﻿52 BULLETTX 1136, U. S. DEPARTMENT OF AGRICULTURE. 



the air through the system. The blower is usually placed outside 

 the kilu in an operating room at one cud, discharging and returning 

 through. duets running the full length of the kiln. The. boating 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 walls 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 unformity. 

 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 flat-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. 



Recent 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 



