WOOD DRYING SYSTEMS
Drying Systems
Convection drying systems consistently feature
- a source of energy to drive diffusion of water in wood and evaporation of water from the wood surface,
- a source of airflow to remove the evaporated water from the system, and
- often a source of vapor to provide control over humidity and drying rate.
In the air, the sun and wind provide energy and air flow. In a drying kiln, hot water, steam hot oil or hot air may supply heat, which usually are distributed in the kiln chamber by via a “heat exchanger”. A heat exchanger is a grill of pipes containing a hot medium, which exchanges heat with the kiln atmosphere. Air flow in kilns is provided by circulating fans. Vapor can be supplied by steam generators or by water sprays. To remove water from a kiln, vents are provided to release wet air and to take in dry air from the outside atmosphere.
Dried Quality
Drying processes must produce material of consistent moisture content free of internal stresses. This is difficult to achieve when the material entering the process is initially variable. Variation can be derived from difference in wood priorities (species, permeability), thickness and drying after sawing and before kiln drying.
- Poor kiln design;
- poor airflow;
- poor process control and
- poorly trained operators; can all cause process variation.
Good kiln drying practice requires consistent processing practices applied
to uniform material. The consequences of poor drying can be expensive. In
some products e.g. structural sawn wood, moisture content at the time of
utilisation may not be critical. However for some products such as flooring,
the consequences can be serious.For example, the shrinkage rate, or 'unit
shrinkage' of E. pilularis (Blackbutt), is 0.38% per 1% change in moisture
content. If for example, 100mm wide strip flooring is laid down at 4-percentage
points moisture content above the equilibrium for the sight of use, and
if the floor is say 10m wide. Then this floor will reduce in width by:
- Shrinkage = 4 X 0.38mm/100mm X 10m = 152mm
In such cases as flooring and other stability sensitive products (e.g. furniture), wood is often brought to equilibrium in the kiln by applying equivalent equilibrium temperature and humidity conditions at the end of a drying cycle, to the proposed sight of use. Kiln operators use a psychometric chart to determine what conditions are required.
High Temperature Drying
Drying of plantation grown softwood has been developed in Australia and New Zealand to deal with distortion that occurs due to the variation in grain angle associated with juvenile and mature wood. At the time of the first harvests of plantation grown softwood, drying processes at the time (primarily air drying or low temperature kiln drying) produced wood that was unusable due to distortion. During the 1970’s experimental work applying drying temperature above the softening point of lignin and large restraining weights took advantage of the viscoelastic properties of wood to produce straight timber. The process has undergone major development to the present day and is unique in world terms. Drying temperature up to 200ºC are applied in some systems, and material that is exclusively juvenile wood can be dried to a quality fit for use as structural framing timber.
