KILN DRYING HANDBOOK. 35 
another only in the moisture contents at which the changes in tem- 
perature and humidity are to be made. It has also been found 
desirable to make specific recommendations for the drying of dif- 
ferent grades and sizes of various species. Thus, while the basic 
principles in the construction and use of both hardwood and soft- 
wood schedules are identical, there is a slight difference in arrange- 
ment. 
The softwood schedules are used as follows: Find in the species 
table (Table 5) the size and kind of stock to be dried and note the 
schedule and division given opposite it. Use this division without 
reference to any of the other divisions in the schedule. Suppose 
4/4 Douglas fir is to be dried. The table shows two schedules, 00(>-IV 
and 00-IV, for 4/4 to 6/4 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 at the foot of Table 4 stating 
that in drying vertical-grain flooring strips the temperature may 
be raised 10° F. higher than the schedule after the stock has dried 
down to 25 per cent. Therefore, if the 4/4 Douglas fir is flooring 
strips, use Schedule 000-IV, Table 4, raising the temperature to 
200° F. at 25 per cent and to 210° F. at 13 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 use with low grades of stock. Schedules for low grades are 
being developed by the Forest Products Laboratory. 
The initial entering-air humidity of 85 per cent given in the soft- 
wood schedules is an ideal which can be maintained only under the 
most favorable conditions. With fast-drying woods, the humidity 
of the air increases rapidly in its passage through the lumber and, 
unless the circulation be very rapid, air entering the lumber at 85 
per cent humidity may become saturated before it leaves the pile. 
This causes uneven drying. Further, differences in temperature in 
various parts of the kiln cause comparatively wide variations in dry- 
ing rate at high humidities. It is, therefore, impractical to use such 
high initial entering-air humidities in kilns which do not have very 
fast circulation and very uniform temperature throughout. Lower 
initial entering-air humidities must then be used in kilns with slow 
circulation and in kilns 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, and only a small portion of the lumber is subjected 
to the low humidity. 
In drying thin stock of a number of species, particularly southern 
pine and Douglas fir, it is possible to secure first-class results with 
lower initial entering-air humidities (as low as 70 per cent) even 
in kilns with extremely rapid circulation. 
The operator will need to experiment more or less to determine 
the particular initial entering-air humidity which will give the best 
results with the particular stock to be dried and the equipment 
available. 
