FOREST PRODUCTS I.ABORATORY t FOREST SERVICE
U. S. DEPARTMENT OF AGRICULTURE
SPECIAL METHODS OF SEASONING WOOD
HIGH-TEMPERATURE DRYING: ITS APPLICATION
TO THE DRYING OF LUMBER
During World War I, superheated-steam kilns were used in the
Pacific Northwest, and very rapid drying rates were reported. Some
green. 1-inch softwoods were dried to 10 percent moisture content in
24 hours, at drying temperatures as high as 230 F. in an atmosphere
of steam. The relative humidity of the steam was regulated by control-
ling the dry-bulb temperature and maintaining a wet-bulb temperature at
the boiling temperature of water (212 F. ). The method is applicable to
1-inch Douglas-fir, true firs, western hemlock, ponrlerosa pine, southern
yellow pine, basswood, and sapwood of sweetgum. So far as is known, it
is not suitable for other hardwoods or for softwoods that have a tendency
to collapse. The reduction in drying time was well worthwhile, but the
severe drying conditions caused such rapid deterioration of materials used
in kiln construction that the use of superheated-steam kilns was discontinued
in the United States.
While there has been a recent revival of interest in the use of superheated-
steam kilns, the principle of drying lumber by this method was recognized
as early as 1867, when U. S. Patent No. 64, 398, "Apparatus for Drying
and Seasoning Lumber by Superheated Steam," was granted to C. F. Allen
and Luther W. Campbell, Aurora, Ill. U. S. Patent No. 1,268,180 was
granted to H. D. Tiemann, Madison, Wis. June 4, 1918, for a superheated-
steam kiln in which circulation was induced by four pairs of steam-spray
lines arranged in such a manner that the direction of circulation could be
Within the past several years, considerable publicity has been given to
three German high-temperature kilns. Early forms were constructed of
steel or of masonry. Later, a welded construction was developed that con
sisted of an aluminum lining, about 5 inches of glass wool, and a sheet-steel
shell. Such construction is vaportight and provides for a reduction in heat
loss and less deterioration of the metal. Ordinarily, such kilns have a
capacity of 2, 000 to 3"-O0-XLboard feet of lumber, and they are used prim-
arily in small industila i p ---.- __
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---N 2 3 1954
Rept. No. 1665-1 (Revised May 1954)
t Maintained at Madison 5, Wisconsin, in cooperation with the University of Wisconsin
Circulation in these high-temperature kilns is stimulated by disk
fans or by centrifugal fans. The disk fan is suitable when the resist-
ance to flow is low, and the centrifugal fan when the resistance is high.
The disk fans are reversible. The fan axes may be parallel to or
horizontally or vertically perpendicular to the long axis of the kiln (6).1
The motors are mounted outside the kiln.
Customarily, air-dried softwoods are loaded into a kiln in the morning,
the vents and fresh-air intakes are closed, the steam-spray valve is
opened unless the evaporated moisture is sufficient to maintain the
desired relative humidity, the fans are started, and the heat, either
steam or electric, is turned on for several hours, during which the
temperature rises to 230 or 239* F. The temperature is maintained
for 4 to 10 hours, and then the heat is shut off automatically. In some
cases, the vents and the fresh-air intakes are partially opened as soon
as the maximum temperature is reached, but ordinarily they are not fully
opened until the heat supply is shut off. The wet-bulb temperature may be
maintained at 21 2* F. but it is generally somewhat lower. The fans are
in operation during the entire kiln run, which is about 24 hours. Ordinar-
ily, these kilns are not equipped with wet- and dry-bulb recorder-control-
lers. In some cases, however, the dry-bulb temperature is thermostatic-
Basically, water vapor is superheated when, at any pressure, its tem-
perature exceeds that of the saturated vapor. When the wet-bulb tempera-
ture is below 212" F. at atmospheric pressure, both air and water vapor
are present. When the wet-bulb temperature reaches 212 F. no air is
present. Further heating of the vapor results in superheated steam at
atmospheric pressure. In other words, if the wet-bulb temperature in a
kiln is below 212 F. the kiln may be called a superheated-vapor kiln.
On the other hand, if the wet-bulb temperature is 212 F. and if the dry-
bulb temperature is above 212 F. the kiln may be called a superheated-
steam kiln. If, however, the dry-bulb temperature is above 212* F., the
term "high-temperature" may be applied to the kiln, irrespective of
whether the wet-bulb temperature is at or below 212* F.
In order to release heat for evaporation in a kiln at temperatures below
the boiling point of water, the temperature of the air and vapor is lowered
The air, of course, can give up heat for evaporation as long as its tempera-
ture is higher than that of the wood to be dried. The vapor, on the other
hand, can, as its temperature drops, supply heat for evaporation only until
the point of saturation is reached. At the standard atmospheric pressure
-Underlined numbers in parentheses refer to references at the end of the
Rept No 1665-1
of 14. 696 pounds per square inch absolute, or zero pound gage, the
temperature of saturated steam is 212 F. Wood in contact with such
steam could be heated, but the water in it could not be evaporated.
Relative humidity may be defined as the ratio of the actual vapor pressure
to the pressure at saturation at the given temperature. For example, at
a dry-bulb temperature of 150 F. and a wet-bulb temperature of 132* F.,
psychrometric curves commonly available to dry-kiln operators show
that the dew point is 130 F. and the relative humidity is about 60 percent.
In steam tables, which are commonly given in handbooks and textbooks on
mechanical engineering, physics, or chemistry, the saturated-vapor pres-
sure at 130 F. is 2. 222 pounds per square inch absolute. The correspond-
ing saturated-vapor pressure at 150 F. is 3.718 pounds per square inch
absolute. The relative humidity is then 2. 222 = 59. 8 percent, which checks
with the percentage determined from the curves as described above.
Suppose now, that saturated steam at 14. 696 pounds per square inch absolute
is superheated from 212 F. to 230 F. while the atmospheric pressure
remains unchanged. In other words, the steam is superheated 18 F.
Referring again to steam tables, we find that at 212" F. the vapor pressure is
14,696 pounds per square inch absolute, and at 230 F. the vapor pressure
is 20. 780 pounds per square inch absolute. The relative humidity is then
20.780 = 70. 7 percent.
Most kiln operators are familiar with the relationship of temperature and
relative humidity to equilibrium moisture content for temperatures below
the boiling point of water at atmospheric pressure. A number of investiga-
tors, notably in Germany (5) and in Australia (3), have performed experi-
ments to determine the equilibrium moisture content of wood exposed at
atmospheric pressure to superheated steam at temperatures up to 300 F
Eisenmann (2) determined corresponding values for pressures up to 3-1/2
atmospheres, or about 51 pounds per square inch absolute. For tempera-
tures at least up to 248 F. the determinations at atmospheric pressure
correspond well with estimations arrived at by the mathematical extrapola-
tion of curves prepared at the U. S. Forest Products Laboratory and dated
May 21, 1926.
Kollman (5) states that drying by superheated steam is suitable for softwoods
and beech, but not for green hardwoods, such as oak, which checks and
collapses readily. If they are air dried, or kiln dried at a moderate tempera-
ture to 25 percent, they can be further kiln dried at a high temperature
Rept. No. 1665-1
without checking In superheated steam at 120" C. (248 F. ), the
equilibrium moisture content of wood is 4. 5 percent, and at 130" C.
(266" F. ) it is only 3 percent Kollman regards these conditions as
too severe and recommends a maximum temperature of 115* C (239*
F. ) In this case, the equilibrium moisture content is about 5 5 per-
cent. He, as well as Egriez (1), considers a range of 110* to 1150 C'
as being most expedient At 110" C. (230- F ) the equilibrium moisture
content is 7 percent.
Rates of Circulation
Various investigators advo ate rates of circulation of from 2 to 10 meters
per second (394 to 1, 970 feet per minute); 2 to 3 meters per second (394
to 590 feet per minute) are most commonly mentioned Among the factors
affecting the rates are, presumably, differences in species, in initial
moisture content in temperature, in relative humidity, and in length of
air travel. Because high temperatures result in high rates of drying, it
is to be expected that the rates of circulation mentioned for foreign kilns
would be higher than those used in American kilns, in order to carry the
moisture away from the lumber as rapidly as it comes to the surface.
Energy Required for Drying
Keylwerth (4) indicates that, with a superheated-vapor kiln, the usual
range of energy required is 1. 2 to 1. 5 kilowatt-hours per kilogram (2. 2
pounds) of water ev.,apc.atedi and that the corresponding range is 2 to 4
kilowatt-hours with low-temperature drying. He attributes the lower energy
requirement to the lower heat loss, lower heat capacity of the construction
materials, and the shorter drying time in the superheated-vapor kiln
Effects of Hih -Temperature Drying on Wood Properties
Keylwerth (4) found that samples of red beech dried in superheated steam at
115' C (239" F. ). as compared with those dried at 65* C (149" F ), had an
equilibrium moisture conr.tent value about 2 percent lower at 12 percent
moisture content, also, they had somewhat higher modulus of rupture, maxi-
mum crushing strength and modulus of elasticity In impact bending and in
Rept. No. 1665-1
tension perpendicular to the grain, however, the samples dried at
115* C. had somewhat lower values. The radial and tangential shrink-
ages were reduced 24 and 20 percent, respectively. Graf and Egner
reported, according to Eisenmann (2), that pine dried at 115* C. (239"
F. ) shrank and swelled only 70 percent as much as air-dry pine.
According to Kollman (5) the discoloration (presumably chemical brown
stain) in pine was very slight when the green lumber was dried at tem-
peratures up to 70" C. (158 F.).
Available information concerning the effect of high-temperature drying
on casehardening is not very detailed. General reports indicate, however,
that casehardening resulting from this method of drying is not of commer-
cial importance in Germany.
(1) EGNER, K.
1951. Drying of timbers at temperatures above 100 C.
Holz als Roh- und Werkstoff Vol. 9, No. 3, pp. 84-97.
(2) EISENMANN, EUGEN
1950. Present position of the hot steam drying of wood and its
Holz-Zentralblatt, Vol. 76, No. 47, pp.- 503-504; also
Vol. 76, No. 48, pp. 509-510.
(3) GRUMACH, M.
1951. The equilibrium moisture content of wood in superheated steam
Commonwealth Sci. and Indus. Res. Organ. Canberra,
Australia, Progress Report No. 5, 6 pp.
(4) KEYLWERTH, RUDOLF
1952. High-temperature drying installations.
Holz als Roh-und Werkstoff, Vol. 10, No. 4, pp. 134-138.
(5) KOLLMAN, FRANZ
1952. Investigations of the drying of sawn pine timber at elevated
Svenska Traforkningsinstitutet. Tratekniska avdelningen,
Sweden, Meddelande 23. 40 pp.
(6) STEMSRUD, FINN
1953. High temperature drying of timber.
Norsk Skogindustri, October, p. 336.
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