in kiln loads delays their drenching of the lumber in the lower portion of such loads. It is possible for sparks from outside to be drawn in through the cold-air intake and start a fire, but this rarely happens. It is the opinion of the Forest Products Laboratory that the fire hazard is not greater in a forced-circulation kiln than in a natural-circulation kiln, because as a rule the temperatures are lower and the relative humidities are higher in the former than in the latter; also there is less danger of "hot spots" in a forced-circulation kiln.

Of the questionnaire replies, only 35 reported on the type of kiln buildings; those at 8 mills were of wood and at 1 wood and cement, the other 26 being brick, tile, cement, or various combinations of these materials.

Wooden walls compare favorably with other types in heat insulation; they may be double, with an interior air space, or built up solid 6 or 8 inches thick by spiking dimension of proper width on top of each other. In almost any type of wall there is considerable heat and moisture loss by evaporation. This is reduced by keeping the interior surface well coated with a special type of asphalt paint adapted to resist the corrosive action of the vapors inside the kiln often heavily laden with wood acids. All ironwork inside the kiln is protected with a similar paint. Galvanizing is worse than useless, as the zinc is promptly eaten away, leaving the iron subject to more rapid corrosion than plain uncoated pipe. A good boiler or stack paint should be used on radiation pipe to avoid destruction by high heat. However, in using any paint on radiation pipes it should be borne in mind that it retards heating. From a practical point of view oil and graphite are said to be more effective in preventing rust and do not retard radiation appreciably.

THE KILN OPERATOR

Kiln-drying results depend largely upon the kiln operator, and he should be given every facility for control of results in that department. He should have facilities for keeping the equipment in good operating condition, and his showing that certain repairs or improvements will be profitable should have prompt action. He should not be expected to put more product through the kilns than they can handle with good results.

The matter of kiln-drying degrade should rest with the operator, and the management should be informed as to what the degrade percentage is; the operator should demonstrate what part of it is unpreventable and what can be avoided by more careful operation.

The kiln operator should have a well-defined program of comparative tests, in order to discover possible improvements in operating methods. This report suggests some possibilities in that direction.

PROGRESSIVE AND COMPARTMENT TYPES OF KILNS

The progressive type of kiln is largely used at sawmills generally, though the use of compartment kilns is increasing, and it is stated that in the Inland Empire the majority of installations are of that type.

The progressive kiln will first be considered; but in these paragraphs the two types will be compared for better understanding of their essential differences.

The questionnaire replies reported a total of 225 progressive-kiln units or rooms and a total of 228 compartment-kiln units. The distribution of each in the various manufacturing regions was follows:

In either progressive or compartment type there may be a battery of rooms built side by side, but each room is an independent kiln unit. In the progressive kiln these rooms will be long in relation to their width, 84 to 135 feet or more, and each room will have doors at both ends, practically the entire width and height of the kiln, for receiving lumber at one end and discharging it at the other. The lumber is often piled crosswise of the kiln, and the kiln is made of width to accommodate the longest lengths of lumber to be kiln-dried in it. There is a 3-rail track laid throughout the kiln and through both doorways; the lumber loads are each supported on three cross bunks, one on each track, having small roller-bearing flanged wheels running on the rails. The tracks are usually given a slight downward incline to facilitate moving the lumber. At stated periods one or two loads of lumber are taken out of the kiln at the dry end, the entire line of lumber moved along, and one or two new loads run in at the green end; thus each load moves progressively through the kiln. Steam pipes are arranged under the lumber in a pit beneath the tracks, generally fed from the dry end, where the pipes are hottest and more numerous; they are gradually reduced in number toward the green end, and sometimes none of them run all the way through. In some cases the steam pipes are fed from the green end and in others at the middle of the kiln. Fresh air from outside is admitted under the pipes at the dry end, is heated as it passes up between the pipes, and circulates toward the green end, becoming cooler and more moist toward this end, where some of it passes through air ducts near the ground level or part way up the side, into and up through ventilating stacks to the exterior atmosphere. This outlet air is still considerably warmer and lighter than the outside air, and it is this difference in weight, or "head," that induces the draft through the stacks and promotes the interior air circulation of the kiln; however, only a limited part of the circulation in a natural-circulation kiln is due directly to ventilation, as by far the major portion is due to differences in temperature within the kiln. In a progressive type of kiln the air is usually hottest and dryest

at the dry end and coolest and most moist at the green end, because of moisture evaporated from the lumber and heat lost in the process; and though it receives some fresh heat from the steam pipes along the way? it is usually so graduated as not to overcome entirely this graduation of conditions in the air stream. There are other kinds of progressive kilns than the typical one described; the feature common to all is the progressive movement of lumber through the kiln, and its partial removal and replenishment from time to time. (Fig. 44.)

Of the progressive kilns reported in the questionnaires, 23 installations (not units) were single-track, 18 were double-track, and 1 had four tracks to a room. Endwise piling was used in 28 and crosswise in 16. In 3 the kiln length ranged between 84 and 96 feet; in 11, 100 to 106 feet; in 17, 118 to 120 feet; in 6, 124 to 135 feet; and in 2, 170 to 176 feet.

FIGURE 44.-Conventionalized longitudinal section of progressive kiln, with recirculating-duct steam sprays at green end for initial steaming, and with a steam spray in the recirculating duct which may be used for both humidity control and boosting air circulation. Arrows indicate some recirculation in the pit besides that in the recirculating duct

In the compartment type of kiln all the lumber is put in at one time, is left until all is seasoned, and is removed at the same time. Conditions within the kiln are varied to suit the condition of the stock-milder heat and greater humidity at first, gradually increasing heat and reducing humidity as the lumber dries. This corresponds roughly with the graduation of conditions throughout the length of the progressive kiln but is under more exact control. The compartment kiln may be long or short and may have doors at both ends for convenience, but doors at one end are sufficient for both charge and removal of stock. Circulation in a flat-stocked compartment kiln is usually sidewise-up through the heating pipes and through the lumber (or perhaps sidewise through the lumber), if piled with an adequate chimney, and down at one or both sides, then up through the heating pipes again. Movement of air in this direction is facilitated by having the lumber endwise instead of crosswise in the kiln, in which case there are but two tracks under the load, with a bunk on each lengthwise of the lumber, and crossers to support it. There may be two or even three shorter bunks on each track instead of one longer bunk. In some compartment kilns there are double tracks for two lines of lumber lengthwise; in others crosswise piling is used.

In the questionnaires reporting use of compartment kilns the lengths were 52, 66, 82, 84, and 90 feet in one installation each; ranged from 100 to 104 in 12; 3 were 120 feet, and 1 each 125 and 130 feet. One track to a room was used in 17 and double tracks in 4. Those in which the lumber loads were endwise numbered 15, and crosswise, 6. The lumber was edge-stacked instead of flat-piled in 4 of the above end-piled installations, and in 1 of the crosspiled.

Progressive kilns require a continuous line of material of uniform drying rate, and it is easiest to meet this condition in sawmill kilndrying, which may be considered their natural field of use. They are also simpler, less costly to install, and require less supervisory control than compartment kilns. The only direct comparison between the two types of kilns for the kiln-drying of western softwoods that is available appears in a course of kiln-drying tests of the Forest Products Laboratory, Vancouver, British Columbia, prepared by J. H. Jenkins; in lecture 4, page 2, the following conclusions are stated as the result of kiln-drying degrade studies made by the British Columbia Products Laboratory in cooperation with the British Columbia Lumber and Shingle Manufacturers' Associations:

1. There were fewer charge kiln loads tested than progressive, but the min mum loss in the best progressive kiln tested was much greater than the maxi. mum loss in any charge kiln tested.

2. The average kiln-drying loss for all progressive kilns tested was $3.30 per thousand feet, while for charge kilns the average loss was only 78 cents per thousand feet.

3. The greatest losses occurred in drying wide stock. The average loss in drying 1 by 8, 10, and 12 inch S4S fir finish was $4.74 per thousand feet for all stock dried in progressive kilns. For similar stock dried in charge kilns the average loss was $1.08 per thousand feet.

4. The superior showing of the charge kiln is not, however, attributed entirely to the kiln-drying system. The operation of the charge kilns tested was a very important factor. Unless operated properly a kiln, whether it be charge or progressive, will not give good results.

PROGRESSIVE KILNS

Of the questionnaire replies, 17 state the dry and wet bulb readings at the dry end of progressive kilns. One reply (redwood) indicated a temperature of 155° F.; 7 ranged in temperature from 180° to 198°; and 9 ranged from 200° to 210°. Of the wet-bulb readings, 3 corresponded with relative air humidities of 20 or below and with a balanced moisture in wood of 2 to 2.9 per cent; 12 ranged from 30 to 45 per cent relative air humidity and 3 to 4.9 per cent of moisture balance in wood; and 2 corresponded with 49 and 51 per cent relative humidity of air and with 5.3 and 5.5 per cent of moisture in wood. Only the three driest readings were seriously low, and they were in southern hardwoods, Douglas fir, and California pine.

In such a kiln there is usually a slow movement of air in the pit from the green end toward the dry end, which may bring back a little humidity with it. Some of the dry-kiln manufacturers try to make this circulation more active by installing a return-air duct in the pit for recirculating and reusing some of the air. However, in actual practice, as determined by recent tests, it was found that because of excessive friction this natural circulation was not sufficient;

but the installation of jet blowers and fans to create an artificial propulsion of the air brought satisfactory results. A sufficient volume of recirculation will suitably increase the humidity at the dry end.

Much of the same result may be accomplished by adding humidity to the heated air at the dry end from a steam spray. Placing this spray in the fresh-air intake may not be efficient, because the cold air will not take up sufficient moisture.

A practical difficulty with either of these expedients is that increasing the humidity of the hot air at the beginning of its circulation decreases its drying effect on the lumber, which also decreases its impulse to circulate; and the air movement may slow up to a degree that seriously affects the volume of kiln output.

Of the progressive kilns reported in the questionnaire replies, 20 were supplied with recirculating ducts and 21 with steam sprays so located (at or near the dry end or in the recirculating duct) as to be adapted to increasing humidity in the kiln.

There are a number of kilns of this kind drying lumber under conditions that are too severe, even when limited by the rate of speed at which the lumber is moved through the kiln, and which, if the amount of steam supply is modified, slow down their drying rate to such a degree that the lumber must remain in the kiln longer or comes through with only a part of it sufficiently dry, and with a more thorough drying on the outside than in the interior. This means that such kilns must be operated at temperatures too high for the amount of moisture that can be held in the air to give the best quality of drying results. Cutting down the ventilation should help, but in a leaky kiln not much control can be obtained in this

way.

The longitudinal circulation of air in a progressive kiln is primarily above the lumber from the dry end to the green end and below the lumber from the green end to the dry end.

Movement through the lumber is mostly a local movement and is largely vertical-ascent of hot air through the pipes and up through the lumber at the dry end and descent of air through the lumber at the green end. As more air must come in under the pipes to support this movement, there is probably some downward movement at the sides of the kiln into the pit, and, in addition, there is probably also a movement of air backward from the green end in the pit under the pipes. The pit usually runs to the green end, if the pipes do not; if it stopped short of the end, this recirculation would be somewhat clogged.

At some points in the length of the kiln the vertical movement of air in the lumber may be downward through the pipes, and this reheated air will be diverted and flow upward at the sides instead of through the lumber, entering the lumber from the top or sides. This reversed movement is likely to take place where the lumber is green and quickly cools the air, adding weight to it and causing its descent. The same thing is often noticeable in a compartment kiln, for a time, after it is freshly charged with green lumber; the circulation will be downward through the lumber and up at the sides until the lumber becomes well heated, when the circulation will revert to its normal cycle.