Why the drying time of a kiln load of lumber is affected by air velocity

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Material Information

Title:
Why the drying time of a kiln load of lumber is affected by air velocity
Series Title:
Report ;
Physical Description:
3 p. : ill. ; 27 cm.
Language:
English
Creator:
Torgeson, O. W
Forest Products Laboratory (U.S.)
University of Wisconsin
Publisher:
U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory
Place of Publication:
Madison, Wis
Publication Date:

Subjects

Subjects / Keywords:
Kilns   ( lcsh )
Lumber -- Drying   ( lcsh )
Air flow   ( lcsh )
Genre:
federal government publication   ( marcgt )
non-fiction   ( marcgt )

Notes

General Note:
Caption title.
General Note:
"August 1954"--Cover.
General Note:
"Information reviewed and reaffirmed"--Cover.
General Note:
"In cooperation with the University of Wisconsin"--Cover.
Statement of Responsibility:
by O.W. Torgeson.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 029510164
oclc - 757547101
System ID:
AA00020684:00001

Full Text




WilY TH I UYINy TIAf COF A IILN
ICAU O)F LIJMII IS AIITCTIM
iY AII VELOCITY
Information IUcvictucd and Ieaffirmcd
August 1954













LU




No. U12C9






UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVICE
FOREST PRODUCTS LABORATORY
Madison 5,Wisconsin
In Cooperation with the University of Wisconsin
















Digitized by the Internet Archive
in 2013










http://archive.org/details/timeofk00fore





WiHY THE DRYING TI.LL OF KILH LC.D OF LULrBR IS


AFFECTED BY AIR VELOCITY


By 0. W. TORriESON, Engineer





Only recently have there been any scientific studies made regarding
the effect of air velocity on the drying rate of lumber in a dry kiln.
Not more than 10 to 15 years ago, air velocities as low as 25 feet per
minute were recoruended by kiln engineers for slow-drying hardw.oods, and
velocities of 75 to 100 were"copsidered very high. Studies! made at the
Forest Products Laboratory during the past few years have i-Ldicated that
the rates which might be called optimum from a drying time standpoint are
much higher than these, ranging from approximately 500 to 800 feet per
minute depending on the length of air travel, the drying schedule, and the
drying rate of the lumber when in a green condition.

In addition to the collection of considerable empirical data, the
studies have helped to clarify the reasons why air velocity affects drying
time. '.'hen the conditioned air from the fans enters the load, it is uni-
form in ter-.erature and relative humidity and, conse-uently, the entering
air edges of the lumber dry at approximately their rin:i:ium rate. As the
air progresses through the spaces between board surfaces, heat is used for
evaporation and moisture in vapor form is added to the air. A -rd.tion in
temperature and relative humidity is immediately established, (1) trans-
versely frr.n the wood surface to the center of the air stream and, (2)
longitudinally from the entering to the leaving-air side of the load.


(1) Transverse gradation of temperature and relative h',.iidity

The transverse gradation ol temperature and relative humidity across
the air stream is caused by more or less laminar flow which permits a
stratification of lo,-; temperatures and high humidities next to the vwood sur-
facet Any means of breaking up this stratification by causing a greater
turbulence of flow vill result in greater ei'fici-nricy of heat transfer and
consequently faster and more uniform dry-jingo On by such conplrto mixing
of the air as it progresses through the ](-4 can the v.xin"- dcryingr capacity
-. of the conditiojc.l air be obtl .ned. Increased tu-'bu.lCn'-e of flow' can be
caused by increa-.rs in air velocity or by introducing obstructions or
interruptions in the air channel .walls. This will be illustrated later by
presenting some of the data collected.

-Results in the drying of sugur maple given in LIimsogr, h R1264, obtainable
from the Forest Products Laboratory, hj'dison, is.


R1269


Agriculture-i i2dison





(2) Longitudinal gradation of temperature and relative humidity

The average longitudinal gradation of temperature and relative
humidity extending from the entering-air side to the leaving-air side of
the lor'-d is dependent entirely on the amount of heat used from the total
amount supplied and, therefore, is a function of air circulation and dying
rate. Given any two, the third variable can be computed. All three of
these can be measure'1 satisfactorily, and the computations have been simpli-
fied by the construction of a chart- showing the amount of air needed to
evaporate 1 pound of water for a 1 temperature drop across the load.


Eymarj merts Show the Effect of Space Piling and
I-r.." Aiy V3loci5. y on ing Time


The first of two drying experiments on 1 by 8-inch northern red oak
was made to show that greater turbulence of flow (and therefore greater
transverse uniformity of air conditions) can be obtained by means of spaces
between the edges of the boards. The spaces tend to break up the stratifica-
tion of the air stream.

Three runs were made untrder the following constant drying conditions;
temperature -- 115 F., relative humidity -- 85 percent, air velocity --
260 feet per minute, sticker thickness -- 1 inch, width of pile 9 boards.
The three runs differed only in the spacing of the boards. In one run, the
boards wore piled edge to eclde, in the secon.1 the boczards were spaced 1-1/2
inches apart, and in the third., groups of three boards were piled edge to
edge leaving a 6-inch space between the third and fourth, and the sixth and
seventh boards. The results showed (1) that the drying lag across the solid
load took the more or less nctTal progressive shape, (2) that the average
lag from board to board vas cos3 Lderably reduced by the 1-1/2 inch spacing,
(5) that the effect of the 6-inch spaces was surprisingly large in respect
to the drying rates of the fourth and seventh boards, and (4) that the
average drying time of the entire load in drying from 80 to 40 p.-rcent
moisture content was approximately the same whether eight 1-1/2-inch or two
6-inch spaces were used, but the soli'. pile took approximately 15 percent
more time.

The second experiment was designed to show that increased turbulence
of flow is caused alho by increased air velocity thereby lessening the need
for space piling. In this experiment, six groups were dried under the fol-
lowing constant drying conditions; -teanerutre *.- 115 F., relative humidity
- 80 percent, sticker thickn.:i'ss -- inch, wid'-h of pile 9 boards.
Three air velocities of 100, 300, and 600 feet per rinute were used and
each of these groups consisted of two vTns one of which was solid piled
(edge to edge) and the other was piled with 2-inch spaces between edges.
2
-Given in Mimeograph R1266, obtainable from the Forest Products Laboratory,
MIdison, Wis.


-2-





The results shownn graphically in Figure 1) indicate (1) that such.
spacing of boards was very beneficial in respect to dryir.j tir,.e at air
velocities as low as 100 feet per minute, (2) that at600 feet per minute
the turbulence of flow caused by such a high velocity '-,as not appreciably
increased by the spacing of the boards and, as a result, such spacing was
not beneficial for this velocity and possibly would not be beneficial for
higher velocities, and (3) that, from a drying time standpoint, an optimum
air velocity of at least 500 or 600 feet per minute is indicated for the
early stages when kiln drying .vcen 1-inch red oak.


R1269


-3-

















/48 ---------------/--
//


16 --



14



//
u4 /
10/


14j -__-- >_ _-^4


I0 -- ----
t. / / I






6 z _600



4
LEGEND
PILED EDGE TO EDGE
---2-INCH SPACES BETWEEN BOARDS
Z



0 1--
0 I 2 3 4 5 6
LENGTH OF AIR TRAVEL (FEET)
` 39200 F
Figure i.--Effect of air velocity and space piling on drying time across
a 6-foot wide pile of i by 8-inch northern red oak in drying
from 80 to 40 percent moisture content under a temperature of
x115 F., and a relative humidity of So percent.




UNIVERSITY OF FLORIDA


3 1262 08866 6382