Kiln drying

“ Kiln-drying which is an artificial method, originated in the effort to improve or shorten the process, by subjecting the wood to a high temperature or to a draught of heated air in a confined space or kiln. ”

“ Moisture tends to transfuse from the hot towards the cold portion of the wood.
11. Collapse of the cells may occur in some species while the wood is hot and plastic. This collapse is independent of subsequent shrinkage.
Theory of Kiln-drying
The dry kiln has long since acquired particular appreciation at the hands of those who have witnessed its time-saving qualities, when practically applied to the drying of timber. The science of drying is itself of the simplest, the exposure to the air being, indeed, the only means needed where the matter of time is not called into question. ”

“ Kiln-drying or hot-air-seasoning is a much more rapid process than air-seasoning and is now in common use, Fig. 57. The drying is also more complete, for while air-dried wood retains from 10% to 20% of moisture, kiln-dried wood may have no more than 5% as it comes from the kiln. ”

“ There are two distinct ways of handling material in dry kilns. One way is to place the load of lumber in a chamber where it remains in the same place throughout the operation, while the conditions of the drying medium are varied as the drying progresses. This is the "apartment" kiln or stationary method. The other is to run the lumber in at one end of the chamber on a wheeled truck and gradually move it along until the drying process is completed, when it is taken out at the opposite end of the kiln. ”

“ The Moist-air kiln is better adapted to the drying of tight cooperage stock, while the Blower kiln is almost universally used throughout the slack cooperage industry for the drying of its products.
For the drying of heavy timbers, planks, blocks, carriage stock, etc., and for all species of hardwood thicker than one inch, the Moist-air kiln is undoubtedly the best.
Both types of kilns are equally well adapted to the drying of 1-inch green Norway and white pine, elm, hemlock, and such woods as are used in the manufacture of flooring, ceiling, siding, shingles, hoops, tub and pail stock, etc. ”

“ In either the blower or moist-air type of dry kiln, however, it is absolutely essential, in order to secure satisfactory results, both as to rapidity in drying and good quality of stock, that the kiln be so designed that the temperature and humidity, together with circulation, are always under convenient control. ”

“ Seasoning and kiln-drying is so important a process in the manufacture of woods that a need is keenly felt for fuller information regarding it, based upon scientific study of the behavior of various species at different mechanical temperatures and under different mechanical drying processes. The special precautions necessary to prevent loss of strength or distortion of shape render the drying of wood especially difficult. ”

“ Hardwood lumber is commonly air-dried from two to six months, before being kiln-dried. For the sake of economy in time, the tendency is to eliminate yard-drying, and substitute kiln-drying. Kiln-drying of one inch oak, takes one or two weeks, quarter-sawn boards taking one and a half times as long as plain-sawn. ”

“ Even after machinery had made rapid and startling strides on its way to perfection, the dry kiln remained practically an unknown quantity, but gradually, as the industry developed and demand for dry material increased, the necessity for some more rapid and positive method of seasoning became apparent, and the subject of artificial drying began to receive the serious attention of the more progressive and energetic members of the craft. ”

“ Lumber thus treated is apt to be more or less weakened by the action of the heat upon the fibers of the wood, which causes thousands of minute fractures, and in many cases the life and the elasticity of the lumber is destroyed. The results of kiln-drying depend largely upon the kiln, and upon the skill with which the lumber is piled, the heat applied, and the rapidity of evaporation of the moisture regulated. ”

“ This method does not so completely dry the lumber as to make kiln-drying unnecessary, but it does materially reduce the time and expense of that process. From time to time, so much of the lumber as is about to be put into veneered products is taken from the stacks and placed in a kiln where the drying is completed and the lumber readied for use. The veneers are imported from three countries. They are received in bundles and are kept in that form in piles, separated as to specie, in petitioner's plant for use as needed in the day-to-day operations of the plant. ”

“ Thorough grinding and mixing are of the utmost importance, as otherwise the cement ultimately produced will be unsound and of inferior quality. The drying of the slurry is generally effected by the waste heat of the kilns, so that while one charge is burning another is drying ready for the next loading of the kilns. The kilns commonly employed are “chamber kilns,” circular Loading the kiln. structures not unlike an ordinary running lime kiln, but having the top closed and connected at the side with a wide flue in which the slurry is exposed to the hot products of combustion from the kiln. ”

“ The varieties of corn that will best bear transportation by sea without kiln-drying, are those which contain a large proportion of oil, as the Golden Sioux, the King Philip or Northern Eight-rowed Yellow, the Dutton, the Browne, the Rhode Island White Flint, &c., but the flour or meal, made from these sorts is not so pleasant to the taste of those unaccustomed to its use, as that made from the soft farinaceous varieties of the South and West, which will be greatly improved, and preserved free from mustiness, by the process of kiln-drying. ”

“ When wet wood is piled in the sun, evaporation goes on with such unevenness that the timbers split and crack in some cases so badly as to become useless for the purpose for which it was intended. Such uneven drying can be prevented by careful piling, keeping the logs immersed in a log pond until wanted, or by piling or storing under an open shed so that the sun cannot get at them.
Experiments have also demonstrated that injury to stock in the way of checking and splitting always develops immediately after the stock is taken into the dry kiln, and is due to the degree of humidity being too low. ”

“ Generally speaking, if the kiln is properly constructed and operated, from two to four days for each inch in thickness of soft wood, and from two to three times as long, at a lower temperature, for hard wood, is usually enough to extract the moisture. It is, however, best to allow the lumber to stay in the kiln, at a moderate temperature, from three days to two weeks after the moisture is extracted, in order to harden and cook the solids of the sap, as by so doing the lumber is not so liable to be influenced by moisture in the future; this is the effect that long weather-drying accomplishes. ”

“ It is probable for the sake of economy, air-drying will be eliminated in the drying process of the future without loss to the quality of the product, but as yet no effective method has been discovered whereby this may be accomplished, because nature performs certain functions in air-drying that cannot be duplicated by artificial means. Because of this, hardwoods, as a rule, cannot be successfully kiln-dried green or direct from the saw, and must receive a certain amount of preliminary air-drying before being placed in a dry kiln. ”

“ Any repairs can also be at once done to any one of the chambers as it gets emptied in its turn. In every process of treating clay or marl, with a view to drying it in the open air or to burning it, it is absolutely necessary to carry out these operations gradually at the risk of cracking and splitting the goods. This kiln, from its very gradual action and absence of sudden changes of temperature, produces no “wasters” from these causes, and the bricks can also be burnt in a much more moist state than in ordinary kilns, as the heat is very gradually brought to bear upon them. ”

“ Operation of the "Blower" Dry Kiln
It is essential that the operator be supplied with proper facilities to keep a record of the material as it is placed into the drying room, and when it is taken out. An accurate record should be kept of the temperature every two or three hours, for the different thicknesses and species of lumber, that he may have some reliable data to guide him in future cases.
Any man possessing ordinary intelligence can operate dry kilns and secure satisfactory results, providing he will use good judgment and follow the basic instructions as outlined below:
1. ”

“ First we must understand that as long as the lumber is moist and drying, it will always be cooler than the surrounding air, the amount of this difference being determined by the rate of drying and the moisture in the wood. As the lumber dries, its temperature gradually rises until it is equal to that of the air, when perfect dryness results. With this fact in mind it is clear that the function of the lumber in a kiln is exactly analogous to that of the ice in an ice box; that is, it is the cooling agent. ”

“ 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. ”

“ One method of drying was finely to bed the kiln with wheat straw laid on the hair-cloth, the hops being spread 8 inches thick over this, 'and then you shall keepe a fire a little more fervent than for the drying of a kiln full of malt,' the fire not to be of wood, for that made the hops smoky and tasted the beer, but of straw! Worlidge, strangely, recommended the bed of the kiln to be covered with tin, as much better than hair-cloth, for then any sort of fuel would do as well as charcoal, since the smoke did not pass through the hops. ”

“ Six to ten days additional kiln-drying is allowed them.
The usual temperature for kilns is from 158 to 180 degrees Fahr. Hardwoods lose moisture so slowly that to place them in the kiln directly from the saw would cause them to shrink very unevenly and hence make them subject to serious “checks”.
Lumber is frequently steamed to prevent its checking and “case hardening” while being kiln dried.
127. Lumber Terms and Measurements. —“Clear” lumber is lumber which is free from knots and sapwood.
“Dressed” lumber or “surfaced” or “sized” lumber is lumber which has passed thru the planer. ”

“ When once the wood has been raised to a high temperature through and through and especially when the surface has been rendered most permeable to moisture, drying may proceed as rapidly as it can be forced by artificial circulation, provided the heat lost from the wood through vaporization is constantly replaced by the heat of the kiln. ”

“ 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. ”

“ Down-draught kilns usually give a more regular fire and a higher percentage of well-fired bricks; and they are more economical in fuel consumption than up-draught kilns, while the hot gases, as they pass from the kiln, may be utilized for drying purposes, being conducted through flues under the floor of the drying-shed, on their way to the chimney. ”

“ A kiln car of edge-stacked lumber, properly piled, is made up of alternate solid sheets of lumber and vertical open-air spaces, so that the hot air and vapors rise naturally and freely through the lumber, drying both sides of the board evenly. The distribution of the heat and moisture being even and uniform, the drying process is naturally quickened, and there is no opportunity or tendency for the lumber to warp. ”

“ The temperature and humidity within the kiln, taken alone, are no criterion of the conditions of drying the pile of lumber if the circulation in any portion [162] is deficient. It is possible to have an extremely rapid circulation of air within the dry kiln itself and yet have stagnation within the individual piles, the air passing chiefly through open spaces and channels. Wherever stagnation exists or the movement of air is too sluggish the temperature will drop and the humidity increase, perhaps to the point of saturation. ”

“ The drying or seasoning of woods, more especially of certain species which have been found difficult to season, has received a good deal of attention. A new type of kiln, invented by a Forest Service man, has been devised to season such woods as the eucalyptus, which has always been very difficult to handle in drying. ”

“ It is quite a difficult matter to give specific or absolute correct weights of any species of timber when thoroughly or properly dried, in order that one may be guided in these kiln operations, as a great deal depends upon the species of wood to be dried, its density, and upon the thickness which it has been cut, and its condition when entering the drying chamber. ”