Seasoning of Wood Part 17

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 circ.u.mferentially 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 alb.u.men.

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 alb.u.men 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 succ.u.mbs, while the alb.u.men 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 part.i.tion 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 pa.s.ses 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).

[Ill.u.s.tration: Fig. 30. Section through United States Forestry Service Humidity-controlled Dry Kiln.]

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 pa.s.sing chiefly through open s.p.a.ces 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 pa.s.sed upward through most of the s.p.a.ces 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 pa.s.s 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 a.s.sist 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, const.i.tutes 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.