78 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
jets, the circulation should be reversed periodically, and the design 
must secure short travel of the steam through the lumber. } : 
Two types of superheated-steam kilns were developed and 
marketed several years ago. In one the heating coils and the steam- 
jet lines are under the lumber piles. In the other the heating coils 
are on the side walls and the steam-jet lines, four in all, are located 
above and below the coils, 
PILING LUMBER FOR KILN DRYING 
Lumber to be kiln dried is usually piled in layers with several 
narow strips, called either stickers or crossers, between adjacent 
layers—the layers are also called courses. Sometimes short stock, 
like spoke billets, handles, and shoe-last blocks, is simply dumped 
into the kiln without any attempt at orderly arrangement. ‘This 
method, however, is likely to cause irregular drying unless small 
amounts only are dried at a time. The piling and the stickering of 
the lumber should provide suitable air passages around the surfaces 
of each piece in the charge, and should furnish sufficient support to 
the lumber, with whatever restraint is necessary, to make it dry as 
straight as possible. 
For drying in a progressive kiln, the lumber is always loaded on 
trucks, or bunks, and is run through the kiln on rails; the track is 
usually pitched downward, so that gravity will assist in moving the 
loads toward the dry end. Similarly, large compartment kilns are 
usually provided with rails, to permit running the lumber in on 
trucks, but many small kilns have no such provisions, and in them the 
lumber is piled on horses or equivalent supports. 
The two general ways of piling lumber are called, respectively, 
horizontal or fiat piling and vertical or edge piling. Each of these 
may be divided into cross and end piling. (Pl. 15, A and B.) In 
cross piling, the boards run across the kiln, and in end piling they run 
along it. | 
FLAT PILING 
The manner of piling must. be adapted to the characteristics of the 
kiln. The design of the forced-circulation type more or less hmits 
the form of piling suitable for it. In natural-circulation compart- 
ment kilns having heating coils running lengthwise of the kiln, the 
lateral circulation that occurs is generally in the plane of the cross- 
section; end piling is best fitted for this condition, since with that 
method the air moves parallel to the stickers. If cross piling is used 
with cross circulation the stickers, which are then at right angles to. 
the movement of the air, practically block the circulation and hence 
extra-wide spacing between adjacent boards or pieces in each layer 
must be maintained if satisfactory results are to be secured. in 
those compartment kilns, however, in which an individual heating 
coil crosswise of the kiln is provided for each pile, cross piling is 
required. 
CIRCULATION 
The circulation in natural-circulation progressive kilns is complex, 
so that obtaining a general longitudinal movement of air through the 
lumber piles is difficult if not impossible. A large but variable part 
