Kiln Drying

Air Seasoning and Kiln Seasoning of Timber

The presence of water in wood

Under ordinary conditions, all wood contains some water, and the amount of water contained in wood at a particular time is known as its moisture content.

The moisture content (MC) of a piece of timber is the weight of water contained in that timber expressed as a percentage of its own weight.

MC % = (Weight of water/Oven dry weight) x 100

MC can be more than 100% and frequently is. Freshly cut plantation grown pine may consist of something like one and a half-parts water to one of wood substance. MC in this case would be 150%.

Green-off-saw timber from hardwood saplings may have a moisture content of about 100%. While the moisture content of mature hardwoods may be in the region of 50%.

Water is the conductive fluid of the living tree. It takes minerals from the soil to the leaves where they are used in the production of sugars and starches - the food of growing trees.
This watery solution is known as sap, a term which is sometimes confused with resinous exudations. However, sap is actually water containing a comparatively small amount of dissolved material, mainly salts. These mineral salts are conducted up to the leaves via the sapwood, and the sugars and starches move in solution through the inner bark back to the roots, nourishing the living cells on the way.

The heartwood, or zone of non-living wood cells, also contains a considerable amount of sap, but no appreciable movement of this sap takes place in the heartwood zone.

Why Season Timber?

In most cases the moisture content of timber in service is in the vicinity of 10-15%. This is a long way removed from the green-off-saw moisture condition mentioned above and, obviously, a considerable amount of drying has taken place.

Whenever timber dries, it shrinks, and seasoning is undertaken mainly to "pre-shrink" timber. This prevents unacceptable changes in size occurring after installation. water from timber so that the amount of water in the timber is in balance with the moisture in the atmosphere.

Timber seasoning is defined as "drying timber to a moisture content suited to the condition and purpose of use".

Shrinkage (Dimensional Change)

A number of terms need to be understood:

Free Moisture, Combined Moisture and Fibre Saturation Point

A single fibre, or cell of wood, is like a thin tube tapered at both ends. The cavity contains moisture known as free moisture. Within the framework of the cell wall are molecules of water called combined moisture. When the free moisture has left the cells, and only combined moisture remains, the wood is at fibre saturation point (FSP)

As the timber seasons, the free moisture leaves the cell cavity before any of the combined moisture leaves. It is relatively easy for free moisture to leave the cells and air-drying is usually quite fast and economical down to, or just a bit below, fibre saturation point. The molecules of bond water however, are ionically bonded (magnetically attracted) to the cell wall molecules and energy is required to break these bonds. A long and progressively slower process of air-drying can supply this energy, but drying below fibre saturation point is best done in a heated kiln.

Moisture Loss and Shrinkage

There is hardly any change in size down to fibre saturation point, but from there on, as the combined moisture begins to leave, the cells start to harden, and shrinkage commences. The exact moisture content of FSP varies with different species. In most cases it lies between 25 to 30%.
There is no point in drying timber down to only 20% or so and claiming that it is seasoned. By this stage, only a small portion of the potential shrinkage will have taken place.

Expansion can also occur when timber begins to re-absorb moisture after being over-dried. The change in size (shrinkage or expansion) is directly proportional to the moisture content change (loss or gain) below fibre saturation point. Because of this direct relationship, The term unit shrinkage can be defined as the percentage change in dimension following a moisture content change of 1%. Unit shrinkage is a fairly important property particularly for species with low fibre saturation points. Cypress Pine is an example of a timber with low shrinkage from green to dry, but its FSP is also low (22%) and so it has moderate unit shrinkage of 0.3%, ie for every 1% change in MC a change in dimension of 0.3% can be expected.

Shrinkage in Different Directions

Shrinkage along the length of a piece of timber is normally insignificant but shrinkage across the grain is important.

Shrinkage across the grain can be sub-divided into:
(1) Radial Shrinkage - observed in the radial direction when a board is quarter sawn; and
(2) Tangential Shrinkage - observed when a board is back sawn.

Radial shrinkage is always less than tangential because the medullary rays (horizontal bands which radiate from the pith towards the bark like the spokes of a wheel) prevent shrinkage in the radial direction. Tangential shrinkage is usually about twice as much as radial shrinkage.
Unless special sawing techniques are used, most boards sawn from a log are predominantly back-sawn. This is the direction of greatest shrinkage and should be taken into account when shrinkage calculations are being made using values from shrinkage tables.

Collapse

Collapse is defined as abnormal and often irregular shrinkage occurring above FSP. In certain species (some Eucalypt) the cells cannot withstand the capillary forces set up when free water leaves the cell cavities and, and as a result they collapse.

In quarter-sawn boards, collapse occurs as a ribbed appearance termed "wash boarding". In back-sawn material it shows by excessive shrinkage and in squares it shows as diamonding where the growth rings are oblique, or as hollowing of the face.

Because shrinkage, and as a consequence face-checking, is less in the radial direction than tangential direction, the species which collapse are best used as boards when they are predominantly quarter-sawn.

Quarter sawing gives a lower saw recovery than back sawing, however quarter sawing is necessary in order to obtain a higher seasoned recovery.

Reconditioning

This is a process involving a steam treatment designed to remove collapse. For best results, the timber should be between 15-18% moisture content when it is reconditioned.

Reconditioning usually involves loading the kiln charge into a special chamber and injecting low pressure steam, to maintain a temperature of approximately 90oC for about six hours.

Equilibrium Moisture Content

Timber in use always contains some moisture, and equilibrium moisture content (EMC) is the moisture level reached when there is a balance or state of equilibrium between atmospheric moisture (humidity) and the moisture in the timber. Humidity changes daily, from high on wet days, to low on very dry days. EMC attempts to follow these changes but the rate of MC change in timber is much slower than the rate of humidity change in the atmosphere. The rate of response to changing humidity also varies, depending on the species, the thickness of timber and the surface coatings used. It can be said that there is no true equilibrium, only a continual attempt to follow the changes in humidity. EMC varies slightly with species, but for practical purposes only one value need be observed at a particular locality and that value is the long run average MC at that locality. EMC is the moisture content to which timber should be dried so that changes in size after installation are kept as small as possible.

Ultimately, it is the responsibility of the specifier/builder/consumer, to order accordingly, when seasoned timber is required. Specialty products, such as polished flooring, panelling and furniture, and whether the building is to be air-conditioned or not are all very important in the final decision of the appropriate MC.

For example, consider a brush box floor: 100 mm wide boards, supplied at 12%, could squeeze up due to expansion during wet weather, when the EMC rises to 14%, by 0.7 mm (unit shrinkage x MC change). During dry weather (EMC of 10%), a gap of 0.7 mm would appear at the joints. A gap of this size would hardly be noticed. However, if the floor had been laid at 16% the gap due to the shrinkage in dry weather could be 2.1 mm (6 x 0.35 mm = 2.1 mm). If laid at 8%, total expansion of the floor during wet weather could push the walls out.

Not all timber needs to be seasoned. Green hardwood framing has been used for years. Although shrinkage across the grain occurs as the framing timber dries, good building practice can cope with the loss of size. But timber for furniture, flooring and panelling must be seasoned to a moisture content equal to the average of the extremes it will reach in service. 

Humidity, Temperature and Air Circulation

Humidity

Humidity is the general term used to describe the presence of water vapour in air. There is a known limit to the amount of water vapour that air can hold at any particular temperature and the higher the temperature the more vapour it can hold.

Water vapour is a term, which unfortunately, can convey two conflicting meanings. Strictly, vapour is invisible. It is the gaseous form assumed by a liquid when heated. However, water vapour is popularly used to describe visible droplets of water suspended in air, such as seen in a mist or in exhaled breath on a cold day.

Similarly, a double meaning applies to the word steam. Strictly, steam is the gaseous form of water heated to boiling point, but it is also popularly used to describe the visible clouds of water droplets such as seen when steam escapes from the spout of a kettle or from around the door of a steaming chamber.

Water vapour is used here in the sense of the invisible, gaseous form of water. The amount of vapour present is expressed either in terms of absolute humidity or relative humidity.

Absolute Humidity

Absolute humidity is usually expressed as the weight of water vapour in a unit weight of air, ie as grams of vapour per kg of dry air.

Relative Humidity

Relative humidity (RH) is a measure of the amount of vapour in the air at any particular temperature, expressed as a percentage of the vapour that can be carried by the air when it is saturated at that temperature. The RH is zero when the air is completely dry and 100% when it is saturated.

RH is the more useful term where drying of timber is concerned and it is important to recognise the significant effect that temperature has on RH.

If a sample of air is heated and no additional moisture (vapour) is added to it, its RH will decrease. If the same sample of air is cooled below its original temperature and no moisture is extracted from it, than its RH will increase (in both instances the absolute humidity remains the same).

Dew Point

Dew point is the lowest temperature at which air can hold a given quantity of vapour. When air with a certain absolute humidity is cooled to the stage where its RH becomes 100%, this temperature is called dew point. Any further cooling would cause some of the vapour to condense as water droplets. This is what happens when dew forms on the grass. It is also what happens when steam or mist or clouds can be seen.

Measurement of RH

RH is a measurement of the drying potential or drying ability of the air and is usually obtained using dry bulb and wet bulb thermometers. A dry bulb thermometer is simply an ordinary "mercury-in-glass" thermometer. A wet bulb thermometer is an ordinary dry bulb thermometer which has a cloth wick wrapped around the bulb. The end of the wick is dipped into a small bottle of clean water. It absorbs the water and the water evaporates into the surrounding air.
Heat known as "latent heat of vaporisation" is required to change liquid water at a certain temperature into water vapour at the same temperature. In the case of a wet bulb thermometer the latent heat necessary to change the water on the wick into water vapour, comes from the surrounding air and from the thermometer bulb itself. This loss of heat reduces the temperature of the bulb and a lower temperature is indicated on the thermometer. The temperature reading on the wet bulb and the temperature reading on the adjacent dry bulb are then applied to humidity tables, to give a reading of RH.

The difference in temperature between the two thermometers is called the wet bulb depression (WBD).

Evaporation from the wick is fast on a dry day. This makes the WBD greater (or wider) than on a day of high humidity when evaporation is low. Evaporation from the wick also depends on temperature. Increasing the dry bulb temperature (DBT) of air, reduces its RH and this increases the evaporation rate. Canvas water bags are a practical example of the action of latent heat of evaporisation. Water seeps through the bag and evaporates. However, to evaporate, the water requires latent heat, and some of this heat is drawn from the water in the bag, thereby cooling it.

A stack of timber left to dry provides another example. The temperature on the leeward side of an air-drying stack in hot weather is usually pleasantly cool because heat has been drawn from the air, to supply the latent heat necessary to evaporate the moisture coming from the timber.

Temperature

Temperature is a measure of the level of heat in a solid, a liquid, or a gas.

Any increase in the temperature of the drying atmosphere increases the rate of evaporation from the surface of timber because:
(a) the rate of transfer of heat to supply the latent heat of vaporisation is increased:
(b) the RH is decreased resulting in the moisture holding capacity of the air being increased.

Any increase in the temperature of the timber being dried increases the rate of transfer of moisture from the in side to the outside of the timber. This is because of the increase in vapour pressure (or driving force) of the moisture in the wood, which increases the moisture diffusion rate through the wood to the surface.

Good air-drying depends very much on the prevailing climate. It is relatively easy and economical to remove the free moisture in the cell cavities of the wood by air-drying.

However, below fibre saturation point (FSP) as the combined moisture in the cell walls is removed, the rate of air-drying becomes progressively slower. It becomes more difficult to remove the combined moisture and, of course, air-drying can never dry timber below the prevailing EMC. If lower moisture content or faster drying below FSP is required, than kiln drying must be used. It is common practice to kiln dry after preliminary air-drying. In conventional kiln drying, the dry and wet bulb temperatures (and RH) can both be controlled so that timber is seasoned with a minimum of drying degrade in as short a time as possible. Drying conditions for the high temperature drying of softwoods are very severe.

These species can tolerate such conditions, but for most hardwoods there has to be a controlled balance between RH and temperature. Evaporation from the surface may be too fast if the RH is too low. This can cause drying stresses to develop.

Drying schedules are designed so that the rate of evaporation from the surface is usually a little ahead of the rate of moisture transfer from the centre of the surface. In a solar kiln, the daily increases in temperature, reduces the RH. The temperature increase also improves moisture transfer within the timber. These kilns rely on an increase in temperature to reduce RH.

The principle of dehumidifier kilns is to reduce the RH of the kiln atmosphere by condensing moisture on the evaporator coils of a refrigeration (air-conditioning) unit. The water, which condenses on the coils, is run through a drain to the outside.

EMC and Psychrometric Charts

Equilibrium moisture content (EMC) was defined as the moisture level reached in timber when there is a balance or state of equilibrium between atmospheric humidity and the moisture in the timber. Clearly EMC will change with changes in RH and temperature.

A psychrometric chart or psychrometric table relates dry bulb temperature, dry bulb depression and RH. The chart also includes corresponding EMC values.

Some examples of this chart (shown below) are:
(1) at Dry Bulb thermometer (DBT) of 300 and Wet Bulb Depression (WBD) of 60 the RH is 60% and the corresponding EMC is 10.5%.
(2) at DBT of 800 and WBD of 200 the RH is 40% and the EMC is 5%.

 

Absolute humidity and dew point temperature are not of great practical importance as far as kiln drying is concerned, but it is interesting for example, to compare the very large increase in the actual moisture held in the air (its absolute humidity) by raising the temperature.

Compare 300 DBT and 50 WBD = 67% RH and
18 g/kg absolute humidity.

With 700 DBT and 100 WBD = 61% RH and
148 g/kg absolute humidity.

Air Circulation

Adequate air circulation is necessary for successful timber seasoning because air is the transfer medium, taking heat to the timber and removing moisture from it.

Good access to the breeze provides sufficient circulation for air-drying. Care must be exercised when operating an air-drying yard, to take best advantage of the prevailing breeze. Stacks should be raised from the ground and they must not be placed too close together, or near buildings and trees which might create a wind shadow. They should be placed parallel to the direction of the prevailing wind.

Airflow must be uniform and adequate through all parts of the stack. Airflow is measured between sticker spaces on the air-exit side of the stack.

Recommended airflow rates for various kiln-drying systems are as follows:

• High temperature drying 5m/sec
• Conventional drying 2m/sec
• Solar and dehumidifier drying 1m/sec.

The importance of Relative Humidity, Temperature and Air Circulation to timber seasoning can be summarised as follows.
(a) The average EMC for an area or for a set of kiln conditions is strongly dependent upon the average RH (or WBD).

(b) The WBD is a measure of the drying potential of the air.

(c) Heat known as latent heat of vaporisation is required to change water from the liquid state to the vapour state.

(d) An increase in air temperature will:

• Reduce RH thereby improving the drying potential of the air;
• Increase the supply of latent heat required for evaporation of moisture from the timber;
• Increase the movement of moisture from the centre of the timber towards the outside.

(e) Adequate and uniform air circulation is required to transfer hot dry air to the timber and remove cooler moist air from the timber.

The correct combination of RH. temperature, and air circulation is vital to the economics of any seasoning operation and to the avoidance of drying degrade.

Testing Moisture Distribution

"Moisture distribution" introduces a new term, "moisture gradient". When timber begins to dry, moisture is evaporated from the surface layers, which soon fall below fibre saturation point.

If the moisture distribution of a piece of green timber, which has been drying for a short period is examined, it will be found that the moisture content varies from its highest value at the centre of the pieces to its lowest at the surface. This condition is known as a "moisture gradient". A check of the moisture gradient can usually show what sort of drying has been given to the piece.

The gradient is said to be steep and the drying conditions have been severe when the moisture content decreases rapidly from the centre. For small gradients, the drying conditions have not been severe. The importance of maintaining relative small moisture gradients throughout the seasoning process will become more evident when we consider the formation and prevention of drying stresses, particularly in relation to kiln drying.

There are three types of moisture meters. Electrical moisture meters, Resistance-type meters and capacitance-type meters. The resistance-type meters are the more convenient moisture meter to use. Capacitance-type meters are often used in the plywood and veneer industry because they do not damage the surface of the timber and they can be as accurate in the very low moisture content range.

Some very cheap rudimentary meters are available on the market, however these should be avoided. There is not much that can go wrong with most of the reliable brands, but if ever there are doubts about meter readings, instruments can be checked by using a set of standard resistances.

Factors Affecting Readings

Correction data for species and for density are required when using the resistance type meter and the capacitance type meter respectively. Before using any moisture meter, the availability and reliability of the correction data applicable should be established.

Special thought should be given to the fact that the resistance type meter will always register the moisture content at the wettest level between the electrodes. Even if nail electrodes are driven into the centre of a piece of timber, say 75 mm thick, only the surface reading will be indicated if the timber has been recently surface wet.

Moisture meters are normally calibrated for testing timber at 200c and the importance of using the temperature corrections, particularly during summer when the day temperature is likely to be over 300c. Corrections must be made for temperature before the species corrections are made.

Providing the precautions and general hints for correct use of the moisture meter are kept in mind, the instrument can be a great aid as a fast and relatively efficient method for indicating the moisture content of most timbers.

The most important precaution in the use of moisture meters is that the readings should not be taken for granted. Meters are used to predict the uncorrected MC from a reading of electrical resistance. It is then necessary, and also most important, to apply corrections for temperature and for species, in that order.

A certain amount of reliability or accuracy is always lost when predictions are determined from corrected readings with resistance type meters. For example a reading of 10% would reflect an actual moisture content between about 9.5% to 10.5%.

• A reading of 15%, actual moisture content between about 13.5% to 16.5%
• A reading of 20%, actual moisture content between about 17% to 23%
• A reading of 30%, actual moisture content between about 22% to 38%.

As you can see, the higher the reading the less accurate. It is fortunate that this reliability is at its best where moisture content testing is most critical, ie in the region below about 20%.

Providing moisture meters are used intelligently they can be of great benefit, particularly in checking timber that is supposed to have been dried. They can probably be used alone, in say a timber yard or a building site, but in a timber seasoning operation they should be used in conjunction with the oven dry method of moisture content determination

 

Notice :

I regret I have lost the information regarding the author of the above and am, therefore, unable to link to the original.

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