/ FOREST PRODUCTS LABORATORY t FOREST SERVICE
U.S. DEPARTMENT OF AGRICULTURE
SPECIAL METHODS OF SEASONING WOOD
VACUUE-i DRYII'C OF 0 ....'O .
...... .. I_. *1. _
Many different processes have been proposed for using a vacuum to dry wood.-.
Charles Howard was granted a patent for a vacuum process in 1893 and numerous
variations have been patented since. No vacuum process for dryi.n, lumber has
come into general use either here or abroad, except the steaming and vacuum
process used to condition poles and other forms of pine timber prior to
preservative treatment. Although amazing results have been obtained by
drying lumber in small experimental vacuum-drying equipment, there are
several limitations to the use of the process in large-size commercial
equipment. Because of these limitations, the Forest Products Laloratory has
never thoroughly investigated this process,.
A vacuum is merely a reduced air pressure, and the principles of drying are
no different at a very low pressure than at any other pressure. The use of
a vacuum alone is not effective for rapidly dryin- wood. Drying of wood in-
volves two processes, movement of water to the surface of the wood and
removal of that water from the surface. A vacuum maintained by continuously
withdrawing the vapors from a drying chamber materially aids removal of the
water from the surface for a short time, but considerable quantities of heat
are required to evaporate water. The evaporation of water from wood in a
vacuum quickly cools the wood so that evaporation becomes very slow in spite
of the vacuum. Furthermore, except at the very start, the rate of drying of
wood is governed by the rate of Loisture movement from the interior to the
surface. A vacuum has very little effect on the rate of moisture diffusion
through wood. The vacuum processes proposed for drying wood, therefore'c, have
incorporated some means of heating the wood before the vacuum is applied.
Hot air, hot water or other liquids, and steam have been used for heating.
A large load of lumber cannot be heated uniformly by ordinary means during
the vacuum period because there is no air or other substance in the vacuum
to carry heat from the heating elements to the lumber by conduction or
convection. There is no practical way of heating the lumber in the middle
of a large load by contact or radiant heat. Although radiant heat may
produce amazing results on a few boards in small apparatus, the only heat
available in the center of a lar&e load is heat stored in the wood during
the heating period. The maximum amount of heat possible to store in wood by
heating with any means is limited, depending on the maximum temperature used,
the density of the wood, and the amount of water in the wood. The amount of
evaporation that can be accomplished by one heating-and-vacuum period,
therefore, also is limited.
When the heating medium is withdrawn and the vacuum applied, some of the
liquid water present in the cell cavities can be forced out of pervious wood
by mechanical action, because of the differ-ntial in pressure between the
wood and the vacuum. This difference can be increased by steaming or other-
wise heating at pressures above atmospheric. The amount of water removed by
this mechanical action is limited and is negligible for refractory woods
t Maintained at Madison 5, WIbconsin in po,-" e/ University of Wisconsin
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3 1262 08924 3405
through which water does not diffuse readily, such as sweetgum heartwood.
There refractory woods are also very resistant to moisture diffusion eyen
at temperatures above the boiling point of water,
Most of the water evaporation also takes place during the first part of the
vacuum, resulting in a rapid cooling of the wood. There is no advantage in
prolon-in" the vacuum more than 2 hours, because, at the low temperatures
rt:ached, evaporation is very slow.
Bc-ause of the limited amount of drying possible with one heating-and- vacuum
cy.-'le, a number of cycles of alternating heating and vacuum must be used to
dry wood to low moisture content values. When the liquid water originally
present in the cavities of the wood cells has been taken out of the wood at
approximately 3u percent moisture content, no further drying by mechanical
action o:curs. The rate of heating of the wood and the diffusion of water
through the wood becoi-.e slower and slower as the moisture content is
lowered so that the vacuum process becomes increasingly inefficient. If
steam is ucod as a heating medium, a point of equilibrium is reached where
the moisture absorbed by the wood during the heating is equal to the amount
relLoved during the vacuum, and no further drying occurs. It would be ideal
to use humidified air as a heating medium, controlling the conditions so
that little or no moisture is taken from or given to the wood during the
heating, but such conditions are difficult to control in the vacuum process.
In addition to the limitations of the process, the fact that the equipment
is expensive works a&airnst commercial utilization. Also, the woods which
dry rapidly in a vacuum can be dried rarnidly in an ordinary kiln.
FOST PRODUITTS LA3BORATOR3Y
11ADISON 5, '"ISCONSIT
.r.pt. 't o. -16 65 .