﻿42 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 



humidity, largely because the original moisture content is low. How- 

 ever, the effect of the drying schedule upon the properties of the 

 glue must be taken into consideration. It has been found possible 

 to dry panel stock at a constant temperature and a constant humid- 

 ity, the latter corresponding to a moisture content about 3 per cent 

 below that to which the panels are to be dried. Thus, if the panels 

 are to come down to 10 per cent, a humidity corresponding to about 

 7 per cent would be used. (See Fig. 3.) At a temperature of 125° 

 F.. which is considered suitable for this work, the humidity corre- 

 sponding to 7 per cent moisture is about 43 per cent. A tempera- 

 ture of 125° F. and a humidity of 43 per cent will dry the average 

 half-inch panel down to 10 per cent moisture in a few horn's; and if 

 the stock be left in the kiln considerably longer, no particular dam- 

 age will result, since the drying rate below the desired 10 per cent 

 will be increasingly slow. 



BENT STOCK. 



Bent stock of various kinds may be dried according to the lumber 

 schedules applying to species and thickness, but caution must be 

 exercised in the matter of steaming, since the excessive use of steam 

 in the early stages of the drying is very apt to result in straightening 

 out the stock. 



SUPERHEATED-STEAM DRYING. 



All of the schedules so far presented are adapted to use in " air " 

 kilns. It is possible to accomplish drying in superheated steam, 

 and several types of superheated-steam kilns are now in use. Live 

 steam superheated by means of coils carrying high-pressure steam 

 is turned into the kiln. The degree of superheat, or the temperature 

 above the boiling point at atmospheric pressure, governs the drying 

 rate, and no further humidity control is needed. Drying tempera- 

 tures usually range between 225° and 240° F., depending upon the 

 class of wood being dried and upon the boiling point at atmospheric 

 pressure. Sometimes an unusual amount of air is mixed with the 

 steam in the kiln, with the result that the drying capacity of the at- 

 mosphere is correspondingly increased. Such cases are indicated by 

 a wet-bulb reading below the boiling point, and so a lower degree of 

 superheat must be carried. 



Species now being dried commercially by superheated steam are 

 principally Douglas fir and western hemlock. Other species for 

 which it may be suitable are western yellow pine, sugar pine, eastern 

 white pine, southern yellow pines, most of the spruces, and some of 

 the true firs. In some of these woods a certain amount of darkening 

 of the surface, especially of the sapwood, may be expected. 



DRYING PERIODS. 



The extreme variability of the drying time with individual lots 

 of stock and with different types of equipment, added to the variable 

 time consumed in steaming and conditioning treatments, makes a 

 tabulation of dr} r ing time of doubtful value. About the fastest dry- 

 ing time for lumber of which the Forest Products Laboratory has 



