62 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
content would be used. (Fig. 4.) At a temperature of 120° F., the 
humidity corresponding to 8 per cent moisture content is about 50 
per cent. A temperature of 120° F. and a humidity of 50 per cent 
will dry the average half-inch panel down to 10 per cent moisture 
content over night, and no particular damage will result if the stock 
is left in the kiln appreciably longer, since the drying rate below 
the desired 10 per cent will be increasingly slow. 
Table 24 shows several combinations of temperatures and relative 
humidities with which a moisture content of 6 to 12 per cent may be 
obtained in freshly glued plywood within a reasonable drying period. 
TABLE 24——Combinations of temperatures and relative humidities suitable for 
drying freshly glued plywood panels to varicus desired moisture content values 
Relative humidities for use with the 
: ; temperatures indicated 
Moisture content desired, per cent 
100° F.. | 210°. || 120 See 
Per cent | Per cent | Per cent | Per cent 
21 22 24 26 
“SEES ea Rs ms | enn a RUN Sell yiy ESO Perey AE i 
Tee WENA 2 Ss SERS, TA Eee fo ST Un ner OE ON ail PES 29 31 34 
FoR TS CR SGA ER SN EE ES Ee ee ee SEE AS eR ee 33 35 37 41 
OEE RS ee Oe Red OAL bal a een ean nae a - - 46- 48 50 53 
Adie ved A or Sed tzcence (oe bi peee ree tent coe karen 58 59 61 65 
| 
MISCELLANEOUS FEATURES OF DRYING 
OSCILLATING SCHEDULES 
In the usual drying schedule, the temperature and the humidity 
are kept practically constant over comparatively long periods of time 
and changes in them, when made, are relatively small. In another 
type of drying schedule, commonly known as an oscillating schedule, 
the temperature or the humidity or both are made to fluctuate regu- 
larly between fixed limits, that is, to oscillate, over quite a wide range 
at comparatively short intervals, the control being automatic and 
based either upon fixed time intervals or upon the temperature and 
the humidity within the kiln. Experiments with various oscillating 
schedules in small fast-circulation kilns have in general failed to 
demonstrate any marked superiority of these schedules over the or- 
dinary steady schedules. The makers of oscillating control appa- 
ratus, however, claim that these schedules do reduce the drying time 
and do reduce the amount of drying stress in the lumber. Most of 
the oscillating controllers now in use are on natural-circulation kilns, 
and possibly the increased amount of circulation within the kiln that 
results from the use of large quantities of steam in the steam jets 
during the periods of increasing humidity tends to produce faster 
and better drying than that obtained with the original circulation. 
DRYING BY SUPERHEATED STEAM 
All of the schedules so far presented are adapted to “air” kilns 
only. Kilns of other types are possible. Superheated steam is ¢a- 
pable of absorbing moisture, the amount that it can take up being 
dependent upon the degree of superheat, and it is therefore a drying 
