Seasoning of Wood by Joseph Bernard Wagner (bill gates best books txt) π
Keeping especially in mind the arrangement and direction of the fibres of wood, it is clear at once why knots and "cross-grain" interfere with the strength of timber. It is due to the structural peculiarities that "honeycombing" occurs in rapid seasoning, that checks or cracks extend radially and follow pith rays, that tangent or "bastard" cut stock shrinks and warps more than that which is quarter-sawn. These same pecu
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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 kilnβor any other apparatus, for that matterβdepends upon intelligent operation.
Operation of the "Blower" Dry KilnIt 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 KilnWhile 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.
Fig. 31. Section through a typical Moist-air Dry Kiln.
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 coilsβthus reducing the temperatureβa dry kiln for drying hardwoods, that will not stand the maximum degree of heat.
Fig. 32. Single Pipe Heating Apparatus for Dry Kilns, arranged for the Use of Live Steam. For Low Temperatures.
Fig. 33. Double Pipe Heating Apparatus for Dry Kilns, arranged for the Use of Live Steam. For Medium Temperatures.
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 KilnTo 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."
Fig. 34. Vertical Pipe Heating Apparatus for Dry Kilns; may be used in Connection with either Live or Exhaust Steam for High or Low Temperatures.
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 MethodAt 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 TypesAll 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
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