Infrared radiation


Material Information

Infrared radiation
Physical Description:
Mixed Material
Forest Products Laboratory (U.S.)
USDA, Forest Service, Forest Products Laboratory ( Madison, Wis )
Publication Date:

Record Information

Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 29235076
oclc - 237055358
System ID:

Full Text


1 ..... -

The successful use of infrared radiation for hardening paints and enamels
on metal has caused some speculation that it might be used to advantage in
seasoning wood. In drying paints or enamels on metal by this method a
battery of lauLps is directed toward the object so that radiation from
adjacent lamps overlaps to provide uniform coverage. The conduction of the
metal and movement of the object past the lamps tends to equalize surface
temperatures rapidly.

Seasoning of lumber, however, presents a basically different problem be-
cause the heat must penetrate the wood without raising the surface te: pera-
ture to such a point that drying defects occur. When wood is heated by
infrared radiation the depth of penetration is slight and heat applied in
this manner is carried into the wood by conr. actionn only.

Since infrared radiation penetrates wood only to a slight degree it does
not appear advantageous to use it as a source of heat for w**ood
because the rate at which the interior wood heats will depend upon the sur-
face temperature and not upon whether the surface temperature was
established by absorption of infrared radiation or by contact with hot cir-
culating air. The time required for heating wood in air in a dry kiln to
the desired drying temperature is relatively small in comparison with the
total drying time. To heat green wood 1 inch thick to the dkiln toeiperature
in a modern forced-air circulation kiln takes only approximately 1 hour, but
several days or weeks may be needed to dry the lumber because of the slow
diffusion rate of moisture at permissible kiln temperatures and relative
humie.ities. All this time the air is throughout the lumber pile
and reaches all surfaces of each piece. Heat supplied by infrared radiation,
in contrast, would reach only the surfaces in direct exposure.

To apply infrared radiation to all surfaces of each board would, see.:ingly,
require thp single-file, board-by-board passage of the lumber through a
tunnel on a traveling chain, a hig-hly impractical ani costly method that
would have to compete in output with the conconly provided kiln capacities
of 20,000 to 60,000 board feet that are standard in the process of drying
lumber by heated air,

In drying lumber by any method, temperatures and relative humidities must be
under accurate control for moct woods. Green oak, for example, is particu-
larly susceptible to surface checking caused by initial relative humidities
below 0J0 percent and green 6ak that is 2 inches or more in thickness,
especially in a forced-air circulation kiln, should have an initial relative
humidity as high as 85 or 90 percent. Oak is also susceptible to honey-
combing caused by the use of temperatures much above 115 F, during the time
the moisture content of the core is above 30 percent. Some species, such as
basswood dry easily and higher temperatures and lower relative humidities
can te used.

Rept. No.51665-4

t Maintained at Madison 5, Wisconsin in cooperation with the University of Wisconsin


3 1262 08924 3413

The r,-l::-.tive humidity can:,ot be controlled within the desired limits at the
surface of the *ood wken th.e surface is heated to a temperature greater than
that of the %rrour.dinr.c- air because the wood then heats the adjacent air
la'er .:.d thus lo'..-er: the relative humidity at the wood surface. With
i:.frar'd radi:.tticr., t'.e wood surface is always heated above the temperature
of t:,e m rro'.r.iing air. :Relative humidity decreases rapidly as temperature
in,;rea:'es, St-irtinrf' with a temperature of 80 F. ani a relative humidityy of
50 perc,--:.t, ti.e relative humidity of the air when heated to 1500 F. will be
o:.l: 11 per'-.e:.t.

It si,j.t *seem hat infrared radiation is well suited for the drying of
veneer because v-.-err is so thin, but here again practical procedures do not
IL,.. '-.'.c.-.zIves to this method of heating. In drying veneer, rEstraint must
be provide, a'ainzt wrinklin.g and bucklin' by' drying it between heavy wire
tesh, spr!L,,-r, closely spa.,ed iron rolls, or plates. When these driers are
used t.. vc:-.,er comes fro.- them reasonably flat nd in good condition for
storage, shipment, or manufacture into plywood. In drying with infrared
r-diatior., the restraining merhr.nisa would interfere with the application of
ht:at or it wo-;ld be difficult to maintain the lamps if used with. the drying
equip_:en.t now available.

Even tho'..-h practical solutions were found for the difficulties of using
infrarc' radiation for the seasonir.g of wood, tr.e cost of electricity would
bt *-rcater in most localities than the cost of steam.

!.'ARCH 1947

? :o...66C-4