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Drying Wood with High Frequency Electric Current by Helmuth Resch
This monograph aims at providing understanding of the many ways to employ
special drying methods using high frequency electric currents for wood and
wood products. It offers a broad coverage of research and development over
many decades from the original concept to industrial applications. The
physical process of dielectric heating has advantages of rapid and
relatively uniform heat transfer, resulting in high drying rates and
avoidance of various drying defects, including any significant casehardening
and oxidative discoloration of wood. Heat transfer to and evaporation of
moisture from wood is shown in dependence on the dielectric properties of
materials. Because wood is heterogeneous, these properties vary not only
with the frequency of the current and the field orientation, but also with
the moisture content, temperature, and density of wood. Considering these
parameters and the specific heat of the material, the selection of a
frequency can be made for heating specific products and the power absorption
and feasibility of a system can be estimated. The text traces the historical
roots of using high frequency current for heating and drying of wood.
Principles of and formulas describing dielectric heating and the dielectric
properties of wood give a background useful for understanding parameters of
specific applications, energy transfer, and power consumption. Early
equipment development for continuous and batch dryers using radio
frequencies and microwaves is reported from work in Russia, USA, Canada,
Japan, and Europe. The greatest emphasis is placed on the method of
combining radio frequency heating with vacuum drying. Research during the
last decades lead to industrial installations designed mainly for drying of
lumber and timbers in vacuum kilns. This development provided a positive
picture for higher value products. On the other hand, the continuation of
research with microwaves promises to make use of their unique properties.
Some ideas are advanced on achieving rapid and economic drying by combining
other heat transfer methods with high frequency heating along production
lines. Finally, the present understanding of the drying mechanism, the
technical feasibility, and economics are considered. A broad listing of
relevant literature provides the reader with a multitude of references. Data
are presented in international and American units.
University of Natural Resources and Applied Life Sciences, Vienna, Austria
Society of Wood Science and Technology, Madison, Wisconsin, USA ISBN
978-0-9817876-1-9
Cost: $30 + postage (Check payable to SWST; Visa, Mastercard)
Order from: Order online at <http://www.swst.org/reschBook.html>
http://www.swst.org/reschBook.html or from SWST, One Gifford Pinchot Drive,
Madison, WI 53726; email: <mailto:vicki@swst.org> vicki(a)swst.org Phone:
608-231-9347; Fax: 608-231-9592
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Dave Cown,
Senior Scientist, Wood Quality
Coordinator, IUFRO Division 5 (Forest Products)
Scion (Trading name of Forest Research Institute Ltd.)
Private Bag 3020
49 Sala St. Rotorua
New Zealand
Ph: +64 7343 5525
Mobile: 021 187 4022
Fx: +64 7343 5507
www.scionresearch.com
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