89 BULLETIN 1136, U. 8. DEPARTMENT OF AGRICULTURE 
blower kilns are designed for downward circulation through the 
lumber, while in internal-fan kilns the movement of the air may be 
either upward or downward as the operator chooses. The arrange- 
ment of heating coils and of other elements in most natural-circula- 
tion kilns tends to produce upward circulation, whereas the evap- 
oration of moisture does the opposite. The preponderance of one 
over the other determines in large measure the direction that the 
air actually takes at any given time; when drying green stock it is 
usually downward during the beginning of the run and upward to- 
ward the end, and it generally is upward throughout the final drying 
of previously air-dried stock. In forced-circulation kilns the eifect 
of evaporation on circulation is largely overshadowed by the efiect 
of the mechanical apparatus. 
Considered from the standpoint of circulation the direction of 
piling of edge stacked stock makes little difference; the circulation 
must be vertical with either cross piling or end piling. The present 
tendency is to use end piling and single-track forced-circulation 
compartment kilns. 
STICKERING 
With edge stacking as usually practiced only three stickers are 
used for each layer, and the boards do not receive support enough 
to prevent warping. Comparable tests made on southern pine lum- 
ber showed that flat piling with seven rows of stickers gave sub- 
stantially less degrade from planer splitting than did edge stacking 
with only three rows of stickers. Bee 
Edge stacking, with the special equipment that makes it profit- 
able, is limited to large softwood operations, principally in the 
Pacific Northwest and other western lumbering regions. ‘There are, 
however, a number of installations in the South. 
PILING PRACTICE 
Both cross piling and end piling are used in natural-circulation 
progressive kilns. Edge stacking also is employed to a slight extent. 
Although there seems to be no definite correlation between the 
method of piling the lumber and the ventilation openings, regard- 
less of how the piles are arranged care must be taken to provide 
ample space for recirculation through them if good drying 1s to 
result. One type of progressive kiln with steam-jet blowers 1s 
arranged to force the air through internal ducts from the green end 
of the kiln back to the dry end. This system can be used with either 
cross piling or end piling. ; 
End piling is the logical arrangement of lumber in a cross-circu- 
lation kiln, but many such kilns, particularly the older ones, are 
equipped for cross piling. On the other hand, several new types 
of cross-piled natural-circulation kilns have recently been developed. 
In them each pile of lumber is arranged, together with the heating 
coils, the steam spray lines, and the flues, to secure active recircu- 
lation of air through the piles; each of these piles has a set of 
heating coils of its own, also placed crosswise of the kiln. Such recir- 
culation is similar in its general features to that through the pile in 
Figure 10, though of course its direction is lengthwise of the kiln 
instead of crosswise. 
