Accelerated kiln schedules

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Title:
Accelerated kiln schedules
Physical Description:
Book
Creator:
McMillen, John M
Forest Products Laboratory (U.S.)
University of Wisconsin
Publisher:
USDA, Forest Service, Forest Products Laboratory ( Madison, Wis )
Publication Date:

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aleph - 29337500
oclc - 230748712
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AA00020626:00001

Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
    Main body
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
Full Text
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UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVICE
FOREST PRODUCTS LABORATORY
Madison 5, Wisconsin
In Cooperation with the University of Wisconsin

ACCELI2ATID KILN SCH UULES
July 1951

US
ENTOTAGC

No. 01901







Biy
Forest t lroduts Lab1'r; t, o rst 8trvi:e
I, I~epazrctnt- cif Agricu ,Jire
Very great advancements in the field of the kiLn drying of wood have been
made in the last: 10 years. These advancement include increased under-
standing and operation of th machine used th<. dry kiln; a great increase
in the number of properly trained. kiln operate )rs, with authority to use
their special knowledge; and the development of new, economically sound
kiln schedules. During World War II the Forest Products Laboratory had a
number of crews in the field testing dry kilins to certify them for use in
drying lumber under the temperature requirements of AYemy-1avy specifications.
These crews found very few kiLns that actually met the requirements. But
with electrical measuring instruments it was comparatively simple to make
slight modifications so that the kilns could attain the uniform conditions
required by the specifications After the war, the Laboratory undertook a
program of training kiln operators in a series of demonstrations. These
were held both at the Laboratory and in the field, with three being held in
the New England area. As a result of these demonstrations, more than 700
persons have been trained in kiln operation.
It is the third phase of the major advance in the field of kiln drying that
is the subject of this paper, the new Forest Products Laboratory schedules
for the kiln drying of wood that permit kiln operators to accelerate drying
without the hazard of excessive seasoning defects.
In February the new Forest Products Laboratory schedules for kiln-drying of
wood were published 3 These schedules and a full explanation of them also
have been published in one of the trade journals.4 The schedules were com-
piled by 0. W. Torgeson from all the information collected at the Forest
Products Laboratory on this subject during the past 14 years This informa-
tion includes experience with the previous schedules published in 1937,
results of intensive research on schedules for individual woods, studies of
the stresses that develop in wood as it dries, and an analysis of the most
advanced schedules that were being used in industry. The new schedules incor-
porate the principles of acceleration enunciated by R. C Rietz,
Paper presented at Vermont Wood Products Conferece, Northfield, Vt May 24,
1951,
2Maintained at Madison, Wis., in cooperation -ith the University of Wisconsin.
Torgeson, O. W ., Schedules for the Kiln Drying of Wood Forest Products Labora-
tory Report D1791. Februaary 1951.
4Torgeson, O. W., New Schedules for Kiln Drying Wood (Chicago) April 1951.
2Rietz, R. C., Accelerating the Kiln Drying of Hardwoods. Southern Lumberman,
July i, 1950
Rept. No. D1901 Agriculture-Madison




nature

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separate from the temperature schedules. Figure 3 shows, in condensed- form,
the entire scheme of the new Forest Products Laboratory shdlsfor -i
kiln drying of wood. The upper part of figure 3 shows the te:mpe)rature-
schedules, and the lower part the humidity schedules.
Which of these schedules to use for a specific item is given in tlhe jInd-
of Forest Products Laboratory Report No. D1791. Schedules for hardwoods-
are listed in one table, and schedules for softwoods are listed in anothe,-r
table. Schedules listed for special items, such as dimension stock andu
squares, may be different from the schedules for lumber of the same species
because of differences in seasoning characteristics and differences in use
requirements.
There are 14 different temperature schedules, ranging from T!, at very 'Low
temperatures for color control, to T14, with high temperatures for -use with
southern pine, true firs, and other easy-drying softwoods, Some pairs of the
schedules are identical except for the final temperatures. For instance, T3
and T4 both start with ii0* F., and succeeding steps are 1200, 1300, and
140 F. In T4, used for northern red oak 4/4 stock, the final temperature is
180' F. T3, with final temperature of 1600 F., is recommended for 8/4 northern
red oak. Experiments have shown that the slightly lower final temperature is
safer from the standpoint of honeycomb checks. Other schedule pairs are
schedules 5 and 6. 7 and 8. 9 and 10. Schedules 11, 12, 13., and 14 are each
distinct. All temperature schedules call for making the first change in
temperature at 30 percent moisture content, the second at 25 percent moisture
content, the third at 20 percent moisture content, and the last at 15 per-,---,+
moisture content. There are only five temperature steps.
The lower part of figure 3 shows the humidity schedules. In Report No, D1791
there are six classes of humidity schedules depending upon original green
moisture content. In figure 3 all six classes have been condensed into one
table. This figure shows that the scheme of lowering the wet-bulb depression
is the same in each of the six separate classes. Classes A. B, C,' D, E, and F
differ from each other only by the initial moisture-content values and the
values at which changes in relative humidity are made. For schedule A the
first change is made at 30 percent, for schedule F the first change is male at
70 percent, and for the other schedules the changes grade in between these
values. One of the biggest advantages in the new schedules is the making of
the first change in relative humidity when the wood loses about one-third of
its green moisture content. The D humidity schedules, for exa mrple, are for
woods having green moisture-content values between 80 and 100 percent. In this
class are the humidity schedules recommended for northern red oak. The first
change in relative humidity comes at 50 percent moisture content, a second
change is made at 40 percent moist-are content, and other changes at 35, 50., and
25 percent.
Each class of humidity schedules consists of eight sets of wet-bulb depressions.
For instance, Dl, D2, D3, D4, and so on. The initial condition called for by
D2 is a wet-bulb depression of V F. Showing humidity in terms of wet-bjlb
depression is another way the new schedules differ from the old, In the,- old
schedules both the dry-bulb temperature and the wet-bulb temperature wer given.
In the new humidity schedules it would not be possible to give actual .wet-buIlb
temperatures because the humidities are used with different combinatio)ns- of dry-
bulb temperatures. The most practical way to express humidities in sceue
this type is to use the wet-bulb depressions.
Rept. No. D1901 -3-




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of the wettest one-half of the total number of samples shoul-d be, I t -)
govern the changes in kiln conditions. The schedules are set ap on
moisture content of heartwood, Heartwood usually takes longer to dry than
sapwood and is more susceptible to seasoning defects.
Figure 4 shows the new schedule for the kiln drying of 4/4 northern
oak in the graphical form. This is made up from the results of the kiln
drying of two charges of red oak in the Forest Products Laboratory's
semicommercial kilns in the kiln-drying demonstration last February. The
initial temperature is 110* F, and that temperature is held unt-ii an average
moisture content of 30 percent is attained by the wettest half of t ,- kiin
samples. Reduction of relative humidity is started at an average moisture
content of 50 percent. Another drop is made when the average moisture content
of the wettest half of the samples gets to 40 percent. The second drop is of
greater magnitude than the first. At 35 percent moisture content the relative
humidity is dropped still more, and so'on, until at 25 percent moisture content
the relative humidity is 10 percent, This is quite different from the old
schedule for oak. An equilibrium moisture content of 2 percent on the surface
furnishes a greater incentive for the moisture to move from the insideto the
outside of the piece. Temperature gradually steps up, starting at 30 percent
moisture content. At 15 percent it goes to a maximum of 180' F,
If the wet-bulb temperature were held constant and the same scheme of relative-
humidity lowering were obtained by raising dry-bulb temperature, there would
be danger of causing serious internal checking, because high temperatures would
be used before the wood in the cores of the pieces would be down below the fiber
saturation point. Prolonged heating of wet wood at high temperatures weakens
it greatly, so that when maximum internal tension stresses develop during the
last stages of drying, the wood no longer is strong enough to resist them,
Drying, to the point of equalizing and conditioning, in the two oak runs
combined in figure 4., took 26 days. By the old schedule it took 31 days.
The runs with the new schedule were started at a much higher moisture-content
value, 87 percent. The present schedule would dry partially air-dried oak
(70 percent moisture content) in 21 daye. TAe difference between 31 days and
21 days is a saving of one-third the during time. In some instances it may be
possible to cut drying time for oak still further; but for the time being
schedule T4-D2 is as far as the Laboratory will go in mak ng general recommenda-
tions to the public for 4/4 northern red oak.
The Laboratory schedules will, in some cases, require modification; they cannot
be used blindly,, because of differences in the characteristics of the wood, in
the local production and selling practices,, in the degree of care in kiln opera-
tion, and in kiln characteristics. Modifications of the standard schedules
should be based on careful observations of the stock during drying and on the
quality of drying desired, It should be understood that there are some varia-
tions in seasoning characteristics within each species and that identical
results cannot always be obtained. For some of the less refractory species
more severe temperatures or relative humidities can be used to reduce drying
time without causing excessive increases in the amount of checking. For
instance, quarter-sawed material has less tendency to surface check than plain-
sawed and, consequently, lower initial relative humidities can be used.

Rept. No. D1901

-5-




1t Ptltd rma, t io ,n or data. Jowcve.r,raildertesfoth
~U~riard~ idI~~ houl notbe mad(- ml hne cnb a~i
:u' ~civ~ :t~ps 1 3aseQ upon close- obse _rvaton ( of s h eairo h
- k ~ ~ i r~in The aborato(ry has' )JLui' ree~ly 1ernd7,t h
i~~~~ ~ 1 rnrzef /4ctowouoi is satisifa-t-ry i c r02crcultio
kiln Ak it Jiedules for 4/4 southern pine ardDilsfrare_
catif~it~r Inf'act, for the pine and f'ir, an oeaoswo eLbrtr
tranedin ri~ fits kiln-drying demonstrations, was ab 7o~ie a tepratu're
!i,--c on i number h-igher than prescribed in the inde!(x. Th.is does nt metan
tha th Laoraoryis going to change its schedules frtrs odbti
m~asta niiulkiln operators should look for coppo7,rtu e tor mdif
tne~ s~idL.s ~ndget faster drying without causingobetnal dyn
ITs -ire. a' intended primarily for use in compartment kilns with% fast
uc c~r~ ~'. Air velcities of 200 to 350 fee-tprmitehrgL e
~~~aa ~~ om~aon When air velocitie3 art- m'hls hntitei
ti~ii ~-t-~L h~r'fsion can be increased. sorehc o ntr- ,cjto
ki ins s -bbdprsions can be inc;reased as much' as 0t
ITn ~ r~kiln, tnec green end is ;sually operat-d at a rat__-.r Low
t-me~a~.:~ a~ high relative humidity as compared to hc cn.tosa h
, rid sorn .rogressive kilns, )nly tedry end i. ,--teprau
s~~~~Jf .uniyotrol The initial. and final conditins ir hs ~e~e
an beu- s. a gu)tde in selecting the proper odtoc rtngenad
dry end.ofth, ~ For refractory itmthpre would bi dange-rir sn
tie e : thi_- way because the intermediate c onrti~ ar -11*l to
h~mr, sve in progressive kilns. In such case_ s, r oew r ore_
)nservati~r mepera'. and relative-humidity schedule may bereqire-d
instead rf'~o. r ciinmended f or c ompartment ki Ins.
T! th i pa- .atep as been made to cover thorougIrly 'he ~nra on
jcitir f nw orstProducts Laboratory schedi.-s- fcr -I r Y~x ryn g
(twou.anJ o t rig~,as an example, the use- of cum- -'~ue I-2 or
., lie t ok Tepaper has tou-Ched bri--fly) -i. aon cf~mjr
nsd )crsinshtdui application as outliLed n T!es Prout 7, T Iocra
F r Kij~~~l Orh.r poink.s covered by-, thiL: oti~~uene o
ccxT ~tr~cn: an nedfor proper lcatior of c-t l bulbtS, f in.al
m~i r--r~ itr~crenntand equalizing and fodtinn t'am sfr
un~f:mi, -_ ioist-ir -crnt and case-hardening re-lief
(>~~Cft~ne sedl an be obandfrom teFrs rdc Labocratocry,

H~jt N :,




_ _~

Table 1.--Combined temperatur and humidity : dnls
for kiln dryng green '1 r-c. oak

:R latiive :Equli brim
. i mdity-: m. tur*
:: en r.

Temper-:Humidity:Moisture content:Temper- : Hui.ity :
ature : step : class D : ature : schedule :
step : number :----------------:schedule: 2 :
number : :Moisture content: T4

1 : 1I
1 : 2
1 : 3
1 : 4
2 : 5
3 : 6
4 : 6
5 : 6

------- -------- --------:-----------
: From : To : Dry : Wet-bulb : Wet :
: biilb :depression: bulb:

OF.
4 :
5 :
8 :
14 :

:Percent
:Initial
: 50
: 40
S35
: 30
: 25
: 20
: 15

:Fercent:
: 50 :
: 40 :
: 55 :
: 30 :
: 25 :
: 20 :
: 15 :
: Final :

O~i

Percent :

Percent
17.5
16.2
13,3
9.9
5.4
2.0
2.6
3.3

106 :
105 :
102 :
96 :
90 :
80 :
90 :
130 :

110
110
110

87
84 :
75 :
60 :
31 :
10 :
14 :
26 :

120
130
140
180

-Relative-humidity and equilibrium-moisture-content

values corresponding to the

given dry-bulb temperatures and wet-bulb depressions are given in a chart in
Forest Products Laboratory Report No. D1791, EquiLibrium moisture content is
that moisture content to which wood will come if exposed long enough to that
particular set of conditions.

Rept. No. D1901




Figure l.--Kiln conditions and drying curve, 4/4 northern red oak,
using old Technical Note 175 schedule 7.
ZM 85901 F

Rept. No. D1901




OR 3U L R3 Tz-mp RF
AVE19AGE- OF 5 VETTEST
(V/ SAMPLE-
o 50 /00 1I50 200 0 o {
7, M 85901 F DRrIIvG T'lvTE-




Figure 2 --General pattern of new Forest Products Laboratory schedules
for kiln drying of hardwoods.
ZM 84898 F

Hept. No. 01901




~_iii;;;;;; ; ~_

i __

I i I I i i I I D Y BUL B r"EMPER~iA 7Z/RRE
RAISED WHEHN fREE
WAT 7~ER IS EVAPORATEDa
RELAIVE HUMIDITrY LOWERED CONDI TIONIN
WHErN E= 0. 7
I OUA L IZNG Ppf
DERADE CONT~i^ROL ---------~ ACCLERATE DRYING
7M 848()8 T7ME VA/ARIES W1 7-H SPECIEiS A ND TCKESS




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C+
O
O
Figure 3.--Forest Products Laboratory schedules for the kiln drying of wood (1951).

ZM 87228 F




- -------------------------------- '- -~;;; ~ ~

TEMPERA TURE SCHEDULES (DRY-8ULB TEMPERA TURES)
MOISTURE TEMPERATURE SCHEDU LE NUMBER
OMSTEP T- T-2 T-3 T-4CONNT T-5 T-6 T-7 T-8 T-9 T-IO T-II T-12 T-13 T-14
NO. FROM ITOT-IIT1T13T4
% OF. F. O F. F. F. OF. F. F. F. "F. OF. F. OF. OF.
I INITIAL 30 100 100 I10 I10 120 120 130 130 140 140 150 160 170 180
2 30 25 105 I10 120 120 130 130 140 140 150 150 160 170 180 190
3 25 20 105 120 130 130 140 140 150 150 160 160 160 170 180 190
4 20 15 115 130 140 140 150 150 160 160 160 170 180 180 190 200
5 15 FINAL 120 150 160 180 160 180 160 180 160 180 180 180 190 200

I I

SCHEDULES (WET-BULB DEPRESSONS)

HUM/DITY

GREEN MOISTURE CONTENT CLASSES

BELOW 40 I 40 60

60-80

80- 00
D
MOISTURE
CONTENT
FROM TO
INITIAL 50
50 40
40 _..35
35 30
30 25
25 FINAL

100- 120 ABOVE 120% X

E
MOISTURE
CONTENT
FROM TO
INITIAL 60
60 50 O
50 40
40 35
35 30
0 FINAL

F
MOISTURE
CONTENT
FROM TO
INITIAL 70
70 60.
60 50
50 _40
40 35
35 FINAL

MOISTURE
CONTENT
FROM TO
INITIAL 40
46 35-
35 35
30 25
25 20
20 FNAL

MOISTURE
CONTENT
FROM TO
% 7.
INITIAL 30
30 25
25 20
20
20 15
15 IO ...
I0 FINAL
', z M 87?PS F

MOISTURE
CONTENT
FROM TO
INITIAL 35
35. 30
30 1 25
25 20
20 15
I6 FINAL

2
O.
4
5
8
14
30
50 -

_3
.
_7
II
19
35
50

4
. ... ... . .
OF. OF.
7 10
10-o 14
15 20
25 35
40 50
50 L50

_
F.
15
20
30~
50
50

7 8

OF.
20
30
40
50
50
50

OF.
25
35
50
50
50

*F.
3
4.
6
10
25
50

I
2
3
4
5
6

STEP
NO.

HUMIDITY SCHEDULE NUMBER




Figure 4.--Kiln conditions and drying curve 4/4 northern red oak,
using new schedule T4-D2.
zM 86987 F

Rept. No. DI901




200i T4 D2 t
160--- -
140 -
/00
- -_ -_ - - - - -
-1- 4------
0 2 4 6 8 /0 12 14 16 18 20 22 24 26 28
DR YING TIME (VA r~j
7m 86987 F




UNIVERSITY OF FLORIDA
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