The Kiln Drying of a Load of Timber

The Kiln Drying of a Load of Timber

The guidance in this section is arranged to follow, as far as possible, the sequence of events during a typical kilning operation. The basic recommendations in the drying schedules appended are conservative and in normal circumstances can be expected to result in satisfactory drying. However, with kiln drying the overall aim is to dry timber to an acceptable standard in the shortest possible time and the guidelines are therefore not intended to be rigid. Often there may be scope for further reductions in drying times.

Preparation of the Load

i.  Examination of the Timber

Although kiln drying conditions are primarily selected on the basis of timber species and sizes to be dried, the initial condition of the timber is also an important consideration which often influences the way the timber should be dried. It is therefore suggested that an examination of the timber should be carried out before drying to establish:

i.  The range of moisture content

ii.  The extent of any degrade associated with previous drying

iii.  Any presence of fungal infection

iv.  Any presence of wood boring insects

v.  The general quality of the timber

In addition such an examination may allow assessment of the extent of other timber imperfections such as discolouration caused by exposure to the weather or by chemical staining. In countries which import timber, examination of the timber received is particularly important as this will already have been subjected to partial drying.

As well as providing guidance for correct drying, an examination of the timber enables the selection of appropriate samples of wood for monitoring the progress of drying and provides information which may be used in estimating kiln drying times (see Appendix C).

ii.  Stacking and Handling

The importance of correct piling of timber for kiln drying cannot be over-emphasised. Before beginning to pile a kiln load, ensure that the foundation of cross bearers on the kiln floor or trolley is firm, strong and absolutely level. An irregular base may permanently distort several layers at the base, or even the whole kiln load if the material is thin or of a species which is liable to distort. Normally at this stage, consideration should be given to the positioning and incorporation of the withdrawable samples of timber in the load. It is recommended that end cleats already attached to the timber should be removed before kilning.

The usual method of piling is to arrange the boards in horizontal layers one above the other, separated as in air drying by a series of piling sticks. These sticks should be made of dry, straight-grained timber reasonably free from resin or other substances that might stain the boards. They are best made from species which do not tend to distort in normal kiln conditions. Sticks 25 x 25mm in cross-section have proved to be a suitable standard size for general drying, but 22 x 22mm sticks are suitable for use in kilns with moderate to fast air circulation. When piling material less than 25mm thick, 19mm sticks may be used to increase the output of the kiln and in some circumstances (e.g. when drying partly dried or thin refractory hardwoods) it is possible to use 13mm sticks. The smaller sticks can be cut with greater width to increase strength and reduce loss by breakage. All sticks should be kept as clean and as dry as possible and handled with reasonable care to minimise wastage. If a number of different sizes of stick are in use it is important to ensure that these are segregated to avoid the accidental use of different sizes on the same layer of a load, as this will increase the risk of distortion.

Each vertical line of sticks should be fully supported by a bearer or longitudinal stringers on the floor of the kiln or on the kiln trolley. Those at each end of the stack should be as near the end as possible and the spacing between the main rows of sticks should be varied according to the timber that is to be dried. Most hardwoods which show little tendency to distort when in thicknesses of 50mm and upward, may be piled with the vertical rows of sticks spaced at maximum intervals of 900mm, but for thinner boards it is advisable to reduce the space to 600mm. Those hardwoods which tend to distort appreciably during drying should have sticks at intervals of not more than 600mm. Boards of these species (25mm or less in thickness) are best piled with the sticks spaced no more than 450mm apart. For timber species in which the quality or grain of the wood could cause it to be particularly prone to various forms of distortion, spacing no wider than 300mm are recommended. When longitudinal stringers are not used, the sticks must be correctly positioned in vertical lines directly above the bearers if distortion degrade is to be kept to a minimum.

In all modern kilns it is recommended that the boards or planks of wood in each layer should be placed edge to edge. An exception should be made when drying squares or scantlings more than about 65mm thick where, if gaps of about 10mm are left between the pieces in each layer, significant drying from the edge faces can occur.

The adoption of a definite routine helps to achieve satisfactory piling. On completing each layer of timber the sticks should be correctly aligned. When the next layer of timber is added some of the sticks may be displaced slightly and these should be returned to their correct positions before piling is continued; realignment under several layers of wood may not be possible without risk of damaging the sticks. Piling guides are sometimes used to ensure that the sticks are placed in vertical alignment and that the vertical sides of the pile are kept even.

Although the width and height of load may be restricted by the size of the kiln door, the aim should always be to fill the length and height of the kiln. The width of the air passages between the load and the side walls should be as recommended by the kiln manufacturer to give uniformity of air flow.

When a variety of lengths has to be piled and it is not possible to combine various lengths to make up the full length of the kiln load it may be convenient to place the longest pieces at the bottom and to reduce the length of the load as the height increases, keeping one end of the load flush. Alternatively, if sorting of lengths is not practical or not economic, which is often the case, a load of uniform length may be built by box piling the timber (Figure 2).

The ends of timber, especially thin materials, are liable to distort badly if unsupported. When it is impossible to avoid incorporating long pieces which overhang the ends of the pile, special supports for these should be erected. This does not represent an ideal solution because as the load dries and shrinks, the level of the stack falls while the height of the support remains relatively unchanged, inducing a slight upward sweep at the ends of the projecting pieces. Within the kiln load, short stick lengths should be inserted both under and over the ends of those pieces which do not quite reach the vertical lines of piling sticks.

For the drying of any timber which is known to be particularly prone to distort, it is recommended that the top of each kiln load should be weighted down with concrete slabs or other suitable non-ferrous material as this will minimise the amount of degrade in the otherwise unrestrained top layers. With timber which is liable to distort it may be necessary to restrict the size of the load.

In all types of kiln in which the circulating air returns in a vertical direction to the inlet, the sides of the load should be made as even as possible. Pieces which jut out appreciably will tend to act as deflectors causing an excess of air to pass through one or two spaces at the expense of others (Figure 6, below). An advantage of the side fan kilns with horizontal air flow is that an irregular-faced load has no adverse effect on the circulation.

Figure 6 Effect of irregular pile face on air circulation

Two other points should be noted in the piling of loads for horizontal flow kilns. The gaps between the face of the load and the end wings of the fan boxes should be as small as possible or should be blanked off to prevent excessive short-circuiting. Secondly, the sticks should be arranged so that there is a vertical tier opposite each end wing to ensure that air entering the load traverses the full width before re-entering and being pulled back into the fan (Figure 7, below). Figures 8B and 8C show how incorrect piling can adversely affect the air circulation.

Whenever possible, different species and thickness should be piled in separate kiln loads. In a mixed load containing thick or refractory timbers and more rapidly drying timbers there is a risk that the former will remain insufficiently dried or that the latter will tend to become over dried. Where mixing is absolutely unavoidable the kiln treatment has to be based on the behaviour of the least tolerant material. It is therefore advisable to pile the faster drying timber on the upper part of the stack so it can be removed independently before it becomes over dried. Even so, the relatively mild conditions to which the more tolerant material is subjected will result in the drying time being longer than if it were dried separately to the appropriate schedule.

Figure 7 Piling of timber in side fan horizontal flow kilns

In any kiln the circulating air will always have a tendency to by-pass the timber through spaces inadvertently formed above, below or along the load. It is recommended that this short-circuiting should be minimised by using canvas curtains or baffles made of wood or other suitable material. Occasionally the volume of timber to be dried may be less than a full load for the kiln available. In such cases the width of the load can be reduced so that it approximately fills the height of the kiln.

Certain timber items cut to standard sizes can conveniently be piled without the use of a large number of sticks. For instance, small furniture parts such as chair leg squares can be self crossed (Figure 8, below). Here the individual pieces are so small that they can be used in place of the normal piling sticks without fear of restricting the air flow. In order that the individual stacks which make up a complete load should be stable when drying shrinkage occurs, sticks should be introduced at intervals across the full width of the load.

Figure 8 Cross-piling of dimension stock

The cost of the handling of timber constitutes quite a large part of the total cost of kiln drying, particularly in the case of material which only requires a short time in the kiln. A number of mechanical aids to piling are available, but the size of the kiln installation and the type of timber being dried usually govern the level of mechanisation which is both practical and economical.

Fork lift trucks may often be useful when the material being dried is of a reasonably uniform length and can be piled as setts approximately 900mm to 1200mm wide and of similar height. These sets can be fork lifted on top of one another and side by side to build up the full kiln load. They can either be deposited on trolleys and then run into the kiln in the usual manner, or built up directly on the kiln floor in kilns designed with full length side doors for this purpose. A slight disadvantage of the use of fork lifts is the tendency for piling sticks to become displaced during transport, especially when the setts are built of thin material. This can be reduced by introducing extra sticks in the bottom two or three layers roughly above the points where the forks will make contact.