b;z
-T--C-
Bridgtes,
G-1\,4-
\Nhrite
raditionally
mostgrainis storedin circular
bins
t h a t a r e c o n v e n i e nf ot r g r a i nh a n d l i n a
gnd
management.
Withtheexcessgrainproduction
andgovernment
loanprograms
of the pastfewyears,
s a v ec o m p l e m e n t et hde i r r o u n d
s o m ep r o d u c e r h
storages
withrectangular
structures
andcoveredpiles.
practices
Although
the management
usedwithround
storages
arestillrequired
forthesealternative
storage
methods,
in rectangular
structures
theymaybe more
difficult
to implement
andmorecriticalin obtaining
a
quality
end-product.
practice
Aeration
isonemanagement
recommended
in bothcircularand rectangular
storages.
The basic
reasons
for grainaeration
arethesameregardless
of
alv,
'a*,
''a
|
+',^^.
JU UUrUrdr rypu.
I to prolongthe storagelifeof the grain.
I to reducegrainlossdue to moldand decay.
I to controlinsectinfestation.
However,
the problemsassociated
withthe designand
useof aerationsyslemsin rectangular
buildings
callfor
differentguidelinesthan those for round storages.
Thesestoragetypesusuallyarenotas uniformin terms
of airflowas theirroundcounterparts,
and the aeration
design criteriahave evolvedfrom generalrules of
thumb.
The purposeof this publication
is to give the grain
producera betterunderstanding
of thesecriteriaand
instructions
for applying
themin rectangular
storages.
-rrd
D-G-
Thesearemoreprevalent
in flatstorages
as shownin
.1.
Figure
Someairflowmovement
willbeexperienced
in
thedeadzones,
butitmaybelessthanhalfofthedesign
rate.To offsetthis problemin flat storages,
0.2cfm/
bushelshouldbe usedas theminimum
designairflow
rate.This is twicethe 0.1 cfm/bugenerally
recommended
forcircular
storages.
Anotherconsideration
in providing
a moreuniform
airflowpatternrelatesto the grainpeakthatgenerally
(Figure
occursinflatstorages
1),Sincetheairwillflow
towardthepathof leastresistance,
themorelevelthe
grainsurface
the moreeventheairflowdistribution
is
withinthegrainmass..
F i n e sm a y a l s o c a u s em o r ea i r f l o wd i s t r i b u t i o n
problems
inflatstorages
thanin circular
bins.Circular
storages
are oftenfilledusinggrainspreaders
which
tendto distribute
thefinesmoreevenly
throughout
the
grainmass,butthismethodof fillingis notpractioal
in
rectangular
structures.
Therethe finesconcentrate
underthefillingauger,
causing
uneven
airflow
at these
pointsinthegrainmass.
DEAD SPACE
University
G R A I NS U R F A C E
t
\\
Airdistribution
is a keyfactorintheeffectiveness
of
anygrainaeration
system.
ldeally,
theairflow
should
be
distributed
uniformly
throughout
thegrainmass.However,in any storagethereare somedeadzonesand
areaswherethe air movement
is lessthandesired.
Orzerl-rr--llts
OEAD
SPACE
\
itl,
AERATION
DUCTS
DEAD
SPACE
F I G U R E1 . A i r d i s t r i b u t i o ni n f l a t g r a i ns t o r a q e .
. College
. Cooperqtiwe
of Kentucky
of Agriculture
Extension
o Horne Econornics
o 4-H . Developrnent
Agriculture
Service
Forany givenrectangular
storage,
the amountof
required
aeration
depends
onthetypeofairdistribution
pattern,
thespeedwithwhichthegrainis to be cooled
and to someextentthe investment
the oroducer
is
willing
to makeforthispractice,
Design
ofsuchsystems
includes
determination
of the numberof ducts,duct
spacing,
ductsizing,
ducttype,fanselection
andsystem
operation.
Ingeneral,
thedesignrecommendations
set
forthinthefollowing
sections
willpertain
toon-floor
duct
systems.
D uct Spacing and Number
Thenumberof ductsis generally
determined
bythe
spacingrequired
for uniform
airflow.
Twogeneralrules
of thumb(illustrated
in Figure2) areusedfor spacing
ductsin a flatstoragearea:
1. Thespacingbetweenductsshouldbe lessthanor
graindepth.
equalto
themaximum
Ascanbeseenfrom
thedimensions
showninFigure
2,Dl should
always
be
lessthanor eoualto H1.
2. Thelongest
airpathserved
bya givenductshould
be
path,From
lessthanor equalto 1.5timestheshortest
Figure2, X1 + Y1 shouldbe lessthanor equalto 1.5
times31foranyductinthesystem.
Ascanbeseenfrom
thefigure,
thiswillbea function
oftheslopeofthegrain
surface.
fu,r;
H1
"rr
tr,
l
FI G U R E2 . l l l u stra ti oonf th e g e n e raspacing
rulesfor aerationductsin flat grainstorages.
Forlevelgrainstorages
the requirednumberof ducts
by dividingthe appropriate
can be determined
storage
by the graindepthand then roundingup to
dimension
the nextwholenumber.This will ensurethat the duct
spacrngis lessthan the graindepth.Care shouldbe
takenin usingduct systemsdesignedfor levelstorages
in situations
wherethe grainsurfacehasa peak.These
systemsare seldomadequateif the surfaceof the grain
thata key
is slopedmorethanlen percent.Remember
elementin systemdesignis uniformair distribution
the grainmass,
throughout
2
Anotherruleof thumbin determining
duct spacingis
thatthe lengthof the non-perforated
ductsectionat the
fan shouldbe aboutthe sameas the distancefromthe
f i r s t d u c t t o t hsei d eo ft h es t r u c t u r(eX " il n F i g u r e
2).This
shouldalso be the distancefrom the end of the duct
farthestfrom the fan to the end wall of the structure
nnnncitp
tho
fanc
Duct Sizing
Oncethenumberof ductsandthespacinghavebeen
d e t e r m i n e dt ,h e i n d i v i d u adl u c t d i a m e t e r sm a y b e
chosen.Generally
thiswillbe a f unctionofthenumberof
bushelseach duct is to aerate,the designairflowrate
and the design velocity.lf the ducts are to run
perpendicular
to the peak, then each duct will be
requiredto aeratethe same amountof grain,and all
ducts will be the same size.This will also be true
regardless
of duct directionif the storagehas a level
surface,lf the ducts are to run parallelwith the grain
peak, the center ducts generallywill aerate more
bushelsandtherefore
shouldbe larger.
It is important
to properly
sizetheductssothatuniform
throughout
the
airflowand pressurewillbe maintained
tube.Typicallythe grainfarthestfromthefanwillbe the
lastto cool.lmproper
or
sizingof eithertheductdiameter
the fan may resultin excessivefrictionlossesand
moretime
unevenairflowin the duct,therebyrequiring
for the fan to coolthe grain.
To facilitate
theductdesignsothata uniformpressure
the recommended
duct designair
drop is maintained,
velocityfor a positiveaerationsystem(duct under
pressure)is 2500 fpm and for a negativeaeration
systems
system(ductundersuction)1500f pm.Positive
regainwhich
in generalwillexperience
a staticpressure
allowsthem to operateat a highervelocityand still
provideuniformair distribution.
Oncethe ductdiameterhas beenselectedbasedon
the systemdesignvelocity,the duct lengthmust be
The duct lengthshouldprovideenough
considered.
perforated
surfacearealo ensurethattheexitvelocityof
the air enteringthe grainfromthe duct is lessthan25
fpm. This value is recommendedto preventlarge
pressuredropsas the air entersthe grainmass.lf the
exit velocityis greaterthan this value,then the duct
diametershouldbe enlargedto increasethe surface
forairflow.Generally
for roundducts,only
areaavailable
B0 oercentof theactualsurfaceareashouldbe usedin
to allowforthe oortionof theduct
the abovecalculation
in contactwiththe floor.
Table1 presentscross-sectional
areasand sur{ace
Table2
areaperft of lengthforcommonductdiameters.
givesapproximate
pressures
for
different
designstatic
graindepthsand types.The followingdesignexample
uses values from these tables to demonstratethe
principles
discussed
above.
2
DESIGN
EXAMPLE:
Supposethe
center
ductinFigure
round
in
needed
toaerate
10,000bushels
usinga
duct a
positive
pressure
system.
Required
cfm= 10,000
bu x 0.2cfm/bu= 2,000cfmfor theduct.
fpm = 0.8ft2.
Ductcross-sectional
area= 2,000ctm/2,500
ar ea and s ur fac e
TABLE 1. Cr oss- sectional
areaper ft of lengthfor roundand half-round
ductdiameters.
Duct
Diameter
(in)
CrossSectionalArea
(fr2)
rouno
From Table 1 a cross-sectionalarea of 0.8 ft2requires
of 14inches.
A 12-inch
diameter
would
a ductdiameter
require
largerthanthe2,500fpmfor a
an air velocity
positive
system.
Recalculate
the air velocity
usingthe 14-inchduct
.l
witha cross-sectional
areaof .069squareft.
ft2= 1871tpm.
Air velocity= 2,000cfm/1.069
Nowdetermine
theminimum
oerforated
surface
area
to maintain
an exitvelocityfromthe ductof 25 fpmor
tess.
Minimumsurfacearea= 1871cfm/25fpm = 14.8lt2.
FromTable1 the surfaceareaperft of lengthfor a
14-inch
ductis 2.93.(Thisincludes
thefactorof 0.8for
contactwiththefloor).Usethisvalueto determine
duct
length.
perft length= 25.5ft.
Length= zl.8ltz/2.931t2
Thisindicates
thatifa 14-inchroundductisto be used
for thissituation,
the duct musthaveat least25.5ft of
perforated
surfacein length.lf the duct lengthis less
to make
than25.5,thenthediametermustbe increased
the exitvelocitylessthanthe maximumvalue.lf a halfan 18roundduct is to be usedin this samesituation,
inchductwouldbe required,
butthe minimumperforated
lengthshouldbe 38.3ft to satisfyall airflowrequirements.
Generally
the maximumductlengthshouldbe limited
to 100ft or lessbecauseof severepressuredropsthat
may be encounteredbeyondthis distance.One alternativefor buildingsmorethan 100 ft longwouldbe to
branchf romthecenterwithtwoductsor useductsf rom
both ends of the building,treatingeach half as a
separatebin.
Duct Types
Twotypesof above-{loor
ductsaregenerally
available:
Metalductswillbefoundin bothround
metalandolastic.
andhalf-round
diameters
ranging
from6-36inches.
The
numberof diametersfor the plastictype is smaller,
rangingfromB-24inches.
Severaltradeoffsmust be consideredin selection.
Firstandforemostistheamounto{openarea.Generally
metalductshaveperforatrons
equaltoaboutten percent
plasticdrainage
of theirduct surfacearea.In contrast,
pipegenerallyhas a maximumperforated
area in the
6
o
10
12
14
15
16
18
20
24
30
36
.196
.349
.545
.785
1.069
1.227
1.396
1.767
2.182
3.142
4.909
7.068
half
rouno
.098
.174
.273
.392
.535
.614
.698
.884
1.091
1.571
2.454
3.534
Surlace Area
Per ft of length
(fi2ntl
roundA
1.26
1.68
2.09
2.51
2.93
3.14
3.3s
3.77
4.19
5.02
6.28
7.54
half
round
.79
1.05
1.31
1.57
1.83
1.96
2.09
2.36
2.62
3.14
qoa
4.71
AThesurfaceareaperft of lengthfor roundductsis reducedby a
factorof 0.8for contactwith the floor.
range of threeto four percent.This should be considered
a minimum
range.
Caution:Usingductswithlessthan
reducethe
this amountof openareamay drastically
effective
tothegrainundernormaldesign
cfm delivered
conditions.
inductselection
include
strucOtherconsiderations
but
metalis stronger,
turalandcostfactors.
Structurally
plastic
remove.
Initially
to
install
and
islighter
andeasier
on howlongthe
theplasticis cheaper,
butdepending
facilityis to be used,metalductsmightbe a better
investment.
Someplasticpipeis soldwith a fabric
"sock"to keepkernelsawayfromthe perforations.
pressure
Whenusingthistypeofduct,a positive
system
is recommended
to prevent
finesandforeign
material
fromclogging
theduct.
System Operation
A negativeaerationsystemmeansthat air is pulled
downthroughthe grainmass by the fan. In a positive
forcesair up throughthe grain
systemthe fan pressure
mass.Eitherway is acceptable,
and each has some
advantages.
A negativesystempreventscondensation
on the roof
as thegrainis cooledandallowsuseof solarheatinthe
roof when warmingthe grain.One disadvantage
with
thismethodisthatthebottomlayersof grainarethelast
to cool,andit maybe difficultto knowwhentheaeration
processis completely
finished.
Positiveaerationsystemsallowthe additionof grain
layerswithoutrewarmingor coolingthe grainthat is
alreadythere.lt is alsoeasierto tellwhenthe grainis in
propercondition
becausethetoplayeristhelastto cool.
There is some evidencethat positivesystemsgive
?
betterair distribution
than negativeones,and basedon
(2500fpm
the maximumair velocityrecommendations
vs. 1500 fpm) the positivesystemsgenerallyrequire
smallerductdiameters.
Fan Selection
Thebestwayto determine
theproper
fanisto usethe
perf
manufacturer's
fan ormance
dataandselect
thefan
forthedesign
mostappropriate
conditions
inthestorage
Thismeansthatoncetheairflows
structure.
havebeen
for each duct,a measureof the static
determined
oressure
mustbe known.
Staticpressure
isthepressure
against
whichthefan
push
pull
must
theair.Therequired
or
staticpressure
is
a function
of thegraindepthandtype,theairvelocity
through
theductandanyairlossesinthefanentrance
Thisvaluewillvaryfromductto duct
andtransition.
geomelrywill not be identical
the
flow
since
air
in all
p a r t so f t h e g r a i nm a s s .H o w e v e ri,f t h e d e s i g n
r e c o m m e n d a t i ofnosr d u c t s i z e sa n d a i r f l o w sa r e
pressure
followed,
thentheapproximate
requirements
grain
in
Table
2
for
various
types
may
beusedin
shown
fanselection.
The mostcommonfantypeusedfor aeration
is the
type.Thesetypesrequirea relatively
axialor propeller
and operatewell in the static
low initialinvestment
pressure
range(threeto fourinchesof water)thatis
required
formostaeration
systems.
Againitisimportant
to usethe manufaclurer's
datain fan selection.
There
TABLE 2. Approximatedesignstaticpressures
for differentgrain types (Hellevang,1984)
Grop
Type
Grop
Depth
(t0
Deslgnalrflow ratesln cfm/bu
0.10
0.20
0.50
Stallcpre$ure (lnchesof watef)
Wheal
10
15
20
25
30
0.74
1.02
1.45
2.00
2.66
0.98
1.58
2.39
3.50
4.91
1.70
3.31
5.90
9.13
't2.60
Barley
Oats
Sunflower
10
't5
20
25
30
0.62
0.79
1.01
1.31
1.69
0.76
1.10
1.61
2.21
3.O7
1.19
2.08
3.50
5.56
7.70
10
15
20
25
30
0.56
0.62
0.73
0.86
1.04
0.61
o.77
1.01
1.31
1.72
0.83
1.31
2.06
3.09
4.40
Corn
Soybeans
EdibleBeans
Astatic pressurebasedon Shedd'sdata with a packingfactor of 1.5 plus 0.5
inches ot water for entranceand duct loss.
may be largedifferencesin the airflowdeliveryof fans
with the same horsepowerrating.In general,for the
samehorsepower
a largediameteraxialfan willmove
(belowtwoinches)while
moreairat lowstaticpressures
t h e s m a l l e rd i a m e t e rw i l l o e r f o r mb e t t e ra t h i o h e r
pressureranges.
Aerationsystemsare designed
to providethe most
possible
to thegrainmass.
uniformairflowdistribution
Any designmust also take into accountthe grain
handling
and unloading
systemusedin the structure.
Above-floor
duct systemsentailuniqueproblemsin
handling
thatarenotcommonto roundstorages.
Theproducer
is encouraged
to develop
that
designs
will provideeasyaccessto the building
andfacilitate
whilestillproviding
adequate
handling
in the structure
airflow.The generaldesignrulesstatedaboveallow
flexibility
in thesesystems,
andproducers
shoulduse
thesein creating
designfortheirindividual
a workable
structure.
To aidgrainproducers
in designof thesesystems,
a
c o m p u t em
r o d e l( B r i d g e se t a l , , 1 9 8 8 )h a s b e e n
for
developed
to provideaerationdesigninformation
Thismodelgenerally
rectangular
storages.
considers
levelor peakedgrainmassesin rectangular
storages
duct
andallowsthe userseveraloptionsin changing
Persons
forinputdesigns.
inlerested
sizesandspacing
in usingthismodelto designan aeration
systemare
encouraged
to contacttheirlocalcountyExtension
for availability
of this
agentor Extension
specialist
program.
compurer
REFERENCES
Bridges,T.C.,D.G.Overhults,
S.G.McNeilland G.M.
White.19BB.
An aerationduct designmodelfor flat
g r a i n s t o r a g e .T r a n s a c t i o n so f t h e A S A E 3 1 ( 4 ) :
1283-12BB.
Brook,Roger.1979.Aeration
systemsfordrygrain.AEIS
Department,
MichNo.391.Agricultural
Engineering
iganStateUniversity,
EastLansing,Ml 48824.
Hellevang,
K. J. 1984.Cropstoragemanagement.
AE7 9 1 , A g r i c u l t u r aEl n g i n e e r i n D
g e p a r t m e n tN, o r t h
DakotaStateUniversity,
Fargo,ND 58105.
for grain
Peterson,
WilliamH. 1986.Designprinciples
aeration in flat storages.IFEC Fact Sheet No. 9,
l l l i n o i sF a r m E l e c t r i f i c a t i oCno u n c i l ,A g r i c u l t u r a l
1304W.Pennsylvania
Ave.,
Engineering
Department,
U r b a n al ,L 6 1 8 0 1.
1960.Aeration
of Agriculture.
UnitedStatesDepartment
of grainin commercialstorages.MarketingResearch
ReportNo.178.
Educational programs of the Kentucky Cooperative Extension Service serue all people regardless of race, color, aqe, sex, religion, handicap, or national origin.
ExtensionSeruice,Universityof KentuckyCollegeof Agriculture,Lexington,and KentuckyStateUniversity,Frankfort.
lssued 1_g1. 1 M