OVERVIEW

What Is Cyclic Drying?

• The most exciting revelation I have experienced during a lifetime of commercial lumber drying commercially using every conventional drying prescription available!
   

• At the same time, it is rather embarrassing to me that it appears that it has taken me almost a lifetime to learn what should have been obvious to me and the world from the outset. All the same, there is some satisfaction in having unraveled the mystery before others.
   

• Basically speaking, it is a kiln drying prescription that captures the positive and natural aspects of air seasoning and conventional drying practices without the negatives.
   

• It encourages regular cool down periods (phases) during the drying cycle. At the same time, this does not mean that extra process time is required compared to conventional drying!
   

• Ours and the findings of others conclude that improved quality outcomes are achievable by this process due to its superior stress relief effects. (details later)
   

• Suggested reading: Independent test results. “Improving The Quality Of Timber From Red Beech (N.Fusca) By Intermittent Drying. Langrish, Keey & Kumar. 1992.

A normal day and night in the life of a solarola kiln; solar heating only.

Why does this simple prescription work?

• The analogy of a modern sawmill manager working long hours is a good one. Exposed to constant stress, the manager will ultimately fail or work less than optimally without regular periods of rest. Conventional drying schedules aimed at pushing wood property limits constantly to extremes produce similar, less-than-optimal performance outcomes.
   

• One of, or perhaps the most important element of successful drying is *moisture differential. (* the difference in moisture between the inner core and outer case of individual boards)
   

• When the outer surface is dried more quickly than the core, the outer surface may shrink at a greater rate than the core and in some cases can directly cause costly splits and cracks. Inevitable stresses occur as a consequence of wood being heated and drying from the surfaces, even when drying is done well.
   

• A conventional kiln schedule typically increases temperature progressively and relentlessly throughout an entire drying cycle.
   

• The constantly increasing (unnatural) temperature increases create situations where, under “ideal” conditions the wood structure is ALWAYS close to BUT NOT EXCEEDING a point of wood failure.
   

• To maintain this stress at safe levels, exacting standards of control are required to maintain high humidity conditions to ensure that case mc does not exceed safe limits that have often been ascertained by rigorous laboratory testing. Small departures from the set conditions are typically destructive in quality terms. Wood is in constant state of stress, living on the edge at times for weeks or months on end.
   

• Methods employed to control high mc condition, case moisture levels and wood stress are numerous, including expensive energy-absorbing steam and water spray systems. They are generally successful in achieving what they set out to achieve, albeit costly and wasteful.
   

• The cooling phase of cyclic drying and air seasoning creates a balancing effect to the heat up phase. This process uses water extracted from the wood itself to maintain safe case mc levels. As activation heat energy and drying rate reduces faster at the case than the core of timber sections, the core continues to liberate moisture faster until it reaches an equilibrium temperature with the case. This creates a water queue effect at the surface (case). This process redistributes moisture naturally from within the lumber, greatly reducing stress to levels markedly below conventional drying levels. At the same time, cyclic cooling increases kiln humidity, creating a second external method of case re-saturation and stress reduction. Combined, both factors are incredibly effective for stress relief.
   

• In practice, our methods normally include three distinctly different schedule phase conditions over the course of a single 24 hour solar cycle. In some instances, the second equalization or third resaturation phases actually see the fastest drying least-dense samples actually gain mass and equalize against their more dense counterparts that lose moisture to target levels.
   

• Further work by us suggests very strongly that practice of this natural prescription of drying may in fact allow for faster than conventional drying rates AND better quality. At this point, we can at least support the fact that cyclic drying times can at least be equal to conventional drying.
   

• The implications of this fact have important flow-on ramifications that go well beyond the huge advantage of producing more from less timber in terms of quality-value and volume yield by reducing drying process wastage. (Splits and cracks, collapse and internal checking) The future implications of our findings on the world lumber industry in general are substantial; not only for solar or hybrid kilns like ours but also low-medium temperature kilns in general.