MECHANISM OF AIR CIRCULATION IN LOVSTED
DRY KILNS
t%
C. LO VS TED 8 COMPANY, INC.
4000 WEST MARGINAL WAY
SEATTLE 6, WASHINGTON
OFFICES IN
VANCOUVER B.C. HONOLULU T.H.
SAN FRANCISCO LOS ANGELES MANILIA R.P.
SEATTLE
rLTA:A.la.61T. OY AIR C:LflGUTATICra
LovsT1-:D ULMER RaY EILNS
Ti-c?ifv;.T
The brisk an3 uniform air circulation in the kilns is the k•ynove
to successful drying of lumber on a commerical scale
There is definite trend in modern lumber drytn toward the use or
high air velocities.,
In this report however, no attempt will be made to discuss effeot
of air velocit; on drying rate of lumber in dry kilns. This report will be principallS one covering mechanism of air circulation
in modern kilns. – oreover the dry kilns with internal disc fans,
and crosswise circulation will bc considered only.
,
All rroblems involved will be treated from the standpoint of these
engaged in the r3anufacture of dry klin equipment.
N6M.
mum- IN DRY YILNS
wrosanveopm ,.....wae• ...........w...m...*ge....oes.e.a....ewamos*.ovreaames.
In any dry kiln one will find an afrway or duct prolidod for
air circulation. In modern disc fan kilns with air recirculation
this duct is not stra1 ,1th but form a ring circuit to rettur n air
to the fans. At a certain point of this duct there is always a
rartition installed across the duct with fans in the openings
(see Fig.
c
g
FA
A -A
5
A
T 0-
This partition cart b
either of straiglat
zigzag pattern. SOMOtimes oven it i s built
with more complication
--it all derendo on do
sign„, But It is important •that this raztiticz
be built in all kilos ly,;
disc fans,„
%psi : 7,)(;,„
v
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• I
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V.10
tF
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f,
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AIR FLOW IN THE DRY KILNS
When the fans are standing still-in a cold kiln the air pressure
in the duct is the same throughout. But when the fans begin to
force air through the openings in fan partition, they build up
pressure on one side of this. partition and vacuum on the other
side.
The air in the kiln starts immediately to move from points of
higher pressure to the points of lower pressure.
The total pressure difference across the fan is termed "total
head" and this total head is necessary to start and maintain the
air flow.
The total head consists of two components:
(1) Static head,which is spent on pressure drops re q uired to
overcome resistances to air flow in the air transmitting
system.
(2) Velocity head, which is the portion of total head required
to produce velocity of flow.
TEE STATIC AND VELOCITY YEADS ARE LUTUALLY COMERTIBLE
The velocity head depends on the square of the velocity of flow.
Velocity of flow in its turn de p ends on cross-section of flow.
If, fce instance, the cross-section of a duct is increased twice,
the velocity will decrease in its magnitude twice also.
Let us assume that velocity V 1 . 600 FPM (see Fig. 1) and velocity
V2 - 300 FPN. The velocity heads are different also. The velocity
head in duct enlargement "D" will be four times smaller than in
duct portion marked "C". The difference will be converted into
static head. Conversely, when the area is reduced, the static head
is partially =averted into velocity head. This conversion, hcwever, as it will be shown later, is always accompanied by a certain anount of static head loss.
STATIC VELOCITY AIM TO?dL BEADS ARE MEASURED IN
IKCHES OF WATER COLITrai
TIII
I.M.NWESOMV*11a.
Theze heads may be readily measured by water gauges communicating
with the duct in the manners shown in Fig. 2.
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Te OTEPCOME OP7RUCTI;MS•IN
A.:: it was ezplained before.,.thc, veloity Ii
rake th air
and atatic prc-sure is sl y nt tc j( yroe CD.I -th:2::)ugh the :j .,d
obstructions. For a parti.cular c.fcr
fn other mcrds a definite'
pressure drop is required to ut air thrcu1i th'3
a.T2 IwIca
to the fans. The pressue drops in the air trQnsnittirc, system
are due to the • friction losses and dynamic 'oases.
(1) The friction losses are the result of friction of air Jjarticles against duet walls era internal friction between air
particles.
The pressure drop required to overcame 'the friction less
increases with velocity hae,d, roughne ,se of duct walls length
of duct s and decreases witL. Llcroase of duct cross-section.
(2) The dynamic ..)..OBEIG3 ars caused by euddea changc:S of di-.7ecUor.
of flow, or its velocity eind by eColy formations,
The prassure drops carpersailg for ramie losses in the
system a;e usually Expressed a nultplos of 'iolocity head •
(7/4000)-. 12he actLal losses. are of coura 3.f 3s6 o: sta"-clo.•
hoad,„ The velocity hea,cts vbe knew row for a given air flcv,
depends entirely upon the orosa-ect.loAd area of duo
The following are arproitimate prnssuase Crops due to the different dynamic losses
(a) Pressure drop due to abruot turn ( ( AO
tgYi eln4y./ 01.
,
..
cic WE'VEITS-17951-1---Th caribLned dyr.amic eld fricItick.
to•aa
elbow are usually expressed as eivalent to the fricti c,a
loss in c lnti Lo of similr BtragfaZ; auet.
I
(b) PrasstuK, drop
StV031713
to dvnaJdo
abrupk.lynel
1 6trierdu by 111-411117...)
467:VC air
otta±..‘ atm 1-ion
lc 2E3
The pressure drop in this case is expressed in numbers
of velocity heads lest across this obstruction.
C
Va
LP
..131•17nanrSVIOatirem4-4,S,Sley,-.2,==.
.
3
2.5o4
)2 $uctm.oF
4'0,131 4
Flq•
(c) Pressure drop due to dynamic losses l Aii?.en air stream
•
rs—dii-rd-ed —ir=
.., -i and eac-h half is ..abicaaL,
to iiinTel.v73
'3.v
M 7, . womawsx ra--vaa.--,,,rov. ■.-_.e.,-. ,Awn,APMOLI■ 31719R••••.....0,f9, wtiti.;74,1-,..V.
i
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1-175-.6-cr900
---"r".176rn Air Conditioning, Feating and Ventilating" by Carrier, Churns and Grant)
The pressure drop in this
case is, expressed in numbers
of velocity heads lost in
this obstruction,
P3
/ V
L 75;4 %
017: yilAV
4040
(d) Pressure dron__
due to sudden duct enlarement
(*INE
T51
EDDY FOR ran7,501,4r)
C6
When V2/V1
7"
,s0.0
4ge
n
ti
`*"..
etram aws. VAN.
Ne.
‘.1
•
410
0.8 - e& P4 = 0.04 (V1/4000)2inCh.•
= 006 -
- 0.16
0.5 -
0.25
7= 0.4 7
= 0.36
0.2
= o,64
ft
',-■ ■•=ma (pa:.
tt
ft
Fa C1)..
ft
Pressure drop duo to sudaen duet Contraction:
C.Luiqe,
CLIDY 17(0
117a 0;45
45E40 -,rtot
= 0,2 -. A P5 7:
I'd 11Sn Va.
0,29
0
•••••
-:-
• (71
17
0024
—.
+OM
ft
.1011
•nr. e.00.74
a 37
0.19
A
.rat.
F14-. 7.
tt
ft
0.09
fi
Tiff= AIR TRANITTTNG SYST:0413 OF W3RTILD2OLTELFILNS
(1) Mullarype Dry Kiln -- Two track kiln with overhead
fans Shdwxi
/74: k77-7.§.7,2z-7/7,tvizz---/77-92x-22A-7z4z,vivr,,--9wzg;p97
R 8.
.- Reversible R.H. and 1,441, disc fans on one long shaft installed
lengthwise. Fens are equally spaced and their size and number depends upon required circulation.
Fans facinr, lengthwise of the kiln blow air directly into
one another and zigzag fan partition turns air flow crosswise. Then air flow makes two 90 0 turns goes through both
lumber lcads r and again turns 90° twice to get back to the
fans.
Shaft with fans may be located either bcluN the lumber
lcads 9 or at their oides o or above them, az shoun.
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Rov,x47s:12)3.e
end
3an3 on :Dne shaft mowIted lon:;t1iL Fans are eclufaly E.tp e d azd thoir slzs ; ard 1=70
Cpt::fld upon x:aquircl oft' cluan,
f) failz- oT 7'i6M;
h.(1.Tap177,
7
foYi nousdls
00'3.7 kiln typos,
CSS
Iland
cir
-1,1e
ia opcnlna
fasd.ft to h:,)21-.
hols
furpoisa
a5
the bafrio
pa-ztlons
V.7f::: fans build up pzessur in
thQ eiz
houolaga
thy:ouch holo3 in the baffle dvilawal*els into the space ba
the loads. Here th e air atram is diviod into two halvor3,
One half flews through load "A" and the •other through load
"B'' , and. then both halves return to the fans.
When direction-of fan rotation is reversed there is
vacuum in the fan housings aLd pressure above horizontal
baffle. Due to these pressure changes circulation reverses
its direction.
Shaft with fan can be located either above, or below the
lumber loads.
(3) "Direct-Flo" Tyne Kiln
.....%
h ---11 -‘
1
II
rtp,...0
t..... ...r.- r-r-'-'12-.:7-----"-- ii
1 -,,,
zi
......17.1
_ ...or ... . *.E1=$.°;,,,
Reversible fans in
this type of kiln are
facing crosswise.
. Each fan is mounted
on its own shaft and
is driven by individual motor.
Uniform and ample
circulation is obtained
497$71e.WMWii t PrNW:tkleiPAVOW
by
having disc fans
ro
0 „
properly spaced the
ful3 length of kiln building. Size and number of fans are
governed by required circulation.
In this type of kiln the fans move air crosswise.
Air streams do not collide as in previous two types, but
are parallel. All these insure uniformity of air circulation.
The fans may be located on the side, above or below the
lumber loads.
tle build single track, two track, and three track compartments.
CCMPARISCW OF LOVSTED AT7:' CIRCULATION SYSTZMS
AIR
L=MOA,M11,0111...i*.•,...reC.LASZ,17.
Any air circulation o3iatram in dry kiln must meat the following
requirements:
(a)
Adequate aftx flow ";tough the 3..V.M7 CCQZ0-00 loast be
Obtained with the smallest power in p ut.
(b)
The a:21.1' flow through the lumber must be unif*rm.
Thus, the best air circulation system is the system whjiah is
capable of producing economically the seine but more uniform
air flax through lumber load in dry kiln
(
PCX/ 0,7 i
1 'VCA r
r'Cd
r' 0 ft
et- "Ili
ti 071
"
Certain laws Govern the performance) of fans. The air
delivery of a given fan, as well as its necessary power
in p ut, depenlo upoo, the static pres9ure againsti7hich
fan is corking and also on the fangs
In order to compare air circulation systems in Lovsted,s
three typos of dry kiln, let us assume:
(a) That kiln lengths in all cass are the same and equal
to 40 ft.
(b) In each kiln there are 5 - 60" dia. disc fans installed
on 8 ft. centers.
(c) Each fan is driven with speed necessary for delivery
16,000 cm of air against obstructions to air flan in
each system.
The pressure drops in systems, as they are shown on Fig. 8,
9 and 10, are tabulated below:
Pressure drop in inches of
rThuI1EF—r-rouT5fe
Pressure drops because of
type
with single
Eiln
Track Performance
(; H.r..iction losses
0.00185"
0,0503"
1
2 1Nnamic losses due tl
Ir-o lahre of air flow
leGtion„ and deceleration
and acceleration of flaw.
0.0801"
0.0201"
0,.0931"
to fan arrangement
0.1068"
0.1o8tf
None
pressure
[
0,1268n
u„.„14,37,n
WaY,31*
0,-,0902"
I
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• 3 1Du.e
.IIt.S■r,•;•Ren
sev,11,7
wrr.,,,,,rum,-
vsmr
.
MT: Lumber load 8 Vt,
x 10 ft high, 4/4 stock el 1“
The smallest pw.scuzs drop :Is :In t?te air ‘eireulation orLtol of
Double Track Pan with Oina)e Track I)c.:J1---orz:snce, but tho -elocl.ty of air flomIng tiveugh lumbm. coursos Js ens-half of that
in the Diller and "TArct-wlo" ki l ns. In i;Le last two 1:5,1nz
the air volocIty though the ii
i• vs_cotly the same
If we increaCdir velocity through the lunbor in a double
track kiln with single track i y., rforuint,3c to cct it equal
the velocities in the other two kiln typos ti'e pressuro drop
will go up, from 0.:29 v1 to
atci 0.52,6" of water.
The pressure drops due to friction 4t q: C,ynam;c losses in
Nuller and "Direct-21o" kilns are p:alctically the sane. The
total presau::e drop in Muller kiIa as well as in Double Track
kiln wit- sin- 3 Ie track performance is increased much ay the
item called r Fan arrangenent".
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74Z- ; Azo; p6•cr,
02411
,
Double. Track Kiln with
31rZ1 Track 7,7 eroyAllem.c.e.
"Dirc . et-Flo"
I n each
casz
fan
„?. 14"
In :!,71.7, 13.1.7.!I-7,77
17,1U0
60n
6:7,i'3.c. fa, u
e7:41.E1..o1-3 17,2.00 (7,7'_'1 is
16,000
o up
7't
'300 IJ
2
•
this
kln clr..1.1,1
•
r:7..cogin to
, -
In
v.,tor
ac;livor 16, C0
Thore =, $ -fans ln,
16 ,000
5 m 80,000 C:T.! per
curve •
(a)
of
0,1y,
e
r,:."),„'Ifht,iy-
A
r
47.
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,:1,C fi-1..:1
f.I,
' 3t,"1 ,,T ‘ ' I, ,,,,t 4040
1
r
-
d '
i
In estimating dy.aaaic lesson on pcu'o ide of fIrts we
have used method shorn on Fig 4 but reduced oceffL;ient
in formula from 2.5 down to 1.725 to allow for possible
velocity comp7)nent nornal to dirotion of air flow nrol.ued
•by farhl. Voir, vas dons becalm- 217:,.,y disc fan
not. p7o-•
duce puve azIal at-: f,w,, Total p :%.-sure ifirops
in
the above tatios are also the st:::n prealires fLI15 cre
workirr, agair
Tic:--- are as fcalow: •
Muller
•
-" ,,-• .,' ', ..,...,
,
11 ' i`
i
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t...-,; 1, iN C.: LC
4)
•'. ,•''
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4,, r.q,
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r • --„------ .--,,,,4!
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P.
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-,- ',-
r ' --' : 1
-Pr- -•••., -,:,
,:i'''' . '
— '—'''''' —'"--- -v't''• iii )I.
i•
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■.,‘:
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... 4
,
(p) In 'Ito Tk Min viith
pefornanc:), 60"
ci.a.catter disc fan a'',; 25C . Rg4 wIll (1,E . :Lvery 16,000 CFMagainst StatIc p2ossuro 0.13 with pewor 1.34 U,
Thwever, to bring volocity through lumber to the same
nagnitudo as in Muller and "Direc-Flo" kilns fan 1:1M
must be imreased.-up. to 500 RFY with power input
= 52,6 EP.
(c) In "Dixect-Flo" kiln 60" dianoter
fan at 300 RPM
deli7ers 21,500 CP against static prosure = 0.15"
with power input 2.1 EP.
In other words, 4 - 60" fans in "Direct-Flo" kiln will
produce the sane circulation as 5 - 6o rr fans in the
Muller tyce kiln.. Thus -"Direct-Flo" kiln - 4 c 21 / 500 = 86000 CFM
Muller type kiln - 5 x 17,100 = 85,500 Cla
Total power input for kilns 40 ft. lon8, when urn same:
velocity through lead naintained in all throe kilns, and
efficiancy of 1T-belt drive is equal to 0.85, are as
follows:
0)
"Direct-Flo" kiln - 2.,1 x.4/0,i9 x 9.33 U.P.
Muller 'Vpo kiln - 2.6 x 5/09 =1.4.4 U.P.
Double tpack kiln
with „,.--Jacle
track performance
- 5.26 x 5/09 --. 263 IIP.
'",he abovo A...t. p ures .prove that a:5.r circulatier
ion c--/--tn ..,
,^.'" ' a .ts the best vi3taim y
,J4..4
:rOm tile staLlapo
.s1A;
...anrpaaanesmar.,-2•,o17.,..,-,...on,--,,..ow ,,,7,,,atvz,aat,,,avoren.nwlmatve.o..-,,acw,,-,..4, w.n■,,, '..7/1:..,•,T,.4.14e..*.t.e:R.3,..tes"V.R.tv..,
t,i4te.f,i..,,••,,,a■,/,
01
recr.
„ . rea _power in put.
F
, .., )
A
07,
'I,' ',.: ::,,
ti
..
%
•
The Double Track Niln with sirtl track
r1cr.equire2
the greatest pc;v7rinout
ThereZok this type of kiln
usually Meta only an modernizaton jobr.1, when
with minimum cost, to change n47:a3.
oi y:culation in old
narrem two track kilns over to fc!ed eirculat4. In th5.c
case even halved vol ,:.)citya U Laprove kiln pornance vel7
muah.
(2) Uniferni.tv r."7 Oir
In I TIT:oet--flo 7-"i
:'ca
procluco air
n
theE;as.:
.af.3
croated by fazo are Dialael,
In the Mu l liDr Kiln. at16 Two Track 17-11.71
performance fans fac f.,) lencthvise
d:127c.f7,tly into :lie anoth,er,„ Then ai:c ficl
croswise.
1
Tlto
k1 rand
Stv.c,:a6
dc'...:1,7,2;cted
-Peduct Laboratory, vfi)o
• Yr, Goenbil of ,:u . -:calian
stuCtied dry kilns in U,3.„A„, ..ates: : •
The cross-shaft internal-fan dry kiln is the DOSt efficient
typo of coomercial kiln 5,1101)c:ration. ty both in ror ,,ard to
uniformity
of circulation o and quantity of air circulated
.
through timber per uni:of power consumption of the fans."
_
•
CONCLUS:ION
During the past, few years considerable research has bee:. caired
on by various organizations in an attcupt to reduce time, required
for lumber drying. There is now a definito tendency to increase
air velocity through lumber load in the kilns, or, in other wordsp
the voluBe of air circulation. This could be done
speeding
up the kiln fans t because fan delivery is directly proportional
to the fan Rpm. llowevor the required power input inoroa3ot in
proportion to the cube of RPM, creatint; power cost entirely cut
of proportion to the benefits obtained.
by
Therefo::e : there is imperative domand now for a more efficient
reversible disc fan.
The fan air delivery per U.P. of power input can be increased
as folicvs:
(1)
air can be delivered against comparatively snail
given static pressures for the same power input by usinc,
a larger fan and turning it slower.
MOPO
This explains why in. OUT North Coast Lilns we -are using
new 84" diameter disc fans.
(2) Stationary shroud ring installed in fan o pening around the
fan increases air flow for the same power input - or obtains
the same air flow for about 10% loss power cost.
The width of stationary shroff ring depends on number of
blades in the fan and nrojected width of fan at blade t;pc,
(3) Comideable . data is available on the design ofuni-lrootional
axial fans, bo there io definite laz of infolnationthat
could be dirzotly alopKed to the reveroiblo Cf.oc
Co 7,, Lovot
CT.) Oompamy Inc;
hay IlDfi:Dn U
p:s ejo('A; to fLild cut tho facto. ; afferz ellilancy of
x.-:)vorcibletUsc. fans,
°
Sp) O11.
T1) tests ar=z, run under air flow con(711Uon comparable to
*4;ho,ze under which they uoually opate in dry
We hope that la the nz.3.7: future we oh,a:J,1 be iw a pofTl.
to put on the market a new t more efficIont revarailJQ
disc fan.
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