KILN DRYING HANDBOOK 63 
agent. Several years ago two types of superheated-steam kiln were 
developed and were marketed to quite an extent in the Pacific North- 
west, principally for drying Douglas fir and western hemlock; a 
number of them are still in operation. In these kilns live steam, 
superheated by means of coils carrying high-pressure steam, 1s 
turned into the kiln. The degree of superheat, that is, the increase 
in temperature above the boiling point of water at atmospheric pres- 
sure, governs the drying rate, and no further humidity control is 
needed. The drying temperatures in such kilns usually range be- 
tween 225° and 240° F., the exact value in any case depending upon 
the class of wood being dried and upon the boiling point of water at 
the atmospheric pressure within the kiln. Sometimes an unusual 
amount of air is mixed with the steam in the kiln, with the result 
that the drying capacity of the kiln atmosphere is correspondingly 
increased. Such a condition is indicated by a wet-bulb reading be- 
low the boiling point; the remedy is to carry a lower degree of 
superheat. 
DRYING PERIODS 
The extreme variability of the drying time with individual lots 
of stock and with different types of equipment, added to the varia- 
bility of the time consumed in steaming and in other conditioning 
treatments, makes a tabulation of total drying time for usual sched- 
ules of doubtfui value. About the fastest drying time for lumber 
of which the forest products laboratory has record is the drying of 
1 by 4 inch Douglas fir flooring strips in 24 hours; the slowest is 
the drying of some southern oak wagon bolsters, which were in the 
kiln almost a year and then had a 15 per cent moisture content. 
The average periods required to dry several common hardwoods 
are presented in Table 25. While these drying rates can readily be 
secured in kilns having high velocity of circulation, it does not nec- 
essarily follow that they can be duplicated under all conditions. 
TABLE 25.—Average drying time for 1-inch stock kiln-dried green from the saw to 
5 per cent moisture content 
One nae 
ina : ina : 
Species of wood mois- pie Species of wood mois- Pee 
ture ture ne 
content content 
Per cent| Days Per cent| Days 
Dello waloine le ere Ren ee al 80 14-19 || Oak, red, or white:! 
Red gum, flat-sawed_____________- 100 18-24 Northern highland stock_____-_ 80 20-24 
Sea 0) fe boa an Mme WA) ie ene Le ele 100 12-16 Northern lowland stock_-__-__- 80 24-30 
Win oganivpes bee aaa ee ae 80 10-12 Southern highland stock______ 80 24-30 
Suganimaplewse sau Pas ee ie Leek 80 15-22 Southern lowland stock______- 80 32-37 
BSAC Keay alia Ub eee seep ets Leak 80 20-24 | 
1 Plain sawed only; quarter sawed dried under the same schedule takes about one-third longer. 
Maple last blocks can be dried in about 60 days, and walnut gun- 
stock blanks in about the same length of time. Heavy oak wagon 
stock takes from one and one-half to two months per inch of thick- 
ness to dry down to 15 per cent moisture content. ‘The common dry- 
ing times for 1-inch softwoods, such as Douglas fir, the southern yel- 
low pines, and the white pines, run from two to four days, with ex- 
