larly, downward movement of the air takes place between the other side of the load and the other kiln wall. In all four of these types the air movement throughout the lumber pile is intended to be strictly horizontal.

In this connection it is interesting to note that the two internalfan kiln manufacturers and the manufacturer of the external-blower type kiln with reversible circulation have specialized in the drying of softwoods while the external-blower kiln without reversible circulation is used principally in the drying of hardwoods.

These kilns are admirably adapted for the package system of handling, and one manufacturer points out the possibility of mechanical flat stacking. Successful drying is being done where sling-load packages are set one on the other, leaving a vertical chimney in the center and with four 1-inch sling sticks between the units comprising the kiln load. This permits the use of cranes and monorails in building kiln loads and eliminates investments in mechanical stackers. Such practice depends primarily upon the individual manufacturer. It must be obvious that a fan or blower discharging air builds up a certain amount of static pressure, but the kiln manufacturers insist that the amount of static pressure in the cross-circulation type of kiln is very low. In the case of the internal-fan type the manufacturers claim static pressure of less than one-eighth of an inch of water. This estimate is very conservative, for in some of the work which the Forest Products Laboratory has been doing they have found static pressures in the neighborhood of one-sixteenth or one-twentieth of an inch. The total static pressure built up by an external blower, particularly if it has an external heating system, must of necessity be considerably more than this on account of the comparatively high velocities used through the heater and the ducts. With properly designed plenum and vacuum chambers the static pressure required to force the air through the kiln itself may be very low.

In an end-pile compartment kiln the air movement will probably be outward from both sides of the lumber, then down under the pipes and around the circle again; and the central tunnel is particularly useful. In a cross-piled compartment kiln with cross circulation the general horizontal movement of air is lengthwise of the lumber; if a central chimney is present, most of the movement in this direction will be in the tunnel, where the air will spread out toward the sides of the kiln and become diverted into the horizontal spaces; it will move crosswise of the lumber and lengthwise of the kiln until it leaves the lumber and joins the general movement of air circulation outside the load, toward the sides of the kiln and down again. A similar air movement will occur in crosswise loads in a progressive kiln, but modified by the general lengthwise movement of air in the kiln. Piling for progressive kilns is often done without using a central chimney, and horizontal circulation in such piles is from the windward side, supplemented by such air as forces its way through the bottom of the pile between the boards. Use of a chimney in such loads would probably promote more uniform drying, particularly in random-width piles.

While there is considerable literature of kiln-drying, there is very little actual information in it about the proper spacing to use be

tween boards in the pile layers. In The Kiln-Drying of Lumber, by Tiemann (p. 157), the following statement is made:

Another principle of almost equal importance is that the lumber should be piled with reference to the direction of air motion in the kiln. It should be so arranged that the path of least resistance lies in the direction of general air movement in the kiln, and that this path should lie in the spaces between adjacent layers of board.

A little farther on the following is found:

Where the air movement is in a general vertical direction

flat

piling is not at all satisfactory, for the reason that the boards baffle the air currents.

The text does not state how this effect may be reduced, but it is obvious that hot air may ascend from the heating pipes beneath the lumber with least baffling in straight vertical spaces between the boards, and that in random boards, reducing the staggered spacing and concentrating the spaced saved in vertical chimneys will probably be the most effective use of it. Even in piling even-width lumber it is probable that use of chimneys and narrower vertical spacing between the groups of boards between the chimneys will promote air circulation, but no tests on this point have been recorded.

A number of systems have already been mentioned in which the air ascends at one side of the load of lumber and is diverted sidewise through it, being baffled to prevent its crossing above the pile. If the circulation is sufficient, this results in side circulation through all of the horizontal spaces in the pile; and it is obvious that vertical air spacing in the pile becomes of minor importance, and that both chimneys and spacing between boards could be omitted without greatly affecting the rate of drying. This form of circulation is used with some blower kilns, and with the Tiemann water-spray kiln and the Forest Service internal-circulation fan kiln; and in some instances it is recommended that the lumber pile be inclined sidewise with the entering side the high side, on the theory that as air cools and becomes heavier in passing through the lumber its natural course is in a downward direction. There are no known definite tests of comparative results.

The Kiln-drying of Lumber, by Koehler and Thelen, says on page 209:

Flat-piled lumber should be stacked carefully, with sufficient spaces between the edges of the boards and between successive layers. With the air entering the lumber piles from underneath, the spaces between the edges of the boards should be from one-quarter to one-third the widths of the boards. In piling strips several may be placed close together and a relatively wide space left between the groups of strips in the same tiers. If the air enters more or less from the sides and goes horizontally through the piles, less space necessary between the edges of the boards than if the circulation is more nearly vertical.

In Practical Kiln-Drying, by E. U. Kettle, the following statement is made, beginning on page 27:

Now, as regards piling stock on trucks ready for the kiln, I want to register a full-blooded, vigorous protest against overcrowding, close piling, and insufficiency of stickers.

After telling how to start the piles for cross piling and end piling, the author continues:

Then proceed to pile, allowing free air space between edges of boards on the first six courses equal to about 30 per cent of the total width; that is, on a load 6 feet wide allow about 2 feet air space for first six courses. From thereon gradually decrease this percentage of air space until top of the load is reached, leaving an air space or "chimney" in the center of the pile. It will, of course, be understood that the liberal spacing in the bottom of the pile is for the benefit of the lumber above, not the adjacent boards, which as they receive the air at its hottest would be the most rapid drying in any event. Vertical arrangement of the

Photograph

FIGURE 50.-Edge-stacked kiln load of lumber collapsed by
shrinkage in kiln and telescoping of tiers.
courtesy of Forest Products Laboratory

spacing will carry most air through the lower layers with least direct effect of the air on these layers.

In the kiln-drying of softwoods edge piling is used to some extent, usually at large mills, since special equipment must be used for stacking and unstacking, as well as special bunks provided with stake pockets. The boards rest on each other edgewise in vertical tiers with vertical stickers between, so that there are no air spaces between the edges of boards. Two or sometimes three courses of stickers are used, which are not enough to prevent a good deal of warp occurring; and sometimes the system is operated without proper provision for tightening up the load as shrinkage occurs and interferes with uniform drying. (Fig. 50.) Edge stacking

29055°-297

puts more lumber on a load and dries it somewhat more rapidly but has been abandoned for flat piling in some instances because of the high amount of degrade resulting. The general tendency is for the air to move downward through a pile of edge-stacked green lumber because of the cooling of the air resulting from the evaporation of moisture. In reality this movement is not a preference on the part of dry-kiln designers but is a natural movement of the air.

Its

FIGURE 52.-Frame used for forming flaring chimney.
only disadvantage is that the stickers must be recessed in
from the end to make room for the form. This could be
avoided by projecting the tier on either side of the chimney
out a little farther than the rest of the lumber. Photograph
courtesy of Forest Products Laboratory

It seems unfortunate that no definite tests are on record as to the best distribution of the spacing in a kiln load, whether in a center chimney or in even spacing between boards in the layer. The point is illustrated by the two loads in Figure 51. In the load at the right the spacing is all equally distributed; in the load at the left it is all concentrated in the middle tunnel, or as much of it as the tapered form of the tunnel will admit. In this load the upward vertical movement of air is all in the central tunnel, and the movement through the lumber is practically all horizontal movement between the lumber layers, with occasional casual