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STEAM REQUIREMENTS IN
LUMBER DRY INNS
No. '1478
Revised April 1936
FOREST PRODUCTS LABORATORY
MADISON 5, WISCONSIN
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVICE
la Cooperation with the University of Wisconsin
STEAM REQUIREMENTS IN LUMBER DRY KILNS
By
Forest Products Laboratory, 1
— Forest Service
U. S. Department of Agriculture
The basic object in kiln drying wood is to remove part of the moisture
naturally present in it, which if allowed to remain would ordinarily
interfere seriously with its use. As soon as evaporation from the surface of the wood commences, a moisture gradient is established, that
is, the wood has been made drier on the surface than in the interior,
and thereby a movement of the moisture from the interior toward the
surface has been started. As wood dries it shrinks. If the moisture
gradient becomes too great, surface shrinkage will cause checks and
other defects. Hence drying conditions must be so controlled that the
moisture gradient will not exceed that which is safe for the species and
thickness of the stock being dried. Control over drying conditions is
obtained by controlling dry- and wet-bulb temperatures and maintaining
a positive and ample circulation of air. In comparison with drying problems where moisture removal is the only factor the kiln drying of wood
is a complex process for if the drying conditions (temperature, relative
humidity, and circulation) are not properly controlled the wood may be
injured and its value depreciated.
Dry kilns may be divided into two general types: (a) the stationary or
compartment type in which the entire charge of wood is subjected to
approximately the same conditions as to dry- and wet-bulb temperatures
and where such conditions are varied as the drying progresses, (b) the
progressive type in which the charge is moved progressively from the
entering end to the discharge end. The lowest temperatures and highest
relative humidities prevail at the entering end and highest temperatures
and lowest relative humidities at the discharge end.
Kilns may also be either of the natural or forced circulation type. In
natural circulation kilns, air movement is caused by differences in
-Maintained at Madison, Wis. , in cooperation with the University of
—Maintained
Wisconsin.
Report No. 1478
-1-
Agriculture -Madison
temperature and in forced circulation kilns by means of blowers, fans,
steam jets, or water sprays. Most dry kilns dispose of the evaporated
moisture by ventilation, that is, by passing out of the kiln some of the
moist, hot air and admitting an equivalent amount of fresh air. No
general comparison of efficiency of kiln types can be made since one
high in evaporating efficiency may produce a product that is poorly dried
and has a high degrade. Also some species may be dried rapidly under
severe conditions with little degrade whereas others must be dried very
slowly under mild conditions to avoid damage.
The useful work accomplished in evaporating moisture from lumber may
be considered as the latent heat of the evaporated moisture. The actual
heat absorption is greater than this. About 30 B.t.u. per pound of dry
wood are absorbed in separating the moisture from the wood when drying
from a moisture content of 30 percent to zero moisture content. In practice wood is seldom dried below 6 to 8 percent moisture content and the
amount of heat required to separate the hygroscopic moisture is only
about 1/3 as much as when drying to zero moisture content. It is necessary, of course, to heat the wood all the way from its initial temperature
to the final temperature of the kiln. The total heat expenditure required
for evaporation only may be considered as the sum of (1) latent heat of
vaporization, (2) heat required to overcome hygroscopic attraction of the
wood, and (3) heat required to raise the temperature of the material and
its contained water to the temperature of evaporation. To this must be
added the amount of heat lost by radiation through walls, ceiling, and
floor; heat required to raise the temperature of the air in the kiln; heat
required for the air entering in ventilated kilns or for reheating the air
in condensing kilns; and the, amount of steam used in steam sprays for
humidifying and for steaming or conditioning treatments. The amount of
steam required to evaporate a known amount of moisture at given conditions can readily be calculated and heat losses can also be calculated
within reasonable limits.
For estimating purposes in ventilated kilns an allowance for heating of
about 5 percent of the gross volume of the kiln space should be made for
the fresh air that is admitted per minute to replace the exhausted moist
air. The steam used for humidification varies enormously and depends
to a considerable degree upon the construction of the kilns and the skill
of the operator. In well-insulated kilns having tight doors and vapor resisting construction, vapor losses are low and a skillful operator can
save steam by careful operation of the exhaust dampers. Preferably,
an automatic device should be used for operating the dampers. In either
case, the object is to use the evaporated moisture from the wood so far
as possible in order to reduce the quantity of steam required for the
Report No. 1478
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control of relative humidity. Steam used in steam sprays may vary from
25 to 100 percent of that used in the heating coils.
The condensation per square foot of radiation in dry kilns, assuming
proper size of supply lines, drain lines and traps, depends upon (1) air
circulation across the coils, (2) steam pressure, and (3) kiln temperature. In natural circulation kilns about 1 square foot of radiation is
allowed for each 6 cubic feet of kiln space above the rail level. At 5
pounds steam pressure the maximum kiln temperature is between 165°
and 180° F. and this temperature occurs when the lumber is practically
dry. For higher temperatures high pressure steam is used. In one
kiln the maximum condensation per square foot at a temperature of
185° F. and 15 pounds steam pressure was 0. 17 pound per square foot
per hour. Only about one-third of this amount was used to maintain a
temperature of 120° F. At 75 pounds steam pressure and 180° F. the
maximum condensation was 0. 39 pound per square foot. In kilns having
forced circulation the amount of radiation required is reduced because
the efficiency of the heating surface is increased.
Table 1 shows the amount of steam required per 1,000 board feet to dry
various hardwoods from an air-dried condition averaging 20 percent
moisture content to a final average moisture content of 5 percent, and
for various softwoods from the green condition to a moisture content of
5 percent. The table is based on well-constructed compartment kilns
utilizing steam to control the humidity and thermostatic devices to regulate the temperature and relative humidity. Where no means of controlling humidity is used, somewhat less steam is required and the quality
of the product is lower. In poorly insulated kilns and kilns with leaky
doors and walls, the steam consumption increases.
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1. -5
Table 1. --Estimated steam consumption for 1-inch lumber
in commercial kilns
Species
:Condition :Time of : Pounds of :Pounds steam : Pounds
: drying : water : per pound of : of steam
: per M
water
:evaporated:
.
: Days : per M : evaporated :
Birch and maple :Air-dried: 6 to 8 .
575 : 2.5 to 4 Douglas -fir
Clears —1 1
Common—
: Green • 3
Green : 2-3/4 :
800 :
515 :
Longleaf pine
: Green :
Oak
:Air-dried: 8 to 12 :
:
2.58
2.44
600
2,064
1,254
:
: 4,000 to
5,500
: 3 to 4.5
.
: 1,800 to
: 2,700
2 to 2.75
2,000
4
: 1,450 to
2,350
Ponderosa pine : Green
4
1,800 : 2
to 2.75
: 3,600 to
5,000
Shortleaf pine
: Green
4
2,550
; 2
to 2.75
: 5,100
Sweetgum
:Air-dried: 7 to 10 :
Western hemlock :
:
Clears— 1
Common—
:
Green
Green
450
1,943 :
1,710 ;
2.5
to 4
to
7,000
to
1,800
: 1,100
2.59
: 5,025
2.55
: 4,353
1
—Davis, Virgil C. Steam Consumption in the Kiln Drying of Western
Softwoods. Jour. of For. Prod. Res. Soc. Vol. IV, No. 5, October
1954.
Report No. 1478