Forest Products Notes on...

Kiln Drying of Timber

A timber drying kiln is usually defined as a heated chamber operating above 50°C with a controlled relative humidity and using forced air circulation – this system aiming to dry wood in an optimum manner with regard to both cost and product quality. However recent innovative technologies have lead to variations on this conventional definition of kiln drying: these systems include de-humidifier drying, solar kilns and high temperature drying.

The main attractions of kiln drying of timber are:

· Stock can be dried to the lower moisture contents necessary for use in certain applications. For example, wood to be installed in a centrally heated house needs to be at approximately 12% if subsequent problems of movement are not to occur - this level of moisture content cannot be produced by air seasoning alone (at least in UK).

· Better control of the drying process is possible when kiln drying timber than when air seasoning - optimum conditions can be maintained throughout all seasons of the year so that moisture can be drawn from the wood at rates appropriate for the species.

· It is possible to produce batches of timber having all its constituent pieces dried to an exact, suitable moisture content.

· The ability to control (and change as necessary) the conditions when kiln drying timber means that the timber can suffer from less drying “degrade” (defects – see later).


In a conventional timber drying kiln the air temperature, relative humidity and air circulation can be controlled to be at optimum levels.

In a conventional timber drying kiln the air temperature, relative humidity and air circulation can be controlled to be at optimum levels:

Air Temperature

Increased temperature increases the rate of moisture movement.

Timber drying kilns can be heated using:

· hot water pipes

· steam heated pipes

· steam injection (this will modify the relative humidity)

· flue gases (although there is the possibility of staining of timber if a heat exchanger system is not used)

· electricity (expensive in many countries)

· gas/oil burners

· sun (“solar drying” using, at its simplest, a glass-house system)

The temperature within a kiln is monitored and controlled using a thermometer or thermocouple system.

Increased temperature also increases the affinity of air for water (it reduces the relative humidity), so to avoid drying defects it is also important to control the relative humidity of the air.

Relative Humidity

The relative humidity within a kiln can be increased using:

· water sprays (in fact producing a very fine mist)

· disc humidifiers (water falls onto a horizontal, rapidly spinning disk)

· steam injection

The relative humidity is reduced by opening “vents” to exhaust “wet” air and draw in drier air.

The relative humidity of the air in the kiln can be measured using a “dry bulb” thermometer and a “wet bulb” thermometer, the latter being having a piece of cloth wrapped around its bulb which is constantly kept damp – this cools the bulb by an amount depending on the “thirstyness” (humidity) of the air - the drier is the air the more it is cooled. The dry and wet bulb temperatures are used to determine the relative humidity.

Air circulation

To maintain the drying process it is important that over-moist air near the timber surfaces is replaced by drier air. In the early days of kiln drying timber, air circulation relied on a natural draught (convection) system, but nowadays forced draught systems are used using fans.

An even rate of air flow of approximately 2 m/s over the boards is recommended, but this can be increased in the latter stages of drying. It is very important that the air is passing over the board surface in all regions of a kiln if stock is not to be produced that is improperly dried: baffles can be used to direct air streams as required.

Kiln Schedules

With the passage of time the rate of drying of wood under constant conditions slows down to rates unacceptable for economic reasons

For this reason it is common practice to periodically harshen the environment within the kiln to maintain an acceptably high (average) drying rate. This is usually achieved by increasing the temperature within the kiln and reducing the relative humidity. A sequence of such different environments is called a Kiln Schedule.

The principle of a drying schedule: here time is given on the horizontal axis and wood moisture content on the vertical axis.

As shown above, at recommended wood moisture contents (and only when the species being dried can tolerate an increase in the rate of drying) the conditions within the kiln are changed (harshened) so that the moisture content of the timber continues to fall at an acceptable rate. Without these periodic changes in the drying environment the rate of drying would become slower and slower as the material equilibrates with the surrounding conditions (shown by the short-dashed lines above).


The change of conditions (usually a raising of the temperature and a reduction in the relative humidity) should only be made once the moisture content of the wood in the kiln has fallen below a specified value. The moisture content of the wood is usually determined by testing samples periodically removed from the kiln. Different schedules have been published for different species (and section sizes).


Below is an example of a kiln schedule (here for European Larch – up to 38 mm thick) Note: Timber mc is % (dry wt. Basis), dry and wet bulb temperatures are degrees Centigrade and Relative Humidity is %:

Stage Timber mc dry wet     RH

1                 Green    60  55.5    80

2                      50     60  54.5    75

3                      40     60  52.0    65

4                      30     65  53.5    55

5                      20     75  56.5    40

Thus stage 3 can only be commenced once the moisture content of the stock has fallen to below 40%.


Generally it is bad practice to mix sizes or species.


As well as periodically checking the moisture content, the stresses being generated in the timber must also be monitored.


Often, to maintain a high average drying rate in a timber drying kiln the stock is over-dried down to (say) 9% before being “conditioned” back up to the desired moisture content of (say) 12%. Such “conditioning” is also useful to ensure a consistent moisture content in the dried stock even if there is a variation in the moisture content of the material before drying commences.

Find out...

How To Avoid Costly Drying Defects in Sawn Timber

Conditioning is also useful in releasing unacceptably high stresses which may have developed in the wood as it dries.


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The principle of a timber drying schedule.

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