Commercial success in drying therefore requires that the substance be exposed to the air in the most efficient manner; that the temperature of the air be as high as the substance will stand without injury, and that the air change or movement be as rapid as is consistent with economical installation and operation. Conditions of success therefore require the observance of the following points, which embody the basic principles of the process: (1) The timber should be heated through before drying begins. (2) The air should be very humid at the beginning of the drying process, and be made drier only gradually. (3) The temperature of the lumber must be maintained uniformly throughout the entire pile. (4) Control of the drying process at any given temperature must be secured by controlling the relative humidity, not by decreasing the circulation. (5) In general, high temperatures permit more rapid drying than do lower temperatures. The higher the temperature of the lumber, the more efficient is the kiln. It is believed that temperatures as high as the boiling point are not injurious to most woods, providing all other fundamentally important features are taken care of. Some species, however, are not able to stand as high temperatures as others, and (6) the degree of dryness attained, where strength is the prime requisite, should not exceed that at which the wood is to be used.
Different Treatment according to Kind
The rapidity with which water may be evaporated, that is, the rate of drying, depends on the size and shape of the piece and on the structure of the wood. Thin stock can be dried much faster than thick, under the same conditions of temperature, circulation, and humidity. Pine can be dried, as a general thing, in about one third of the time that would be required for oak of the same thickness, although the former contains the more water of the two. Quarter-sawn oak usually requires half again as long as plain oak. Mahogany requires about the same time as plain oak; ash dries in a little less time, and maple, according to the purpose for which it is intended, may be dried in one fifth the time needed for oak, or may require a slightly longer treatment. For birch, the time required is from one half to two thirds, and for poplar and basswood, from, one fifth to one third that required for oak.
All kinds and thicknesses of lumber cannot be dried at the same time in the same kiln. It is manifest that green and air-dried lumber, dense and porous lumber, all require different treatment. For instance, Southern yellow pine when cut green from the log will stand a very high temperature, say 200 degrees Fahrenheit, and in fact this high temperature is necessary together with a rapid circulation of air in order to neutralize the acidity of the pitch which causes the wood to blue and discolor. This lumber requires to be heated up immediately and to be kept hot throughout the length of the kiln. Hence the kiln must not be of such length as to allow of the air being too much cooled before escaping.
Temperature depends
While it is true that a higher temperature can be carried in the kiln for drying pine and similar woods, this does not altogether account for the great difference in drying time, as experience has taught us that even when both woods are dried in the same kiln, under the same conditions, pine will still dry much faster, proving thereby that the structure of the wood itself affects drying.
The aim of all kiln designers should be to dry in the shortest possible time, without injury to the material. Experience has demonstrated that high temperatures are very effective in evaporating water, regardless of the degree of humidity, but great care must be exercised in using extreme temperatures that the material to be dried is not damaged by checking, case-hardening, or hollow-horning.
The temperature used should depend upon the species and condition of the material when entering the kiln. In general, it is advantageous to have as high a temperature as possible, both for economy of operation and speed of drying, but the physical properties of the wood will govern this.
Many species cannot be dried satisfactorily at high temperatures on account of their peculiar behavior. This is particularly so with green lumber.
Air-dried wood will stand a relatively higher temperature, as a rule, than wet or green wood. In drying green wood direct from the saw, it is usually best to start with a comparatively low temperature, and not raise the temperature until the wood is nearly dry. For example, green maple containing about 60 per cent of its dry weight in water should be started at about 120 degrees Fahrenheit and when it reaches a dryness of 25 per cent, the temperature may be raised gradually up to 190 degrees.