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Out-of-door Seasoning
The most effective seasoning is without doubt that obtained by the uniform, slow drying which takes place in properly constructed piles outdoors, under exposure to the winds and the sun. Lumber has always been seasoned in this way, which is still the best for ordinary purposes.
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.
The present methods of air-seasoning in use have been determined by long experience, and are probably as good as they could be made for present conditions. But the same care has not up to this time been given to the seasoning of such timber as ties, bridge material, posts, telegraph and telephone poles, etc. These have sometimes been piled more or less intelligently, but in the majority of cases their value has been too low to make it seem worth while to pile with reference to anything beyond convenience in handling.
In piling material for air-seasoning, one should utilize high, dry ground when possible, and see that the foundations are high enough off the ground, so that there is proper air circulation through the bottom of the piles, and also that the piles are far enough apart so that the air may circulate freely through and around them.
It is air circulation that is desired in all cases of drying, both in dry kilns and out-of-doors, and not sunshine; that is, not the sun shining directly upon the material. The ends also should be protected from the sun, and everything possible done to induce a free circulation of air, and to keep the foundations free from all plant growth.
Naturally, the heavier the material to be dried, the more difficulty is experienced from checking, which has its most active time in the spring when the sap is rising. In fact the main period of danger in material checking comes with the March winds and the April showers, and not infrequently in the South it occurs earlier than that. In other words, as soon as the sap begins to rise, the timber shows signs of checking, and that is the time to take extra precautions by careful piling and protection from the sun. When the hot days of summer arrive the tendency to check is not so bad, but stock will sour from the heat, stain from the sap, mildew from moisture, and fall a prey to wood-destroying insects.
It has been proven in a general way that wood will season more slowly in winter than in summer, and also that the water content during various months varies. In the spring the drying-out of wood cut in October and November will take place more rapidly.
SECTION XI
KILN-DRYING OF WOOD
Advantages of Kiln-drying over Air-drying
Some of the advantages of kiln-drying to be secured over air-drying in addition to reducing the shipping weight and lessening quantity of stock are the following:
1. Less material lost. 2. Better quality of product. 3. Prevention of sap stain and mould. 4. Fixation of gums and resins. 5. Reduction of hygroscopicity.
This reduction in the tendency to take up moisture means a reduction in the "working" of the material which, even though slight, is of importance.
The problem of drying wood in the best manner divides itself into two distinct parts, one of which is entirely concerned with the behavior of the wood itself and the physical phenomena involved, while the other part has to do with the control of the drying process.
Physical Conditions governing the Drying of Wood
1. Wood is soft and plastic while hot and moist, and becomes "set" in whatever shape it dries. Some species are much more plastic than others.
2. Wood substance begins to shrink only when it dries below the fibre-saturation point, at which it contains from 25 to 30 per cent moisture based on its dry weight. Eucalyptus and certain other species appear to be exceptions to this law.
3. The shrinkage of wood is about twice as great circumferentially as in the radial direction; lengthwise, it is very slight.
4. Wood shrinks most when subjected, while kept moist, to slow drying at high temperatures.
5. Rapid drying produces less shrinkage than slow drying at high temperatures, but is apt to cause case-hardening and honeycombing, especially in dense woods.
6. Case-hardening, honeycombing, and cupping result directly from conditions 1, 4, and 5, and chemical changes of the outer surface.
7. Brittleness is caused by carrying the drying process too far, or by using too high temperatures. Safe limits of treatment vary greatly for different species.
8. Wood absorbs or loses moisture in proportion to the relative humidity in the air, not according to the temperature. This property is called its "hygroscopicity."
9. Hygroscopicity and "working" are reduced but not eliminated by thorough drying.
10. 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. Otherwise, where hours, even minutes, have a marked significance, then other means must be introduced to bring about the desired effect. In any event, however, the same simple and natural remedy pertains,—the absorption of moisture. This moisture in green timber is known as "sap", which is itself composed of a number of ingredients, most important among which are water, resin, and albumen.
All dry kilns in existence use heat to season timber; that is, to drive out that portion of the "sap" which is volatile.
The heat does not drive out the resin of the pines nor the albumen of the hardwoods. It is really of no advantage in this respect. Resin in its hardened state as produced by heat is only slowly soluble in water and contains a large proportion of carbon, the most stable form of matter. Therefore, its retention in the pores of the wood is a positive advantage.
To produce the ideal effect the drying must commence at the heart of the piece and work outward, the moisture being removed from the surface as fast as it exudes from the pores of the wood. To successfully accomplish this, adjustments must be available to regulate the temperature, circulation, and humidity according to the variations of the atmospheric conditions, the kind and condition of the material to be dried.
This ideal effect is only attained by the use of a type of dry kiln in which the surface of the lumber is kept soft, the pores being left open until all the moisture within has been volatilized by the heat and carried off by a free circulation of air. When the moisture has been removed from the pores, the surface is dried without closing the pores, resulting in timber that is clean, soft, bright, straight, and absolutely free from stains, checks, or other imperfections.
Now, no matter how the method of drying may be applied, it must be remembered that vapor exists in the atmosphere at all times, its volume being regulated by the capacity of the temperature absorbed. To kiln-dry properly, a free current of air must be maintained, of sufficient volume to carry off this moisture. Now, the capacity of this air for drying depends entirely upon the ability of its temperature to absorb or carry off a larger proportion of moisture than that apportioned by natural means. Thus, it will be seen, a cubic foot of air at 32 degrees Fahrenheit is capable of absorbing only two grains of water, while at 160 degrees, it will dispose of ninety grains. The air, therefore, should be made as dry as possible and caused to move freely, so as to remove all moisture from the surface of the wood as soon as it appears. Thus the heat effects a double purpose, not only increasing the rate of evaporation, but also the capacity of the air for absorption. Where these means are applied, which rely on the heat alone to accomplish this purpose, only that of the moisture which is volatile succumbs, while the albumen and resin becoming hardened under the treatment close up the pores of the wood. This latter result is oft-times accomplished while moisture yet remains and which in an enforced effort to escape bursts open the cells in which it has been confined and creates what is known as "checks."
Therefore, taking the above facts into consideration, the essentials for the successful kiln-drying of wood may be enumerated as follows:
1. The evaporation from the surface of a stick should not exceed the rate at which the moisture transfuses from the interior to the surface.
2. Drying should proceed uniformly at all points, otherwise extra stresses are set up in the wood, causing warping, etc.
3. Heat should penetrate to the interior of the piece before drying begins.
4. The humidity should be suited to the condition of the wood at the start and reduced in the proper ratio as drying progresses. With wet or green wood it should usually be held uniform at a degree which will prevent the surface from drying below its saturation point until all the free water has evaporated, then gradually reduced to remove the hygroscopic moisture.
5. The temperature should be uniform and as high as the species under treatment will stand without excessive shrinkage, collapse, or checking.
6. Rate of drying should be controlled by the amount of humidity in the air and not by the rate of circulation, which should be made ample at all times.
7. In drying refractory hardwoods, such as oak, best results are obtained at a comparatively low temperature. In more easily dried hardwoods, such as maple, and some of the more difficult softwoods, as cypress, the process may be hastened by a higher temperature but not above the boiling point. In many of the softwoods, the rate of drying may be very greatly increased by heating above the boiling point with a large circulation of vapor at atmospheric pressure.
8. Unequal shrinkage between the exterior and interior portions of the wood and also unequal chemical changes must be guarded against by temperatures and humidities suited to the species in question to prevent subsequent cupping and warping.
9. The degree of dryness attained should conform to the use to which the wood is put.
10. Proper piling of the material and weighting to prevent warping are of great importance.
Requirements in a Satisfactory Dry Kiln
The requirements in a satisfactory dry kiln are:
1. Control of humidity at all times. 2. Ample air circulation at all points. 3. Uniform and proper temperatures.
In order to meet these requirements the United States Forestry Service has designed a kiln in which the humidity, temperature, and circulation can be controlled at all times.
Briefly, it consists of a drying chamber with a partition on either side, making two narrow side chambers open top and bottom.
The steam pipes are in the usual position underneath the material to be dried.
At the top of the side chambers is a spray; at the bottom are gutters and an eliminator or set of baffle plates to separate the fine mist from the air.
The spray accomplishes two things: It induces an increased circulation and it regulates the humidity. This is done by regulating the temperature of the spray water.
The air under the heating coil is saturated at whatever temperature is required. This temperature is the dew point of the air after it passes up into the drying chamber above the coils. Knowing the temperature in the drying room and the dew point, the relative humidity is thus determined.
The relative humidity is simply the ratio of the vapor pressure at the dew point to the pressure of saturated vapor (see Fig. 30).
Theory and Description of the Forestry Service Kiln
The humidities and temperatures in the piles of lumber are largely dependent upon the circulation of air within the kiln. 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 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.
When in large kilns the forced circulation is in the opposite direction from that induced by the cooling of the air by the lumber, there is always more or less uncertainty as to the movement of the air through the piles. Even with the boards placed edge-wise, with stickers running vertically, and with the heating pipes beneath the lumber, it was found that although the air passed upward through most of the spaces it was actually descending through others, so that very unequal drying resulted. While edge piling would at first thought seem ideal for the freest circulation in an ordinary kiln with steam pipes below, it in fact produces an indeterminate condition; air columns may pass downward through some channels as well as upward through others, and probably stagnate in still others. Nevertheless, edge piling is greatly superior to flat piling where the heating system is below the lumber.
From experiments and from study of conditions in commercial kilns the idea was developed of so arranging the parts of the kiln and the pile of lumber that advantage might be taken of this cooling of the air to assist the circulation. That this can be readily accomplished without doing away with the present features of regulation of humidity by means of a spray of water is clear from Fig. 30, which shows a cross-section of the improved humidity-regulated dry kiln.
In the form shown in the sketch a chamber or flue B runs through the center near the bottom. This flue is only about 6 or 7 feet in height and, together with the water spray F and the baffle plates DD, constitutes the humidity-control feature of the kiln. This control of humidity is affected by the temperature of the water used in the spray. This spray completely saturates the air in the flue B at whatever predetermined temperature is required. The baffle plates DD are to separate all entrained particles of water from the air, so that it is delivered to the heaters in a saturated condition at the required temperature. This temperature is, therefore, the dew point of the air when heated above, and the method of humidity control may therefore be called the dew-point method. It is a very simple matter by means of the humidity diagram (see Fig. 93), or by a hygrodeik (Fig. 94), to determine what dew-point temperature is needed for any desired humidity above the heaters.
Besides regulating the humidity the spray F also acts as an ejector and forces circulation of air through the flue B. The heating system H is concentrated near the outer walls, so as to heat the rising column of air. The temperature within the drying chamber is controlled by means of any suitable thermostat, actuating a valve on the main steam line. The lumber is piled in such a way that the stickers slope downward toward the sides of the kiln.
M is an auxiliary steam spray pointing downward for use at very high temperatures. C is a gutter to catch the precipitation and conduct it back to the pump, the water being recirculated through the sprays. G is a pipe condenser for use toward the end of the drying operation. K is a baffle plate for diverting the heated air and at the same time shielding the under layers of boards from direct radiation of the steam pipes.
The operation of the kiln is simple. The heated air rises above the pipes HH and between the piles of lumber. As it comes in contact with the piles, portions of it are cooled and pass downward and outward through the layers of boards into the space between the condensers GG. Here the column of cooled air descends into the spray flue B, where its velocity is increased by the force of the water spray. It then passes out from the baffle plates to the heaters and repeats the cycle.
One of the greatest advantages of this natural circulation method is that the colder the lumber when placed in the kiln the greater is the movement produced, under the very conditions which call for the greatest circulation—just the opposite of the direct-circulation method. This is a feature of the greatest importance in winter, when the lumber is put into the kiln in a frozen condition. One truckload of lumber at 60 per cent moisture may easily contain over 7,000 pounds of ice.
In the matter of circulation the kiln is, in fact, seldom regulatory—the colder the lumber the greater the circulation produced, with the effect increased toward the cooler and wetter portions of the pile.
Preliminary steaming may be used in connection with this kiln, but experiments indicate that ordinarily it is not desirable, since the high humidity which can be secured gives as good results, and being at as low a temperature as desired, much better results in the case of certain difficult woods like oak, eucalyptus, etc., are obtained.
This kiln has another advantage in that its operation is entirely independent of outdoor atmospheric conditions, except that barometric pressure will effect it slightly.
KILN-DRYING
Remarks
Drying is an essential part of the preparation of wood for manufacture. For a long time the only drying process used or known was air-drying, or the exposure of wood to the gradual drying influences of the open air, and is what has now been termed "preliminary seasoning." This method is without doubt the most successful and effective seasoning, 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.
Within recent years, considerable interest is awakening among wood users in the operation of kiln-drying. The losses occasioned in air-drying and in improper kiln-drying, and the necessity for getting material dry as quickly as possible from the saw, for shipping purposes and also for manufacturing, are bringing about a realization of the importance of a technical knowledge of the subject.
The losses which occur in air-drying wood, through checking, warping, staining, and rotting, are often greater than one would suppose. While correct statistics of this nature are difficult to obtain, some idea may be had of the amount of degrading of the better class of lumber. In the case of one species of soft wood, Western larch, it is commonly admitted that the best grades fall off sixty to seventy per cent in air-drying, and it is probable that the same is true in the case of Southern swamp oaks. In Western yellow pine, the loss is great, and in the Southern red gum, it is probably as much as thirty per cent. It may be said that in all species there is some loss in air-drying, but in some easily dried species such as spruce, hemlock, maple, etc., it is not so great.
It would hardly be correct to state at the present time that this loss could be entirely prevented by proper methods of kiln-drying the green lumber, but it is safe to say that it can be greatly reduced.
It is well where stock is kiln-dried direct from the saw or knife, after having first been steamed or boiled—as in the case of veneers, etc.,—to get them into the kiln while they are still warm, as they are then in good condition for kiln-drying, as the fibres of the wood are soft and the pores well opened, which will allow of forcing the evaporation of moisture without much damage being done to the material.
With softwoods it is a common practice to kiln-dry direct from the saw. This procedure, however, is ill adapted for the hardwoods, in which it would produce such warping and checking as would greatly reduce the value of the product. Therefore, hardwoods, as a rule, are more or less thoroughly air-dried before being placed in the dry kiln, where the residue of moisture may be reduced to within three or four per cent, which is much lower than is possible by air-drying only.
It is probable that for the sake of economy, air-drying will be eliminated in the drying processes of the future without loss to the quality of the product, but as yet no method has been discovered whereby this may be accomplished.
The dry kiln has been, and probably still is, one of the most troublesome factors arising from the development of the timber industry. In the earlier days, before power machinery for the working-up of timber products came into general use, dry kilns were unheard-of, air-drying or seasoning was then relied upon solely to furnish the craftsman with dry stock from which to manufacture his product. 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.
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. In so doing, time is saved and a certain degree of control over the drying operation is secured.
The first efforts in the way of artificial drying were confined to aiding or hastening nature in the seasoning process by exposing the material to the direct heat from fires built in pits, over which the lumber was piled in a way to expose it to the heat rays of the fires below. This, of course, was a primitive, hazardous, and very unsatisfactory method, to say the least, but it marked the first step in the evolution of the present-day dry kiln, and in that particular only is it deserving of mention.
Underlying Principles
In addition to marking the first step in artificial drying, it illustrated also, in the simplest manner possible, the three underlying principles governing all drying problems: (1) The application of heat to evaporate or volatilize the water contained in the material; (2) with sufficient air in circulation to carry away in suspension the vapor thus liberated; and (3) with a certain amount of humidity present to prevent the surface from drying too rapidly while the heat is allowed to penetrate to the interior. The last performs two distinct functions: (a) It makes the wood more permeable to the passage of the moisture from the interior of the wood to the surface, and (b) it supplies the latent heat necessary to evaporate the moisture after it reaches the surface. The air circulation is important in removing the moisture after it has been evaporated by the heat, and ventilation also serves the purpose of bringing the heat in contact with the wood. If, however, plain, dry heat is applied to the wood, the surface will become entirely dry before the interior moisture is even heated, let alone removed. This condition causes "case-hardening" or "hollow-horning." So it is very essential that sufficient humidity be maintained to prevent the surface from drying too rapidly, while the heat is allowed to penetrate to the interior.
This humidity or moisture is originated by the evaporation from the drying wood, or by the admission of steam into the dry kiln by the use of steam spray pipes, and is absolutely necessary in the process of hastening the drying of wood. With green lumber it keeps the sap near the surface of the piece in a condition that allows the escape of the moisture from its interior; or, in other words, it prevents the outside from drying first, which would close the pores and cause case-hardening.
The great amount of latent heat necessary to evaporate the water after it has reached the surface is shown by the fact that the evaporation of only one pound of water will extract approximately 66 degrees from 1,000 cubic feet of air, allowing the air to drop in temperature from 154 to 84 degrees Fahrenheit. In addition to this amount of heat, the wood and the water must also be raised to the temperature at which the drying is to be accomplished.
It matters not what type of dry kiln is used, source or application of heating medium, these underlying principles remain the same, and must be the first things considered in the design or selection of the equipment necessary for producing the three essentials of drying: Heat, humidity, and circulation.
Although these principles constitute the basis of all drying problems and must, therefore, be continually carried in mind in the consideration of them, it is equally necessary to have a comprehensive understanding of the characteristics of the materials to be dried, and its action during the drying process. All failures in the past, in the drying of timber products, can be directly attributed to either the kiln designer's neglect of these things, or his failure to carry them fully in mind in the consideration of his problems.
Wood has characteristics very much different from those of other materials, and what little knowledge we have of it and its properties has been taken from the accumulated records of experience. The reason for this imperfect knowledge lies in the fact that wood is not a homogeneous material like the metals, but a complicated structure, and so variable that one stick will behave in a manner widely different from that of another, although it may have been cut from the same tree.
The great variety of woods often makes the mere distinction of the kind or species of the tree most difficult. It is not uncommon to find men of long experience disagree as to the kind of tree a certain piece of lumber was cut from, and, in some cases, there is even a wide difference in the appearance and evidently the structure of timber cut from the same tree.
Objects of Kiln-drying
The objects of kiln-drying wood may be placed under three main headings: (1) To reduce shipping expenses; (2) to reduce the quantity necessary to maintain in stock; and (3) to reduce losses in air-drying and to properly prepare the wood for subsequent use. Item number 2 naturally follows as a consequence of either 1 or 3. The reduction in weight on account of shipping expenses is of greatest significance with the Northwestern lumbermen in the case of Douglas fir, redwood, Western red cedar, sugar pine, bull pine, and other softwoods.
Very rapid methods of rough drying are possible with some of these species, and are in use. High temperatures are used, and the water is sometimes boiled off from the wood by heating above 212 degrees Fahrenheit. These high-temperature methods will not apply to the majority of hardwoods, however, nor to many of the softwoods.
It must first of all be recognized that the drying of lumber is a totally different operation from the drying of a fabric or of thin material. In the latter, it is largely a matter of evaporated moisture, but wood is not only hygroscopic and attracts moisture from the air, but its physical behavior is very complex and renders the extraction of moisture a very complicated process.
An idea of its complexity may be had by mentioning some of the conditions which must be contended with. Shrinkage is, perhaps, the most important. This is unequal in different directions, being twice as great tangentially as radially and fifty times as great radially as longitudinally. Moreover, shrinkage is often unequal in different portions of the same piece. The slowness of the transfusion of moisture through the wood is an important factor. This varies with different woods and greatly in different directions. Wood becomes soft and plastic when hot and moist, and will yield more or less to internal stresses. As some species are practically impervious to air when wet, this plasticity of the cell walls causes them to collapse as the water passes outward from the cell cavities. This difficulty has given much trouble in the case of Western red cedar, and also to some extent in redwood. The unequal shrinkage causes internal stresses in the wood as it dries, which results in warping, checking, case-hardening, and honeycombing. Case-hardening is one of the most common defects in improperly dried lumber. It is clearly shown by the cupping of the two halves when a case-hardened board is resawed. Chemical changes also occur in the wood in drying, especially so at higher temperatures, rendering it less hygroscopic, but more brittle. If dried too much or at too high a temperature, the strength and toughness is seriously reduced.
Conditions of Success
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.
It is exceedingly important that the material be practically at the same temperature throughout if perfect drying is to be secured. It should be the same temperature in the center of a pile or car as on the outside, and the same in the center of each individual piece of wood as on its surface. This is the effect obtained by natural air-drying. The outside atmosphere and breezes (natural air circulation) are so ample that the heat extracted for drying does not appreciably change the temperature.
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.
It is evident that to secure an even temperature, a free circulation of air must be brought in contact with the wood. It is also evident that in addition to heat and a circulation of air, the air must be charged with a certain amount of moisture to prevent surface drying or case-hardening.
There are some twenty-five different makes of dry kilns on the market, which fulfill to a varying degree the fundamental requirements. Probably none of them succeed perfectly in fulfilling all.
It is well to have the temperature of a dry kiln controlled by a thermostat which actuates the valve on the main steam supply pipe. It is doubly important to maintain a uniform temperature and avoid fluctuations in the dry kiln, since a change in temperature will greatly alter the relative humidity.
In artificial drying, temperatures of from 150 to 180 degrees Fahrenheit are usually employed. Pine, spruce, cypress, cedar, etc., are dried fresh from the saw, allowing four days for 1-inch stuff. Hardwoods, especially oak, ash, maple, birch, sycamore, etc., are usually air-seasoned for three to six months to allow the first shrinkage to take place more gradually, and are then exposed to the above temperatures in the kiln for about six to ten days for 1-inch stuff, other dimensions in proportion.
Freshly cut poplar and cottonwood are often dried direct from the saw in a kiln. By employing lower temperatures, 100 to 120 degrees Fahrenheit, green oak, ash, etc., can be seasoned in dry kilns without much injury to the material.
Steaming and sweating the wood is sometimes resorted to in order to prevent checking and case-hardening, but not, as has been frequently asserted, to enable the material to dry.
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.
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.
A fresh piece of sapwood will lose weight in boiling water and can also be dried to quite an extent in steam. This proves conclusively that a high degree of humidity does not have the detrimental effect on drying that is commonly attributed to it. In fact, a proper degree of humidity, especially in the loading or receiving end of a kiln, is just as necessary to good results in drying as getting the proper temperature.
Experiments have demonstrated also that injury to stock in the way of checking, warping, and hollow-horning always develops immediately after the stock is taken into the kiln, and is due to the degree of humidity being too low. The receiving end of the kiln should always be kept moist, where the stock has not been steamed before being put into the kiln. The reason for this is simple enough. When the air is too dry it tends to dry the outside of the material first—which is termed "case-hardening"—and in so doing shrinks and closes up the pores of the wood. As the stock is moved down the kiln, it absorbs a continually increasing amount of heat, which tends to drive off the moisture still present in the center of the stock. The pores on the outside having been closed up, there is no exit for the vapor or steam that is being rapidly formed in the center. It must find its way out some way, and in doing so sets up strains, which result either in checking, warping, or hollow-horning. If the humidity had been kept higher, the outside of the material would not have dried so quickly, and the pores would have remained open for the exit of moisture from the interior of the wood, and this trouble would have been avoided.
Where the humidity is kept at a high point in the receiving end of the kiln, a higher rate of temperature may also be carried, and in that way the drying process is hastened with comparative safety.
It is essential, therefore, to have an ample supply of heat through the convection currents of the air; but in the case of wood the rate of evaporation must be controlled, else checking will occur. This can be done by means of the relative humidity, as stated before. It is clear now that when the air—or, more properly speaking, the space—is completely saturated no evaporation can take place at the given temperature. By reducing the humidity, evaporation takes place more and more rapidly.
Another bad feature of an insufficient and non-uniform supply of heat is that each piece of wood will be heated to the evaporating point on the outer surface, the inside remaining cool until considerable drying has taken place from the surface. Ordinarily in dry kilns high humidity and large circulation of air are antitheses to one another. To obtain the high humidity the circulation is either stopped altogether or greatly reduced, and to reduce the humidity a greater circulation is induced by opening the ventilators or otherwise increasing the draft. This is evidently not good practice, but as a rule is unavoidable in most dry kilns of present make. The humidity should be raised to check evaporation without reducing the circulation if possible.
While thin stock, such as cooperage and box stuff is less inclined to give trouble by undue checking than 1-inch and thicker, one will find that any dry kiln will give more uniform results and, at the same time, be more economical in the use of steam, when the humidity and temperature is carried at as high a point as possible without injury to the material to be dried.
Any well-made dry kiln which will fulfill the conditions required as to circulation and humidity control should work satisfactorily; but each case must be studied by itself, and the various factors modified to suit the peculiar conditions of the problem in hand. In every new case the material should be constantly watched and studied and, if checking begins, the humidity should be increased until it stops. It is not reducing the circulation, but adding the necessary moisture to the air, that should be depended on to prevent checking. For this purpose it is well to have steam jets in the kiln so that if needed they are ready at hand.
Kiln-drying
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. It is the usual custom in these kilns to maintain one end of the chamber moist and the other end dry. This is known as the "progressive" type of kiln, and is the one most commonly used in large operations.
It is, however, the least satisfactory of the two where careful drying is required, since the conditions cannot be so well regulated and the temperatures and humidities are apt to change with any change of wind. The apartment method can be arranged so that it will not require any more kiln space or any more handling of lumber than the progressive type. It does, however, require more intelligent operation, since the conditions in the drying chamber must be changed as the drying progresses. With the progressive type the conditions, once properly established, remain the same.
To obtain draft or circulation three methods are in use—by forced draft or a blower usually placed outside the kiln, by ventilation, and by internal circulation and condensation. A great many patents have been taken out on different methods of ventilation, but in actual operation few kilns work exactly as intended. Frequently the air moves in the reverse direction for which the ventilators were planned. Sometimes a condenser is used in connection with the blower and the air is recirculated. It is also—and more satisfactorily—used with the gentle internal-gravity currents of air.
Many patents have been taken out for heating systems. The differences among these, however, have more to do the mechanical construction than with the process of drying. In general, the heating is either direct or indirect. In the former steam coils are placed in the chamber with the lumber, and in the latter the air is heated by either steam coils or a furnace before it is introduced into the drying chamber.
Moisture is sometimes supplied by means of free steam jets in the kiln or in the entering air; but more often the moisture evaporated from the lumber is relied upon to maintain the humidity necessary.
A substance becomes dry by the evaporation of its inherent moisture into the surrounding space. If this space be confined it soon becomes saturated and the process stops. Hence, constant change is necessary in order that the moisture given off may be continually carried away.
In practice, air movement, is therefore absolutely essential to the process of drying. Heat is merely a useful accessory which serves to decrease the time of drying by increasing both the rate of evaporation and the absorbing power of the surrounding space.
It makes no difference whether this space is a vacuum or filled with air; under either condition it will take up a stated weight of vapor. From this it appears that the vapor molecules find sufficient space between the molecules of air. But the converse is not true, for somewhat less air will be contained in a given space saturated with vapor than in one devoid of moisture. In other words the air does not seem to find sufficient space between the molecules of vapor.
If the temperature of the confined space be increased, opportunity will thereby be provided for the vaporization of more water, but if it be decreased, its capacity for moisture will be reduced and visible water will be deposited. The temperature at which this takes place is known as the "dew-point" and depends upon the initial degree of saturation of the given space; the less the relative saturation the lower the dew-point.
Careful piling of the material to be dried, both in the yard and dry kiln, is essential to good results in drying.
Air-dried material is not dry, and its moisture is too unevenly distributed to insure good behavior after manufacture.
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.
Elm will naturally weigh less than beech, and where the wood is close-grained or compact it will weigh more than coarse-grained wood of the same species, and, therefore, no set rules can be laid down, as good judgment only should be used, as the quality of the drying is not purely one of time. Sometimes the comparatively slow process gives excellent results, while to rush a lot of stock through the kiln may be to turn it out so poorly seasoned that it will not give satisfaction when worked into the finished product. The mistreatment of the material in this respect results in numerous defects, chief among which are warping and twisting, checking, case-hardening, and honeycombing, or, as sometimes called, hollow-horning.
Since the proportion of sap and heartwood varies with size, age, species, and individual trees, the following figures as regards weight must be regarded as mere approximations:
POUNDS OF WATER LOST IN DRYING 100 POUNDS OF GREEN WOOD IN THE KILN
========================================================================= Sapwood or Heartwood outer part or interior ========================================================================= (1) Pine, cedar, spruce, and fir 45-65 16-25 (2) Cypress, extremely variable 50-65 18-60 (3) Poplar, cottonwood, and basswood 60-65 40-60 (4) Oak, beech, ash, maple, birch, elm, hickory, chestnut, walnut, and sycamore 40-50 30-40 =========================================================================
The lighter kinds have the most water in the sapwood; thus sycamore has more water than hickory, etc.
The efficiency of the drying operations depends a great deal upon the way in which, the lumber is piled, especially when the humidity is not regulated. From the theory of drying it is evident that the rate of evaporation in dry kilns where the humidity is not regulated depends entirely upon the rate of circulation, other things being equal. Consequently, those portions of the wood which receive the greatest amount of air dry the most rapidly, and vice versa. The only way, therefore, in which anything like uniform drying can take place is where the lumber is so piled that each portion of it comes in contact with the same amount of air.
In the Forestry Service kiln (Fig. 30), where the degree of relative humidity is used to control the rate of drying, the amount of circulation makes little difference, provided it exceeds a certain amount. It is desirable to pile the lumber so as to offer as little frictional resistance as possible and at the same time secure uniform circulation. If circulation is excessive in any place it simply means waste of energy but no other injury to the lumber.
The best method of piling is one which permits the heated air to pass through the pile in a somewhat downward direction. The natural tendency of the cooled air to descend is thus taken advantage of in assisting the circulation in the kiln. This is especially important when cold or green lumber is first introduced into the kiln. But even when the lumber has become warmed the cooling due to the evaporation increases the density of the mixture of the air and vapor.
Kiln-drying Gum
The following article was published by the United States Forestry Service as to the best method of kiln-drying gum:
Piling.—Perhaps the most important factor in good kiln-drying, especially in the case of the gums, is the method of piling. It is our opinion that proper and very careful piling will greatly reduce the loss due to warping. A good method of piling is to place the lumber lengthwise of the kiln and on an incline cross-wise. The warm air should rise at the higher side of the pile and descend between the courses of lumber. The reason for this is very simple and the principle has been applied in the manufacture of the best ice boxes for some time. The most efficient refrigerators are iced at the side, the ice compartment opening to the cooling chamber at the top and bottom. The warm air from above is cooled by melting the ice. It then becomes denser and settles down into the main chamber. The articles in the cooling room warm the air as they cool, so it rises to the top and again comes in contact with the ice, thus completing the cycle. The rate of this natural circulation is automatically regulated by the temperature of the articles in the cooling chamber and by the amount of ice in the icing compartment; hence the efficiency of such a box is high.
Now let us apply this principle to the drying of lumber. 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. Similarly, the heating pipes in a dry kiln bring about the same effect as the articles of food in the ice box in that they serve to heat the air. Therefore, the air will be cooled by the lumber, causing it to pass downward through the piles. If the heating units are placed at the sides of the kiln, the action of the air in a good ice box is duplicated in the kiln. The significant point in this connection is that, the greener and colder the lumber, the faster is the circulation. This is a highly desirable feature.
A second vital point is that as the wood becomes gradually drier the circulation automatically decreases, thus resulting in increased efficiency, because there is no need for circulation greater than enough to maintain the humidity of the air as it leaves the lumber about the same as it enters. Therefore, we advocate either the longitudinal side-wise inclined pile or edge stacking, the latter being much preferable when possible. Of course the piles in our kiln were small and could not be weighted properly, so the best results as to reducing warping were not obtained.
Preliminary Steaming.—Because the fibres of the gums become plastic while moist and hot without causing defects, it is desirable to heat the air-dried lumber to about 200 degrees Fahrenheit in saturated steam at atmospheric pressure in order to reduce the warping. This treatment also furnishes a means of heating the lumber very rapidly. It is probably a good way to stop the sap-staining of green lumber, if it is steamed while green. We have not investigated the other effects of steaming green gum, however, so we hesitate to recommend it.
Temperatures as high as 210 degrees Fahrenheit were used with no apparent harm to the material. The best result was obtained with the temperature of 180 degrees Fahrenheit, after the first preliminary heating in steam to 200 degrees Fahrenheit. Higher temperatures may be used with air-dried gum, however.
The best method of humidity control proved to be to reduce the relative humidity of the air from 100 per cent (saturated steam) very carefully at first and then more rapidly to 30 per cent in about four days. If the change is too marked immediately after the steaming period, checking will invariably result. Under these temperature and humidity conditions the stock was dried from 15 per cent moisture, based on the dry wood weight, to 6 per cent in five days' time. The loss due to checking was about 5 per cent, based on the actual footage loss, not on commercial grades.
Final Steaming.—From time to time during the test runs the material was resawed to test for case-hardening. The stock dried in five days showed slight case-hardening, so it was steamed at atmospheric pressure for 36 minutes near the close of the run, with the result that when dried off again the stresses were no longer present. The material from one run was steamed for three hours at atmospheric pressure and proved very badly case-hardened, but in the reverse direction. It seems possible that by testing for the amount of case-hardening one might select a final steaming period which would eliminate all stresses in the wood.
Kiln-drying of Green Red Gum
The following article was published by the United States Forestry Service on the kiln-drying of green red gum:
A short time ago fifteen fine, red-gum logs 16 feet long were received from Sardis, Miss. They were in excellent condition and quite green.
It has been our belief that if the gum could be kiln-dried directly from the saw, a number of the difficulties in seasoning might be avoided. Therefore, we have undertaken to find out whether or not such a thing is feasible. The green logs now at the laboratory are to be used in this investigation. One run of a preliminary nature has just been made, the method and results of which I will now tell.
This method was really adapted to the drying of Southern pine, and one log of the green gum was cut into 1-inch stock and dried with the pine. The heartwood contained many knots and some checks, although it was in general of quite good quality. The sapwood was in fine condition and almost as white as snow.
This material was edge-stacked with one crosser at either end and one at the center, of the 16-foot board. This is sufficient for the pine, but was absolutely inadequate for drying green gum. A special shrinkage take-up was applied at the three points. The results proved very interesting in spite of the warping which was expected with but three crossers in 16 feet. The method of circulation described was used. It is our belief that edge piling is best for this method.
This method of kiln-drying depends on the maintenance of a high velocity of slightly superheated steam through the lumber. In few words, the object is to maintain the temperature of the vapor as it leaves the lumber at slightly above 212 degrees Fahrenheit. In order to accomplish this result, it is necessary to maintain the high velocity of circulation. As the wood dries, the superheat may be increased until a temperature of 225 degrees or 230 degrees Fahrenheit of the exit air is recorded.
The 1-inch green gum was dried from 20.1 per cent to 11.4 per cent moisture, based on the dry wood weight in 45 hours. The loss due to checking was 10 per cent. Nearly every knot in the heartwood was checked, showing that as the knots could be eliminated in any case, this loss might not be so great. It was significant that practically all of the checking occurred in the heartwood. The loss due to warping was 22 per cent. Of course this was large; but not nearly enough crossers were used for the gum. It is our opinion that this loss due to warping can be very much reduced by using at least eight crossers and providing for taking up of the shrinkage. A feature of this process which is very important is that the method absolutely prevents all sap staining.
Another delightful surprise was the manner in which the superheated steam method of drying changed the color of the sapwood from pure white to a beautifully uniform, clean-looking, cherry red color which very closely resembles that of the heartwood. This method is not new by any means, as several patents have been granted on the steaming of gum to render the sapwood more nearly the color of the heartwoods. The method of application in kiln-drying green gum we believe to be new, however. Other methods for kiln-drying this green stock are to be tested until the proper process is developed. We expect to have something interesting to report in the near future.[1]
[Footnote 1: The above test was made at the United States Forestry Service Laboratory, Madison, Wis.]
SECTION XII
TYPES OF DRY KILNS
DIFFERENT TYPES OF DRY KILNS
Dry kilns as in use to-day are divided into two classes: The "pipe" or "moist-air" kiln, in which natural draft is relied upon for circulation and, the "blower" or "hot blast" kiln, in which the circulation is produced by fans or blowers. Both classes have their adherents and either one will produce satisfactory results if properly operated.
The "Blower" or "Hot Blast" Kiln
The blower kiln in its various types has been in use so long that it is hardly necessary to give to it a lengthy introduction. These kilns at their inauguration were a wonderful improvement over the old style "bake-oven" or "sweat box" kiln then employed, both on account of the improved quality of the material and the rapidity at which it was dried.
These blower kilns have undergone steady improvement, not only in the apparatus and equipment, but also in their general design, method of introducing air, and provision for controlling the temperature and humidity. With this type of kiln the circulation is always under absolute control and can be adjusted to suit the conditions, which necessarily vary with the conditions of the material to be dried and the quantity to be put through the kiln.
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. Any dry kiln in which this has not been carefully considered will not give the desired results.
In the old style blower kiln, while the circulation and temperature was very largely under the operator's control, it was next to impossible to produce conditions in the receiving end of the kiln so that the humidity could be kept at the proper point. In fact, this was one reason why the natural draft, or so-called moist-air kiln was developed.
The advent of the moist-air kiln served as an education to kiln designers and manufacturers, in that it has shown conclusively the value of a proper degree of humidity in the receiving end of any progressive dry kiln, and it has been of special benefit also in that it gave the manufacturers of blower kilns an idea as to how to improve the design of their type of kiln to overcome the difficulty referred to in the old style blower kilns. This has now been remedied, and in a decidedly simple manner, as is usually the case with all things that possess merit.
It was found that by returning from one third to one half of the moist air after having passed through the kiln back to the fan room and by mixing it with the fresh and more or less dry air going into the drying room, that the humidity could be kept under convenient control.
The amount of air that can be returned from a kiln of this class depends upon three things: (1) The condition of the material when entering the drying room; (2) the rapidity with which the material is to be dried; and (3) the condition of the outside atmosphere. In the winter season it will be found that a larger proportion of air may be returned to the drying room than in summer, as the air during the winter season contains considerably less moisture and as a consequence is much drier. This is rather a fortunate coincidence, as, when the kiln is being operated in this manner, it will be much more economical in its steam consumption.
In the summer season, when the outside atmosphere is saturated to a much greater extent, it will be found that it is not possible to return as great a quantity of air to the drying room, although there have been instances of kilns of this class, which in operation have had all the air returned and found to give satisfactory results. This is an unusual condition, however, and can only be accounted for by some special or peculiar condition surrounding the installation.
In some instances, the desired amount of humidity in a blower type of kiln is obtained by the addition of a steam spray in the receiving end of the kiln, much in the same manner as that used in the moist-air kilns. This method is not as economical as returning the moisture-laden air from the drying room as explained in the preceding paragraph.
With the positive circulation that may be obtained in a blower kiln, and with the conditions of temperature and humidity under convenient control, this type of kiln has the elements most necessary to produce satisfactory drying in the quickest possible elapsed time.
It must not be inferred from this, however, that this class of dry kiln may be installed and satisfactory results obtained regardless of how it is handled. A great deal of the success of any dry kiln—or any other apparatus, for that matter—depends upon intelligent operation.
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. When cold and before putting into operation, heat the apparatus slowly until all pipes are hot, then start the fan or blower, gradually bringing it up to its required speed.
2. See that all steam supply valves are kept wide open, unless you desire to lengthen the time required to dry the material.
3. When using exhaust steam, the valve from the header (which is a separate drip, independent of the trap connection) must be kept wide open, but must be closed when live steam is used on that part of the heater.
4. The engines as supplied by the manufacturers are constructed to operate the fan or blower at a proper speed with its throttle valve wide open, and with not less than 80 pounds pressure of steam.
5. If the return steam trap does not discharge regularly, it is important that it be opened and thoroughly cleaned and the valve seat re-ground.
6. As good air circulation is as essential as the proper degree of heat, and as the volume of air and its contact with the material to be dried depends upon the volume delivered by the fan or blower, it is necessary to maintain a regular and uniform speed of the engine.
7. Atmospheric openings must always be maintained in the fan or heater room for fresh air supply.
8. Successful drying cannot be accomplished without ample and free circulation of air at all times.
If the above instructions are fully carried out, and good judgment used in the handling and operation of the blower kiln, no difficulties should be encountered in successfully drying the materials at hand.
The "Pipe" or "Moist-air" Dry Kiln
While in the blower class of dry kiln, the circulation is obtained by forced draft with the aid of fans or blowers, in the Moist-air kilns (see Fig. 31); the circulation is obtained by natural draft only, aided by the manipulation of dampers installed at the receiving end of the drying room, which lead to vertical flues through a stack to the outside atmosphere.
The heat in these kilns is obtained by condensing steam in coils of pipe, which are placed underneath the material to be dried. As the degree of heat required, and steam pressure govern the amount of radiation, there are several types of radiating coils. In Fig. 32 will be seen the Single Row Heating Coils for live or high pressure steam, which are used when the low temperature is required. Figure 33 shows the Double (or 2) Row Heating Coils for live or high pressure steam. This apparatus is used when a medium temperature is required. In Fig. 34 will be seen the Vertical Type Heating Coils which is recommended where exhaust or low-pressure steam is to be used, or may be used with live or high-pressure steam when high temperatures are desired.
These heating coils are usually installed in sections, which permit any degree of heat from the minimum to the maximum to be maintained by the elimination of, or the addition of, any number of heating sections. This gives a dry kiln for the drying of green softwoods, or by shutting off a portion of the radiating coils—thus reducing the temperature—a dry kiln for drying hardwoods, that will not stand the maximum degree of heat.
In the Moist-air or Natural Draft type of dry kiln, any degree of humidity, from clear and dry to a dense fog may be obtained; this is in fact, the main and most important feature of this type of dry kiln, and the most essential one in the drying of hardwoods.
It is not generally understood that the length of a kiln has any effect upon the quantity of material that may be put through it, but it is a fact nevertheless that long kilns are much more effective, and produce a better quality of stock in less time than kilns of shorter length.
Experience has proven that a kiln from 80 to 125 feet in length will produce the best results, and it should be the practice, where possible, to keep them within these figures. The reason for this is that in a long kiln there is a greater drop in temperature between the discharge end and the green or receiving end of the kiln.
It is very essential that the conditions in the receiving end of the kiln, as far as the temperature and humidity are concerned, must go hand in hand.
It has also been found that in a long kiln the desired conditions may be obtained with higher temperatures than with a shorter kiln; consequently higher temperatures may be carried in the discharge end of the kiln, thereby securing greater rapidity in drying. It is not unusual to find that a temperature of 200 degrees Fahrenheit is carried in the discharge end of a long dry kiln with safety, without in any way injuring the quality of the material, although, it would be better not to exceed 180 degrees in the discharge end, and about 120 degrees in the receiving or green end in order to be on the safe side.
Operation of the "Moist-air" Dry Kiln
To obtain the best results these kilns should be kept in continuous operation when once started, that is, they should be operated continuously day and night. When not in operation at night or on Sundays, and the kiln is used to season green stock direct from the saw, the large doors at both ends of the kiln should be opened wide, or the material to be dried will "sap stain."
It is highly important that the operator attending any drying apparatus keep a minute and accurate record of the condition of the material as it is placed into the drying room, and its final condition when taken out.
Records of the temperature and humidity should be taken frequently and at stated periods for the different thicknesses and species of material, in order that he may have reliable data to guide him in future operations.
The following facts should be taken into consideration when operating the Moist-air dry kiln:
1. Before any material has been placed in the drying room, the steam should be turned into the heating or radiating coils, gradually warming them, and bringing the temperature in the kiln up to the desired degree.
2. Care should be exercised that there is sufficient humidity in the receiving or loading end of the kiln, in order to guard against checking, case-hardening, etc. Therefore it is essential that the steam spray at the receiving or loading end of the kiln be properly manipulated.
3. As the temperature depends principally upon the pressure of steam carried in the boilers, maintain a steam pressure of not less than 80 pounds at all times; it may range as high as 100 pounds. The higher the temperature with its relatively high humidity the more rapidly the drying will be accomplished.
4. Since air circulation is as essential as the proper degree of heat, and as its contact with the material to be dried depends upon its free circulation, it is necessary that the dampers for its admittance into, and its exit from, the drying room be efficiently and properly operated. Successful drying cannot be accomplished without ample and free circulation of air at all times during the drying process.
If the above basic principles are carefully noted and followed out, and good common sense used in the handling and operation of the kiln apparatus, no serious difficulties should arise against the successful drying of the materials at hand.
Choice of Drying Method
At this point naturally arises the question: Which of the two classes of dry kilns, the "Moist-air" or "Blower" kiln is the better adapted for my particular needs?
This must be determined entirely by the species of wood to be dried, its condition when it goes into the kiln, and what kind of finished product is to be manufactured from it.
Almost any species of hardwood which has been subjected to air-seasoning for three months or more may be dried rapidly and in the best possible condition for glue-jointing and fine finishing with a "Blower" kiln, but green hardwood, direct from the saw, can only be successfully dried (if at all) in a "Moist-air" kiln.
Most furniture factories have considerable bent stock which must of necessity be thoroughly steamed before bending. By steaming, the initial process of the Moist-air kiln has been consummated. Hence, the Blower kiln is better adapted to the drying of such stock than the Moist-air kiln would be, as the stock has been thoroughly soaked by the preliminary steaming, and all that is required is sufficient heat to volatilize the moisture, and a strong circulation of air to remove it as it comes to the surface.
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.
The selection of one or the other for such work is largely matter of personal opinion.
Kilns of Different Types
All dry kilns as in use to-day are divided as to method of drying into two classes:
The "Pipe" or "Moist-air" kiln; The "Blower" or "Hot Blast" kiln;
both of which have been fully explained in a previous article.
The above two classes are again subdivided into five different types of dry kilns as follows:
The "Progressive" kiln; The "Apartment" kiln; The "Pocket" kiln; The "Tower" kiln; The "Box" kiln.
The "Progressive" Dry Kiln
Dry kilns constructed so that the material goes in at one end and is taken out at the opposite end are called Progressive dry kilns, from the fact that the material gradually progresses through the kiln from one stage to another while drying (see Fig. 31).
In the operation of the Progressive kiln, the material is first subjected to a sweating or steaming process at the receiving or loading end of the kiln with a low temperature and a relative high humidity. It then gradually progresses through the kiln into higher temperatures and lower humidities, as well as changes of air circulation, until it reaches the final stage at the discharge end of the kiln.
Progressive kilns, in order to produce the most satisfactory results, especially in the drying of hardwoods or heavy softwood timbers, should be not less than 100 feet in length (see Fig. 35).
In placing this type of kiln in operation, the following instructions should be carefully followed:
When steam has been turned into the heating coils, and the kiln is fairly warm, place the first car of material to be dried in the drying room—preferably in the morning—about 25 feet from the kiln door on the receiving or loading end of the kiln, blocking the wheels so that it will remain stationary.
Five hours later, or about noon, run in the second car and stop it about five feet from the first one placed in the drying room. Five hours later, or in the evening push car number two up against the first car; then run in car number three, stopping it about five feet from car number two.
On the morning of the second day, push car number three against the others, and then move them all forward about 25 feet, and then run in car number four, stopping it about five feet from the car in advance of it. Five hours later, or about noon, run in car number five and stop it about five feet from car number four. In the evening or about five hours later, push these cars against the ones ahead, and run in loaded car number six, stopping it about five feet from the preceding car.
On the morning of the third day, move all the cars forward about six feet; then run in loaded car number seven stop it about four feet from the car preceding it. Five hours later or about noon push this car against those in advance of it, and run in loaded car number eight moving all cars forward about six feet, and continue in this manner until the full complement of cars have been placed in the kiln. When the kiln has been filled, remove car number one and push all the remaining cars forward and run in the next loaded car, and continue in this manner as long as the kiln is in operation.
As the temperature depends principally upon the pressure of steam, maintain a steam pressure of not less than 80 pounds at all times; it may range up to as high as 100 pounds. The higher the temperature with a relatively higher humidity the more rapidly the drying will be accomplished.
If the above instructions are carried out, the temperatures, humidities, and air circulation properly manipulated, there should be complete success in the handling of this type of dry kiln.
The Progressive type of dry kiln is adapted to such lines of manufacture that have large quantities of material to kiln-dry where the species to be dried is of a similiar nature or texture, and does not vary to any great extent in its thickness, such, for instance, as:
Oak flooring plants; Maple flooring plants; Cooperage plants; Large box plants; Furniture factories; etc.
In the selection of this kind of dry kiln, consideration should be given to the question of ground space of sufficient length or dimension to accommodate a kiln of proper length for successful drying.
The "Apartment" Dry Kiln
The Apartment system of dry kilns are primarily designed for the drying of different kinds or sizes of material at the same time, a separate room or apartment being devoted to each species or size when the quantity is sufficient (see Fig. 36).
These kilns are sometimes built single or in batteries of two or more, generally not exceeding 40 or 50 feet in length with doors and platforms at both ends the same as the Progressive kilns; but in operation each kiln is entirely filled at one loading and then closed, and the entire contents dried at one time, then emptied and again recharged.
Any number of apartments may be built, and each apartment may be arranged to handle any number of cars, generally about three or four, or they may be so constructed that the material is piled directly upon the floor of the drying room.
When cars are used, it is well to have a transfer car at each end of the kilns, and stub tracks for holding cars of dry material, and for the loading of the unseasoned stock, as in this manner the kilns may be kept in full operation at all times.
In this type of dry kiln the material receives the same treatment and process that it would in a Progressive kiln. The advantages of Apartment kilns is manifest where certain conditions require the drying of numerous kinds as well as thicknesses of material at one and the same time. This method permits of several short drying rooms or apartments so that it is not necessary to mix hardwoods and softwoods, or thick and thin material in the same kiln room.
In these small kilns the circulation is under perfect control, so that the efficiency is equal to that of the more extensive plants, and will readily appeal to manufacturers whose output calls for the prompt and constant seasoning of a large variety of small stock, rather than a large volume of material of uniform size and grade.
Apartment kilns are recommended for industries where conditions require numerous kinds and thicknesses of material to be dried, such as:
Furniture factories; Piano factories; Interior woodwork mills; Planing mills; etc.
The "Pocket" Dry Kiln
"Pocket" dry kilns (see Fig. 37) are generally built in batteries of several pockets. They have the tracks level and the lumber goes in and out at the same end. Each drying room is entirely filled at one time, the material is dried and then removed and the kiln again recharged.
The length of "Pocket" kilns ranges generally from 14 feet to about 32 feet.
The interior equipment for this type of dry kiln is arranged very similiar to that used in the Apartment kiln. The heating or radiating coils and steam spray jets extend the whole length of the drying room, and are arranged for the use of either live or exhaust steam, as desired.
Inasmuch as Pocket kilns have doors at one end only, this feature eliminates a certain amount of door exposure, which conduces towards economy in operation.
In operating Pocket kilns, woods of different texture and thickness should be separated and placed in different drying rooms, and each kiln adjusted and operated to accommodate the peculiarities of the species and thickness of the material to be dried.
Naturally, the more complex the conditions of manufacturing wood products in any industry, the more difficult will be the proper drying of same. Pocket kilns, are, therefore, recommended for factories having several different kinds and thicknesses of material to dry in small quantities of each, such as: |
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