Fresh cut wood contains a great deal of water. If the water is not removed, the wood can’t be used to produce a high quality finished product. The equipment that is used for drying wood is termed as wood drying machinery or wood drying equipment. Properly dried wood sells for a higher price and is much easier to work with than wood that hasn’t been dried. When wood is dried right and suitably stocked in wood drying rack afterwards, it machines better, glues better, and finishes better. Drying also improves the strength of the wood, kills infestations, hardens pitch, preserves colour, reduces weight and controls shrinkage. Wood that is not dried under controlled wood drying methods is prone to warping, staining, and other degrade that diminishes its selling price and workability.
Classification of Woods based on Drying
The timbers are classified as follows according to their ease of drying:
Highly refractory woods: These woods are slow and difficult to dry if the final product is to be free from defects, particularly cracks and splits. They require considerable protection and care against rapid drying conditions to obtain best results.
Moderately refractory woods: These timbers show a moderate tendency to crack and split during seasoning. They can be seasoned free from defects under moderately rapid drying conditions.
Non-refractory woods: These woods can be rapidly seasoned to be free from defects even by applying high temperatures in an industrial wood drying kiln.
Working of Wood Drying Machinery
The main aim of a wood dryer is to increase the thermal value of wood. In the process of drying, the removal of moisture out of materials is carried out by means of exhalation and withdrawal of generated streams. All wood dryers are classified, generally by the ways of warmth admission to the surface of the drying materials. There are contact, convection, radiation, sublimation, high-frequency and combined dryers. Convection dryers are more effective for dispersed materials as a desiccative agent has a continuous contact with wood. Depending on the material characteristics and the size of fragments, there are different constructions of the dryers drumming, belt, with a “boiling” layer, aero fountain and others. The speed of drying process depends on the temperature of the wood drying kiln machinery, relevant speed of its moving, specific surface of the drying fragments and physical characteristics of the material. At the beginning of the drying process, the humidity of the material is constant to a larger extent. After a certain period of wood drying time when the concentration of moisture at the material surface goes down as the result of exhalation, the period of falling drying speed and the temperature of the material begins to increase. As moisture content goes down the temperature of the dryable fragments approximates to the temperature of a desiccative agent. Thus, there are some limits for the temperature of the used desiccative agent in connection with the possibility of fine dispersed dry wood fragments ignition.