Air Circulation
Air circulation is of the utmost importance, since no drying whatever can take place when it is lacking. The evaporation of moisture requires heat and this must be supplied by the circulating air. Moreover, the moisture laden air must be constantly removed and fresh, drier air substituted. Probably this is the factor which gives more trouble in commercial operations than anything else, and the one which causes the greatest number of failures.
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.