7P.c
U. S. Department
u'1
ti tKOOR1l -
of Agriculture, Forest Service
FOREST PRODUCTS LABORATOR Y
In cooperation with the University of Wisconsi n
MADISON, WISCONSI N
1
BRAIN WORK IN KILN OPERATIO N
By C. V. SWEET
Specialist in Kiln Drying
Approved Copy File d
3 — 6-2Q
BRAIN WORK IN K=T,N OPERATION
There
is
no phase of kiln drying that stands out
mdre clearly as essential to success or directly responsible for failure than the intelligence and attention that i s
iLren to the operation of the kiln . Those who have watche d
with particular interest the progress of the practice o f
kiln drying lumber, especially under the stress of war conditions, have seen that the greatest difference between success and failure has been in the operation more than in th e
kilns themselves, whether it has been with airplane stock ,
green oak vehicle stock, ship timbers, or wooden legs . Poo r
kilns and good operation have often produced as good results ,
as good kilns and poor operation . But in the long run, th e
big problems of artificially seasoning lumber will be solved
only by a happy combination of the two factors . It takes a
good kiln and good operation to obtain really good results .
It has frequently been' brought to the attentio n
of the Forest Products Laboratory of the United States For wit aervice .'at Madison, Wisconsin, that with a given typ e
of drf
n ; .,iandling similar kinds of stock according t o
the same rtc amended schedule, one concern has dried its ma`
terial with Pract
i1 . .' 011
-.
ther
ed a
loss of 40 to 50
er• Rien .1-
,' -. r, ;
.
L6
2 59
Al &
.
I I
If .t' art of kiln drying lumber has come throug h
its recent trials trimmed of some non-essential ideas an d
b-enef ited by new conceptions, the sum total Of our informa tion concerning it has been greatly increased . On the othe r
hand, if there is still a vestige of the old notion tha t
"this kind of kiln will dry lumber in half the time with les s
checking than that kind of kiln", a big load of inertia mus t
still be moved .
Emphasis on the operation of the dry kiln mean o
the injection of the human, intelligent, interested elemen t
into kiln drying which is too frequently absent in the methods of many concerns . The kind of kiln operation that i s
possible at the hands of an ex-lumber piler whc has becom e
too old to work efficiently anywhere .else, or of a ma i -wh e
has grown up with the company and. in his 'later yeara deserve s
an easy job ' , or of a fireman at the plant who has a few
e-X -
tra hours to fill in .- that kind of kiln` -ope?:atiion d:ees not ..
enter into this consideration .
The kiln drying of lumber, unless it, be th, ,dfyin g
of air-seasoned softwoods, is an art involving speciaal ; 4ml .
edge, skill and good workmanship . Without detracting• f-rya .
the credit of the practical lumberman, it' iaay'be $sAcl
tr t
the greatest advances in kiln drying have cone in the las t
few years . This advance, reflected in new methods of seas o
ing, is the outgrowth of accumulated scientific knowledge o f
L6
259
_2 .
1
r
1
+ 4
I
the physical and methmical proper't -es of Woa@ . . T Pi sizmess _fui dry kiln operatcr today has to think of, characteristic s
of wood in terms which are unfamiliar and in many instance s
dis astef ul to the colder -cyoe of practical lumberman .
As a matter of fact this ccnception of the role of
cpe •ation ' in kiln drying is opposed to a more or less genera l
commercial tendency to make kiln drying an automatic proces s
from which the el em'ent of personal j ud .gme+'t is remeved a s
much as possible tiereby attributing to dry kiln's
tai- ,
selves whether of, ope particular design or aniethex verve ,mye .•- ti
tic, secret capacity- to dry lumber perfectly with the mi 34inum
I
watching by a competent operator.. Or often lumber i : . siiOse d
.p
to be dried successfully by some so-called "p_atefrt .dd process . ."_,
L
Sad experience show that it is impossible to' . i t 1! :r mbea?y % •~ :
L
dry kiln like a potato in an oven and take it out all done to r a queen's taste .
It is not uncommon to read in advertisements i n
trade journals that this
or
that kind of dry kiln or proces s
dries lumber without checking, honeycombing, or casehardening .
Such a statement is the danger signal,
A simple state-
ment that temperature and humidity conditions are flexibl e
and under the immediate control of the operator would inspir e
more confidence .
The most satisfactory types of
kilns
yet develope d
are those with featUres of automatic temperature and humidit y
L6
2 59
y •' 1
LR
i
control which make the'operator a complete master of hi s
kilns in that any conditions which he regards beneficial t o
the lumber may be maintained hour after hour, or day after
day, until he considers it desirable t8 change such condit 3 oils
The kiln operation itself can never become automatic although the control of drying conditions may well b e
that, and efforts to bring about automatic control of suc h
phases are most decidedly on the right track,
A brief an d
general discussion of some of the features of kiln operatio n
which are of utmost importance may throw additional ligh t
on the importance'of the skilled operator as apart from th e
kiln .
RelationBetweenMethod ofPiling and
Uniformity of Dryin g
The first opportunity for an exhibition of skil l
in kiln operation is in the adoption of the proper syste m
of piling the lumber in the kiln, It may be safely
said
that comparatively few kilns are operated with any though t
as to whether the
piling
suits the kiln or not . Because o f
the very definite relation that exists between method o f
piling and the circulation of air in the kiln, this subjec t
is to be as strongly emphasized as careful temperature control and regulation of the humidity .
+. 1
First and foremost, lumber is piled in the dr y
kiln so as to dry uniformly and as quickly as possible .
There is no more effective way of bottling up the circulation in a kiln and slowing up the drying rate than by piling in lumber across the path of the moving air .
The pil- ,
ing should be such as to offer the least obstruction to' a
free and positive circulation of air within the kiln . Methods of piling which lead to complete utilization of al l
space and in greatest economy of handling the lumber throug h
the kiln are important features to be sure, but entirel y
secondary and subordinate to the method that conforms t o
the circulation system of the kiln . The method of pilin g
must be such as will throw the "stickers" parallel to th e
predominant air currents and never perpendicular to an d
obstructing it .
End piling in a progressive type of kiln, whic h
depends for its circulation on the passage of air from th e
hot, dry end to the gre
cool end places great obstructio n
to free and uniform passage of air and is obviously wrong .
Cross piling in a type of kiln in which the circulation i s
vertical-lateral as in many types of ventilated and condenser kilns, is just as serious in that the normal air current s
are obstructed .
This point is illustrated clearly by a case o f
practical experience . Some 4/4 green maple was loaded into
J
I'
ti
'I
ar-tae.
N
i
I
PLED CROSSWISE rx w /METRELATAW JIM/4/AR /NEAR
Ff/RN/TVRC
PLANT IN NEW VAWC STATC
rr .
. :
1A
'
1
1
'
11
'
t
I• :
1
1
T
.1
.1
1,
.
r
-
Sketches Of representative commercial installa tions of a type of progressive kiln in which the circula1x~ili:n l
tion of air is from end to end . Note that i
the "stickers" oppose and obstruct the intended clroulatiO m
Both end and cross piling are often used in this
type of kiln, but cross piling is distinctly, superior to
end piling, from the standpoint of circixlation . This il Iustrates the lack of attention to a very important item .
of kiln drying .
r
1
_
I
I
f
I
x 115 l1
'Q ' •~
, t-
II ,°1
I1 A
~:
~ .1
'
.1 I
R II 1 •
_ ~1 'I•
' I II •
Oa
u , 111 Ih
u
I
•■
NI . I
II
"I ,rl '
.5 . ~ ;.
.1
0 1
IL 1
■
I Ir
1
Sketches of representative commercial installa :
tions of a type cf compartment kiln in which the circulaof air is up in the center and down and out at the sides ,
Cra g s Riling places greatest obstruction to this syste m
of circulation,
At one plant the lumber is piled crosswies_e and at the other is piled endwise . There is only on e
of 'ure two methods which can be right . Another illustration of failure to give attention to circulation in th e
kiln .
V
1
1-
k
J•
n
II
L I '
I'
C
.
.
1•
1
r . :'
I II 1 -
I ~~
.
• -
,1
a dry kiln having vertical-lateral circulation -. Part of th e
stock was end piled and part of it cross piled .
Temperatur e
readings were taken daily at different parts of both piles .
After the run was well started the difference between th e
hottest and o-oolest portion of the end-piled stock was 1 00 ,.
while the difference in the cross-piled stock was 58 0 .
had a marked effect on the uniformity of drying .
Thi s
The end, .._
piled stock dried from 30 per cent to a maximum moisture con tent of 6 per cent in 10 days while it took 13 days for th e
Pto-ck piled crosswise to dry from 30 per cent down to 12 pe r
cent . T1'is shows that there is a right and wrong way
lUMber to be piled in every type of l ~ .l y,•
for ,
Zo ' Ao Pe, : inter`
ed in even, rapid drying, this point is of great practica l
significance .
Know.le_dge . .ofMtuaiConditions in Kil n
Prevents Losses in Stoc k
After the lumber is properly piled in the kiln
and the heat turned on, the competency of the operator agai n
comes into play . It is essential that the operator knew
accurately the drying conditions in every part of the 41n:.
The determination of the conditions to which the lumber is ,
actually subjected is a eritical -phase of kiln . o}pe al orl.;
Wi th t , ~kn
edsA available to the olp•eTa-to:r
4wOg-t • n+e
'~~
An actual record of temperature conditions during the kiln drying of heavy, green oak at an auto bod y
plant in Ohio, Note the great difference in temperatur e
ie eadings obtained from the recording instrument in on e
part of the kiln and from a standard thermometer in anothe r
p art, The operator of the kiln did not know under wha t
condition the lumber was actually being dried, Result wa s
loss of 70 per cent,
If one part of the kiln it warmer
be ruined in any dry kiln .
than another, that fact should be known by the operator an d
If one end of the- kiln rune a t
allowances mane therefore .
lower humidity than the other, that fact should be known an d
reckoned with .
When measurements of temperature and humid-
ity are taken they are too often taken by sticking an instrument through a hole in the big door and assuming that all
parts of the . kiln read the same as near the door . The temperatiare and humidity taken at any one part o_
in other parts
Ito
~
.off
the kiln una r
,r
less the circulation ia' more perfect than in mo-Of t
The kiln . operator- should know tie . b.elaswi ' lot his'
■
kiln when lumber is . being dried in it as an engineer Itnow e
-
the correlate-d workings of his gas. engine .
actual. condition ,of the lumber in the -Mans
ought to be as easily- and broadly - recogni
is ate
.
I
t
►
.
kiln op
1
ax, , • '
tors as, the fact that the amount of fire 34er &, oiler i s
the- meaisure of the-amount of heat produced, and vat there i s
a most flagrant disregard of this fundamental 1)0.0 i ; an many
kiln -drying - operatooz -s . Ordinary p-rarctice invOr e,s 'in toper atioip based on the number of clays , that Use.. lxlmber ha s
J
inY
9
ti
~
I
I
~
I
I
i fl_ I r
1_ 1=
~t71
Y
the kiln . The variation in the original condition of th e
lumber is so great, the difference in the drying conditio ire
in the kiln are so pronounced that this method is un•certaifl .
This practice When applied to the commercial drying of heav y
green oak has resulted in exorbitant losses .
The whole dry kilning operation is linked up wit h
the amount of moisture in wood and obviously the operatio n
of the kiln must be on that basis .
It is a well established fact that there is a
point in the drying process around which the entire dryingi
It is the point which marks th e
dividing 19een green and partially seasoned wood . Up
to frd
0101, i' drying condition must be moderate and con-
stant and after which the drying may take place more rapidly .
Technically this point in the drying is called the fi -
io.ew saturation point and practically it c9rresponds roughl y
to moisture content of 25 per cent based on the bone dry
weight of the wood .
There is no way of knowing when the lumber in the
ltirto
*1rAqW[
~
I
how much.'4agpistgOto tie is left in the
p-ort
a and no
-
. ~.
stgnifi
} ce- in
this
•d .
connect
knowin g
The one criterion of regulation is the moistur e
content of the lumber and its fitness to withstand mor e
severe drying conditions and never the number of days tha t
the lumber has been in the kiln .
It Pays to Watch Behavior of Lumbe r
]hiring Kiln Dryin g
Brain work in connection with kiln operation show s
up more than anywhere else in watching the effect of the drying on the condition of the stock itself . To go into a worm
moist kiln, especially on a hot summer day, and look for bur face checks and other defects is not the most agreeable thin g
in the world but it is worth while when a kiln load of expensive lumber is at stake, In the old-time kiln operation t o
spend any time in the dry kiln was never thought of, in fact ,
in most kilns there is no way to get into the kiln except th e
main door be opened, thereby letting out the heat and exposing the lumber .
A furniture man when consulted regarding his trouble s
in kiln drying green oak was urged to cut a small door in th e
large kiln ' door so that the operator could get in easily t o
watch the condition of the stock from day to day . His contention in opposition was that a small door was a nuisance ; tha t
it was continuously getting out of repair and that it so
r
wea, ,erred the big door ti
thre-e~years ,
it
pieces in two o r
1
~t him around $1000 . '
The gook in his dry-ni.
He c_oult. have bui *-:fir
w kiln dooms
ien the old one was to o
weak, for, l00,,,bt t t ..e lumber in the ktl* , , $.ccording to thi s
business man's reasoning was not worth the
a !batter
' =miT or .
It i s
.
recoil that his $1000 worth of tom was a tota l
loss, but -that his
ition .
doris still in good ,
What 'gre some- of the thixigs th
i,etf be seen ,b y
tally all of
watching the stbiek? And the answer is : Pra
the things that result in io'ssed off" lumber during kiln drying .
Symptoms`'can _'l e seen and they can Ie -fought just as a physi cian fights the syrmpto
of disease . If the temperature i n
the kiln is too high and the lumber too green ,the stock arheck s
and ca2ehardens■ . Maybe the class of work is such that a few n
,
surface checks do mot- count .
the cheeks will ca•os-e
Msbe •the opera
r,rin:g the x
aii.nd
of the
Ea r tha t
r
closes these surfae•e c-heeks•, Uau ly a compressio?i -
.t
.1-i1
th e
surface and- a tension bra the inside w1 ch'3s c m-%t,c c1 • -ol aharden• ,'r f`;
ing . And the force 'which is -powerful enough to
surface ' checks is alO-o po rerfu_i eri
9
ose
gh to cause a -
rupturing of the fIb,erS or ho eyoothbing, wh- ., happens
r' r+ `
,IPi
. J: `
:
wlv
~'r' f i
-pied to centiY~- . When' gieck s" 'under th e
s compre gsi'®b, t
ia.
b ,v to detne-lro , '
: 4
71
;
Stages in the development of honey-combing from
a surface check which has developed early in the dryin g
and allowed to close up without proper steaming treatmen t
.ri the dry kiln`. The application of steam at the proper
s . me prevents honey-combing .
The sketches are to be regarded as imaginar y
cross-sections cut through the same check during successive stages in the drying . .
at the surface - the danger sign is Dint .
T%l operttor mus t
be there to see the sign .
W71's e Steaming is Both a Pound o f
PreventionandOunce of Cur e
The remedy to be applied to 1umbe T. Which showis de fects and danger signs during drying is a comparatively re cent one, not cowpl,stely understood but of broad s apt)O.icat:imn, '
It consists 'of the application of saturated ,
and great value .
steam for a period of time and at a temperature wh4oh ,Lm . the
judgment of the operator suits the _case at hand . The ap04:4gov.J. .i 7 ,
tion of this curativ
principle =IPA, be understood before ap-
plied because, like most effeOtive triatnemts, it can work (let riment as well as benifit .
This principle maybe used at the beginning off' th e
run to heat the . lumber through to the center befoTe . 4An g
from the outside begins ; also in the case of partially se gIO&
ed stock to relieve the tendencies toward casehardening which
have been set up during air drying . As such it ip often *QUIP
preliminary steaming . A steaniing or sweating of this .a-tu,ilo
is usually for a period of about 6 hours per every Awh i n
thickness and at a temperature of only about 25° higher tha n
the temperature at which the run is to start .
'lL
1,
.
of .steami.n.A_ fur , .ik piras
.
,".,.
•
L6
259
of
7M
I
.11~
The
practic e
days at the: i et&rt. .i:g a
1~ '
big waste of drying time ; does not accomplish more than ca n
be accomplished in a few hours and actually damages the stock .
To kill mold that develops on the lumber in the kil n
when the drying conditions are mild, an application of saturated stream for an hour at a temperature of 165-180 degree s
is most effective .
The most valuable application of the remedy is t o
lumber which has become casehardened to the extent that th e
wood fibers tend to give way to the stresses set up or whe n
the surface checks show indications of "pinching in . "
It i s
necessary to steam at a temperature of 16o-180 degrees
for
a
period of one to several hours, depending on the severity o f
the case as judged by the operator . This saturated steamin g
at high temperature moistens the outside of the wood and reduces the stresses and the hard bony surfaces that have bee n
set up in the wood fibers . With these stresses removed, th e
danger+ of honeycombing is gone and the drying may continu e
according to schedule until perhaps the stresses become serious again ; necessitating another application of steam . When
the lumber is ready to be removed from the kiln and it is severely• Gasehar1lenedr with the teideric to: Warp after machinin g
upanr :
in, the milop-s and
itEsawing, prap .er application of saturate d
steam re r'ves the casehardening s ei- 4p a►d' greatly improves th e
ual t
..
..12-
Operation of Kiln Most ImportantElemen t
in Kiln Dryin g
Operation means knowing the exact temperature any ,
humidity. at ' which the lumber is actually drying all- Over . the
e
kiln,
,
it means knowing how the wood is reacting to the con -
ditions as told by symptoms such as checking, casehardenin g
4.
and moisture distribution .
I";
,When these things are appreciated and . knowri• .•accurately, the art of kiln drying lumber will have taken a step .
forward `and more general commercial success will' be realized.
I.
Y