drift of air through spaces in the layers where these exist. These spaces are, however, purposely staggered so they will not build up into vertical passages in which a local vertical movement might occur that would interfere with the through horizontal movement of air. This effect is clearly shown in the right-hand load, where it is assumed that the upward movement of hot air, at least in the bottom of the load, will predominate and produce a through movement. As air can not move through a crossing in cross direction in both directions at the same time, the horizontal movement can proceed only from one crossing to the next, diverted from one upward current

FIGURE 53. Completed load piled with tunnel form shown in
Figure 52. When the edges of the chimney or tunnel are
irregular more air is diverted into some layers than others.
Photograph courtesy of Forest Products Laboratory

and swept into the next adjoining current, as shown by the arrows; and the cross circulation is therefore an indirect or baffled circulation, although as already stated the cross circulation must do most of the drying because it comes in contact with the largest surface of the lumber. It would seem that the left-hand load would have the freest air circulation throughout, and that this condition would not be greatly improved by adding more space between the boards, which by building up vertical subcurrents would add the baffling effect obtained in the other load.

In the questionnaire replies, five of the mills using center chimneys concentrate all the vertical spacing in them and use none between

boards; and one uses a 1-inch spacing. This general effect may be seen in Figure 52, illustrating the use of a form for shaping the center chimney, and in Figure 53 another load built in the same way. Another mill uses a space of 3 to 4 inches, reducing the top quarter of the load to 2 inches. Two southern pine mills use spaces of 4 and 5 inches, but neither uses center chimneys.

A center chimney is particularly useful in compartment kilns where the heating pipes are under the lumber; but only 8 of the questionnaire replies show its use in compartment kilns, 5 being in California pine, 1 in southern hardwood, and 1 in northern hardwood. In the West the chimneys are flared, and one northern hardwood mill also uses this form.

The lumber manufacturer can make tests by placing in the same charge loads piled by the two systems illustrated and observing the results; also, by piling some loads halfway between the two systems, with vertical tiers of spacing between the boards in addition to the central tunnel. The figures are drawn on the assumption of endpiled stock and cross circulation up through the lumber, out toward both sides and down at the sides to below the pipes again.

STICKERS AND STICKING IN DRY-KILN PILING

Dry-kiln stickers used for inch lumber are usually an inch in thickness, and 12 or 2 inches thick for thicker stock. As the sticker crossing dries more slowly, narrow stickers are always desirable, and 2 inches is wide enough for softwoods, as the pile, of course, is not so high as outdoor piles. Some woods are undesirable because of a tendency to leave sticker marks on the lumber; pine, Douglas fir, western fir, spruce, and basswood are all good species for stickers. Stickers are used more liberally in kiln piling than in outdoor piling. Some softwoods that kiln-dry fairly straight with few stickers may develop considerable degrade in going through the planer. This is true of southern pine; but figures have been published on test runs (Southern Lumberman, December 17, 1927, p. 167) by the Forest Products Laboratory at a southern mill, on the same stock with four sticker tiers and with nine sticker tiers, showing results as follows:

Where lumber is piled for crosswise position in kiln it is piled across 3 bunks, 1 for each of the 3 lines of rail, and these can give direct support for only 3 lines of stickers; but it is possible to start 2 lines of stickers side by side over each bunk and gradually spread them out as they are carried up, until the desired spacing is secured. The load illustrated in Figure 54 has 7 lines of stickers in the longer

FIGURE 54.-Kiln load piled with vertical chimney for progressive kiln. Notice also good method of supporting seven sticker lines (in longer portion of load) from only three crossers; also stickers in the middle of the bottom of the two A-angles for spacing the boards apart, though not for supporting any weight. The car stands on a transfer for which the coupled motor truck supplies power, and also carries a winch and cable for moving cars. Photograph courtesy of Forest Products Laboratory

bottom portion, all deriving support from the 3 bunk crossers, though only 3 are directly over the crossers. Figure 55 is also included here to show the difference between piling for compartment kilns and for progressive kilns at this plant, where both types of kilns are in use. Keeping stickers in nice alignment takes less labor if some form of mechanical guide is used. Figure 56 shows a rather elaborate frame for this purpose, with counterweights permitting the frames to be lifted as the load height increases. Figure 57 shows a kiln load of redwood which was piled in this frame, in which the regularity of the sticker courses is very evident.

It is regarded as good general practice in both hardwoods and softwoods to put the end stickers out square with the ends of the lumber, and this seems to be useful in reducing any degrade through checking and splitting. Often the bunks are not placed clear out at

FIGURE 56.-Frame with movable sticker guides, used in piling kiln loads. Elevated tram beyond is convenient in reaching tops of loads. Photograph courtesy of Forest Products Laboratory

the ends, and the end stickers are started on them and slanted gradually out until they are at the ends of the lumber.

Stickers, of course, should be of uniform thickness; but it is of even more importance that the lumber in each course should be of uniform thickness. A thin board between two thicker ones is not held down across its width by the sticker and the weight of lumber above, even if the sticker sags down at the middle of the board; and if it does sag, it tends to produce a cup in the lumber resting over that point. More even sawing is the best remedy for this condition, but it can be considerably alleviated by care in arranging the boards in piling.

Box piling is used more in dry-kiln piling than in outdoor piling but is not always used where it should be, nor is it always properly done. A number of diagrams in dry-kiln literature show it in

correctly, because they show short and long lumber piled in the same tier. (Fig. 21.)

In box piling the length of the pile is the length of the longest lumber, and the outer tiers, and usually also the tiers next the center chimney, are built entirely of the long lengths. A sufficient number of other tiers to accommodate the proportion of short lumber are started, each tier flush at one end and short at the other, of course not reaching the end sticker at the short end; and each tier is carried up throughout with its own length of lumber. Of course, if there is any half-length lumber, two lengths can be laid in a tier of long, with proper sticker support for the inner ends. In yard box piling the short stock is usually all laid flush at the front end

and short at the rear, but in kiln box piling the flush ends are usually alternated at the two ends, and at either end the flush ends are as far as possible alternated with the short ends, in adjoining tiers.

End stickers not reached by the boards in the course below naturally have no support at that point; and if a long board were laid over the short board, the weight of the end would come on the unsupported sticker and bend and perhaps break it. A single failure of a short board to reach the end sticker could be cured by putting a short block of the same thickness under the sticker, and this is sometimes done; but this practice can not be carried to any considerable extent because of the added labor.

Even with box piling the ends of long lumber undoubtedly get more air than the ends which do not come out to the end stickers,