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PURCESS MCP SMALL STCI
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
In Cooperation with the University of Wisconsin
Digitized by the Internet Archive
TECHNIQUE OF DEVELOPING A DRYING PROCESS
FOR SMALL STOCK
0. W. TORGESON, Emgineer
In drying small pieces of rood, the handling cost is an im-
oortant factor. One possible drying procedure is to dump the nieces
loosely into a crib and force air through the voids. This process has
been used to some extent and, in general, has proven to be fairly
Recently the opinion of the Forest Products Laboratory was
asked concerning the dryin- procedure for cocobolo handles. Cocobolo
is a Central American wood that has long been used in the cutlery trade
for knife handles. The principal commercial sources are Costa Rica,
Nicaragua, and Panama. It has a dark color, fine texture, and dense
structure and contains an oil that tends to waterproof the wood and
keep it in shape after manufacture. Prolonged or repeated i.mersing
in soapy water is reported to have little effect on the wood except to
darken its color, an important advantage in kitchen and butcher knives.
It is also used for small tool handles, brush backs, and in musical
and scientific instruments.
The crib method of drying was su.--.-:-sted, but no data were
available showing frictional and other losses in forcing air through
such a load. The problem was considered to be of sufficient general
interest to justify a small amount of work to determine the pressure
drop and air velocity through crib loads of various lengths in the
direction of air travel.
The blocks used were 21/32 by 1-1/32 by 4-3/4 inches in size
and were dumped loosely into a duct a-rproximately 2 by 2 ft-t in cross
section. A one-half inch mesh screen at each end retained the blocks
in a vertical position and the distance between the screens rf-presented
the length of air travel through the load. This distance was varied
from 1 to 5 feet in one-half foot intervals, and for each loading, fan
speeds of 220, 440, 660, 890, and 1,170 revolutions per minute nre use-
to obtain various pressure drops across the load. Readings of the drop
in pressure across the load were obtained by means of two static oresyur'
tubes connected to a micromanometer. Air volumes and air velocities were
obtained from anemometer readings taken in a connecting duct. These were
adjusted according to cross-sectional areas to give the velocity of the
air as it entered the load and then further adjusted according to the
percentage of voids to obtain the average air velocities within the lond.
The percentages of voids were obtained by counting the number of pieces
in each crib load.
Figure 1 is a photograph of a crib load of blocks as located
in the test duct.
The results are shown graphically in figure 2 and 3.
The curves of figure 2 show the effect of length of load on
fan delivery and pressure head at several fan speeds. The shape of the
curves indicates an entrance loss and, therefore, relatively big losses
for short loads. The limitations of the fan to increase the pressure
head against added resistances are shown also by the flattening of the
pressure curves as the length of air travel increases.
The single curve in figure 3 shows the amount of voids as a
function of length of air travel. The change in the amount of voids was
due to the fact that the pieces against the screens were supported in
such a way as to increase the voids in those areas, but as the length
of load increased this effect decreased and for long loads the percent-
age of voids was practically constant at approximately 55 percent.
The curves of figure 3 show the effect of length of load on
air velocity for a series of pressure drops between 0.1 and 1.0 inch
of vater. The curves on the left show the velocity of the air in the
duct before entering the load and those on the right are the same
values adjusted according to the percentage of voids to show the
average velocity within the load.
Figure 3 indicates that a considerable amount of air volume
will pass through a crib load of small blocks even under small pressure
drops across the load. It is therefore concluded that the pressure drops
obtainable with a disk fan are satisfactory provided the resistance of
the system is not too great and that the kiln and baffles are sufficiently
tight to prevent excessive leakage.
Under the same pressure drop, the air delivery through a
5-foot load was approximately one-half of that through a 1-foot load.
The flattening of the curves indicates, however, that the length of load
could be increased greater than 5 feet with relatively small decreases
in air delivery.
Figure l.--A crib load of blocks in the test duct.
The return duct on the o-posite side
is used for air measurements.
0 2 3 4 5 0 2 3 4
AIR TRAVEL (FEET) AIR TR,4VEL (FEET)
OF BY i1 BY 4.-INVCH BLOCKS
WHEN DELIVERING AIR THROUGH LOOSE PILE
k 37964 F
0 1 2 3 4
AIR TRAVEL (FEET)
5 0 1 2 3 4 5
AIR TRAVEL (FEET)
AIR VELOCITY THROUGH LOOSE PILE OF BY lj BY 43-INCH BLOCKS
UNDER VARIOUS LENGTHS OF LOAD AND PRESSURE DROPS ACROSS LOAD
U 37965 F
Calculation of Air Needs Based on Assumed Drying
The Laboratory has practically no information concerning proper
schedules for cocobolo'. End checking and color =mist be considered. The
schedule in table 1 is believed to be conservative, but is given mainly
as a basis for calculating the air needs:
Table l.--Kiln drying schedule
Stage of : Moisture : Temperature : Relative : Theoretical
drying : content : : humidity drying
Percent : 0 F. : Percent Hours
1 35 to 25 120 : 0 17
2 :25 to 15 : 130 0 :6 2g
3 15 to 3 : 140 : *10 : 69
Total........: 35 to 3 :..... .. .... .........: ...... .... 114
A'final moisture content of 3 percent is given because,
from reports, a lo.' iroisture content is desirable from a manufacturing
standpoint. To obtain a moisture content lower than this in a reason-
able drying time would re-quire temperatures above 140 F.
It is believed that the drying rate of cocobolo is cer-
tainly not faster than that of .oak, and, because of its greater density,
cocobolo may dry even slower. The drying rate of oak, however, was
used in calculating the time needed for each stage of drying under the
In order to compute 'air needs for the various states of
drying, the oven-dry weight of cocobolo was assumed to be 60 pounds
per cubic foot. Within 1.-square Tfoot of crib 5 feet long having 5
percent voids, the volume of wood would be approximately 5 x 0.45 or
2.25 cubic feet, and the.weight would be 2.25 x 60 or 135 pounds. The
total moisture loss per minute during each stage of dryir.- is given
in table 2.
Table 2.--Average drying rate
: : ..~.... ,
Stage : Moisture Oven-dry : Moisture : Drying : Average
of : content weight of : loss : time, : moisture
drying : loss wood : : loss. per
------------------- --------- ----------S--------.........---
: Percent : Pounds : Pounds : Minutes : Pound
1 10 135 : 13'5 : 1,020 : 0.0132
2 10 135 : 13.5 : 1,6g0 : .0O
3 12 .135 : 16.2 : 4,140 .0039
A second assumption must be made regarding the proper temper-
ature drop across the load to avoid an excessive drying lag on the leav-
ing air side. A 4-degree temperature drop is assumed to'be satisfactory
in this respect for each of the three stages of drying.
At the temperatures and relative humidities given in the
schedule, the first stage of drying requires 61,000 cubic feet of air
to evaporate 1 pound of water with a 1 F. temperature drop across the
load; the second stage, 62",000 cubic feet';' and the third stage'," 3,000
cubic feet. The air requirements for each stage are computed in table 3
by multiplying by the average amount of moisture evaporated per minute
and dividing by the 4-degree selected temperature drop.
Table 3.--Air requirements
Stage of dry- : Air volume through
ing .:.1 square foot of
cross section of
per minute .
1 i : 202
As 1 square foot of crib area was taken as a basis, these values
represent also the velocity of the entering air.
Referring again to the left hand series of curves in fi,'ure 3,
an entering air velocity of 202 feet per minute with a 5-foot air travel
requires a pressure drop across the load of practically 0.6 inch of
water. Table 4 shows the pressure drop requirements for each of the
three stages of drying with a 5-foot air travel:
Table 4.--Pressure drop requirements
Stage of : Entering air : Pressure drop
drying : requirements : requirements
: Feet per minute: Inch of water
1 : 202 : 0.5
2 124: .21
3 : 62 : .07
The computed drying periods are based on nn infinite air
velocity and are the minimum under the assumed drring characteristics
of the wood. They do not take into account the lag in drying due to
the temperature drop across the load, and therefore, the actual
average drying time for the load may be somewhat greater than those
shown. For the same reason, air velocities hizhLr than those com-
puted will result in temperature drops less than 4 degrees and will
bring the average drying time of the load more nearly to that
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