Function and calculation of ventilation in drying compartments


Material Information

Function and calculation of ventilation in drying compartments
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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Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 29511493
oclc - 757554012
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Full Text

Infurmatin Ucvkit td and RU lffirmcd
March 1954

No. P1265

Madison 5, Wisconsin
In Cooperation with the University of Wisconsin

Digitized by the Internet Archive
in 2013


IN DRn:, COP..'.;7T


0. W. TORGESC2', Engineer

In kiln drying ',;ork, ventilation should not be confused with circula-
tion. Ventilation refers to the passa.: of air into and out of tne drying
compartment to dispose of the evaporated moisture. Circulation refers to
the internal movement of air which is necessary to carry heat to the dr;i"..:
surfaces and to- replace tne neat lost through the roof, walls, and doors.
Ventilation is necessary to maintain scheduled relative humidity conditions,
while circulation is needed so that t'ese conditions can be maintained, as
uniformly as practical, within all parts of the compartment.

In kiln drying green lumber, a great deal of heat would be wasted if
all the air necessary for a satisfactory rate of air circulation were drawn
from the outside, heated, humidified, and tnen discharged after having passed
through the load'only once. For that reason, most of the circulation should
be provided by recirculation and the amount of fresh air taken in should be
sufficient to absorb only the evaporated moisture when raised to kiln cordi-

Modern kilns are equipped with fans to recirculate the air, but these
may act also as exhaust fans by taking in and discharging air through vents
located on both vacuum and pressure sides. The desired humidity is maintained
by opening or closing these vents either manually or automatically. A sepa-
rate ex.hauist fan and motor, automatically operated by a hygrostat or by a
wet bulb thermostat, can be used also. Steam sprays are used ordinarily to
humidify the air in case of overventing or if, because of leakage, the amount
of evaporated moisture is insufficient to maintain the desired humidity. In
an automatic system the sprays should not come on until after the vents are
closed or the exhaust fan is shut off.

The problem of estimating the amount of venting needed involves a
knowledge of the drying rate, and of the outside air conditions as well as
those within the kiln. The basis of the calculation is that the amount of
incoming air must be such that, when heated to the kiln temperature and
mixed with the evaporated moisture, the relative humidity is raised to that
called for by the kiln schedule.

The first step is to estimate the amount of moisture loss per minute.
This value when divided by the difference in moisture associated with one

Report 17o. R1265

unit of inside and outside air, gives the amount of fresh air to be taken in
per minute. :.' sing 1 pound of dry air as the unit, the c-onFlication due
to volunme charges is eliminated. ,xr.re3sed in terms of an equation:

V1 V2

A = .'.)unt ef dry air to be toker, in per minute (pounds)
R = Drying rate (pounds of water per minute)
Vi = Amount of vapor per pound of dry air in kiln (po.rz:.ds)
V2 = Amount of vapor per pound of dry air outside (pounds).

The last step is to convert "A" to volume of air by multiplying by
the volume (cubic feet) of 1 pound of dry air plus its associated vapor at
the temperature and'humidity of the outside air.

Air properties in tabular form are given usually for either dry air
or fully saturated air and those for partly saturated air must be estimate I
or computed. Figures 1 and 2 present these values graphically and can be
used with sufficient accuracy for computations such as these. To illustrate
their use in computing the volume of outside air to be taken in, certain
assumptions will bp made and the corresponding values and results tabulated.
To show that the nPeded amount of outside air varies with outside air condi-
tions, two possible winter and two possible summer conditions will be as'umeJi.
The winter and summer conditions will be represented by relative humidities
of 90 and 50 percent, respectively, under a winter temperature of 20 F. an-i
a summer temperature of 80 F. As the need for venting at any one set of air
conditions "'aries directly with the drying rate, only one rate will be given
in the illustration. A rate of 0.5 pound per minute will be assumed as being
that possible from 1,000 board feet of 1-inch sugar maple during the initial
stage of drying ur.ndr a temperature of 130 F. and a relative humidity of 80
percent. It will be assumed also that all the evaporated moisture is r.Lxied
with the incoming fresh air.

The results of the computations given in table 1 show that 108 cubic
feet is the mc:iim'r, volume of fresh air needed under these particular
conditions. Fresh air at '0 F. and 90 percent relative humidity, when heated
and humidified to 130c F. and 80 percent humidity, increases in volume from
1L.O cubic feet per pound of dry air to 16.9 (fig. 2). Using this ratio of
expansion, the 108 cubic feet of fresh air is incrE..sLd to-130, which, if
passed through 1,0`0 board feet of L/h lumber piled on 1-inch stickers to
a width of 4 feet, results in an air velocity of 6 feet per minute through the
load. This is approximately only 2 percent of the air velocity cnrr-.n in
present day fan kilns.

If both the kiln temperature and humidity are lowered until the amount
ef vapor per pound of dry air is the same as that of the outside air, then
the amount of needed venting as computed above becomes infinitely large.
This, of 'course, is a fallacy because, as the jrount of venting increases,
more and ,ore of the evaporated moisture passes directly out of the kiln, ,r.d

Report No. R1126-


less and less remains to be mixed with the inco.min.:*; air. For that reason
the amount of evaporated moisture to be mixed with the fresh air should be
reduced by a percentage based somewhat on the ratio of ventilation to circu-
lation rates, and the limit is reached when items B and C in table 1 become
zero, meaning that the air passes directly thro'-.-h the kiln with no recircu-
lation occurrin-g. For this reason the accuracy of the results as computed
is greatest where the required venti:-I is only a small part of the total air
circulation through the load, but becomes increasingly erroneous as the cor-
puted value approaches the desired circulation rate which represents thc
upper limit of venting.

To summarize:

(1) Ventilation refers to the disposal of the evaporated moisture by
vent in .

(2) Circulation refers to the rate of air ilovement through the load.

(3) Th amount of needed ventilation is governed by the drying rate
and by the difference in t>.e amount of vapor per pound of dry air within and
without the kiln. Ordir.arily the needed ventilation i3 only a small part of
the desired circulation and recirculating fans should be provided which are
independent of the venting system.

(4) The fresh air requirements as computed here are greater than
needed because some of the evaporated moisture is vented without being re-
circulated. The greater the amount of venting, the greater becomes the
error, -he limit is reached when the needed ventilation is equal to the
desired circulation rate. In this case, the proper kiln condition is 'at-
tained simply by heating the outside air to the kiln temperature and no r -
.irculation is necessary or possible. *',here the desired kiln condition is
such that the unit amount of vapor in the kiln is less than that outside, a
condensing system must be employed to remove the evaporated moisture.

(5) Best economy can be secured by automatically controlling the vent
dampers or ventilating fan to prevent overventing and, therefore, unnecessary
heat losses.

Report No. RP1265


Table 1

: 1

2 3

: 4

- - - - - : - - - - - : -:- - : - - - - -
Kiln Outside conditions

130 F. : 20 F. : 20 F. : 80 F. : 80 F.
:80 percent:50 percent:90 percent:50 percent:90 percent
Relative : relative relative : relative : relative
Humidity : humidity : humidity : humidity : humidity

(A) Amount of :(Figure l):(Figure 1):(Figure l):(Figure 1):(Figure 1)
vapor per pound
of dry air 0.085 : 0.001 : 0.002 : 0.011 : 0.020
---------------- ------- ---------- ---------- ------------S
. . . S: . . .
(B) Amount of : :(1A 2A) :(1A 3A) :(1A 4A) :(1A 5A)
vapor per pound
of dry air to : 0.084 : 0.083 0.074 0.065
be added

(C) Amount of :
moisture added : 0.5
to fresh air
per minute :

(D) Amount of : :(IC -+ 2B) :(1C + 3B) :(1C 4B) (lC + 53)
Ary air to be ,
taken in per : 5.95 : 6.02 : 6.75 7.69

(E) Volume of :(Figure 2):(Figure 2):(Figure 2):(Figure 2)
1 pound of dry
air plus vapor 12.10 : 12.11 13.83 : 14.03
(Cubic feet).

(F) Amount of :(2D x 2E) (3D x 3E) :(4D x 4E) (5D x 5E)
fresh air to be
taken in per : 72.00 73.00 : 93.00 : 108.00
minute per
1,000 board :
feet : :
(Cubic feet)

Report ';o. R1265


S/ - -- ------
0.32 -

< 026 --- -/ -t--

k 0. ..... .......24 .

o -. -. - .-..

0.14 0.. )

0.04-- o ----- -


0 /0 20 30 40 50 60 70 80 90 /00

Figure l.--Weight of vapor (pounds) per pound of dry air at various
combinations of temperature 3r.d relative humidity and a
barometric pressure of 29-92 inches of mercury.

/3 .-.0-

--- -- __----- 10 --------- -------__________

/ L ______- ---7---- 60__--------------_

0 o
_________________________-------- 4Q7-----0-
-J60 i

-t---- 10
------------------ 0 --------

30 40 50 60 70

80 90

I I I II i t1111111111 11 I111111111111111111111 111
3 1262 08866 6259


re- 2.--Volume of air alnd vapor (cubic feet) per pound of dry air
at various com irations of temperature and relative hu-
miditty and a i arornet ric pressure of 29.92 inches of mercury.

z 4 3655 F