Vapor Drying: the Artificial Seasoning of Wood in Vapor of Organic Chemicals

The first commercial application of the principle of utilizing vaporized media other than steam or fixed gases to the drying of wood was effected in 1945 at the Spartanburg, S.C. plant of the Taylor-Colquitt Wood Preserving Company. The process employs any of a large number of organic solvents that are immiscible in water and have an initial boiling point between 212?F. and 400?F. The semi-commercial pilot plant used in the development of the vapor-drying process consists of a conventional pressure impregnation unit which was adapted for vapor drying by installing the four units required for such a conversion, viz., an evaporator, condenser, separator, and condensate return tank. In carrying out vapor drying tests, the poles of other timbers in each charge are weighed individually and measured to obtain length and other dimensions. With round timbers, diameters are taken at butt, midpoint and top. A check on the total charge weight is obtained by means of the cylinder scales when the charge is placed in it. After introduction of vapor to the cylinder is begun, readings are taken at half hourly intervals to determine water removal, vapor temperatures, and internal temperatures of the wood. The water removal is determined directly by separating from the drying agent all water received from the condenser in a half hour-interval. The separator used in this instance is a cone-shaped tank, similar to a separatory funnel with sight glasses at the bottom of the cone for making the batch separations. During the vacuum, after completion of the cycle of heating in vapor, these readings are continued. At the end of the vacuum the charge is removed from the cylinder for weighing and measuring, unless the drying is to be followed immediately by impregnation. Although the rapidity of drying by means of the vapor process is accomplished by exposure of the wood to vapors condensing at relatively high temperatures, the results obtained from the standpoint of incidence of checking and splitting and other factors tending to degrade the finished product are as a rule much better than those obtained by conventional seasoning or conditioning practices. The amount of shrinkage obtained in most species is normal, and this has led to the conclusion that, although drying stresses comparable to those produced by other methods are developed in the wood, the duration of stress applied to given laminae is so short, because of the rapid drying, that the strength-time limit of resistance of the wood to failure is not exceeded. Detailed records are given of operating conditions and test results of 35 charges of southern pine poles dried in the pilot plant.

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