22 BULLETIN 552, U. S. DEPARTMENT OF AGRICULTURE. 
KILN-DRYING. 
Lumber is kiln-dried when there is need for seasoning it quickly, 
or when the manufacturer does not wish to cany large stocks in his 
yard. A kiln is used also to further dry partially air-seasoned or 
even fully air-seasoned material, for special uses. 
The main problem in kiln-drying lumber is to prevent the moisture 
from evaporating from the surface of the pieces faster than it is 
brought to the surface from the interior. When this happens the 
surface becomes considerably drier than the interior and begins to 
shrink. If the difference in moisture content is sufficient, the sur- 
face portion opens up in checks. 
The evaporation from the surface of wood in a kiln can be controlled 
to a large degree by regulating the humidity, temperature, and 
amount of air passing over the wood; and a correctly designed kiln, 
especially one for drying the more difficult woods, should be con- 
structed and equipped in a way to insure this regulation. 
A dry kiln may consist simply of a box in which lumber can be 
heated, or of a good-sized building or group of buildings (battery) 
containing steam pipes, condensers, sprays, and various air pas- 
sages capable of adjustment to regulate the amount of ventilation. 
The elaborateness of the kiln depends, of course, mainly upon the 
value of the lumber that is to be dried. For lumber worth $100 
per 1,000 board feet, it obviously will pay to use more careful dry- 
ing methods than in handling material valued at $20 or $25 per 
1,000 board feet. 
TYPES OF KILNS. 
Kilns for drying lumber may be divided into two general classes 
(fig. 16): (a) compartment kilns, and (b) progressive kilns. In 
compartment kilns the conditions are changed during the drying 
process, and all lumber in the kiln is dried at one time. The condi- 
tions at any time during drying are uniform throughout the whole 
kiln. In a progressive kiln conditions at one end differ from those 
at the other, and the lumber is dried progressively by being passed 
through the kiln. Compartment kilns are used when it is desired 
to dry lumber of various sizes and species, while progressive kilns 
are used where uniform stock is handled. 
The methods of operation generally used in lumber kilns are: (a) 
natural ventilation, (b) condensing, and (c) superheated steam. 
In kilns operating by natural ventilation, the humidity or dampness 
is controlled by the use of escaping steam and evaporated moisture. 
Circulation in progressive kilns is largely longitudinal and in compart- 
ment kilns transverse. Moist air is allowed to escape from the kiln. 
In condensing kilns the humidity is controlled by recirculating the 
air, which has taken up water from the lumber, across water pipes 
