
4. Timber is sterilized in the kiln due to its specific conditions which kill insects and fungi.
5. Timber is dried under highly controlled conditions so chances of development seasoning defects are limited.
6. Better use of temperature
7. The yield of seasoned wood is continuous.
1. Large capital investment is involved, so not economical.
2. There is always a transpiration problem to the area where there is a seasoning kiln.
3. Transportation costs are also there.
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