KILN DRYING HANDBOOK 29 
The most important use of steam jets, as aids in producing eircu- 
lation, is in kilns that lack other means of forcing circulation, While 
they are commonly so placed as to stimulate recirculation within the 
kiln, they are occasionally located in the intake flues or in the outlets 
to augment the outside circulation. A usual arrangement of them 
is in rows along the sides or down the center of the kiln. The me- 
chanical efficiency of steam jets is very low in comparison to that 
of fans or blowers, but other considerations often outweigh this fact. 
Water sprays, as used in the water-spray kiln, produce circula- 
tion (assisted by the heating coils) through a combination of tempera- 
ture difference and of mechanical means. Located in rows along 
the sides or in the center of the kiln, near the bottom and pointing 
downwards, they both cool the air and drive it downward by impact. 
It passes over the heating coils after leaving the sprays, then through 
the lumber, and again through the sprays. 
MEASUREMENT AND CONTROL OF CIRCULATION 
Although the temperature and the humidity best for a particular 
drying condition may be specified with certainty, the amount of cir- 
culation desirable is not so easily disposed of. While rapid, uniform 
circulation does induce faster and more uniform drying and also 
permits better control of the drying conditions than slow, irregular 
circulation, it becomes increasingly difficult to secure uniformity as 
the speed of circulation rises, beyond certain limits, and producing 
and maintaining high circulation rates add to the cost of operation. 
Economic considerations naturally make desirable the range in rates 
of circulation that yields the greatest return on the total investment. 
RATE OF CIRCULATION 
Prescribing one specific rate of circulation as the best for all dry- 
ing conditions is impossible, because circulation requirements vary 
widely ; considerable difference in them may exist even during a single 
kiln run. A number of detail factors have an influence in determin- 
ing the rate of circulation proper for any particular set of conditions; 
among the most important are the following: (1) Species of wood, 
(2) grade of lumber, (3) previous seasoning of lamber, (4) size of 
lumber, (5) purpose for which lumber is to be used, and (6) length 
of air travel through lumber pile. These factors of course have an 
important bearing upon the selection of the drying schedule for the 
problem in hand. Rapid rates of circulation and high-humidity 
drying schedules have proved themselves to be paying investments in 
drying both green hardwoods and green softwoods. One of the out- 
standing developments in seasoning practice within the last four 
years has been the progress made in the use of high-circulation rates 
in the drying of certain southern and western softwoods. Fates as 
high as 150 feet per minute through the lumber charge are not 
unusual. Rapidly drying softwoods having large amounts of mois- 
ture when green require a circulation of about 100 to 150 feet per 
minute through the lumber stacks with a length of air travei not 
over 5 to 7 feet if the most effective drying schedules are to be em- 
| ployed successfully and if the kiln degrade is to be kept to a mini- 
mum. Green hardwoods, which as a rule dry more slowly than soft- 
