52 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 
the air through the system. The blower is usually placed outside 
the kiln in an operating room at one end, discharging and returning 
through ducts running the full length of the kiln. The heating units 
may be in a box located at the blower, or they may be arranged in 
almost any desired form in the kiln proper. Humidity may be in- 
creased by means of a steam jet located in the return duct and de- 
creased by opening a fresh-air intake also in the return duct. One 
manufacturer prefers to decrease the humidity in his blower kilns 
by using canvas curtains to form the outer walls of the flues. Be- 
tween these curtains and the side wails of the kiln are ventilated pas- 
sages about a foot wide. Moisture transfuses through the curtains 
from the inside out, and is carried away on the ventilating current of 
air. This air may be drawn from the operating room and exhausted 
through a chimney. 
The rate of circulation in blower kilns may be increased indefi- 
nitely, but beyond a certain point it is difficult to maintain unf ormity. 
A few forced-circulation kilns in which the circulation is produced 
by fans located in the kiln itself have been used for the drying of 
lumber and veneer; and several such types of kilns are being de- 
veloped at the Forest Products Laboratory. In one of these the fans 
are all mounted on a single shaft running lengthwise of the kiln and 
driven by a motor located outside. Office fans and other self-con- 
tained motor-driven fans have also been used with considerable suc- 
cess. There are several points of special interest in this type of 
forced circulation, of which ease of installation and reversal of cir- 
culation are foremost. Periodical reversal of the circulation pro- 
duces faster and more uniform drying. Humidity in these kilns 
may be controlled by any one of several methods, but usually steam 
alone is sufficient, as leakage keeps the humidity sufficiently low. 
Figure 10 is a diagrammatical cross section of an internal-fan kiln 
of the compartment type, arranged for fiat-end piling. The double- 
pointed arrows illustrate the path of the air through the lumber; 
the direction of air travel may be reversed at will by reversing the 
direction of rotation of the fan shaft.. This shaft extends the length 
of the kiln and has fans mounted upon it at intervals of about 7 
feet. These fans are so housed that when the direction of rotation 
is such that the air movement is upward through the central flue and 
downward along the side walls, the air enters the fans through suit- 
able openings in the side walls of the housings and is deflected 
upward after passing through the fans. The double distributors 
serve to distribute the air uniformly along the width and length 
of the central flue, reducing the velocity appreciably at the same 
time. 
Kecent tests have shown that a very uniform, fast circulation of 
air may be obtained in this type of kiln with a surprisingly small 
power consumption. 
SUPERHEATED-STEAM KILNS. 
The superheated-steam kiln is comparatively simple in construc- 
tion and operation. Provision must be made for high-pressure 
steam for heating coils and jets; the circulation must be reversed 
periodically; and the kiln must be designed for short travel of the 
steam through the lumber. One type of superheated-steam kiln was 
