Kiln drying wood can play a crucial role in the production high quality, square-edged timber as kilns can allow for precise control of factors influencing the drying process: temperature, relative humidity and air circulation.
A wood drying kiln is usually defined as a heated chamber operating at a controlled temperature, usually above 50 degrees Centigrade, with 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 using dehumidifier kilns to dry wood, solar kilns and using high temperatures to kiln dry wood.
The main attractions of kiln drying wood:
Importantly however, all the above potential advantages of kiln drying wood will only acrue if the kiln is operated correctly. If a kiln is not operated carefully and correctly great damage can be very quickly inflicted on the wood within the kiln severely reducing its value.
In a conventional wood drying kiln the air temperature, relative humidity and air circulation can be controlled to be at optimum levels:
Increased temperature increases the rate of moisture movement. Wood drying kilns can be heated using:
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 costly drying defects it is also important to control the relative humidity of the air.
The relative humidity within a kiln can be increased using:
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 a thermometer 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 the "wet bulb" is cooled. The dry and wet bulb temperatures are used to determine the relative humidity.
To maintain the optimal rate when kiln drying wood it is important that over-moist air near the wood surfaces is replaced by drier air. In the early days of kiln drying wood, 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 passes over the board surfaces 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.
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 when kiln drying wood 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.
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 wood 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 kiln schedules are available for kiln drying wood of different species (and section sizes): see U.S. Dept. of Agriculture, Forest Products Dept. "Dry Kiln Schedules for Commercial Woods - Temperate and Tropical"
Below is an example of a kiln schedule (here for European Larch – up to 38 mm thick) Note: Wood moisture content is expressed as the weight of water in a piece of wood divided by the weight of the piece once dried to a constant weight in an oven operating at greater than 100C, - expressed as a percentage: this is known as the wood moisture content expressed as the "percentage oven dry basis". Dry and wet bulb temperatures are in degrees Centigrade and Relative Humidity is as a percentage:
| Stage | Wood Moisture Content (% o.d. basis) | Wet Bulb Temp. | Dry Bulb Temp. | Relative Humidity (%) |
|---|---|---|---|---|
| 1 | Green | 60 | 55.5 | 80 |
| 2 | 50 | 60 | 54.5 | 75 |
| 3 | 40 | 60 | 52 | 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 40%
Generally it is bad practice to mix sizes or species when kiln drying wood.
As well as periodically checking the moisture content, the stresses being generated in the stock must also be monitored if the serious defect of case-hardening is to be avoided.
Often, to maintain a high average drying rate 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. Conditioning is also useful in releasing unacceptably high stress which may have developed when kiln drying wood.
The commonest type of equipment used for kiln drying wood are the “heat and vent” type “compartment kilns” which dry wood in batches:-
These can be categorised by the location of the fans used to circulate the air. Thus there are:-
These ulilise a number of fans on a longitudinal driven shaft (below, top) or on one or more shafts running across the kiln (below, lower):-
The cross-shaft type is the most popular used in the UK for kiln drying wood. One or more fans are used to circulate the air through the stickered stack of timber.
These are also popular:-
Kilns should always be run full to reduce air "short-circuiting”, when air fails to pass through the stack of wood.
Originally, compartment systems used for kiln drying wood were built of brick (cavity-wall), but these suffered from mortar degradation due to the acids released from the wood. Nowadays, they are usually constructed from double skinned sheet aluminium or steel, well insulated with glass wool. From a point of view of energy conservation it is imperative that there are good seals around the doors. The fans can be external or internal.
Here an operator periodically checks the moisture content of the drying wood and changes to the next schedule stage as necessary. A good, reliable operator is vital!
Here the change in drying environment is made at pre-set times (there is no automatic measurement of the moisture content of the wood).
Automatically self-correcting systems. Here the moisture content of the wood is measured electrically and the equipment automatically selects the next stageof the drying schedule when required.
There is increasing computerisation when kiln drying wood and (fortunately) increasing importance being given to quality control.
Find out how to avoid common, costly drying defects such as Case Hardening and other value reducing Drying Defects.
Also, it is well worth considering the The Advantages of Kiln Drying Timber Compared to Air Seasoning of Timber.