On 24 installations there are steam-spray pipes at the green end for an initial steaming of the lumber, but it is regularly used only at 16 mills, 10 in southern pine, 3 in fir, and 1 each in redwood, northern hemlock and hardwood, and North Carolina pine.

The continuous schedule of a progressive kiln is somewhat broken into by cooling the kiln when the doors at both ends are opened for taking out dry lumber, moving the charge forward, and adding additional loads of green lumber. There was considerable difference of opinion as to the time required for the kiln to get back to its usual operating temperature after this cooling. The time was estimated at under 3 hours in 15 replies, as 3 to 13 hours in 3, and as 24 hours in 2. Kiln doors are of wood at 11 mills, of wood with asbestos lining at 4, of wood lined with roofing paper at 6, of uninsulated steel at 3, and of steel insulated with asbestos at 9. At 9 mills the kiln doors are hung on hinges; at 13 patent door carriers are used; at 1 mill the doors elevate, and at 2 they run, with wheels at the bottom, on a track.

Sometimes the first layer of heavy plank is laid before the load of lumber is started, either to protect it from overdrying in the bottom of the layer or to afford a firm foundation for sticker courses elsewhere than over the bunk supports. Such use was reported by four mills in northern hardwood, North Carolina pine, cypress, and southern pine. One California pine mill also uses this plan on select uppers. Instead of plank, narrow lumber is placed in two or more layers in the bottoms of loads of wider stock at 3 mills in southern pine and 1 in Douglas fir; while 1 southern pine mill, which has hardwood production also, places 3 layers of pine in the bottom of kiln loads of hardwood.

Cross-piled lumber is usually piled on three bunks, each of which constitutes a crosser; but in end piling the bunks can be combined to provide for any desired number of crossers, and many variations were shown in the replies received. One mill spaces crossers 2 feet apart, which would be 9 for 16 feet, and four mills space them 4 feet apart. Only 2 crossers are used at 1 mill, 2 to 4 at 1 mill, 2 to 6 at 2 mills, 3 at 5 mills, 3 to 4 at 1 mill, and 3 to 6 at 1 mill; four crossers are used at 7 mills, 4 to 5 at 1 mill, and 4 to 6 at 6 mills; five to six crossers are used at 1 mill, 5 to 7 at 2 mills, and 7 to 9 at 1 mill. An average of close to 4 crossers is used in redwood, California pine, cypress, southern pine, and Pondosa pine, 5 for Douglas fir, 22% for North Carolina pine, and 8 or 9 for both northern and southern hardwoods. As there is little sticker marking in good kiln-drying, closer sticking can be used to better advantage for softwoods in kiln-drying than in air-drying; and the tests on southern pine kiln-drying at the plant of the Kaul Lumber Co., made by L. V. Teesdale for the Forest Products Laboratory (Southern Lumberman, December 17, 1927, p. 167), showed that the used of nine stickers per course instead of four reduced the degrade loss to less than half. As this was in compartment kilns, the figures will be given under that heading.

Spacing between boards in the layers also varies greatly. The general purpose is to encourage vertical circulation, and the spaces fall into tiers in lumber of even width and are often arranged in tiers in random widths by piling the lumber in close tiers about 12 inches

wide with space tiers between. Figure 45 shows a load mostly of strips with some wider lumber tier piled, and Figure 46 shows similar piling with much wider air spaces. In the replies received 4 mills use 2-inch spaces, and 4 vary the spacing from 1/2 to 3 inches; 8 mills use 4-inch spacing; 5 mills, spacing of 5 to 6 inches; 1 mill, 5 to 8 inches; 4 mills, 7 to 8 inches.

Chimneys are not generally considered as important in piling for progressive as for compartment kilns, especially in cross piling, but 10 mills reported use of chimneys, 9 being vertical and 1 wider at the bottom and flared; 8 running all the way to the top, 1 stopping 6 inches from the top, and 1 stopping halfway in.

FIGURE 45.-Kiln load of random length lumber piled in seven ver-
tical tiers with moderate air vents between. Photograph cour-
tesy of Forest Products Laboratory

Lumber is completely sorted by widths before kiln-drying at only 2 of the reporting mills, in Douglas fir and in western pine, but is partly sorted by widths at 7 other mills. Lumber is sorted by lengths at 16 of the reporting mills and partly sorted at 11, partial sorting separating it into such length as 12 feet down, 14 to 16 feet, 18 to 20 feet, and other divisions.

Mixed lengths are box piled for kiln-drying at 9 mills, partly box piled at 1 mill, and box piled in a modified manner at 1 mill, this

modification consisting of not keeping the separate lengths in separate tiers.

Additional damage is reported in overhanging ends of lumber in kiln loads by 17 mills, while 8 state that little of such damage occurs. Additional degrade at the top of the load, by roof radiation or drip or warp in the unweighted top layers, is reported by 7 mills, while 13 report that such damage is insignificant. One mill uses cover boards to prevent it, and 2 others keep wide lumber away from the top of the pile and also use narrow lumber in the bottom of the pile to reduce degrade damage there. Extra degrade in the bottom is reported by 7 mills, while others say that little added degrade occurs at this point.

FIGURE 46.-Kiln loads of random widths piled in five vertical tiers with four very wide air vents between. Photograph courtesy of Forest Products Laboratory The use of thicker stickers for thicker lumber seems less common in kiln-drying than in yard piling. One redwood mill uses the same stickers for 3-inch lumber but doubles them; and one Douglas fir manufacturer provides thicker stickers for use in kiln piling of 2 inches and thicker and also for finish.

Only three mills reported that lumber is sufficiently graded before kiln-drying so that comparison with the regrading after kilndrying will give an adequate idea of the amount of degrade; but 15 others make occasional tests by grading a certain amount of stock both before and after kiln-drying and thus getting a comparison. Of the replies received, 10 expressed the belief that progressivekiln degrade was under 5 per cent; 9, that it ranged from 6 to 10 per cent; 1, 20 per cent; 1, 48 per cent; and 3 merely stated that it was little.

Figure 54 shows a kiln load piled for a progressive kiln, with center chimney going through to the top, and which may be compared with the similar load in Figure 55 piled for a compartment kiln, in which it will be noticed that the chimney is wider at the bottom and tapers off below the top of the load. Figure 47 illustrates a progressive-kiln load made up of eight unit packages assembled by a crane.

FIGURE 47.-Kiln load composed of eight package units, assembled by
crane. Photograph courtesy of Forest Products Laboratory

Figure 48 illustrates an ingenious controlling chart for two doubletrack progressive kilns, originated by C. A. Rishel, kiln operator for the W. M. Ritter Lumber Co., at Maben, W. Va. By moving the paper slip as the charges on the track are moved along, the chart always correctly shows the position of the entire contents of the kiln, and special stock when it comes out will be applied on its own order instead of perhaps being worked by mistake into something else.

COMPARTMENT KILNS FOR SAWMILL KILN-DRYING

Compartment-kiln operating at the sawmill differs in a number of respects from progressive-kiln operations, and some of these differences have caused the greater use of the progressive kiln in the past. Progressive kilns are not very responsive to control, and the control that is applied is mostly in the degree of heat at the dry end, with

some control of humidity and circulation by the air dampers. The compartment kiln is under direct control all the time and will not operate successfully unless the control is actively exercised. It needs

FIGURE 48.-Control chart for two double-track progressive kilns, devised by C. A. Rishel, kiln operator for the W. M. Ritter Lumber Co. at Maben, W. Va. Four spools of paper are used, one for each tier, and the strips are moved down as the loads are moved through the kilns, the upper row of spaces representing the loading platform and the lower the cooling shed. When a space in the cooling shed is emptied the marker card 2 is used to indicate it. Illustration reproduced from the Ritter employees' magazine

more supervision, but when that is given it will produce better results, although the work of good progressive kilns on softwoods has been accepted as satisfactory.