KILN DRYING HANDBOOK ve 
GENERAL PRINCIPLES OF DRYING WOOD 
The drying of wood is a very complex process, concerning many 
phases of which information is still lacking. It is not essential, how- 
ever, that the operator understand all of the details of the movement 
of moisture through wood and all of the attendant phenomena. He 
may take it for granted, for the time being, that the moisture in each 
piece tends to distribute itself evenly, moving from the more moist 
parts to the drier. ones. 
The movement of moisture through wood is affected by a number 
of controllable external factors. Only two of them need to be 
considered here—the temperature and the humidity of the atmosphere 
surrounding the wood, that is, of the air in the kiln. Circulation of 
this air, adequate in both uniformity and volume, is necessary in order 
to control its temperature and its humidity; in fact the success of a 
kiln-drying operation depends very largely upon the proper regu- 
lation of heat, humidity, and circulation. 
HEAT IN THE KILN 
Heat is required, in kiln drying, to evaporate the moisture from 
the wood, whether the temperature of the kiln is high or low. The 
higher permissible temperatures, however, increase the rate of trans- 
fusion of moisture to the surface of the wood and thus permit more 
-rapid drying. The temperature that is correct for the purpose is 
determined in each case largely by the species of the wood and by the 
thickness and the shape of the individual pieces; these factors are 
modified somewhat by the use requirements for the finished ‘stock. 
Commercial-kiln temperatures range from 100 to 250° F. 
A kiln temperature above that of the surrounding atmosphere 
introduces a problem in the heating of buildings, imposing upon the 
heating system the added burden of replacing promptly the heat lost 
through its walls. The higher the kiln temperature, the greater will 
be the heat losses. 
The relation between the total heat input to the kiln and the heat 
required to evaporate the moisture from the charge in it is highly 
variable, even if steam used for humidification is omitted from the heat 
input. Such factors as species of wood, thickness of lumber, rate of 
drying, character of drying schedule, and type of kiln, have an im- 
portant effect upon this relation. Under the best possible conditions 
it is commercially practical to secure the evaporation of a pound of 
water for about 114 pounds of steam put into the heating coils. 
Average heat efficiency, however, is much lower than this. Economy 
of steam is of importance in many plants, but sacrifice of quality in 
drying seldom if ever pays a dividend. 
SOURCES OF HEAT 
Many methods have been used to heat kilns, and although most 
of them are obsolete or impractical, brief mention will be made of 
the principal ones. | 
Direct furnace heat.—A fire built on the ground or in a crude fire- 
place is the source of heat in the direct-furnace-heat method. The 
