Ventilation in a dry kiln


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Ventilation in a dry kiln
Physical Description:
Mixed Material
Loughborough, W. Karl
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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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 29357363
oclc - 756701527
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Full Text
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wall,.Il' MENIl(ll~t

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No. IP1205

Madison, Wisconsin
In Cooperation with the University of Wisconsian


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A n.-"' point of view for an old problem often helps to solve the
cr-blem. The enrinecring point of view, as distinguished from. the
empiric'il one, 'ill be some.-hat ne-r in v-.'ork on the problems of ventila-
tion ;L..d circulation In a dry kiln, and makinE use of it sho.Uld help
matters. In conformity '.7ith his point of viewr and method of attack, the
enic.i r first determines the la;s that govern an action and then applies
t?.-., td since the r:neral principles covering ventilation in a dry kiln
are 'ell know our first task is merely to select the proper ones.

Althou:n it is trae that the laws governing the flo7 of air are
pcrh-. s less xnd.-rstood than those of almost every othcr branch of
cncn. 'r> for air flo', at pressures as 1o-7 as those found in the d&r
kiln a t.horou:.. uindcrstaunding of thormodynu:"ics is unnccc's'-ry. -.hc
s' J- forila comr:onlly used for the flow of 7.atcr may be used for the
flowi of P-ir without introducir.E-g -iny errors of practical -m.-gnituide; con-
trary.r to an ideca th.rt is all too prevtlnt, thcrc is no Mystcry about the
The -.:ri-ar purposes of this article are to dir.:ct morc Attcnr.tion
to the 'rhblom of ventilation in a dry kiln and to urgc consid-ration
of the problem 7ith the analyticz,.l method of the cngincer. Carrying out
th.cso purposes .-ill require sho'iine how the uci-'hts of air arc .affectcd
by various conditions, such as tc mpcrc tur rclntivo humidity, rund b.aro-
nactric prcssuro. The n, xt stop -ill then be to sh)"- hor the ch s in
might of .ir affect vc.ntilition in dry kilns of various typcs, Morc-
.vcr, t.: function of ventilation and also the bcari:-.g v:-,-.tilr.tion hs'.
o:n clrculrtion in a kiln will be discussed. The mrattor of circ-ilrtion
*-ill b- in a later article.

Since this n-rticlo deals pri-.rily "ith vcntil-tlon, a cler
.dcst :di:-.. of the difference bct-c. ventilation r-.d circullatio:- is
dcsir'.bl. Ventilation is the roncunal of go-'d : ir in a s-bst-..t.i-.lly
closc 3- -cc, such as a d-ry kil;n, b;- the (1.-issior. of fresh air o expulsion of old air. Circulr.tion, on the other h-n.d, Frim-ril:,r -i -3
a (oi-C "rour-.d in circl., -nd it al- .. Involvs the idc. ':f a
as d ist.. shed from a rcpl.acing mov..-,cnt. It is crcly the tr.-vli of
thc air within a closed space,:-.d, ulic v .:t il-.tion, in its'. If h-s
noth'.- to do .ith cith.;r the d.iszion f fresh "ir fror. the outside
:r the expulsion of used-up air from the inside.

r:'bKls.ed-. in 'ood Products, IMovember --nd Decer.ber, 1931.



The weight of dry air in pounds per cubic foot may be calculate,'
from the formula

0.0028862 b
1 + 0.0021758 t (1)

where b is barometric pressure in inches of mercury and t is temperature
in degrees Fahrenheit. A slide-rule calculation gives the weight per
cubic foot of air at 160 F. and a standard barometric pressure of 29.92
inches of mercury as 0,0641 pound per cubic foot. To find the weight of
moist air, formula (1) is modified by the subtraction of a quantity to
take care of the effects of the pressure exerted by water vapor, thus:

0.0028862 b 0.001088 e
1 + 0.0021758 t (2)

where b and t have the same meanings as in formula (1) and e is the vapor
pressure in inches of mercury. For example, the weight of a cubic foot
of saturated air at 160 F. and a barometric pressure of 29.92 inches is
by substitution

0.002S862 x 29.92 0.0010gg88 X 9.6500 0.05637 pound per cubic foot.
1 + 0.0021758 x 160

Fortunately, it is rarely necessary in dry-kiln problems to cal-
culate the weight of air under various conditions. Every handbook on
the subject of heating and ventilating gives in tables and curves the
weights of air in almost every condition of temperature, relative humid-
ity, and barometric pressure. The two formulas just presented will
serve their purpose if they show how the factors involved affect the
weight of air; the higher the barometric pressure the more the air will
weigh, and the higher the vapor pressure and the temperature the less it
will weigh. Figure 1 shows how the weight of dry air changes with tem-
perature, how the weight of water vapor necessary to saturate a cubic
foot of dry air and vapor mixture increases with temperature, and finally
how water vapor displaces dry air in a mixture of saturated air and thus
reduces the weight of the dry air and vapor mixture per unit of volume.

Air at constant pressure, expanding as it is heated, changes its
volume almost exactly in proportion to its absolute temperature; for kiln
temperatures, add 460 to the thermometer reading to get the absolute



temperature on the Fahrpnheit scale. Expressed as a formula for use with
ordinary conr.ercial temperatures, c the volume at any temperature
arnd l1, the volume at any different, temperature tD, have the following

q = qo 460 +t
460 + to j (5)

The relative volume of a given weight of air at different tempera-
ture., is pictured in Ficare 2, B; air at 10O F., for example, has about
21 -r cer.t greater volume rer unit of weight than air at 70 A similar
rel-tion holds for the %-'eights w and 1o of a given volume of air at tw7o
ter-.J.rritues, under constant pressure:

tew(.4 ;.c. t+ t
W6 = w C' + t (4)

.Then water varor is admitted into dry air under constant press-ure
the vapor dLsplaces the air. The amount of air displaced depends on the
pressure :-.f the water vapor, and the total pressure of the m.ixt-ire is
therefore unchar.ged; it is merely the constant sum of the pressures ex-
erted by the dry, air ind the water vator. Thus, becCause water vapor is
lighter than dry air under the same conditions of temperature and pressure,
th,. weight of air per cubic foot is decreased by an aniount equal to the
difference in wei-ht between the -7ater vapor and the dry air that it dis-
Ilaces. Finally, at 212 F. and normal barometric pressure, there is no
dry air present; vapor pressure alone balances the barometric pressure.
FiF-:re 2, A, sh'-.,s the relation for a wide rani-e of temperature.

At 1900 F. a pound of dry air with the vapor needed to saturate
it occuries 3.23 times as much space as a pound of djry air at 70%
(Fig,. 2, A.) This fact has some significance in the determination of the
relative size of the inlet and the outlet ducts of a ventilated kiln.

F:1ire 3 shows the relationship between the space in saturated
air occup'ied by a pound of water in vapor and in liquid form. At 2120 F.
the ratio is 1670. Figure 3 gives some idea of the large volume of air
th:..t must be put through a kiln in removing. 1 pound of water from it,
and how the required amount of air is affected by the kiln temperature.
The cirvc brings out plainly the fact that the lo'-er the temperature is,
the -r--riter the ventilation r.uist bc.



Kiln men often talk about moisture content expressed in percentage
of the weight of the oven-dry wood, but few have tried to picture what
moisture expressed thus means in terms of absolute water. The amount of
water to be removed from a kiln in a given time must be known before the
required ventilation can be determined. A simple calculation will give
us some conception of how much ventilation is required to remove the
water evaporated. Suppose that a kiln charge of 35,000 board feet of
air-dried red oak has an average moisture content of 35 percent when
entering the kiln, and that this lumber will be dried down to a moisture
content of 10 percent. The approximate weight of a thousand feet of
oven-dry red oak, which may be calculated from its specific gravity, is
2900 pounds. In drying 35,000 feet from 35 percent to 10 percent moisture
content it will be necessary to evaporate 35 x 2900 x 0.25 or approximately
25,400 pounds of water. A gallon of water weighs 8.34 pounds and there
are about 50 gallons of water in a barrel. Roughly then, 61 barrels of
water, nearly a carload, must be evaporated from the 35,000 feet of air-
dried oak.


Having discussed in a general way the various conditions that
affect the weight of air, a preliminary matter, we are now in a position
to consider the ventilation in a dry kiln. It is often said that air in
a kiln moves by suction. Ordinarily this is a misconception. Air in a
kiln moves primarily under the action of its own weight. When a volume
of air -rows heavier than the air around it, for any reason, it flows
downward just as a liquid or any other ras would do under the same cir-
cumstances. Adequate appreciation of the effect of the weight of air in
causing movement of air is necessary for a clear understanding of ven-
tilation in a dry kiln.

Suppose, for example, that the temperature of a kiln is 150 F.
while the outside temperature is 60, that the vertical height from inlet
to outlet is 25 feet, and that the outlet is closed so that no air can
escape from the kiln. The weight of a cubic foot of air at 60 F. is
0.0764 pound; hence the weight at 1500 formula (4) is 0.065 pound. The
difference between the weights of a cubic foot of air inside and one
outside the kiln is then 0.0114 pound, and the actual difference in the
weight of the two 25-foot columns of air will be 0.285 pound. The inter-
val column will be driven up against the cover at the top of the chimncy
with a force amounting to 0.285 pound per square foot. When the cover
is removed the hot air will flow into the atmosphere because the pressure
on the lower side of the cover exceeded the pressure on the upper side.
As long as a temperature difference continues, air will continue to flow
from the atmosphere through the kiln and into the atmosphere again, thus
ventilating the kiln.


This illustration points out two things. One is that the greater
t'-.t difference in air temperature between the inside and the outside of
the kiln, the greater will be the ventilating action. The second is that
the' lower the intake with respect to the outlet, the greater the difference
b't: "-.n the weights of the two columns of air and consequently the greater
the ventilation.

An experimental kiln 13 feet high was built at the Forest Products
Laboratory, so that cold air could enter at the top, middle, and bottom
of the kiln wall and could leave the kiln through outlets built at dif-
fer-nt cl'vations in the kiln chimney. In operation, air is permitted to
ent,.r in turn at the there. inlets and each time to escape in turn at one
of the three outlets* Anemomuter readings in the inlet, taken when the
tem,'crature of the kiln was 150 F., tend to show the effect of chimney
height on ventilation. WMhen the air was taken in at the bottom and
allowed to escape at the top the anemometer gave a wind velocity of 360
feet per minute. Reversing the process, allowing the air to enter at the
top and dischar-e at the bottom, made the air velocity so slight that it
would i.:.t turn t.- anenomet..r over.

Yet inferring from. this that kilns that take air in close to the
roof and discharge it near the bottom will not dry lumber is not correct.
A satisfactory location of inle-ts and outlets in a kiln depends vEry
largely on the amount of ventilation rcquircd to remove the moisture
:vaporated from the -,ood. Obviously, the ventilating system in a kiln
used for drying the grcn sapwood of softwoods should be designed so as
to obtain a rapid turnover of air in the kiln. In kiln drying air-dried
hard.;oods, however, the location of the inlet and outlet ducts is relative-
ly unimportant b-causc only a small mount of ventilation is then necessary.
If a kiln can dinchar;-e the moisture evaporated from the lumber that is
all th1cit n-cd be .-xpccted from a ventilating system.

The standard formula

"2 ..h (5)

v",,. -re V is the velocity of the air mover-e.r.t caused by difference in den-
sit.-", the acceleration of gravity, and h the head producing the V1ir
flo", sho"s the factors that must b.' considered in the manipulation of
ventilation darmers d'uring-- the drying operation. Removing large quanti-
ties of air tr-nls to lower the relative hur.idity in the kiln, while a
hii-h rate :.f evaporation tcnds to increase it. Th.- higLcr the, temperatures
ar., the relative b':-.lty, the -r-iter t:-.c ?. da.---rs, in its ss..r.tials, is merely the problem ,f vcn-tilating no :-ore
air than the amount net-ded to reduce, the relative humidity to the value
desir,- '..

'The function of ventilation in a dry kiln is to remove the moit:r.--
that evaporates from the lumber. After the moisture has boon converted
into vapor it must escape from the kiln in some way, either from the doors
or through properly designed and adjusted ventilators.

The quantity of air moved for this purpose is perhaps larger than
we orAinarily think it. For example, our calculation showed that 25,400
pounds of water had to be evaporated from 35,000 board feet of oak in
drying it from a moisture content of 35 percent to one of 10 pcrcen:t.
Since lumber can not be dried to low moisture values if the air leaves
the kiln in a saturated condition, let us suppose that the average tem-
perature and relative humidity of the air as it leaves the kiln of 150 F.
and 50 percent, respectively. The weight of saturated vapor per cubic
foot at this temperature is 0.01030 pound. At 50 percent relative humid-
ity the weight of the vapor will be just half of this or 0.00506 per
cubic foot. Then 5,020,000 cubic feet of dry air must leave the kiln in
order to carry away the evaporated water. Supposing that it takes 12
days to dry this kiln charge, an average of 292 cubic feet of air must
leavc th. kiln per minute, either through cracks around the doors or
through the ventilators.

Possibly the air inlets and outlets in a given kiln do not func-
tion properly, making it easier for the air to escape from the kiln
through cracks at the tops of the doors than through the ducts provided
for the purpose. Now the pressure of the heavier outside air forces the
lighter warm air out of the kiln. If a certain volume of heated air is
forced out, it is forced out by an equal volume of cold air coming into
the kiln, and if the air enters through leaks at the bottom of a door
the temperature near the door will be lower than that in the center of
the kiln. Some operators often think it more expedient to make the doors
tighter than to provide larger air ducts of better design. Spending
money on new doors is no solution of the problem of producing uniform
temperatures in the kiln. The procedure is wrong in principle. This
type of kiln must be ventilated, whether some moisture is removed through
condensation or not. If air does not enter the air intakes satisfactorily
and if the doors are made air tight, what will displace the vapor in the
kiln? It is better for the air to enter around the kiln doors than not
to enter the kiln at all.

On the other hand, the local chilling caused by outside air
entering a dry kiln improperly is bad in every respect. T-king an ex-
treme case for illustration, suppose that the kiln drying the 35,000
feet of oak just mentioned has no ventilating ducts, all the air enter-
ing through leaks around the doors. Then the heating coil nearest the
door will have to supply ten times as much heat as the other coils in
order to warm the entering air and keep the temperature of the kiln charge
uniform. It can not supply so much heat, and in consequence the lumber
near the door chills and fails to dry. Although this illustration ,is
somewhat extreme, the general fact is true of all leaky kilns. Lack of
uniformity in the heating requirements along a kiln rnpk3s it practically



iTmos.i-ie to maintain a uniform temperature throughout the charge. The
solution of the problc is to provide arple intake ducts designed to
distribute the air along the length of the kiln. In this way the heating
r'- .:irc::,r. nts of each unit length of the. coils would at least tend to be
the sn. To prevent air from entering around the doors without providing
a ifl.ce herere it can enter is poor practice.

Because of the effect of tcmpcra-turc on the density of air it is
L-rcGpibic to establish ;7s groat a difference between the weights of
insi i'- and of outside air when the dry kiln is operLted at low tonpcrat-.ires
as r.'hen a high-tu=perature sch,'dule is followed. In other -.ords, it is
difficult to ustablish a rLpid r-.te of air discharge frc, a ventilated
kiln 'when s'-ch a L:iln is o-cratUd at low temperatures. For ,xar.plc, to
c:cr-'.it cir "'t the rate the cross-sectionrl. area of ventil-tor s-.pace
will have to be about one-third larger for a kiln t<-cpcrrturc of 2l?0 F.
tChna fcr one of 150. Hcnc(, it is difficult to operate a vcntil:tud ':iln
sLtisfactorily when the species of wood b-Ling dried demands a lo-tcmper-
ature s ch c duelo.

.tter vapor is lighter thTi the air it displaces. Hence,th n hihor
the relativCe humidityy the grL.o'-ter the rate of ventilation. At a tzmpcra-
ture of 1500 F. and with oth-:r conditions constant, the draft causoc by
saturated air is 60 percent greater than that caused by dry air.


Ventilation, of course, plays some part in the circulation )f a
nat-ral-draft kiln, but often too much is said about where the air leaves
arnd 7"here it enters Lni its effect on circulation; this effect is small.
The f1-riztion of ventilation is to get out of the kiln the moist,:re evar-
orate!1 fr:-. the lum-ber. If it also induces a certain amount of circula-
tion so the better, but that is not the proper function Af ven:tila-
tion; ircr.3asint the ventilating characteristics of the kiln for the ,rine
purpose of sti=alatinE the internal circulation is Fener'illy ExTrensivo
anr.''. ? ;.V- :s is an in-fficierit mieth:d movinrE. air.


The most i-port.At fact to keep in' vhen consiLerirn ven.tila-
tion in a dry kiln is that the air in the kiln moves -inder the. action of
its :'r. weight. Both the desi.-. r a.. the o:-crat:r sl-.culi underst -. id
this clearly. 3--tter kilns *-.ed better dr.'i-.- should come from proper
attention to this one f-ct.

R12. -.5



0.08 a


0 .0 -----------_ __-----------__ _ _ ___

o 0.06

w 0.05

o 0.04




0 -
0 40 80 120 160 200 240

Figure i.--Weight at normal atmospheric pressure and at various temperatures
of -- A.--A cubic foot of dry air. B.--A cubic foot of a saturated
mixture of air and vapor. C.--The vapor necessary to saturate a cubic
foot of dry air.


320 ---.



o //

U 160

120 000



l*J4 ii3 (F

0 40 80 120 160 200 240


Figure a.--A.--Volume at various temperatures of i pound of dry air plus
the volume of the vapor required to saturate the &ir, expressed as a
ratio to the corrempo0ding total volume of dry air a2d 0por at 70 F.
and normal atmospheric preaeure. B.--Volume of i pound of dry air at
various temperatures expressed aa a ratio to the volume at 7o0b F. at
normal atmospbhric pressure.




0 2 ------- _--------------------------------------

o IO
0 100
V) 80





0 40 80 120 160 200 240

Figure 3.--Ratio of the volume of i pound of water vapor to the volume
of i pound of water at the same temperature. The volume of the
water vapor is independent of any air that may be present.

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