Seasoning of Wood

by J. B. Wagner

It is necessary that the air circulate through every part of the kiln and that the moving air come in contact with every portion of the material to be dried. In fact, the humidity is dependent upon the circulation. If the air stagnates in any portion of the pile, then the temperature will drop and the humidity rise to a condition of saturation. Drying will not take place at this portion of the pile and the material is apt to mould and rot.

The method of piling the material on trucks or in the kiln, is therefore, of extreme importance. Various methods are in use. Ordinary flat piling is probably the poorest. Flat piling with open chimney spaces in the piles is better. But neither method is suitable for a kiln in which the circulation is mainly vertical.

Edge piling with stickers running vertically is in use in kilns when the heating coils are beneath. This is much better.

Air being cooled as it comes in contact with a pile of material, becomes denser, and consequently tends to sink. Unless the material to be dried is so arranged that the air can pass gradually downward through the pile as it cools, poor circulation is apt to result.

In edge-piled lumber, with the heating system beneath the piles, the natural tendency of the cooled air to descend is opposed by the hot air beneath which tends to rise. An indeterminate condition is thus brought about, resulting in non-uniform drying. It has been found that air will rise through some layers and descend through others.

Humidity

Humidity is of prime importance because the rate of drying and prevention of checking and case-hardening are largely dependent thereon. It is generally true that the surface of the wood should not dry more rapidly than the moisture transfuses from the center of the piece to its surface, otherwise disaster will result. As a sufficient amount of moisture is removed from the wood to maintain the desired humidity, it is not good economy to generate moisture in an outside apparatus and force it into a kiln, unless the moisture in the wood is not sufficient for this purpose; in that case provision should be made for adding any additional moisture that may be required.

The rate of evaporation may best be controlled by controlling the amount of vapor present in the air (relative humidity); it should not be controlled by reducing the air circulation, since a large circulation is needed at all times to supply the necessary heat.

The humidity should be graded from 100 per cent at the receiving end of the kiln, to whatever humidity corresponds with the desired degree of dryness at the delivery end.

The kiln should be so designed that the proper degree may be maintained at its every section.