General Principles Of Wood

The drying of wood involves a heat and mass transfer phenomenon. When the wood is dried, the surface dries ahead of the interior because of direct evaporation of moisture at the surface. As a result a vapour pressure/ moisture dradient is set up which causes the moisture to move from the interior to the surface movement/diffusion through the wood is complex phenomenon and function of extrenally applied conditions and wood characteristics.

Phase 1

During the initial green stage, surface evaporation is balanced against the rate of outward diffusion of moisture to prevent surface cracking and case hardening. This is achieved by adjusting the equilibrium moisture content(EMC) of drying air without rasing the temperature too high.

Phase 11

Intermediate stage, when EMC of the drying air can safely be reduced to accelerate the drying process. However, temperature should be controlled to guard against other defects such as compression, internal cracking, wraping, etc.

Phase 111

The final stage, when the core has started drying below the fiber saturation point. Now the EMC of the drying can be reduced and the temperature raised without much risk.

During tall the above phases of wood drying, internal stresses can develo suitable measure should be taken to release the stresses to avoid degradation of timber.

The various factors which influence drying rate is described below:

a: Temperature

b: Relative Humidity (RH)

c: Air circulation

d: Spices

e: Initial Moisture Content

f: Thickness

g: Stacking

h: Relationship Between Process Parameter