The drying rate of sugar maple as affected by relative humidity and air velocity


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The drying rate of sugar maple as affected by relative humidity and air velocity
Physical Description:
Torgeson, O. W
Forest Products Laboratory (U.S.)
University of Wisconsin
U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory ( Madison, Wis )
Publication Date:

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oclc - 263182052
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Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
        Page 1
    Original and final moisture content
        Page 2
    Sapwood versus heartwood, temperature of kiln air, and size of stock
        Page 3
    Relative humidity of kiln air
        Page 4
        Page 5
    Velocity of kiln air
        Page 6
    Length of air travel
        Page 7
        Page 8
    Figures 1 to 5
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
Full Text
Ucccmbcr 19J4C
Madison, Wisconsin
In Cooperation with the University of Wisconsin

Digitized by the Internet Arch


in 2013
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0. W. TORGESON, Engineer
Lumber dry kilns now in use vary from natural circulation
kilns with hand control to the latest type of fan kilns with automatic
control of both temperature and humidity. In between are remodeled
kilns equipped with various types and sizes of fans, and various tynes
of control.
Some slow circulation kilns are used for drying ,-reen stock
where rapid circulation would be best, and some rapid circulation kilns
are used for drying air-dried stock ,here a much slo!,er rate would be
Circulation is needed to replace heat losses throu,7n walls
and around doors and -,here these losses are large a brisk rate is help-
ful in maintaining uniform drying conditions., Another important con-
sideration, however, is the supplying of heat for evaporation, and,
therefore, more efficient designing and drying could be accomplished
if more specific information was available concerning the air needs
for various species and items.
Hor long does it take to kiln dry sugar maple? This seems
like a simple Question, but it can be answered satisfactorily only after
many factors are taken into consideration. These factors are: (1)
original and final moisture content, (2) sapwood or heartw7ood, (3) tem-
perature of kiln air, (4) size of stock, (5) relative humidity of kiln
air, (6) velocity of kiln air, and (7) length of air travel. To corre-
late various combinations of these factors and drying time reauires
considerable .knowledge of the effect of each.
Recently thE Forest Products Laboratory completed a series
of kiln runs on the heartwood and sapwood of 1-by 9-inch sugar marle
primarily for the purpose of determining the effect of air velocity on
drying time. The runs were made in a wind tunnel type of dryin.- unit
within which the temperature, relative humidity, and air velocity ,-pro
maintained constant during each of the various runs. Tht:- boards w ere
piled on 1-inch stickers to a width of 4 feet in the direction of air
travel. The original green moisture content averaged, approximatly,


70 percent and the drying period was from this condition down to a
moisture content of 20 to 30 percent. Only one temperature, 1300 F.,
ras used for all runs, but the relative humidity, because of surface
checking tendencies, ranged from 20 to 76 percent for the sapood
boards and from 76 to 91 percent for the heartwood boards. The air
velocity ranged from 155 to 1640 feet per minute. The data obtained
in these kiln runs are presented here in discussing the seven factors
enumerated in the preceding paragraph.
(1) Original and Final IMoisture Content
The drying rate of a given piece of -ood. is proportional to
the moisture gradient. By gradient is meant the rate of increase in
moisture content from the surface fibers toward the center. This gra-
dient is greatest at the beginning and, under any given relative humidity,
is a maximum only rhen the air velocity is sufficiently high to bring the
surface fibers down to moisture content in eouilibrium rith the surround-
ing air. As the interior dries, the gradient decreases causin corres-
ponding changes in the drying rate.
To illustrate the importance of moisture content, tne average
drying rate of sugar maple under a temperature of 1300 F. and a relative
humidity of 76 percent is given in table 1 for 1C percent moisture con-
tent changes. The highest air velocity run (1,640 feet per minute) is
used to eliminate as much as possible the velocity effect.
The drying rate at any specific moisture value is influenced
by the original green moisture content. For that reason a slight error
is introduced in comparing the drying rates of the saprood which nas an
average original moisture content of 70 percent with that of the heart-
wood which had an average original moisture content of 65 percent. At
55 percent moisture content, this error amounts to an-roximat,,ly 15
percent, but at 20 percent moisture content the error is negligible.
In other words, if the average original moisture content of the heart-
rwood had been 70 instead of 65 percent, the average drying rate between
60 and 50 percent moisture content would have been approximately 15
percent less than that shon.
Under these constant drying conditions, more time was needed
to dry from 30 to 20 percent moisture content than from 70 to 30
percent. This illustrates the necessity of specifying auite definitely
the moisture content limits in estimating drying_ time.



Table l.--Drying time of 4/4 sugar maple under a temperature of 1300 F.,
a relative humidity of 76 percent, and an air velocity of
1640 feet per minute
: Sapwood : Heartrood
Moisture content :--------------------------:------------------------
---------------------: Drying :Average moisture: Drying:Average moisture
From : To : time :content loss per: time :content loss or
: : : hour : : hour
---------: ---------: ---------------
Percent : Percent : Hours : Percent : Hours : Percent
70 : 60 : 3.2 : 3-1 : ----
60 : 50 : 4.4 : 2.3 : 6.3 : 1.6
50 : o : 6.1 : 1.6 : 10.0 : 1.0
40 : 30 : 10.9 : .9 : 20.0 : .5
30 : 20 : 34.9 : -3 : 47.0 : .2
60 : 20 : 56.-3 : 3-3
(2) Sapwood Versus Heartwood
In general, heartwood dries slower than sap-cod, and in the
case of the sugar maple the ratio of drying rates was approximately two-
thirds. This is illustrated by the data given in table 1. Under the
conditions given, the drying time from 60 to 20 percent moisture con-
tent was 83.0 hours for heart,,ood and 56.0 for'sap-ood.
(3) Temperature of Kiln Air
Only one temperature (1300 F.) was used but some data on oak
have indicated that between 1300 and 1600 F. drying rate increases
approximately 2 percent for each 1 decree increase in temperature. On
this basis, the sugar maple sanVwood at 1600 F. rould dry from 60 to 20
percent moisture content in 56-3 or 35 hours. This computed drying
time is only an approximation and is given merely as an illustration
that temperature must be considered in estimating drying time.
(4) Size of Stock
Although only one size was dried in these norticular exeori-
ments, it might be well to explain how the data can be used to estimate



the drying time of other sizes. Drying time is not directly.proportional
to thickness, but,. fqor..a.n. infinite -width, is more -nearly proportional to
the square of the thickness. W idth is-a factor-also because as the
width decreases the edge drying becomes relatively more important as com-
pared to the amount of drying from the faces. A mathematical method of
computing this has been used at the Laboratory and has been found to
check well with empirical methods. Some of these computed ratios are
shown in table 2.
Table 2.--Theoretical drying time of various sizes based on
that of 1-inch stock of infinite width as unity
Thick- :-------------------------------------- -----------------------
ness : 1 : 2 : : : 6 : : Infinite
Inches : Relative time
1 : 0.50 : 0.O : 0.90 : 0.94 : 0.97 : 0.99 : 1
2 : : 2.00 : 2.77 : 3.20 : 3.60 : 3.77 : 4 .00
3 : : : .50 : 5.76 : 7.20 : 7-9 : 9.00
4 : : : 8.00 : 11.0 : 12.80 : 16.00
The values show the relative drying time as compared to that
of 1-inch stock of infinite width. Estimates can be made for narrow
widths only when the air circulates freely around all sides. When
the stock is piled edge to edge in a wide solid layer, an infinite
r idth may be assumed.
(5) Relative Humidity of Kiln Air
Previously, it was stated that the drying rate is proportional
to the moisture gradient. One factor limiting the gradient is tne
equilibrium moisture content at the surface of the mood and this, in
turn, is a function of the relative humidity of the air. The confusing
thing, however, is that the relative humidities at the surfaces and at
the leaving-air side are not the same as that of the conditioned air.
As heat passes from the air stream to the wood surface and is used for
evaporation, a temperature drop occurs which, together with the addition
of the evaporated moisture, results in an increase in relative humidity
at the wood surface. The main air stream is thus affected and by the
time it reaches the leaving air side, it has a lower temperature and
higher relative humidity than when it entered the load. The magnitude
of the difference between the air stream and the surface is governed
mainly by the air velocity -hile that between the entering and


leaving-air sides of the load is governed by the volu:-,e of air suplied
and the length of air travel. Both differences (on cerpendicu r nrd
one parallel to the board surfaces) are affected by the rate at 7rlic!-
the moisture is given off.
This is illustrated Era-khicaly in fi,-,re 1. The Jirvir,. data
.ere collectA from a series of '.eart-ood runs 'here the relative ln,=id-
ity and air 7elocity of the indi-%idual runs 7ere as follow's: 7", 0, ,
and l percent a- 235 feet per minte, and 76, SO, and 6 nerc-nt at
450 and 980 lfct per-minute. On the charts, the line-s- dra-ar throu, h
the data points -ere extended to the zero hing rate line at 100 rer-
cent relative humility.
It might be -ell to mentio here that the moi 7ture content
values identifying the curves in figures 1 to 4 are a7VrEa#- V-9US
that in each case a moisture gradient existed from thc interior of the
-ood to the surface and from the entering to the lealvin7-air sides of
the load. At -the same average moisture content, Iiff-trnc;: in the
slope of these gradients accounted for the differences in drying rate
A comparison of the three charts of fire 1 sho-s that at
60 percent moisture content the relative nudity effect on-drying
rate ander an air velocity of 990 feet -er minute -as quit e different
from that at 235 feet per minute. At a moisture content of 30 percent,
ho-ever, the difference Tas very much less.
At a moisture content of 60 percent, a constant dr~yinz rate o-f
0.49 recent moisture content loss per hour -,ras obtained unfr eacn of
the fol1o-ing drying conditions, 76 perc-nt relative humidity and 2Y7
feet per minute air velocity, 95 nrccent -and 150 feet per cinu'Le, !nr-
91 nercent'and 990 feet per minute. Each of these thre_ conditions,
then, produced the same average effective ecilibriu-n moisture cont- nt
or. the surface of the wood at that particular moisture content of stoz~c.
Figlrre 2 sho-s the drying data for sa- oo-i boards -hen dric-1
under relative humidities from 76 dto-n to 20 percent. The air velocity
es 1,> fct ncr 'iinute, which -as sufficintl high tor prevent any
ar-oreciable humidity rise next to the -ond surface esrecillly at the. lo
moisture content values. The curves, then, represent the relative
humidity effect on drying rate and shor- ho7 much more important it -as
at the high-r moisture values.
"'he curves also sho- that the imnortance of in
tive humidity became increasingly greater as the humidity increased
above 60 or 70 percent. Belo- 6o percent, the humidity effect
became relatively unimportant.

Rl 2 P 5


(6) Velocity of Kiln Air

No kiln-drying time records are complete -:ithout sho-ing the
air velocity ani volume as 77ell as the temperature and relative humidlity.
The imnortanc. of this is. s,-o-'n by the constant drying, rate, curves of
4:i,=e 3. ach curv_ rerresents some definite dryin, rate of a 4-foot
-,ile of I by -inch sugar ma-ple sap-cood boards '-hen -at O -)ercent mois-
tuare content and -hen subjected to various combinations of air volocitv
and humidity-. The chart shors that -Ihen the -ood. 7,as at this 'high
moisture content, the surface r-as subjected to the relative humidity of
the conditioned air only -hen the air velocity, -aes very ni -,h. 2 e 10-
_ns axJ_- -air velocity, a constant dryin 7 raste -~as -:aIntained on~ly
b lo77erinj h relative humidity of the cond-itionod air.
--or inst.-ance, -'hen. the stock -as at GC necrcernt moisture con-
tent an air velccity- of 1,600 feet per minute and a relati-,re hi-midity
of F4 resul-ted in a moisture loss of 2 percent per hour. Ihen the air
velocity 7,as re4daced. to 4CO feet per minute the relative humidity had
to b-e reduced to -_ F -ercent to maintain the same averac,-_, rate.
in both caie3 the effective average humidit-y at 'the wood surface must
have been a-; -roximately 3141 percent but in the case of the lower
velocity the h-umidity varied from the 43' -,ercent of the conditioned
air to the a-,erage of 91t percent next to the roo surface. For this
reason, t_,.6 results in various t, es of kilns an-arently usin- the
s~e drying schedule may vary widely because of a difference in air
ThIn effect of air velocity or, drying rate at definite moisture-
content values is shor-n 'by the curves 'of fi,7 ,re 4v. The ob--ious con-
clusion is that air velocity is most i-zrortant at hi,7:h moisture-content
values, and becomes relatively unim~ortant belo- 30 percent moisture
content. 1For thorou,;tly air-dried stock very little velocity is needed
except to establish uniform drying conditions in all parts of the- k-iln.
Another im-;ortant conclusion can be made from the data as
re s ent inr fi .e 5 Tisgraph shors the avera--e drying time in
hours of a 4-fcot -ide rile of I by S-inch sugar maple sar-ood oad
'-hen drie! from 70 to 25 'perce-nt -_oisture content under a, constant
't, :.-erature of 1300O F and under the in, !iated rp"Iativ nui~ities nd
air velocities. 7he data indicate that much hii-_,ir air velocities are
needea; for high. than for lcm, relative h=daity 7-~ues h r- ason
for t:.sis that, a: hi, h humiI"i ties, changes in the-_ _- -ilibri=m mois-
tur- content of -o d become increasingly greater rit "n increases
in hum=idity, and; as a- result the drying rate is more a-f-f;_teld by the
chan.- sin h,_AMidity brought atout b, an adlitio. of moisture froz the
Too,-- the dro7 in tem-perature across the lo.
:efining o-pti.-:= air velocity. as beine- some velocity beyond
hi cr th e ffect on dr-1,inz time becomes relatively unin,,ort,7nt, the


optimum values for relative humidities of 20, 5-, and 7e rcre nt miat
be selected from the curves of figure 5 as being 200, 4)0, and 00 feet
per minute, respectively. Of course, for slo-er-drying species such as
oak, these values would be lower.
(7) Length of Air Travel
As conditioned air passes-through a load and heat is used for
evaporation, the progressive changes in temperature and humidity re-
sult in changes in drying rate, and, therefore, in a drying las across
the load in the direction of air direction. Under otherwise fixed con-
ditions, the amount of this lag is governed by the length of air travel,
but is not directly proportional. To illustrate; data are riven in
table 3 to show the time to dry from 70 to 40 percent moisture content
at even intervals across a 4-foot pile of 4/4 sugar male sar-ooJ. As
the air velocity effect on the entering-air side is quite different from
that on the leaving-air side, the data given are 'from three separate
air-velocity runs.
Table 3.--Drying time of 4/4 sugar maple saprood from 70 to 40 percent
moisture content at definite intervals acros-s a 4-foot
wide pile under an entering-air temperature of 1300 F,
and a relative humidity of 76 percent
: Length of air travel
Air :------------------------------------------ : Averace
velocity : 0 feet : 1 foot : 2 feet : 3 feet : 4 feet :
:-----------: :--------:
: Entering : : : :Leaving :
: air : : : ir
Feet per : EHours : Hours : Hours : H'-ours : Hours : Hours
minute : :
256 : 15 : 40 : 54 : 63 : 69 : 49
545 : 12 : 24 : 28 : 30 : 31 : 27
852 10 16 : 19 21 22 18
The drying time lag in each run is considerably greater than
that -hich might be expected from the average temcrat-urj dror across
the load. The air, as it enters the load, is uniform in t mroratur-
and humidity, and, consequently, the velocity effect onr, th iiryin rate
of the entering-air edge is relatively small. Te directional forco of
the air, however, prevents a uniform distribution of the e-v.aor.t~ -l
moisture and heat loss, and, as a result, the air at the -avi:--air



edge varies greatly in temperature and humidity from the wood surface to
the center of the air stream. The velocity of the air influences this
nonuniformity and consequently has its greatest effect on the leaving-air
The greatest difference in drying time occurred within the
first 2 feet of the 4-foot air travel and in each of the three runs the
average drying time of the full load was represented by the dryin, time
of the wood located approximately 1.6 feet (or 0.4 of the total idth)
in from the entering-air side. At the highvelocity this decreasing
effect of length of air travel on drying time was such as to indicate
that the width of load could have been considerably greater with only a
small change in average drying time or in that of the leaving-air edge.
The effect at the loT velocity was relatively high and might be taken
as suggesting the desirability of introducing several entering-air
edges within the load by means of vertical flues. In other words, it
suggests that two 4-foot loads of sugar maple sap-ood placed side by
side with a space between might have a lesser average drying time than
one 9-foot load.
Aside from the velocity phase it might be ell to noint out
here that the saprood boards dried without checking even under a rela.-
tive humidity of 20 percent and an a'ir velocity of 1,50 feet per
minute, whereas the.heartrood boards checked some at a humidity as
high as 80 percent. For this reason, if, in saying and kiln drying
4/4 maple, the all-sapood boards could be sorted from the heart-ood
boards and dried separately under a lo= relative humi(dity schedule
then their drying time would-be greatly reduced from that ordinarily
allowed for log run maple..
In general, air needs -are proportional to drying rates. For
that reason, air needs are least for the lower moisture content stock
and for the slower-drying species and items. An exception to this rule
occurs Then relative humiditiesare increased above, a nnroximately 70
percent. Such a procedure reduces drying rate, but reouires a higher
air velocity to prevent excessive increases in the moisture content of
the wood surface and, consequently, excessive loss of drying time. By
inference, then, the most efficient kiln from a dryin' time' stndnfoint'
would be one equipped to furnish a great deal of air at the beginning
and then lesser amounts as the moisture content of stock decreases and
as reductions are made in relative humidity.



O s 450 FTr PR M/N.
% \
p 235 FT, PER MIN. .
0.2-, .., ,, \ ",
70 30 90 /00 80 90 /00 60 90 /00
Ic )7 T
Figure l.-Effect of relative humidity on the drying rate of sugar
maple heartwood at a temperature of 130 deg. F. and air
velocities of 235, 450, and 980 feet per minute.

2_ _ _
0 L___ 1
20 30 40 50 60 70 80 90 /00
- 7~/F
Figure .2-Effoct of relIati,,e humidity on the d~firg rate of sugar
mnaple apwood R.t i temperature of' 1 30 dcp-. F. an an~ a r
-locity of'(34 foet Fer minute.



4 ,o
4 0 ...- -. - -
o0 400 600 800 /000 ZOO /400
, c- .o F
Figure 3.--Effect of air velocity and relative humidty on the mois-
ture content loss per hour of sugar maple sapwood when at
a moisture content of 60 percent and a temperature of
130 der. F.

- 3T60
Figure~~~~40 .....e~t ....... .i ......... .r .h .ri g rt f s g r ma1
~3~p.Q~d~ a ~rnprdur of 307 de.F i lrltv
hum dit 20 yo er e t


70 - v -----
-->0 _
0 200 400 600 800 /000 /ZLO0 1400 1600
Figure 5.--El'fe t of air velocity on ttbe drying time of' sugar maple
sapwocd in dr7;ing from 70 to 25 ,,- roeert moisture contend ,
at -, temperature of 130 deg. F. an relat;-,., hiuiities
of 20, 50, an~d 76 percent.

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