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When a moisture gardient has been set up following surface drying, the difference in vapour pressure between the core and surface may be regarded as the driving force causing the outward movement of moisture. The vapour pressure exerted by water or by wood containing water rises very rapidly with increasing temperature and the rate of increase itself is greater the higher the moisture content of the wood. To illustrate the effect of temperature on drying rate it can be estimated that a piece of wood having a surface moisture content of 16% and a core moisture content of 40%, the vapour pressure gradient across the wood at a temperature of 50 degrees centigrade is four times greater than that at 20 degrees centigrade and at 80 degrees centigrade it is more than eight times as large. In kiln drying, there are further advantages in the use of high temperatures in that the capacity of air for holding water vapour, and hence it's drying potential, increases rapidly with temperature and the amount of air which has to be exhausted, and hence the heast lost in this way, is reduced. Furthermore the rate at which a load can be conditioned to obtain a reasonably uniform final moisture content is increased. Unfortunately, the considerable benefits obtainable by raising the temperature at which wood is dried cannot always be fully realised because there are limits to the temperatures which various species will tolerate without detriment (Pratt 1974).
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