72 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
green end, and in some kilns vents are provided along the length 
so that some of the.air can be exhausted before it reaches the green 
end. There is seldom any provision, however, for regulating the 
temperature along the length of the kiln. 
NATURAL-CIRCULATION PROGRESSIVE KILNS 
The circulation in natural-circulation progressive kilns is very com- 
plex. In the commonest kind of such kilns it is made up of (1) 
longitudinal outside circulation (see “Air circulation in the kiln,” 
p. 26) from the intakes at the dry end to the exhaust flues at the green 
end, (2) longitudinal recirculation from the dry end to the green 
end above the heating coils and in the reverse direction below them, 
and (8) cross recirculation, mostly vertical, through and around 
each pile. The longitudinal circulation takes place largely in the 
passages between the inner face of the kiln and the piles instead 
of through the piles, so that much of the actual drying is done by 
the widespread vertical recirculation. The longitudinal circulation 
serves chiefly to maintain the temperature and the humidity gradients 
and to remove much of the moisture from the kiln. 
The various kiln manufacturers employ a number of different 
arrangements of intake ducts and exhaust flues. In the most common 
one, intakes are provided at the dry end and outlets at the green 
end. In another, no intake openings are provided, and exhaust 
flues are located in one or more rows the entire length of the kiln. 
Although a chamber for preliminary steaming can be formed in 
many natural-circulation progressive kilns by dropping a curtain 
between two trucks near the green end, steaming in such a curtained- 
off space is apt to upset the conditions in the kiln, increasing the 
humidity throughout. Furthermore, trouble may result if stock 
warmed in this manner is not carefully cooled in saturated air to 
the drying temperature before the curtain is rolled up and drying 
is begun. 
FORCED-CIRCULATION PROGRESSIVE KILNS 
Where the longitudinal circulation in a progressive kiln is stimu- 
lated by steam-jet blowers, they are usually placed near the dry end 
of the kiln, pointing toward the opposite end, and may be so ar- 
ranged that they can draw fresh air from the outside as well as 
recirculate the air within the kiln. Many arrangements of disk and 
centrifugal fans are possible, and various ones have been tried out 
trom time to time. At present the most favored arrangement seems 
to be a pair of large disk fans in the end wall of the kiin, at the 
green end. The air drawn through the kiln by the fans may be 
discharged to the atmosphere or returned to the dry end through 
suitable ducts, depending upon the design of the kiln. It should be 
pointed out here that the gradients of temperature and humidity 
along the length of the kiln depend, among other things, upon the 
rate of circulation, and a marked increase in that rate will reduce 
these gradients very materially. 
NATURAL-CIRCULATION COMPARTMENT KILNS 
Although natural-circulation compartment kilns differ consider- 
ably in detail design, most of them are arranged for cross circulation ; 
