Introduction
The aim of drying is "the removal of the water (sap) as economically and with as little damage as possible”.
How much water do we have to remove on drying? Well how much is present in timber when it is removed from the forest?
| Species | Heartwood | Sapwood |
|---|---|---|
| Yellow Birch | 64 | 68 |
| American Beech | 58 | 79 |
| American Elm | 92 | 84 |
| Douglas Fir | 40 | 116 |
| Western Hemlock | 93 | 167 |
| Sitka Spruce | 50 | 131 |
In the above table note the variations in moisture content between species and between heartwood and sapwood. There will also be a variation with the time of year when felled. The drying “load” on a timber kiln may also vary depending on where and for how long a stack of sawn timber has been stored.
In order to understand and better control the wood drying proces it is necessary to be familiar with a number of concepts and definitions:
This is the liquid water in the cavities of wood (the “lumens”). The removal or replacement of this water does not cause shrinkage and swelling or the material.
This is the water contained intimately within the structure of the cell walls of the wood. It is the removal or replacement of this water which causes shrinkage and swelling of the material.
This is defined as the point at which the cell walls are full of water, but there is no water in the lumen. In practice it is very difficult to determine the FSP accurately. Importantly within a piece of wood it is possible that some regions may be above FSP (and so not inclined to shrink) at the same time as others are well below FSP (and so may be shrinking or attempting to shrink).
Wood will always adjust its moisture content so as to be in equilibrium with its surroundings (but not at the same moisture content as it’s surroundings). In fact the “equilibrium moisture content” of wood depends on the “drying potential” of the surrounding air – this is determined by both the “relative humidity” of the air and its temperature. Problems commonly occur when timber adjusts it’s EMC to new conditions! Hence the need to produce products at a moisture content appropriate to the service conditions.
Note:
The percentage Relative Humidity = 100 times the amount of water vapour in a certain volume of air divided by the maximum amount of water vapour the air can hold at that temperature.
Charts exist which enable the EMC to be determined for combinations of relative humidity and temperature.
An understanding of the above fundamental concepts is critical in understanding how and why timber is optimally dried to maximise financial profit.