Uniformity of air distribution in a lumber dry kiln


Material Information

Uniformity of air distribution in a lumber dry kiln
Series Title:
Report ;
Physical Description:
6, 1 p. : ill. ; 26 cm.
Torgeson, O. W
United States -- Forest Service
University of Wisconsin
Dept. of Agriculture, Forest Service, Forest Products Laboratory
Place of Publication:
Madison, Wis
Publication Date:


Subjects / Keywords:
Lumber   ( lcsh )
federal government publication   ( marcgt )
non-fiction   ( marcgt )


Statement of Responsibility:
by O.W. Torgeson.
General Note:
Cover title.
General Note:
"April 1940."
General Note:
"In cooperation with the University of Wisconsin."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 029328644
oclc - 219779829
System ID:

Full Text

April 1940

No. i12(7

Madison 5, Wisconsin
In Cooperation with the University of Wisconsin


"* **H--HO

Digitized by the Internet Archive
in 2013



by 0.. J. iCR'GOLS, Engineer

The advantages of optimum rates of air circulation in a lumber dry
kiln are often lost by permitting nonuniformity of air flow: through the load.
Any pronounced irregularity of air flow will cause a lag in drying in some
areas and will govern eiLther tLhe drying time or the uniformity of moisture

Recently the Forest Pro(lucts Laboratory started a study of aerody-
na'iric problems within a kiln which included a fe, expr.i mnts to determine
the deir-ee to which certain factors influence the unif'ormity of air flow
rirouiih a nile of 1-inch boards. Three factors were st';dled: (1) Uidth of
eterin:' air sauce, (2) projectin;- odges on the onteringn air side, and (3)
thickness of' stickers. Factors (1) and (2) were determined simultaneously
so that the results presented here portray, the com-binFr effect rather than
the independent, effect of these two factors,

The kiln used is i_ feet lon,; and feet wide, and is equipped with
three 3-inch overhead disk fans .' -sre were driven by individual 1.5 hp.
motors at a speed of 553 revolutions per minute. i'o heat wras supplied and
all air velocity readings were taken at ordinary room temperatures by means
of a velocity mettr equipped with an i1-inch duct jet.

(., o idfh of Enterinfr Air ,Spa'ce

The lur1er pil- in this test consisted of' 1-inch lumber 10 feet long
piled on 1-inch stickers to a width of 3 feet and a heivh of 7 feet. The
unt rrin ,, air ed.s projected or recessed from zero to throe-fourths of an
inch movabl, partition was placed in th ent!rn1P air space and shifted
so as to provide ento;rin;' air spaces 12, 1 2c, 2,, and 3C inches wide,
respectively. Another p-ostion was provijded by resti lhe top of the
partition against th,; kiln %-ill And th- bottom aanst the .il 4, thus form-
ir, a d-;fi :cttng baffle w iIth an ,*nt.;rin; air space 30 inches wide at the top
Id 1 inch wide at thi. bottom. Th`, l:;avin airE spnce .as 24 incn"heso wide
throughout th, s,.ri,;s of' tests.

Velocity readin,(s wer t',. cal lines on the leaving air side of the load. For -ach position of the
partition, an average of all roadin.s wac obtained and in addition r,-,:,-s

-Published in thu SouThern Lumberman, Anril 15, l1)1.


above and below this grand average wert averaged separate ly, the difference
b tween those latter representing, approximately, the range between which 50
percent o" the readings fell. This range was divided by two to obtain the
plus and ninus average variation from the grand avera .

The results are given in Table 1. Fh sa indicate that the width of
the entering air space and the. resulting air velocity within that space
gr.ably influ,.iced the uniformraity of air velocity through thu load of lumber.
Apparently thu sloping position of the partition was best. Under this con-
dition the air entered the pile, at approximately the same velocity from top
to bottom and was partly deflected into the openings between layers. As
will b1 illustrated later, much of the variation was caused by the project-
ing edges, but when the circulation was reversed and the entering air edges
w, r, evim, yet considerable variation occurred.

table i.--iffect of width of cntring air space cn uniformity of air flow

Tidth of Av .rage down- Averare Average variation through lumber
entering ward air air pile above and b low avera^'e
air velocity in velocity velocity (includes approximately
space upper area of through 50 percent of' readings)
entering air lumber ----------------------------------
space pil

----------- -------------------------------- -----------------------
(i) (2) (3) (: (5

Inches Ft.per min. :1't.por ;rain.: I't.per min. r e t
Sl, 269 225
12 6o : 251 170 6
S :62) 22 123 4
24 4I-)0 2 6 9-' 32
30 39: 6 9 24
39' top ):
1 bottom): 2: 1)

Column 2 of Yabl 1 gives the velocity of the air ,io delivered from the
fan,- into th.; space betwrun the kiln wa]l ,;.nd lumber piL.. Tlhe velocity
within this space hias a dir-ct and most important effect u-on their uniformity
of air flow throu ':-ut the lumber ni]. T ith a constant volume of air
delivery, the nt-.rin,< air v locity is inv.rsely proportional to the width
of this space. Eva v with th comparatively ample width of L* iachfs, the
uniformity of air flow 1 ,ft mu-h to bo desire, bein lz than hofl of that
se cured with th. 30-inch space and the sloping baffle.
total air deliv-r. of the
Column 3 gives the average air velocity, which i.- a mieasure'of thoG
fans. Llimina ting th, average for 6h c-inch width, where slight errors of
mIasuremonts may have b.en caused by turbulent flog, and using the av.ragc
for 12-inch width as a bas, the fan, delivery was incr.easud < percent
using an l-.'.-inch space, l4 percent using 24 or 30-inch spaces, and 17 p rent

I1,'7 -2-

when the side wall sloped from a width of 30 inches at th, topto 1 inch at
the bottom. These added amounts of air, plus greater uniformity, indicate
the desirability of using an entering air space at least 24 inches wide.

One other effect, not shown in Table 1, is that with the 6-inch space
the hi-hest velocities were through the lower layers, but as the width in-
creased the distribution vertically changed until at the 30-inch width the
highest velocities were through the top layers.

(2) Projecting Edges on Entering Air Side

Uuch of the nonuniformity shown in Table 1 was due to projecting
edges, which deflected air into the spaces above and prevented air frr.
entering the spaces below. In some cases the direction of flow in the spaces
below was reversed, that is, air returned from the leaving air side to the
,nterin- air side. In many other cases the air in spaces below projecting
edges was practically stagnant. This effect was very pronounced when the air
velocity downward in the entering air space was high, but became very much
less as this velocity decreased.

Figure 1 shows layers 16, 17, and 19 (numbering from the top) and
the relative position of their entering edges. The air velocities obtained
under entering air space widths of 6 and 24 inches, respectively, are given
in the separate sketches "A" and "B".

In case "A" it is apparent that, in drying, the lower surfaces of
lay. lr 17 will lag behind the upp tent and increased warp. In case "B," with an entering air space 24 inches
wide the uniformity of air flow is much better, but even here the air
velocity in th.: space below the projecting edge is only 76 percent of that
in the space above.

In other words, uniformity of air circulation is increased consider-
/ ably when the air flows through a lumber pile having the entering edges even;
.nd this effect becoi!is more pronounced as the air velocity in the entering
air space increases.

(3' Thickness of Stickers

The pile arrangfe'-ent and the procedure for this study were practically
the same as for th studi s on entering air width and projecting edges,
except that in this case the entering air edges were vertically aligned and
the sticker thickness's us d 1r 1/2, 3/LI, and 1-1/4 inches. The width
of both entering and leaving air spaces was 30 inches. The air v.lociti s
%vere read on the leaving air side in each sticker space at five locations
along the length of the load.


The following results favor the use of thin stickers: First, the
kiln capacity is increased. Second, the air velocity is increased due to
the decrease in total air space between layers. This increased air
velocity may produce a faster drying rate. Third, the uniformity of air
flow is increased, as shco.,ri by the data presented in Table 2.

Table 2.--Effect of sticker size on uniformity of air flow

Thickness : Average air : Average variation through lumber pile above
of :velocity through: and below average velocity (includes
stickers : lumber pile : approximately 50 percent of readings)

Inches : Ft. per min. : Ft. per min, Percent

1/2 : h5h hi 9
3/h : 389 h9 13
1 367 60 16
1-1/h : 330 75 23

One objection to the use of thin stickers is the additional back
pressure caused by the entrance and frictional losses and the resulting
effect of air delivery and power consumption. it might be thought that
with more lumber in the kiln more air would be needed to provide heat for
the additional amount of evaporation and to replace the amount lost due to
increased resistance. U'ith a possible increase in drying rate and a more
uniform distribution of air, however, the time needed to obtain a given
uniform moisture content might not be increased even if no additional air
is supplied. The economics of the use of thin stickers has not been
determined, but in this particular study an estimate of the power and
air delivery losses was obtained by the use of a watt-hour meter and an
inclined manometer and pitot tube.

Readings of pressure difference were taken on eich side of one fan
at a point 6 inches away from the blades and 5 inches in from the rim.
These readings of pressure differences are comparable within themselves,
but are only approximately correct because of the turbulence of flow espe-
cially on the discharge side where the air is turned abruptly by a baffle.

Readings were taken when the kiln was empty and when cor.letely
baffled as representing the tv o limits of air delivery and pov;er consumption.
additionall readings were taken with the lumber pile in place and piled on
1/2-inch and 1-1/h-inch stickers, respectively, as representinf the two
extremes of the stickers used. i results are given in Table 3 and indi-
cate that by using 1/2-inch in place of 1-1/h-inch stickers (thus increas-
ing the kiln capacity 52 percent) the air delivery was reduce 16 ryercJ-n-t
and the power consumption increased 1 percent." Under these conditions, the


capacity of the fans would have to be increased 68 percent to provide the
same amount of air per board foot as available to the lumber when piled
on 1-1/h-inch stickers.

Table 3.--Effect of sticker size on total air delivery and power consumption

Thickness Static pressure : Total air Total power
of stickers : difference across fan delivered : consumption
--- -- -- -- -- -- - - - - - -- - - -- - -------------
Inches Inch of water : Cubic feet : Kilowatts
1/2............ 0.26 13,200 3.45
1-1/ ........... .2h 15,700 3.4
Kiln mpty...... .23 : 18,000 3.37
Kiln fully
baffled....... .52 1,000 4.10
leakage) :

It is obviously not possible, from the data at hand, to determine the
proper sticker thickness for any given set of conditions. The very evident
advantages of greater kiln capacity and greater uniformity of air flow re-
sulting from the use of thin stickers must be balanced against decreased air
moving efficiency. A reasonable balance for one set of drying conditions
m.ay, be far different from a reasonable balance for an entirely different set
of drying conditions.

In regard to the fan delivery and power consumption, one other item
of interest was disclosed during these tests. The kiln used has a baffle
which deflects the air from the fan 180 upward causing it to make another
180 turn before reaching the lumber below. Tith the kiln empty, the
dropping of this baffle caused the static pressure difference to drop from
0.23 to 0.09 inch of water, the pover consumption to drop from 3.37 to 2.83
kilowatts, and the air delivery to increase from 18,000 to slightly over
20,000 cubic feet per minute. This shows the importance of keeping any
sudden changes in air direction to a minimum.


The following conclusions are applicable only when conditions are
similar to those of these experiments.

1. The width of the entering air space should not be less than 2l
inches. A tentative rule might be set up that the width of the entering
air space should not be less than one-half of th, sum of the spaces between
the layers of boards of a full kiln load of 1-inch lumber. This minimum
width of space needs to exist only at the top of the load and from there

dovm i sloping baffle is beneficial by deflectlng tlhe air into the spaces
between the boards.

2. projecting cdes deflect large volumes of air into the spaces
irjnndiately above then and cause local variations in air velocity even to
the point -her the direction of air travel is reversed throt. the spaces
iurr.diatly below thornem. The higher the air velocity down the entering air
space th greater was this action, but in all cases good returns in unl-
forimity cam be, secured simply by aligning the entering air edges.

5. Unifor.rity in air distribution was improved by reduce':, tU thick-
n ss of stickers. This procedure has the added advantage of increasing the
kiln capacity with only a negligible increase in powt;r consumption. It is
believed that thcs advantages overbalance the fact that some reduction in
air delivery is caused by the additional frictional losses.

SBaffles causing sudden changes in the direction of air movement
result in considerable increases in pow, r consumption and decreases in fan
dl ivw.ry.

1 ?67


l 480 FT PER MI4N.


S */ 17 FT. PER MIN.
4' __________________________________






7 -

FIG. 1

Y .35701 F

10233 FT. PER M41/N.

i I I' ,1I Iii III
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