Quality Drying

Achieving quality drying
while minimising drying time

Optimal timber drying can be described as minimum drying time with minimal drying defects. In order to approach quality drying, we need to differentiate between inherent defects and drying defects in timer. Inherent defects are defects that are present in the timber irrespective of how the drying is performed. Drying defects are defects specifically caused by the drying process.

For the purpose of this article, drying defects are summarised as the following:

     1.  Surface and internal checking
     2.  Distortion
     3.  Discolouration
     4.  Inadequate control of final moisture content

While it is easy to achieve drying quality when sufficient drying time is allowed, the challenge lies in minimising drying time without increased drying defects.

This article will discuss, in brief, the physics of moisture movement during the drying process, stress development and methods to reduce drying time.

Model of moisture transport

Between the wood surface and the air stream, a thin layer is located which is called the boundary layer. Heat and mass transfer happens in this boundary layer. Heat is transferred from the boundary layer to the wood surface while moisture is transferred from the wood surface to the boundary layer. The factors affecting the rate of moisture transport from the wood to the air stream are the following:

1.  Temperature of the air stream
2.  Relative humidity of the air stream
3.  Air velocity
4.  Temperature and vapour pressure of the wood surface layer

Internal moisture movement

Moisture is transported between adjacent layers.

At layer number X the following takes place:

• Moisture is transported into layer no' X from layer no' X+1. The driving force for moisture movement is the partial vapour differential (dP/dx) below fibre saturation point (FSP) and a combination of capillary pressure and vapour pressure differential above FSP.
• At point X, the change in moisture content is determined by the difference between moisture flow into the layer and moisture flow out of this layer.

In order to achieve good drying, a steady flow of moisture to the surface is necessary. The surface layers must not dry faster than that of the rate of the moisture flow to the surface, otherwise the surface layer will dry excessively with respect to the average of the board. This will lead to large moisture gradients and excessive surface stress.

Stress development during drying 

As wood dries below FSP, shrinking starts to take place. Shrinkage is proportional to moisture loss below fibre saturation point.

Going back to our moisture transport model, we notice that there will always be a moisture gradient during drying. This is necessary as the partial vapour pressure differential (which will be zero if there is no moisture gradient) is the driving force behind moisture movement from the core to the surface. Applying the principle of shrinkage to our moisture transport model we will see that a board will start to deform as depicted in the following diagram:

Shrinkage applied to moisture transport

We know that a board retains its rectangular shape, so this is not really possible. The result is that the surface layer will shrink less than it wants to (as driven by the shrinkage when not constrained by the rest of the board). The rest of the board exercises an opposite compressive force to restrain the shrinkage. As a result tensile stress develops in the surface layer and compressive stress in the core.

Stress development during drying

This tensile stress will increase as the moisture loss in the surface layer progresses. After some time the surface stress will reach a maximum and then decrease. At the same time a compressive stress will develop in the core. This happens because there has to be a zero resultant stress in the board. (The sum of the stresses must be zero because the board stays square).

At a certain stage the surface develops into compressive stress and the core goes into tensile stress. This is the point referred to as stress reversal. As can be observed from the stress development diagram, the surface stress continues to go into negative territory while the core stress goes further into tensile stress as drying progresses. At the end of drying there are residual stresses, which need to be relieved. This is done through a process called conditioning.

There is a maximum stress perpendicular to the grain that wood fibres can withstand. This maximum stress is a function of temperature and specific wood properties. If the tensile stress exceeds that maximum allowable stress level, a surface crack or check will appear.

What is the point of all this?

The classical drying schedule for quality-dried timber is typically as follows:

Typical drying schedule

The dry-bulb and wet-bulb trends can be inverted. We observe that there are three drying regimes: careful drying, medium and aggressive drying. In the initial stage, careful drying is practiced and later a more aggressive drying approach can be applied. The general rule of thumb is that we can dry faster once the transition through Fiber saturation point has been made.

Why is it necessary to dry carefully in the beginning? This is crucial to the development of surface checking. If we dry too aggressively in the beginning the surface layer will dry too fast compared to the rest of the board, and we run the risk of excessive stress development.

Why is an understanding of stress development important in our analysis of drying schedules? As explained surface checks occur when the tensile stress in the surface exceeds the maximum allowable stress perpendicular to the grain. We need to do two things:

     1.  Control the level of the maximum stress.
     2.  Refrain from changing to aggressive drying before we passed the maximum stress.

Once the stress peak has passed the surface stress level will go down. Increasing the drying rate will not make the stress levels go up again.

Having knowledge of the position (in time and moisture content) of the peak stress level is crucial to optimising drying. Once we have passed the position of maximum stress, the drying can be changed to aggressive conditions without risk of developing surface or internal checks. According to mathematical models that I have worked on, the maximum stress is predicted to be around 40 to 45% moisture content but this is dependent on the particular drying schedule used.

If a very mild schedule is used the maximum stress happens at 35 to 40%, while using relatively aggressive drying in the beginning will result in a higher stress peak, which will happen earlier.

Distortion

Distortion can be defined as the departure of boards from the original plane causing uneven shrinkage.

Studies have shown that there is very little distortion at 18 to 20% moisture content. Distortion increases significantly with every percentage moisture content below 13% moisture content. The best way to control distortion is to prevent overdrying. The key point here is reliable end-point determination of drying. Preventing overdrying (a) saves drying time (b) increases kiln throughput. The advantages of stack restraint cannot be over emphasised.

Discolouration

Discolouration (brown stain or yellow stain) happens because of a chemical reaction of sugars in the wood (Maillard reaction.) This is aggravated with higher temperatures. The way to reduce discolouration is to dry at lower temperatures, but extreme high air velocity is needed if short drying times are required.

It is a theoretical possibility to increase the temperature at a later stage, once the evaporative plane has receded into the wood.

Alternatively one can deal with discolouration by planning off a layer from the surface of the wood. Studies in New Zealand suggest that the latter approach is the most economical.

Inadequate control of final
moisture content variation

The object of final moisture content control can be stated as follows:

     1.  Drying to the correct average moisture content.
     2.  Minimising the standard deviation (or moisture spread) around the average.

For example: Drying to an average moisture content of 10% with a spread from 5 to 17% moisture content is not an example of good drying.

To achieve accurate control of final moisture content, a reliable method of end point determination is essential. The operator needs a degree of certainty of the average moisture content of a kiln charge before the kiln is stopped and the moisture checked. Stopping kiln charges, checking the moisture content and then restarting the kiln charge, is not optimal drying.

To minimise the standard deviation in moisture content an equalisation schedule is needed for the final stages of drying. This can be implemented as the following: When an average moisture content of 12% is reached, the drying conditions are altered to an EMC (equilibrium moisture content) of 6% moisture content. This condition is maintained for a period between four to 10 hours (depending on standard deviation at 12% moisture content).

The effect of this final equalisation is: the boards at 14 to 15% moisture content will dry down to 11 to12% while the boards at 9% will typically lose only 1%.

What is important here is that the timing of equalisation needs to be accurate. When equalisation is introduced too early (say at 14% average moisture content) we will require 12 to 18 hours for the wetter boards to dry down to below 12% moisture content.

If equalisation is introduced too late we already have overdrying and the associated defects such as distortion.

This process unfortunately requires additional time. The alternative is to use an in-line moisture meter in the dry mill to sort wet boards from the rest of the charge. An in-line moisture meter can mark all the boards above 12% moisture content (or whatever the cutoff point is) for the graders to remove.