KILN DRYING HANDBOOK 51 
causes uneven drying even in the same pile, and the situation, of 
course, is worse over the entire kiln than in any one part of it. Be- 
sides, at high humidities even slight differences in temperature in 
various parts of the kiln cause comparatively wide variations in the 
drying rate. Itis impractical, therefore, to use high initial entering- 
air humidities in kilns that do not have fast circulation and uniform 
temperature throughout. Lower initial entering-air humidities must 
be employed in kilns with slow circulation and also in those lacking 
uniformity in temperature. The slow circulation compensates in 
large measure for the lower entering-air humidity, since the humidity 
rises rapidly as the air passes through the pile; in consequence only 
a small portion of the lumber is subjected to its lowest value. 
In drying thin stock it is possible in the case of a number of species, 
particularly southern pine and Douglas fir, to secure first-class results 
with initial entering-air humidities below 70 per cent in kilns having 
rapid circulation. On the other hand, particularly in wide stock, 
some checking may occur even with initial entering-air humidities as 
high as 70 per cent. Accordingly the operator will need to experi- 
ment more or less to determine the particularly initial entering-air 
humidity that will give the best results with the particular stock to 
be dried and with the equipment available. 
Preliminary steaming of softwoods is the exception rather than 
the rule, but it has been found desirable in several special instances. 
The final humidities in these schedules will be satisfactory for 
many progressive kilns, yet in forced-circulation compartment kilns 
they may at times prove to be too low. 
The final temperatures may be found to be too low, for progressive 
kilns, to give circulation enough to carry off the evaporated moisture. 
The softwood schedules are used as follows: Find in the species 
table (Table 5) the kind and the size of stock to be dried and note 
the schedule and the division given opposite it. Use the division 
thus specified, without reference to any of the other divisions in the 
schedule. Suppose 4/4-inch Douglas fir is to be dried. The table 
shows two schedules, 000-[V and 00-IV, for 4/4 to 6/4 inch 
Douglas fir. The more severe one is for the ordinary run of stock 
and the milder one for wide, flat-grain stock. There is also a general 
note following Table 4 stating that in drying vertical-grain flooring 
strips the temperatures may be raised 10° F.. higher than those of the 
schedules, after the stock has dried down to 25 per cent moisture con- 
tent. ‘Therefore, if the 4/4-1nch Douglas fir is flooring strips of 
this type, use Schedule 000-IV of Table 4, raising the temperature 
to 200° F. at 25 per cent and to 210° F. at 18 per cent. If it is 
ordinary stock, use Schedule 000-IV without change, and if it is 
wide, flat-grain stock, use 00-IV. 
These softwood schedules are not intended for low grades of stock. 
Several special schedules for the lower grades of Douglas fir and 
southern pine are presented on succeeding pages, 
