The most common method of drying is to extract moisture in the form of water
vapour. To do this, heat must be supplied to the wood to provide the latent heat
of vaporisation. There are several ways of conveying heat to the wood and
removing the evaporated moisture.
Nearly all the world's timber is, in fact, dried in air. This can be carried
out at ordinary atmospheric temperatures (air drying), or in a kiln at
controlled temperatures raised artificially above atmospheric temperature but
not usually above 100°C, the boiling point of water. Air drying and kiln drying
are fundamentally the same process because, with both, air is the medium which
conveys heat to the wood and carries away the evaporated moisture.
The factors which will be described are those which affect wood when dried in
air (in the open or in a kiln). There are several other ways in which wood can
be dried, in chemicals, in a vacuum etc. Under these conditions different
factors come into play.
- Vapour Pressure and Relative Humidity
To understand how wood dries in air it is necessary to introduce some
terminology. When air holds the maximum possible amount of vapour, the vapour
exerts what is called the saturation vapour pressure. If the water vapour
present is less than this maximum then the air can take up more moisture. The
ratio of actual vapour pressure to the saturation vapour pressure at any given
temperature, expressed as a percentage, is called the relative humidity (rh).
When a piece of wet wood is exposed to air which is not already saturated
(i.e. its relative humidity is less than 100%), evaporation takes place from
its surface. At a given temperature the rate of evaporation is dependent on
the vapour pressure difference between the air close to the wood and that of
the more mobile air above this zone.
- Temperature
The temperature of a piece of wood and of the air surrounding it will also
affect the rate of water evaporation from the wood surface. With kiln drying,
warm or hot air is passed over the timber and at the start of the drying
process the temperature differential between the air and the wet wood will
usually be large. As a result, heat energy will be transferred from the air to
the wood surface where it will raise the temperature of both the wood and the
water it contains. Water, in the form of vapour, will then be lost from the
wood surfaces, provided the surrounding air is not already saturated with
moisture. This results in the development of a moisture content gradient from
the inside to the outside of the wood. As the temperature is raised this
increases not only the steepness of this moisture gradient, but also the rate
of moisture movement along the gradient and the rate of loss of water vapour
from the surface of the wood.
To illustrate the effect of temperature on drying rate, a piece of wood
with a moisture content of 16% at the surface and 40% at the core will
generally have moisture gradients at 50°C and 80°C which are respectively
four and eight times greater than that at 20° C.
With kiln drying, higher temperatures also increase the capacity of the air
for moisture. An advantage of this is that less air needs to be heated and
exhausted from the kiln. In addition higher temperatures allow more rapid
conditioning of a timber load to a uniform final moisture content.
Unfortunately the considerable benefits obtainable by raising the drying
temperature cannot always be fully exploited because there are limits to the
drying rates which various wood species will tolerate without degrade.
In the drying of many species, especially medium density and heavy
hardwoods, shrinkage and accompanying distortion may increase as the
temperature is raised. So with species which are prone to distort it is normal
to use comparatively low kiln temperatures. A few species are liable to
collapse and/or honeycomb if dried at high temperatures. Many tend to darken
appreciably and, in resinous timbers, drying at temperatures above about 50°
C causes the resin to exude on to the wood surface, although this may not
necessarily be detrimental for all products or uses. Finally, since high
temperature drying may cause a slight loss in impact strength, it is not
advisable to exceed about 60° C when drying timber for items such as tool
handles and sports goods.
- Air Movement
If the air surrounding a piece of wet wood is stagnant and of small volume,
it will soon become saturated and evaporation of moisture from the wood will
stop. Even when there is a continuous stream of air passing over the wood the
layer of air in immediate contact with the wood will move more slowly and have
a higher vapour pressure than the main stream. This is known as the 'boundary
layer effect'. With increasing air velocity in the main stream this effect
decreases and evaporation rates from the wood surface increase, particularly
when the air flow is turbulent rather than laminar. An increase in air speed
can therefore be regarded as equivalent to a reduction of the humidity barrier
near the wood surfaces.
Since air passing through a stack of wet wood gives up heat and takes up
moisture it is bound to be cooler and more humid where it emerges than where
it enters and the drying rate is therefore slower on the air outlet than on
the air inlet side of the stack. The faster the air speed and the narrower the
stack, the smaller is the difference between the two sides. For this reason
fairly high air speeds are desirable in a drying kiln, particularly when the
timber being dried is very wet and loses its moisture readily. In most modern
kilns the uniformity of drying is further improved by reversing the direction
of air flow through the kiln load at regular intervals.
- Movement of Moisture in the Wood
When water evaporates from the surface of a piece of wet wood the moisture
content in the outer zone is lowered and moisture begins to move outwards from
the wetter interior. In practical terms this movement of moisture can be
accepted as being a combination of capillary flow and moisture diffusion, a
process which is resisted by the structure of the wood, particularly in dense
hardwood species. If the rate of water loss by evaporation exceeds the rate at
which moisture from the wet interior can pass to the surface, the moisture
gradient within the wood becomes progressively steeper. As the outer layers
dry below the fibre saturation point their tendency to start shrinking is
resisted by the wetter interior so that stresses develop. If these stresses
become large they can lead to a number of drying defects.
In both air and kiln drying the establishment of a moisture gradient is
unavoidable and indeed desirable, for in any particular piece of wood at a
given temperature the rate of movement of moisture up to the surface is
proportional to the steepness of the gradient. The skill in timber drying lies
in controlling the rate of evaporation to match the rate at which moisture is
reaching the surface; the aim is to maximise the moisture gradient without
damaging the timber.
- Supply of Heat
A supply of heat is required to dry timber. In kiln drying, sensible heat
is needed to raise the temperature of the wood and the water it contains to
the required drying temperature, and latent heat is needed to evaporate the
water. To give an impression of the amounts of energy involved, the drying of
15 cubic meters of timber from green to 20% moisture content by electricity
could require a minimum of 3000kWh. Energy is a major element in the running
costs of conventional kilns which vent warm, moist air to the outside and
therefore recover none of the drying energy input. As the relative cost of
energy rises, efficient heat utilisation has become a more significant factor
and much attention has been given to techniques which might reduce expenditure
on energy. These range from simple upgrading of kiln insulation to the
development of heat pump kilns.
MTC’s
