An Evaluation of Factors Affecting the Rate of Drying of Round Western Hemlock During the Boiling Under Vacuum Process

Experiments were conducted to establish a mathematical relationship between factors which cause the removal of moisture from wood and the moisture removed or condensate collected. Factors which are measurable and influence the amount of water boiled from wood are bath temperature, vacuum, time, size of specimens, initial moisture content and specific gravity. Freshly-cut western hemlock logs ranging from 12 to 18 inches in diameter were obtained in 34-foot lengths. On arrival, they were peeled and sprayed with a water-soluble sap-stain preventative, protected on both ends with an application of resorcinol glue, and covered with canvas to retard evaporation. Before being treated, logs were cut to 16-foot lengths and the fresh cuts were protected with resorcinol glue. Each 16-foot log was treated alone in the retort. A series of 14 experiments were run in which temperatures were maintained constant through a range from 150? to 210?F. The vacuum, which was constant in each experiment, ranged from the lowest noticeable boiling point up to 28 inches maximum. Readings of water boiled from the wood were recorded hourly for approximately 60 hours. A formula to express the amount of moisture removed per cubic foot from the experiments, as calculated by successive approximation, was developed. A comparison of the condensate calculated by this equation with the actual condensate collected from each log shows, except for four logs, that the calculated amount is quite accurate. The large discrepancy between actual and calculated condensate in four logs may indicate that there have been experimental errors or that an additional factor such as heat conductivity must be included in the equation.

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