d e p a r t m e not f a g r i c u l t u r aeln g i n e e r i n g
o
b y J . N . W a l k e ra n d G . A . D u n c a n
l^'
I
r'
Ltrculotlon
in Greenhouses
o f a g r i c u i t u r .ec o o p e r a t i veex t e n s i o sn e r v i c e
u n i v e r s i t oy f k e n t u c k y. c o l l e g e
a g r i c u l t u r e . h o m e e c o n o m i c s. 4 h . d e v e l o p m e n t
near
For maximum plant growth in greenhouses,
and humidity and normalor
optimum levelsof temperature
abovelevelsof carbon dioxide must be maintainedat the
leaf surfaces.These conditionsare influencedby several
factors,Duringdaylighthours,the plantsurfaces
areadding
moistureto tfG air, therebyincreasing
the humidity. Also,
the plantsareutilizingcarbondioxidein the photosynthesis
process,thereby decreasing
the level of carbondioxide in
t h e s u r r o u n d i nagi r .
At night, and on cool days,radiantheatlossfrom the
plant surfacescan resultin a reductionof leaftemperatures
below the surroundingair. Temperaturestratification,due
to the tendencyof warm air to riseand cold air to fall, can
alsooccur in houseswithout positiveair circulation.
lf conditions within the foliage regionsare to be
maintainedat the desiredlevels,the transportof carbon
dioxide and thermalenergyinto the plant canopyand the
transportof water vapor out of the plant canopy must be
provided.This can be accomplished
with positiveair circulation.
TemperatureVariation
In one study where a
continuouslyrunningfan was
used to mix air within a
greenhouse, temperatures
averaged
8oF higherand relative humidities11% lower
(measuredat a point two feet
abovethe ground beds)than
when a convection heating
systemwas usedalone.aThe
improved environment
yield
resultedin an increased
of tomatoes and a lower
incidenceof plant diseases.
J r r e{ 1 r , . , { r
::\.1: .t:i .tt
r r r d r ! r O r ; i r ! - i i i O i r r . - r: i ' r/ r i l r ' :
,r i ri. i
:,rarr'
i ) i ) t|:i tr :i i :)
19tj-i lilt-'11 r:1:..'iilrr: r".t , i" e ii
(arl,ilrlr q,',r,i
I rr ,i lrr,.rii',, .r
,.,:ir.
rrr)1.
e.',
ssLredflurlh.,r;:i.-,.1r1(ilrLrL,rrllirIr]'fs
P o s i t i v ea i r c i r c u l a t i o np e r m i t sh u m i d i t y l e v e l si n
greenhousesto be effectively controlled during cold
the condensation
on the cold glassor
weatherby increasing
plasticsurfaces.
To achievethis, air movementmust transport the moist air to the cool surfaces.6
Whenpositiveair
movementis provided,an outsidetemperature90 to 120
below the inside temperaturein a single covered
greenhouse,or an outside temperature17oF
be I ow the temperatu
re in a double
c o v e r e dh o u s e ,w i l l r e s u l ti n i n s i d e
h u m i d i t i e sb e i n g l i m i t e d t o
90Vo.
In a study of carbon
dioxide (CO2) concentrations
in conventionalglassgreenhouses without air circulation, the daytime CO2 concentrationsin the crop zone
dropped to 83% of the
n o r m a l a i r v a l u ee v e n w h e n
top vents were fully open.l
:1t:- ]t
I'r,,j i\
Humidity Variation
CarbonDioxideVariation
) t
,\, tt
?.t'ti !taa) ai ,r"ri!Jr
irrlrrJ
i.'
,irl,,L
.i:F jr'l:rr'.rar,a
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'1,.
' ,'. ': r.i ,, itf
Ha/:,rlrr ,llrl,ii
rl.r
iid,iifaj
il;
In a different study of heatingsystemperformance,
resultsshowedthat air circulationfans which moved'air
verticallyreducedthe air temperaturegradientsand eliminated the high temperaturebuildup in the ridge areaof a
greenhouse
usingside-wallheatingpipes.3In this case,however,the air patterncreatedby the circulationfansopposed
the thermal induced convectioncurrentscreatedby the
heatingpipes.The net resultwas an increased
temperature
v a r i a t i o na t t h e c r o p l e v e l ,w h i c h w a s ,o f c o u r s eu, n d e s i r able.
,r'r, I i,
fr/ri,
^dtll
!rr.rrrn aal.r,rll.lnt[Ja:oJuie]30
Servce Lln!arsl!tlKeir!a[!Cr'rr:tt:rtLcr!!ll]ri,Lrrr'rol1)n
a.
l
rr:
Jirril6
1!1.1 faoLiprrali'rr!\,l
aidKenl!ckvSlaleUf"!'s1!
rfr'
Fraail!rfl
. i :
i : j r
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L l
AEN_18
Thesereductionsin concentrationwereconsideredto be at
least partly due to the lack of air movement within the
greenhouses.
In another investigationan air flow rate of
100 fpm at a CO2 concentrationof 200 ppm gaveequivalent growth as 300 ppm when the air was still, the difference being attributed to the circulation of CO2 to the
leaves,2
Typical Air CirculationSystems
To evaluatethe adequacyof air circulationpatterns,
severalrecommendedsystemswere installedin a 33 by 90
foot gablegreenhouse
at the Universityof Kentuckyand
the air circulationmeasured.
Basedupon these studies?and the studies of other
relativeto air circuinvestigators,
severalrecommendations
lation systemscan be made. Three primary methods of
creatinga positiveair circulation pattern are recommended
and discussed
asfollours,
Rateof Air Circulation
Ho rizo ntaI Convectio n System:
All these benefitsof air movementare important to
plant growth within greenhouses.
Thoughthe optimum air
velocity has not beenthoroughlyevaluated,it is suggested
that a minimum velocity of 40 fpm shouldbe provided.?
Below this level, air flow is highly erratic and mixing
throughoutthe selectedspacecan not be assured.Such a
velocitywill causeslightleaf movementwith plantshaving
long branchedleaves,suchas tomatoes.Fansare generally
necessary
to obtainthis levelof positiveair movement.
f*l-
The horizontal convectionconctpt wasfirst reported
by Kothsin Connecticut.slt consistsof usingfans 16 to 18
inchesin diameterwith small motorsto developa horizontal air movementpattern.
ln a singlehouse60 feet or lessin length,fanswould
positioned
be
in diagonallyoppositecornersof the house.
Each would direct the air along the sidesof the houseas
shownin Figure1(a),To preventlow velocityregionsin the
f.*
t-
-+
o . S M A L L S I N G L EH O U S E
-+
-+
--+*t
+*
f*J
*+
-+
f*
c . M U L T I P L EL O N G N A R R O W
-l
b. LONG SINGLEHOUSE
-
f*
{l*
{-_
{*
-,
{*-
-+
-+
d. MULTIPLELONG WIDE HOUSES
HOUSES
Figurel. Typical arrangementsforfansforhorizontal convectionaircirculationsystems.
2
AEN_18
centerof the house,the fans shouldbe aimedslightly(10o
to 150)towardsthe center.The fansshouldbe closeto the
For houses
roof and shouldbe wired to run continuously..
to usetwo additionalfans
longerthan 60 feet,it is desirable
to assurecompleteair circulation.The two additionalfans
should be positionedmidway alongthe .houseon opposite
sidesof the houseasshownin Figure1(b).
houses(commonly
For narrow multiple-connected
houses),the air is usually
referredto as ridge-and-furrow
circulateddown one houseand backup the adlacenthouse
as shown in Figure 1(c). Koths indicatesthat, for houses
lessthan 100 feet long, only one fan is neededper house;
for
though for longerhouses,two fans would be necessary
houses24 leet
eachhouse,as shown.For ridge-and-furrow
or morein width, air is usuallycirculatedwithin eachhouse
as shown in Figure 1(d). In this casethe air in adjacent
housesis in oppositedirectionsso the circulationpatterns
betweenadjacenthousesis in the samedirectionwherethe
adjacenthousesconnect,
In the studiesat the Universityof Kentucky, four
16-inchdiameterfans havinga ratedcapacityof 1800cfm
each at 1/8 inch static pressurewere used.The total fan
capacityin CFM l"free" delivery)was approximately1/4
the housevolume.Eachfan motor was 1/4 HP givinga total
connectedhorsepowerof 1 HP. The fanswere pointed150
towardsthe center.The velocitypattern6 inchesabovethe
ground surfaceis shown in Figure 2. The systemproved
highly effectivewith no air velocitiesbeingbelow 40 fpm.
The velocitiesvaried between50 and 200 feet per minute
with the highestvelocitiesoccurringalongthe sidewallsin
sf thefans.
t h ev i c i n i t i eo
Overhead Pertorated Sleeve Systems:
The overheadperforatedtube systemis commercially
available.lt consistsof a fan, or fans,locqtedat the end of
the houseand blowingair through a perforatedpolyethylene tube suspended
the lengthof the house.The tube has
small holesuniformlyspacedalongits lengthto allow air to
into the greenhouse
discharge
space.
For a singlehouse 100 feet or lessin lengthand 30
feet or lessin width, a singletube can be usedasshownin
Figure3(a). The fan capacityin cubic feet per minute (at
1/8 or 1/10 inch static pressure)
should be approximately
1/4 of the housevolume.The cross-sectional
areaof the
'l
tube shouldbe approximately squarefoot per 1000 cfm
of fan capacityand the area of all the holes in the tube
should be 1.5 to 2.0 times the duct cross-sectional
area.In
greenhouse
feet
long
words,
feet
wide
and
other
a
28
80
and havingan averageheightof 9 feet,would havea house
v o l u m eo f 2 O , 1 6 0c u b i cf e e t .T h e a i r c i r c u l a t i o n
fanshould
therefore have a capacity of about 5000 cfm. The sleeve
should have a cross-sectional:
areaof 5 souarefeet which
requirea
s d i a m e t e r ( do) f 2 . 5 f e e t,14l d 3 = S t . f n t t o t a l h o l e
areashould be between7.5 and 10 squarefeet. lf holesare
placed every 2 feet in both sidesof the sleeve,and the
sleeveis 6 feet shorter than the houselength, a total of 74
holeswould be provided(or 37 eachsidel. Eachhole would
needto be approximately2114 inchesin diameter.
The tube shouldbe suspended
aboveheadheight(6.5
to 7.0 feet) and the holes oriented to dischargeair out
towardseachside.With the commercialtubes,the discharge
holes are actuallyslightly below a horizontalline passing
through the center of the duct and therefore the air is
directedslightlydownwardstowardsthe ground.
With houseswider than 30 feet, two or more tubes
are recommendedas shown in Figure3(b). lf the houseis
longerthan 100 to 120 feet, a fan and tube should be run
from eachend asshownin Figure3(c). For multiple (ridgeand-furrow)houses,a tube is placedin eachhouse,providing eachhouseis lessthan 30 feet wide. Additionaldetails
on polyethylenetube systemsare givenin Extensionpublication AEN-7, "Poly-tube Heating-Ventilation
Systenrs
a n d E q u i p m e n t . "T h i s p u b l i c a t i o ni s a v a i l a b l ae t n o c h a r g e
from your county Extensionoffice.
at the Universityof KenIn the experimentalstuglies
tucky, both a singleand a two tube systemwere investigated
in a 33 feet wide greenhouse.
The two tube systemgavethe
performance.
best
lt consistedof two 18 inch diameterfans
through
18 inch diametersleeves.The fan
dischargingair
motors were each 114 HP, givinga total connectedhorsepower of 1/2 HP. The holesin the tubeswere 3 feet apart
and were 2 3/4 inchesin diameter.The air flow pattern6
inchesabovethe groundis shownin Figure4. As indicated,
approximatelV114of the floor areahad air velocitiesbelow
40 fpm, which is consideredthe lowestdesirablelevel.The
total rated capecity of the two fans was only 20% of the
housevolume.lf largerfanswereto be used,higherground
air flow velocitieswould be anticipatedand comparable
performanceto that achievedwith the horizontalconvection systemsmight be obtained.
With the singletube systemin the 33 feet wide house
(wider than recommended),
a largerair flow capacitywas
provided but the velocitiesat the ground were below 40
fpm virtually everywherewithin the greenhouse.This
demonstratedthe lack of ground level circulationwhen a
singletube is usedin too wide a house.
SidewalI VentiIatio n-R* i rculatio n System:
A conceptof usingsidewallmountedfansto recirculate, ventilateand distributeheatwithin a greenhouse
was
developedand describedpreviously.sFor this system,the
fans should have a capacity of from 3/4 to 1 times the
volumeof the houseto providesummerventilationrequirements.The fanscan be operatedat low speedin the winter
AEN_18
\
FAN
7,
Figure2. Air velocity pattern at 6 inchesabove ground level for
horizontalconvectioncirculationsystem.
--1-,!
N
HORIZONTAL
CONVECTION
( F A N S P O I N T E D TOWARDCENTER)
..
I
FAN
e
= BELow40 F.P.M.
I 20
t
D I S P E R S I OTNI M E I . O M I N .
AEN_18
I
__-___+
b . W I D ES I N G L EH O U S E
O . S M A L L S I N G L EH O U S E
----____+
--+
-
---l-
-
f+f
-|TrTT
F
r
or LESS
C. LONG SINGLE HOUSE
d. MULTIPLE HOUSE
Figure 3. Typical arrangementsfor overheadperforated plastic sleeveair circulation svsrems
to provideapproximately
1/2 of theseamounts.Whenrecirculating,the fanspull air across
the greenhouse
andthrough
a chamberon the outsideof the greenhouse
and then dischargeit back into the housealongthe undersideof the
roof of the greenhouse.
A sketchof the unit is shownin
Figure5. A triangularbafflein front of the fan directsthe
majorportionof the air diagonally
acrossthe house.
Air movement tests on a commerciallyfabricated
sidewallventilationunit wereconducted.
The unit hadtwo
fans,eachratedat 8,540cfm at high speed.Eachfan motor
was 3/4 HP, givinga total connectedhorsepower
ol 1 112
HP. This unit, which was positioned10 feet off-centerin
the 33 by 90 foot greenhouse,
had an air movingcapacity
of 52o/o
of the housevolume.The effectiveness
of the unit
in developing
a desirable
air circulationpatternis shownin
Figure5. Only the one corner,which was55 feet from the
unit, had air velocitiesbelow 40 fpm. Therewas a wider
rangeof velocitieswith the sidewallunit than with the
other systems,
the velocities
varyingwith the sidewallunit
from 20 to 350 fpm. The power requirementsfor this
systemwere also higher than for the other systems.However,the systemwas capableof developingeffectiveair
circulationpatternswithin the greenhouse
whilecombining
heatdistributionand 100%ventilationinto one package.
AEN-18
Figure 4. Air wlocity pattern 6 inches above the ground for an
overhead perforated plastic tube air circulation system.
_J^N
(rwo)
O V E R H E AP
DE R F O R A T E D
P L A S T I CS L E E V E
= B E L O W4 0 F . P . M .
D I S P E R S I OT
NI M E O . 8 5 M I N .
AEN_18
Figure 5. Air velocity pattern 6 inches above the ground for a sidewal I ventilation-recirculation system.
_/\
h\
200
25o
I
S I D EW A L L V E N T I L A T I O N
( H I G HS P E ED )
= B E L O W 4 0 F ,P .M .
F A N U NI T
D I SP ER SI O N T I M E I . I M I N .
AEN_18
References
Other Systems:
Two other systemswere studied:e 1) overheadvertical convection units (turbulators),and 2) perforatedplastic
sleeveson the ground. Neither of these systemsmoved air
effectively within the greenhouse.The turbulators were
commercial units rated for 2500 squarefeet of greenhouse
space.The actual spaceservedby the units was only 1485
squarefeet, considerablylessthan recommended.Even so,
low velocities were observedeverywherewithin the greenh o us e ,
The system using perforated plastic sleeveson the
ground consisted of six 11.4 inch diameter sleevesuniformly spacedacrossthe house and running from one end
of the house to the other with 1 inch diameter holes 12
inches apart on each side of the sleeves.The rated fan
capacity was approximately 1/4 the housevolume. As with
the previous system, very little air movement could be
measuredat ground level and the system was judged as
being ineffective.
Summary
Continuous. positive air movement within greenhouses is highly desirable since it equalizestemperature,
carbon dioxide and humidity conditions within the house.
Through improved conditions, healthier plant growth can
be obtained and problems with diseasesassociatedwith
high humidity are lessened.Since high temperatures are
reduced in the upper portions of the house,air circulation
may also reduceheatingcoststo some extent,
For effective air circulation, some form of fan
inducedair movement is necessary.Basedupon experimental studies,the horizontal convection system,where all the
air in the house is moved in a horizontal circular pattern, is
recommended. Such a system is simple in concept and
easily installed. lt is quite adaptableto a wide variety of
types and shapesof greenhouses.
As alternate systems,either a perforated polyethylene
sleevesystem or a sidewall ventilation recirculationsystem
are recommended. Both of these systemsare more difficult
to install properly; but. when correctly installed,they are
quite effective in developing desirable air circulation
patterns.These systemshave the added advantagethat the
system can effectively be used for heat distribution as well
as air circulation. The perforatedsleevesystem is also widely used for the introduction of ventilation air in the winter.
The sidewallsystem, by the use of motorized dampers,can
be used for winter and summer ventilation in addition to
lMorris, L. G.; "Some Recent
Advancesin The Controlof
Plant Environmentin Glasshouses."
Symposiumon EngineeringAspectsof Environment
Controlfor PlantGrowth,
CSIRO, Engineering
Section.Belbourne,
1963.
2Waggoner,P. E., Moss, D. N., and Hosketh,J. D.;
"Radiationin The PlantEnvironrnent
and Photosynthesis."
AgronomyJournal,Yol.55, pages36-39,1963.
3Carpenter,
W. J., and Bark,L. D.; "Temperature
Patterns
in GreenhouseHeating." Florisa Review, January 26,
1967,pages17-19,February2, pages43-45, February9,
pages21-22,February16, pages28,92-95 and February
2 3 ,p a g e9s8 - 1 0 1 , 1 9 6 7 .
4cotter, D. J. and Seay,P. T.; "The Effectof Circulating
Air on The Environment
and Tonnto Growth Response
in
a PlasticGreenhouse."Proceedingsof American Society for
Horticultural*ience, Yol.77, pages643-646,1961.
sCotter,D. J. andWalker,J. N.; "An Automatic
Systemfor
Environmental
Kentucky
Control in PlasticGreenhouses."
Agricultural ExperimentStation, Misc. Bulletin 275-A,
1962.
6Walker,J. N. and Cotter, D. J.; "Condensationand
ResultantHumidity in Greenhouses
DuringColdWeather."
Transactionsof the American Society of Agricultural Engi
nr-rc,Yol. 1 1, No. 2, pages263-266,1968.
Twalker,J. N.; "Ventilationand Air Movementin Greenhouses."4e CongresInternationaldu Chauffageet de la
Climatisation,
Paris,1967.
8Koths,J. S.;
"Air MovementWithin Greenhouses."
Proceedingsof Greenhouse
Constructionand Environmental
Control Seminar,Universityof Massachusetts,
Amherst,
Massachusetts,
January,1967.
ewalker, J. N. and Duncan, G. A.; "Effectivenes of
R e c o m m e n d e dG r e e n h o u s A
e i r C i r c u l a t i o nS y s t e m s . "
ASAE Paper,presented
Region,American
at the Southeast
Societyof AgriculturalEngineers,
SheratonBiltmoreHotel,
Atlanta,Georgia,February4-7,1973.
heat distribution.
Neither the vertical convection system (tu;bulators)
nor the perforated plastic sleeveson the ground are recont
mended for developingeffective air circulation patterns in
greenhouses.
lssued7-73; 10M to 12-75; 5M-782