The best method of piling is one which permits the heated air to pa.s.s through the pile in a somewhat downward direction. The natural tendency of the cooled air to descend is thus taken advantage of in a.s.sisting 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 a.n.a.logous 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 pa.s.s 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 cla.s.ses: 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 cla.s.ses 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 quant.i.ty 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 pa.s.sed 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 cla.s.s 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 quant.i.ty of air to the drying room, although there have been instances of kilns of this cla.s.s, 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 cla.s.s 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.