composting reactors the return of the jedi · oxygen supply throughout the composting media. ......
TRANSCRIPT
Composting Bioreactors DesignIIIReportWinter2011Jamaleddine,Eyad[260282587]Rainville,Cloé[260282662]
DEPARTMENTOFBIORESOURCEENGINEERING
JAMALEDDINE&RAINVILLE2011
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ABSTRACT
Considering the present push towards greener industrial and residential activities,
composting is once again a hot topic amongst Ecological Engineers. Uniform composting
conditions are necessary to ensure the destruction of pathogens andmaintain thewhole
system at the same composting stage, so it is essential to maintain a homogeneous
temperaturethroughoutthecompost.Inthequesttoaccomplishthelatter,anin‐vesselheat
redistributionsystemwasconstructedandtested.Thesystemrequiresnoexternal inputs
of energy, but exploits the principles of conductive and convective heat exchange. Once
composting gets underway and temperature differentials arise within the compost bed,
changes in buoyancy cause water to flow through a closed coil of copper tubing,
redistributing the core heat throughout the medium. Heat is also conducted along the
copper tubing. In the past, a controlled experiment was conducted to test the design. A
statisticalanalysisoftheexperimentalresultsdemonstratesthatthevesselsfittedwiththe
heat redistribution system exhibit lower temperature gradients within the compost bed
thanincontrolvesselswithoutthesystem.Thepresentwilldealwithanairredistribution
systemtobefittedtotheaforementioneddesign.Thelatterwouldpermitwarmairexiting
fromthetopfourinchwholetobecooledandre‐circulatedtothebottomfour‐inchwholeof
the barrel. The overall objective is essential to reduce heat losseswhilemaintained the
replenishingoftheoxygensupplythroughoutthecompostingmedia.
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Introduction
Compostisessentiallythedecayingoforganicmatter.Primarily,amesophilicphase
occurs, followedbyathermophilicphase.Forcenturiesmanhasbeenutilizingthe
latter process to increase soil fertility, reduce organic ordure volumes and treat
contaminated soils. Composting is a practice gaining popularity amongst the
agricultural community, theengineering realmand,onamoregeneral scale, even
with theaverage individual.Essentially, asmoreapplications involveutilizing this
ancient technique, one must account for the numerous limitations that can be
encountered when composting. Of the latter, the inability to ensure that the
compostingmediaisfullycuredafteracertainperiodorensuringthattheentirety
of the media has attained the crucial thermophilic phase, where pathogenic
organisms are destroyed, are limiting factors when considering composting as a
means to an end. More so, the production of volatile fatty acids from
microorganisms is the sourceofunpleasantodors that candeter individuals from
settingupacompostingbin.Theaforementionedlimitingfactorsareduetothenon‐
homogenous nature of compost and the presence of anaerobic digestion within
pockets of the compost media. In the past, a heat redistribution system was
designedinthequesttoredistributethecoretemperatureofthecompostingmedia
uniformlythroughoutthecompostingvessel,withoutanyexternalinputsofenergy.
To do so, the heat produced by the activated microorganisms was uniformly
distributedbyawatertightsystemconsistingofcopperandplastictubingconnected
toaheatercoreplacedatthecenterofthecompostingmixture.Nextwedesigned
theabovementionedheatredistributionsystemandtesteditusingsixtwohundred
literpolyethylenebarrels.Thelatterwasdonebyfittingthebarrelswithafour‐inch
holeatthetopandbottomandameshgridat8inchesfromthebottomtoholdthe
0.15m3ofcompost.Amixtureofdogfoodandwoodchipswasutilizedduetotheir
low cost and availability. To insure the statistical validity of the results, six
compostingvesselswerebuilt,threecontrolsandthreebarrelsfittedwiththeheat
redistributionsystem;astatisticalanalysiswasthenconductedtodetermineifwe
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had obtained valid results. The objective was to test the effectiveness of the
designedsystemtotransferheatthroughoutthecompostmediaandpermituniform
composting throughout the latter, therefore a fully cured final product. Themain
constraintwithinthedesignof theheatredistributionsystemwasthefact thatno
external inputs of energywere to be added; the systemwas to be self‐sufficient.
Results obtained in the past suggested to a 99.5% confidence coefficient that the
heat redistribution system was meeting it’s objective of distributing the heat
uniformlythroughoutthebarrel.Wewerealsoabletosuggestthatthesystemhad
thepotentialofaccelerating thecompostingphaseandproducingacuredproduct
quicker than the barrels not fitted with the heat redistribution system. We did
howevernoticethatasignificantamountofheatwaslostfromthefourinchwhole
at the topof thepolyethylenecontainer.Thesewholes,oneat the topandbottom
wereputinplacetoinsurethattheairwouldcirculatethroughoutthecomposting
bed, favoring aerobic bacterial growth and reducing the production of odorous
gasses.Therefore,inaquesttoreduceheatlossandfavorhighertemperaturesand
a prolonged thermophilic phase we have gone about designing an air exchange
system. This system (AES) should be able to permit oxygen to be replenished
throughout the composting media while minimizing losses through the four‐inch
wholes.Sketchesoftheheatredistributionsystem(HRS)andtheairredistribution
systemcanbefoundinAppendixB,Figure12,andAppendixC,Figure13.
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DesignoftheHRSSystem
FUNCTION
Theheatredistributionsystemutilizestheheatproduced,undertheformof
energy frommicroorganisms, anddistributes it throughout the compostingmedia
permittingtemperaturegradientstobeloweredandthecomposttobeatthesame
compostingphasethroughouttheprocess.Essentially,astheheatercoreplacedat
thecenterofthebarrelisheatedbythemicrobialactivity,thedensityofthewater
withinthelatterdrops.Thedensitygradientofthewaterwithintheheatercoreand
coppertubingcausesthewatertoflowfromtheheatercoretotheplasticpipingand
intothecoppertubingthroughoutthecompostheap.Thisprocessiscalledthermal
driving. Thedifferencebetween the forcesofgravityexertedon the twovolumes
will cause the warmer fluid to rise and the colder fluid to sink. The continuous
warmingofthebarrelisbasedonthisprinciple.Asthemicroorganismswithinthe
compost bin begin to digest the nutrients, heatwill be dissipated and once it has
elevated thewater’s temperature to the proper level, thewarmwaterwill slowly
rise as the coldwater spirals down the copper tubing towards the bottom of the
barrel. Whentheaforementionedoccurs, theheat fromthecenterof thebarrel is
evenlydistributedthroughoutthecompostingmixtureduetothehighconductivity
of the copper tubing, without external inputs of energy. The latter permits the
compost to be at the same microbial phase, whether that be mesophilic or
thermophilic, essentially eliminating pockets of undigested organic mater and
ensuringthatthefinalproductiscompletelycured.Thefour‐inchholes,madeatthe
top and bottom of the barrel and the clearance produced by the grill and V‐bent
steel supports also permit air to flow, through convection, throughout themedia,
permittingaerobicconditions.ThelatterreducestheamountofVOA(volatilefatty
acids) emitted by the compost, therefore reducing the unpleasant ammonia smell
causedbythedecompositionprocess.
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A Solid Works model was designed to better understand the layout of the heat
redistributionsystemanditsimplementationbeforetheconstructionandtestingof
thebarrelsthatwasdone.FromtheSolidWorksdrawingsandpreviouswork,the
construction of the vessels was much facilitated; see figures in Appendix B. The
latteralsoprobablycontributedtothefinalresultsnotbeinghinderedbytechnical
errors. Furthermore another simulation model was designed including the ARS
systemtoestablishabetterreferenceintermsofspaceandsizing.Thelatterwould
beusedtosimplyhaveabetterunderstandingof thewaythesystemwouldcome
togetherwhentheHRSandtheARSsystemareimplemented.NotethatinAppendix
BDrawings1 to6, the lidof the200LiterPolyethylenebarrelswasnot included.
Thelatteristoensurethatthesystemcanbeproperlyseen.
CONSTRUCTION
Asmentionedprevious,thematerialsusedwithintheHRSdesigninvolveda
heatercore,fivefeet(1.50meters)ofcoppertubingandabout2feet(0.6meters)of
braidedplasticpiping.Therefore,forthebarrelsfittedwiththeheatredistribution
system, three five‐foot coils of copper tubing,with an inner diameter of 4/8 inch
(0.01meters)wereusedalongsidethree2‐feetsegmentsof5/8in.(0.0158meters)
inner diameter plastic piping and a 3‐way control valve. A zinc coatedmesh grid
wouldbefittedintoeachofthesixcompostingvessels.V‐bentsteelbarstoholdthe
totalweightofcompostwouldsupportthelatter.Afour‐inch(0.1m)holewouldbe
madeatthecenterofthetoplidofthebarrelandanotherfour‐inchholewouldbe
made 8 inches (0.2m) from ground height, to insure there would be airflow
throughoutthecompostingmedia. Theheatredistributionsystemwasassembled
and tested by inserting the heater core into a water bath and increasing the
temperatureofthewaterbathuntilwatermotioncouldbeobservedthroughoutthe
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clearplastic,carewastakentoensureflowwasoccurringthroughoutthepipingand
the water motion did not only consist of localized turbulence. It had been
determinedthataround35°Cwaterwouldstartflowing.Aftertestingallthreeofthe
heatredistributionsystem,framesweredesignedtoholdthe latterandinsurethe
heatercorewouldbeatthecenterofthetwohundred‐literpolyethylenebarrels,as
shown in Figure 2. The heat redistribution systems were then fitted to their
respectivebarrels.The200Lplasticvesselsweretheninsulatedwithmineralwool
and bubble rap tominimize heat losses from the sides of the barrels. All vessels
were then transported to the Bioresource Engineering Laboratory for further
testing.
Figure1:Picture of the heat redistributionsystem, before being placed into theinsulatedcompostbarrel.
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HRSCALCULATIONSThermalDriving
Thermaldrivingheadistheforcethatcausesnaturalcirculationtotakeplace.Itis
causedbythedifferenceindensitybetweentwobodiesorareasoffluid.Whenwe
have two volumes that are at different temperatures, then the volume with the
highertemperaturewillhavea lowerdensityandhence lessmass. The inverse is
also true, which is why the volumewith a lower temperaturewill have a higher
density and a greatermass. The higher temperaturewill not only bring about a
lower mass, it will also lower the force exerted on the fluid by gravity. The
differencebetweenthe forceofgravityexertedon the twovolumeswill cause the
warmerfluidtoriseandthecolderfluidtosink(Munsonetal.,2005).
The continuous warming of the barrel is based on this principle. As the
microorganismswithin thecompostbinbegin todigest thenutrients,heatwillbe
dissipatedandonceithaselevatedthewaterstemperaturetotheproperlevel,the
warm water will slowly rise as the cold water spirals down the copper tubing
towardsthebottomofthebarrel.
Frictioninthepipes
Twomainfactorswereconsidered:theReynoldsnumberandtheheadlossdueto
friction. The former is necessary to determine whether the flow is laminar or
turbulent and the latter to establish the losses in the systemdue to the choice of
material.Itisimportanttomaintainalaminarflowbecauseitismorestableandit
willlowerthepressuredropinthepipes.Turbulentflowisamuchmorecomplex
process although it should theoretically enhance the heat and mass transfer
processes.
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Materials
HOMOGENEOUSCOMPOST
Twooptionswereavailablefortheorganicwastematerial:dogfoodandchickenor
cowmanure.Dogfoodwaschosenovermanurechieflyforitsconformity.Sincethe
experimentwillberepeatedinthefuture,amoreconformmaterialwasfavoredto
avoiddiscrepanciesbetweenexperimentsandbetweenthe6barrelsthatwereset
up for this design. Additional features include the greatly reduced amount of
pathogenicorganismsanditsFDAapproval(FDA2010).Anotherbeneficialaspect
isthedogfood’swaterabsorptioncapacity.Severalmaterialswerealsoconsidered
for the bulking agent: sawdust, shreddedpaper, straw, andwood chips. Sawdust
wasrejectedsinceweneededamaterialthatcouldprovidestructuretothemixture.
Theuseofshreddedpaperwasaninterestingoptionsinceitpermittedtherecycling
ofoldmaterial,butaswithsawdust,itwouldn’tprovideadequatestructure.When
comparingtheremainingtwomaterials,aswithdogfoodandmanure,theissueof
availabilitywasnoted. Sinceanotherexperiment run is scheduled for this spring,
similar materials must be available at that time. Straw would have been more
difficulttoobtainthanwoodchipsatthatpoint,andifobtainedwouldhavebeenofa
different quality than the fresh straw collected in the fall. Wood chipswere also
favoredfortheirlargersize,providingsuitablestructuretothecompost,aswellas
theiravailabilityandconsistency.
Inordertodeterminethetotalmassofcompostmaterials,avolumeanddensityhad
tobeestablished.Theheightofcompostwaschosentobe26inches(0.6604m)and
thediameterof the200‐litrepolyethylenebarrelwas21 inches (0.5334m).From
this information, the volume of compostmaterial was found to be 0.15m3. The
density of themixturewas assumed to be 550 kg/m3, after consultationwith an
expertonthematter(Dr.S.Barrington, PhD,Agr.Eng.,McGillUniversity),yieldinga
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woodchipsaccountedfor4.9litersofwater(basedoninformationinTable1)and
so44.6litersoftapwatertheoreticallyhadtobeaddedtothecompostmixture.To
ensurethatsucha largequantityofwaterwouldremainwithinoursystemrather
than leak out through the bottom 4‐inch hole, thewood chipswere soaked for 3
daysinawhiteplasticbinwithadepth,width,andheightof0.57,0.86and0.58m.
Oncethematerialswerepurchased,theywereanalyzedinalaboratoryformoisture
content,density,percent total solidsandpercent ash content. Todetermine total
solids,threesamplesofeachmaterialwereweighed,placedinanovenat103°Cfor
24 hours andweighed oncemore (see Sample Calculations, Eq. 5). The remains
were then placed in a furnace, set to a temperature of 550°C for 5 hours, to
determine theash contentofbothmaterials (seeSampleCalculationsEq.6). Ash
contentisexpressedasapercentofthetotalsolids.Thedensitywasmeasuredby
weighing the samples in a crucible of known volume. Characteristics of the final
compostmixturewereanalyzedinthesamemanorasthedogfoodandwoodchips,
however6samplesweretestedinsteadof3.Resultsfromthelaboratoryanalysisof
thecompostmaterialswillbefurtherdiscussedintheAnalysissection.Withthese
results,calculationswereverifiedanditerationswereconductedoncemoretoyield
moreaccuratemassesofeachingredient,basedonmeasuredparameters.
Oncethematerialswerepurchased,threesamplesofeachdogfoodandwoodchips
were analyzed according to the aforementioned methods. The data obtained is
presented in Appendix C: Tables 5, 6 and 7. In tables 2 and 3, results formean
moisturecontent,meantotalsolidscontentandmeanashcontentarepresentedfor
bothdogfoodandwoodchips.Themoisturecontentandtotalsolidscontentwere
close to the values that had been initially assumed. This indicates that the
calculationsmadetoarriveatdesiredmassesdogfoodandwoodchipsbasedona
theoretical C/N ratio respected the characteristics of the chosen composting
materials.However,thedensitieswerequitedifferentwith341.3and162.0kg/m3
for dog food andwood chips respectively. Also, themoisture content of thewet
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A Hewitt Packer Data logger (Model #: 34970A) was used to acquire the
temperaturereadingsfromthethreepre‐determinedheightspreviouslymentioned.
Thelatterwassettotaketemperaturereadingsatfifteen‐minuteintervalsandthe
datawasextractedfromthedataloggereveryday.
The datawas collected for a period of thirty days, running three control barrels,
labeled CX‐# and three barrels fittedwith the heat redistribution system, labeled
HR‐X#(XvariesfromAtoCand#varyfrom1to3)asshowninfigure3.
Figure3:Labelingschematicofthethermocouples.
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Three things are apparentwhen evaluating the graphs above (Fig. 4). One of the
latterwouldbethefactthatthegreenlinesonallofthecharts,representingthetop
thermocouples,seemtohavemoreaggressiveandunpredictablevariationsthanthe
otherlines.Thisisduetotheunforeseeneffectsofcompaction.Itwasnottakeninto
accountthatcompostvolumewouldbereducedtothatextent(70mmdecreasein
height), exposing the top thermocouples. The aforementioned lead the top
thermocouplestomeasureambientairwithinthecompostingvesselsinsteadofthe
actualtemperatureatthetopofthecompostmedia.Anotheraspectworthnotingis
thehigher temperature that thevessels fittedwith theheat redistribution system
(HRS) attain. The preceding is assumed to be due to the heat being uniformly
distributed throughout the composting vessels fitted with the HRS, favoring the
microorganisms of thermophilic nature, permitting the latter to attain full
maturationandintheprocesspermittingthevesselstoattainhighertemperatures.
The third phenomenon that can be observed involves the smaller temperature
differences noticed between the center andbottom thermocouples (Red andBlue
lines respectively) in the vessels fittedwith theHRS, notably between 35 and 50
degrees Celsius. More so, after 400 data acquisitions, it can be noticed that the
temperature starts decreasing in the control barrels, whereas the HR vessels
temperaturescontinuetorise.Thisalsocanbeattributedtotheheatredistribution
systemandwillbediscussedfurtherinthediscussionsection.Abetterdepictionof
the latter can be observed in Figure 5 and Figure 6, where the difference of
temperaturesbetweenthecenterandbottomthermocoupleswereaveragedoutfor
thecontrolandHRSvessels.ItcanbeobservedthattemperatureoftheHRSvessels
donotattainaslargedifferencesasthecontrol,demonstratingthatovertheperiod
ofninedaysand861dataacquisitions,thevesselsfittedwiththeheatredistribution
systemseemtohaveamoreuniformtemperaturegradient.
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ANALYSISOFRESULTS
Havingplottedtheaveragedifferencebetweenthemiddleandbottomtemperature
readings of both the control barrels and the heat redistribution barrels, it is
essentialtoestablishwhetherthesamplemeansfortemperaturedifferencesofthe
control and heat redistribution systems are significantly different. Using a right‐
handone‐tailedtestabouttheequalityoftwopopulationmeans, itwasfoundthat
the average temperature difference in the control barrelswas significantly larger
fromtheaveragetemperaturedifferenceintheheatredistributionsystem. Itmay
be affirmed that the average temperature difference in the heat redistribution
system is in fact smaller than the averagedifference in the control barrelswith a
confidence intervalof99.5%. Thisconfirmsthat thedesign is functional in that it
succeededinwarmingthecompostmixtureinamoreuniformmannerandattaining
highertemperaturesthanthecontrols,withoutanyexternal inputsofenergy.This
testwas basedon the samplemeans: 5.493°C for the control and4.742°C for the
heat redistribution system, and the samplevariances:9.652 for the control, 5.019
for the heat redistribution system. Detailed calculations are presented in Sample
Calculationssetb,AppendixG.
Fromtheaboveanalysis, theHRShasdemonstrateditseffectiveness inpermitting
the barrels to attain higher temperatures than the controls and to have a more
uniform temperature gradient throughout the compostmedia. Thismore uniform
temperature gradient could be effective in destroying pathogenic organisms,
increasing the quality of the final cured product and potentially reducing the
compostingtime. Itshouldbementionedthatevenwithout thetopthermocouple,
the results are valid and statistically sound. The comparisonbetween center and
bottom thermocouple readings to determine temperature distribution and
uniformity is statistically sound. More so, in the controls, midway through the
experiment it seems that the temperature stabilizes and starts to decrease. The
latterismostlikelyduetothecompostnotbeingabletoattainthesecondtierofthe
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thermophilicphasedue topocketsat lower temperatures inhibiting thegrowthof
the required thermophilic bacteria and slowing down the composting process.
Whereas the barrels fitted with the HRS are able to continue their gains in
temperature and alsomaintain, on a general basis, a smaller overall temperature
gap. When evaluating Figure 5 and Figure 6, it was observed that overall the
temperature differencewithin the control vessels is higher than the temperature
difference within the barrels fitted with the HRS. One also has to take into
consideration that, asmentioned previously temperature starts decreasing in the
control vessels around the four hundredth scan,whereas the temperaturewithin
the HRS barrels continues to increase even after the last data scan that was
recorded. ThelattercouldexplainthesecondhalfofFigure5,wheretemperature
differences within the controls seems to drop, but this may be explained by the
generaldecreasewithinthosebarrelsasmicrobialactivitydiminishes.
Relying on the statistical analysis procured in the Sample calculations set b, it is
possible to declare that the heat redistribution system has attained the set out
objective of homogenizing the temperature throughout the composting media,
increasing the speed of composting and permitting the barrels to attain higher
temperatures possibly destroying pathogenic organisms throughout the compost.
The speedof the compostingprocesswill be furtherdiscussed in thenext report,
since data gathering is still underway. The latter gives a strong argument to
continuethedevelopmentandameliorationsprocuredintheimprovementssection
and the possible implementation of the HRS on a larger scale, increasing the
efficiency of composting and it’s applications in a residential, industrial and
commercialbasis.
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RiskAssessment&FailureMode
A limitationof thedesignwouldbe thepotential failureof theheat redistribution
system. Failure could occur during the filling phase, where the dumping of the
compostonto theHRScouldcauseawater leakandhencedisrupt the flow in the
coppertubing.However,carewastakenwhenthelatterwasdone.Moreso,another
factorthatcannotbefullyremediatedfororobservedduringtherunningphaseof
the compostwouldbe the reductionor stoppageof flow throughout thepipingof
theHRS.The lattercouldoccur if anairbubblewere toenter thesystem,causing
blockage,orifakinkwascausedbytheweightofthecompostitself.However,one
must take into consideration that even if there were to be some sort of limiting
factor thatwould cause the stoppage of flow, the high conductivity of the copper
tubing could still permit the heat to be transferred from the center of the barrel,
throughtheheatercore,intothewaterwithinthelatterandthroughouttherestof
thepiping and fluidby conduction.The latter shouldbe taken into consideration,
because uniformity in the temperature gradient does not necessarily imply flow
withinthepiping.ItisimportanttomentionthattheHRSwastestedbyinsertingit
inaheatedwaterbathsetat20°Candincreasingthetemperatureinincrementsof
fivedegreesuntilflowcouldbeobservedthroughoutthetransparentplastictubing,
therefore demonstrating that the design was sound and able, under ideal
circumstance to transmit warmed water throughout the piping. Flow occurred
around35°CinallthreeoftheHRSsystems.Amethodofreducingkinksandleakage
would involve reducing the lengthof theplasticbraidedpiping toavoidexcessive
twistingmotions,andevaluatingthecoppertubing,beforefilling,ensuringthereare
noapparentorhiddenkinksandavoidinganyabruptchangesinthedirectionofthe
copper tubing. Another important aspect tomention is the environment inwhich
thebarrelsarerun.Thelocationwheretheunitswereplacedwasmaintainedata
temperatureof20degreesforthefirsttwodays(192scans)andthenat25degrees
Celsius for the rest of the experiment. The latter should be taken into account in
JAMALEDDINE&RAINVILLE2011
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future runs. To avoid additional discrepancies in the future, a logbook should be
keptwiththedateandtimeatwhichtheBioresourceEngineeringLaboratorywas
accessedsinceitwasbroughttoourattentionthatfellowstudentswhohadworkto
completeintheEngineeringLabwouldleavethedooropeninanattempttoreduce
thefowlsmellsemanatingfromthecompostbarrels.Thebreezecreatedinthebarn
could be responsible for the erratic behavior of the top (exposed) thermocouple
readings. Finally, in terms of risk assessment it is important to have respiratory
protection when dealing with any types of large volumes of compost. For the
purposesofthisexperimentdatawasuploadedonceadayandarespiratorysafety
devicewasused.Anotherimportantsafetyaspectwouldinvolvecontactavoidance
withcompostthatisleftasresidueonthesidesofthebarrels.
ISSUES
DuringthetestingoftheHRS,amultitudeofissueswereencountered.Ofthelatter,
heatlossfromthetopfour‐inchhole,madeforaerationwasprobablyofthehighest
significance. Other issues include the large amount of leachate produced by the
compostandthefactthatithadtobemanuallycollectedandresuppliedtothetop
of thecompostpile.The lossofnitrogenouscompoundsduringthedecomposition
processoccursmainlythroughemissionofgasessuchasNH3andNOx,aspreviously
mentioned. This loss of nutrients may have a significant impact on the nutrient
balanceofoursystem.Sincethecompostvesseliswellisolated,itisassumedthat
themajorityofnitrogenousemissionsareexitingthroughthe4‐inchholeatthetop
of the barrel. Heat is also lost through the same opening. These issues will be
addressedbyrecirculationthewarmairproducedbythecompostbymeansofan
air‐to‐air heat exchanger. Also, the reduction in compost volume was not
anticipatedtobesolarge.
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AirRedistributionSystem
ARSINITIALDESIGN
Todealwiththeheatlossfromthetopfourinchwhole,wehadtofindamethodto
keeptheprocessaerobicwhilereducingheatflowfromthetopofthecontainer.To
achievetheaforementionedanAirRedistributionSystem(ARS)wasdesigned.The
latterwouldsimplyconsistofachimneythatwouldbeable tore‐circulate theair
into the bottom four‐inch hole while maintaining the oxygen supply. The initial
designisdepictedinFigure7.
Figure 7: Sketch of the design III concept.
JAMALEDDINE&RAINVILLE2011
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inflow of ambient, quiescent air due to kinks that might occur during the
constructionorassemblyprocesses.
Since the designed system has no external input of energy, it relies on free
convectiontodrivetheheatexchanger.Thisfreeconvectionoriginateswhenabody
forceactsonafluidinwhichtherearedensitygradients.Theneteffectisknownas
buoyancyforceanditinducesfreeconvectioncurrents(Incroperaetal.,2007). In
this case, the body force is gravity and the density gradient is temperature. The
processbeginsasthewarmair,resultingfromthemicrobialactivity,risesthrough
theinnercylinderduetobuoyancysincethedensityofthewarmairislowerthan
thatoftheambientair.Atthispoint,wehaveassumedthatthewarmairisevenly
distributed throughout the entire cylinder, up until the point where it leaves the
innercylinder throughasimilar4 inchdiameter(101.6mm). The innercylinder
shouldbecomposedofahighlyconductivematerial.Below,inTable5,arealistof
metallicandnon‐metallicmaterialswithrelativelyhighconductiveproperties.
Metal
Conductivity, k (W/m*K) at 330 K
Silicon Carbide 490 Silver 428 Copper (pure) 399 Beryllium Oxide 247 Aluminum (pure) 238 Magnesium 155 Tungsten 141 Zinc 114.3 Iron 77 Tin 65 Commercial Bronze (09% Cu, 10% Al)
52
Chromium steels 48.2 Diamond 2047
Table5:Conductivepropertiesofmetalsandnon‐metalmaterials. Source:Incroperaetal.,2007.
JAMALEDDINE&RAINVILLE2011
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Obviously, diamondandpure silver arenot inourbudget. Silicon carbide canbe
purchased in Canada at a price of 277.00$ for a 50mm by 50mm sheet ():
110’800$/m2.This ismuchmore expensive than copper,which canbepurchased
for26$,fora1ftx2ftsheet(www.whimsie.com/coppersheetwire),around140$/m2.
Thus, copper ismoreaffordableandstillhasveryhighconductivity,k. As for the
outercylinder,thesamepolyethylenematerialthatcompostvessel ismadeofwill
be used, alongwith the same insulatingmaterials, whichweremineralwool and
bubblefoilinsulationwiththermalconductivitiesof0.042W/mKand0.034W/mK
respectively(Incroperaetal,2007,AppendixA).
DETERMININGTHEBOUNDARYLAYER
As the ambient air, approximated at 20 °C, comes into contact with the warm
surfaceoftheinnercylinder,athermalboundarywilldevelopduetothedifference
in temperatures. The fluidparticles coming intocontact themetallic surfacewill
achievethermalequilibriumatthesolidssurfacetemperature.Theseparticleswill
then exchange energy with those adjacent to them in the fluid, creating a
temperature gradient in the fluid. The region in which this gradient occurs is
definedasthethermalboundarylayer,attheleadingedgeofwhichthetemperature
profilewillbeuniform,withT(y)equatingT∞(definedastheambienttemperature).
However,thestandardequationsdonotapplyinthiscase,sincethereisnoforced
convection and the plate, or rather cylindrical surface, is vertical. As a result, the
governingequationwillinvolvethedimensionlessparameterGr(Grashoffnumber),
whose functionmaybe compared to thatof theReynoldsnumber in situationsof
forced convection, and thatmeasures the ratio of buoyancy forces to the viscous
forcesactingonthefluid(Incroperaetal.,2007).
JAMALEDDINE&RAINVILLE2011
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The5mmdistancewillbethelengthbetweentheinnercylinder’ssurfaceandthe
perimeteroftheouterinsulatedcylinder.Itisalsoimportanttonotethestateofthe
fluid.FlowisconsideredlaminariftheproductoftheGrashoff(Gr)andPrandtl(Pr)
numbersarebelow1x109(Incroperaetal.,2007).Inourcase,Gr0.8mandPrwere
1’970’259’723 and 0.706 respectively, which yields a value of 1.3 x109. Since the
valueobtainedisveryclosetothe109 limit, this indicatesthatat theheightof0.8
meters, the flow is beginning to transition from laminar to turbulent. However,
throughout most of its length (0m to 0.7m), the air redistribution system
demonstrates laminar flow patterns, according to the equations previously
mentioned.
Additionally, in order to increase the surface area of the inner, heat conducting
cylinder, twooptionswereavailable:verticallyaligned fins (straight, triangularor
parabolic)orfoldingthecoppersheettocreateripplesalongthesurface.Although
thefinsmightgenerateaslightlylargersurfacearea,itwasmorerealistictocreate
thefoldsonthesurfaceofthecoppersheetthantofirmlyattach60individualfins.
To determine the amount of folds required, the perimeter of the inner cylinder
(319.9mm)was divided by the number of spacings. In the table below, several
spacingswerecalculated.
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Number of Spacings
Width of spacing
(mm)
Length of Fold (mm)
New Perimeter
(mm)
Perimeter with Fins
(mm) 10 31.92 5.00 99.92 346.19 15 21.28 4.43 132.95 361.19 20 15.96 4.12 164.83 376.19 25 12.77 3.92 196.11 391.19 30 10.64 3.78 227.05 406.19 35 9.12 3.68 257.77 421.19 40 7.98 3.60 288.33 436.19 45 7.09 3.54 318.79 451.19 50 6.38 3.49 349.17 466.19 55 5.80 3.45 379.49 481.19 60 5.32 3.41 409.76 496.19 65 4.91 3.38 439.99 511.19 70 4.56 3.36 470.20 526.19 75 4.26 3.34 500.38 541.19
Table 8: Values for the perimeter of the inner cylinder, dependant on the number ofspacingsassigned.
Evidently, the new perimeter has to be larger than the perimeter of the 4 inch
diameter,discardingallspacingsunder46.Onceagain,feasibilityofconstructionis
key;weneedthehighestnumberofspacingspossiblewithoutitbeingtoosmallfor
ustoactuallybuild.Wedecidedon60spacings,yieldinganewperimeterof409.76
mm, a 29% increase compared to the initial 4 inch diameter. From Table 8, the
columnonthecompleterightindicateswhattheperimetercouldhavebeen,hadwe
chosen the impractical fins. It is higher than the folds, however if both are
compared at 60 spacings, the difference is less noticeable than at 15 spacings. A
graphical depiction facilitates the comparison: the perimeterwith folds increases
morerapidlythattheperimeterwithfins(Figure11).
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VELOCITYANDMASSFLOWRATEOFWARMEDAMBIENTAIR
The mass flow rate of the warmed air was also determined. Mass flow rate is a
function of the air density, velocity and cross sectional area of flow (Incropera,
2007).
ρ=1.0682(at328K)A=(πD2)/4=(π*(0.1016m)2)/4=0.008107m2
where:
ν:velocity,m/s g:gravitationalacceleration,9.81m/s2
L:verticaldistancefrombottomofthesurface,m.
ΔT:Temperaturedifference, Ts‐T∞=35KT∞:Ambient“room”temperature,293K
ν=0.6m/s
Hence,themassflowrateofthefluidatroomtemperature(293K)hasavelocityof
0.6m/sandamassflowrateof0.005196kg/sasitcomesintocontactwiththehot
metallicsurfacethattheinnercylinderconsistsof.
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ARSSUMMARY
Theairredistributionsystemconsistsof twoverticalconcentriccylinders,bothof
0.8minheight.Theinnercylinderwillbeconstructedusingathincoppersheetto
enhancetheconductionofheatfromtheexhaustairofthecompostandwillbeleft
openattheend. Theinnercylinderwillhaveastarformationwith60spacingsof
5.32mmeach,providingaperimeterof409.76mmandasurfaceareaofcontactof
327’808mm2(0.328m2).Additionally,thelengthofeachofthe120foldswillbeof
3.41mm.
The outer cylinder will be made of polyethylene, covered in the appropriate
insulation as described above, and the top surfacewill not be left open since the
freshincomingairwillbedirectedtowardsthebottomofthecompostvessel,rather
thanlosttoanopeningatthetop.Toensureinflowoffreshair,theoutercylinder
will be punctured 5 cm intervals from the bottom, 2 cm intervals along the
horizontal, and a well insulated 2 inch diameter piping system will connect the
bottomsectionoftheoutercylindertotwothe2inchopeningsthatwillbepresent
oneitherside,atthebottomofthevessels.SeeAppendixC.
TESTING&SIMULATION
Constructionandtestingoftheairredistributionsystemshouldbeginthissummer.
Theseresultsshouldhelpbetterdeterminetheprecisionofourcalculationsandthe
overallefficiencyofthesystemitself.Duringtheexperiment,weintendoninserting
3thermocoupleswithintheinnercopper,star‐shapedcylinder:thefirstjustabove
thecompostvessel’s4inchopening,thesecondat0.4minheightandthethirdat
theexit(0.8m). Thermocoupleswillalsobeplacedinthesectionbetweenbothof
thevertical cylinders, at thesameheights. The lastpairof thermocoupleswillbe
JAMALEDDINE&RAINVILLE2011
38
measuringthetemperatureoftheairasitleavestheinsulatedpipingandentersthe
compostvesselfromtheopeningatthebottom.
Essentially the testing is a process where calculation methodologies will be
compared to theirempirical standingsandadjustmentswillbemade tomend the
discrepancies.
JAMALEDDINE&RAINVILLE2011
39
EconomicOverview
The fact that the HRS and ARS system can be implemented on a small scale, for
exampleonamunicipallevel,onamediumorlargeindustriallevelforanumberof
reasonsincreasetheprobabilitiesthatsuchasystemmightbecommercialized.The
purposeofourdesignbeingaproofofconceptweconcentratedondemonstrating
thatthesystemwasfunctional.However,consideringtheincreasingpricesof land
filling and the popularity of bioremediation the system could be a cost‐effective
solutionforthelatter.
Note that the approximate cost of the HRS & ARS systems, including the two
hundred litter vessels hovers around 90$, that said one could cut costs and used
recycledmaterial tobuildand implement theCompostingBioreactors.Land filling
in Canada has an average cost of around 85$ per ton, however the total
environmentalcostofthelatterincludingtheeco‐systembenefitsthatalandfillwill
destroy or hinder does not have a set value. Note also that land‐filling will be
subjected to higher taxes in the upcoming years and that the Composting
BioreactorsbearingHRSandARSsystemcouldbeusedforthousandsofcomposting
cycleswithvery littlemaintenanceconsidering therearenomechanicalpartsand
thatitutilizesnoexternalinputsofenergy.Overallthecompostingreactors,onthe
short run,mightbemoreexpensive,however the futurebenefitsaremuchhigher
than the initial cost considering the final product could be used or sold as soil
fertilizerandthefactthatland‐fillingisseenasanoutdatedmethodology.
Assuming that a composting bioreactor can hold 90 kg of compost per run and
requiresthirtydaystocompostthelatterwithaninitialcostof90$andalifecycleof
athirtyrunsthetotalcostfor2700kgoforganicmatteris90$.Thelatterdoesno
includethe labor that it requires.Assumeaperson ispaid12$perhour to fill the
barrelasasecondarytasktotheirmainemploymentandthatittakefifteenminutes
tofill/emptyabarrelthatis180$laborexpensefor30runssumminguptoatotalof
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270$for2700kgtherefore,about100$perton.Nowconsiderthatthelandnotused
by the organic waste is kept as a natural habitat and a part is utilized for the
buildingofahousingprojectorapark.ThelatterwouldofferEco‐Systembenefits
thatthelandfillwouldontheotherhandtakeaway.Thebioremediationcapacities
ofcompostingcouldalsobeutilizedonasmallorlarge‐scaleoperation.
Onalargerscaleassuming10.8tonesoforganicmatter,therefore10800kgneeded
tobelandfilledonaspanof1yearandthatthecompostingbioreactorwouldcost
70$ to mass produce (recycled material). If one were to land‐fill the 10 tones it
wouldcostatotalof920$,whereas10bioreactorscouldhaveanoutputof900kg
per cycle and 10 800kg output per year. The total cost of the reactorswould be
(10*70)+(10*0.25*12*12*2)= 1420$. However, cured compost can be resold at a
priceofapproximately25$/ton.Therefore(10800kg/1000)*25=270$.Thatsaid,
thetotalcostofcompostingthesoilandsellingitwouldcomebacktoalossof230$
includingthehassleofhavingsomeonefillandemptythevessels.
Fromtheeconomicpointofview,keepinginmindthatthereisnodiscounting,the
overallcostofoperatingatwohundredlitervesselseemstobehigherthansimply
sending the contaminated soil to a land fill. However, as taxation increases for
landfilling and as public opinion turns against the latter practice, it will not be a
viable option. Also, the reactors could be built on a larger scale to save time and
money. Also, some items like theheatexchangerwithin theHRSsystemcouldbe
recycled from a junkyard. Amore in debt analysis utilizing discount rates, Initial
Costanalysisandexact itempriceswouldyieldmoreconcurrentdataonwhether
suchaprojectcouldhaveanindustrialormunicipalapplication.
JAMALEDDINE&RAINVILLE2011
41
Conclusion
Havingdemonstratedtoa99.5%confidencelevelthattheHRSsystemisfunctional
andisdistributingtheheatuniformlythroughoutourcompostingmediawecansay
thattheinitialobjectiveoftestingandassessingtheeffectivenessoftheHRSsystem
has been attained. The secondpart being the design of theARS systemwas also
completed. Although the feat of designing and testing theHRS system is a design
project in itself theengineeringprocesswasalsoutilized in thesecondhalfof the
project, as the ARS system required a very rugged mathematical and simulation
intensive design process. One should note that the Composting Bioreactors have
receivedasignificantamountofattention in thepast fewmonths, investmentand
funding for the latter project, including an application for a patent is now a
possibility. Future considerations should involve the testing of theARS system as
done for the HRS system, meaning a three standard to three ARS fitted 200L
polyethylenevesselsshouldberunalongsidetodeterminetheeffectivenessofthe
latter. The ARS system should be able to maintain higher temperatures than the
standardbarrelsforaprolongedperiodoftime.Thereforethedataanalysiswould
compare maximal temperatures attained and the length of the time these
temperatures can be maintained in the standards and in the ARS fitted systems.
Another testcould involve fitting threevesselswith theHRSandARSdesignsand
threeonlywiththeHRS.Eitherofthelattertestswouldrequirearuggedstatistical
analysistovalidatethedataonceitiscompiled.
Inconclusion,onecanwithgreatcertaintydeclarethat fromanacademicpointof
viewtheCompostingBioreactorsareasuccessinthattheproofofconcepthasbeen
attained. From an economical or industrial point of view much testing and
manipulations should be done to better the reactors for larger scale use and to
lowerthecostofthefinalproduct.Inessence,theCompostingReactorsareawork
in progress as any other feasible engineering design; there is much room for
improvement.
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REFERENCES
Barrington,S.,D.Choiniere,M.TriguiandW.Knight.2002.Effectofcarbonsourceoncompostnitrogenandcarbonlosses.BioresourceTechnology83:189‐194.CCME. 2005. Guidelines for compost quality. Canadian Council ofMinisters of theEnvironment.PublicationNo.CCME1340.Ottawa,ON.ISBN1‐896997‐60‐0Duteilleul, P., D.E. Mather and B. Pelletier. 2009. Lecture Notes of the StatiscticalMehtods 1 Course (AEMA 310), Fall 2009 edition, McGill University, MacdonaldCampus.FoodandDrugAdministration(FDA).2010. Animal&Veterinary:FDARegulationofPetFood.http://www.fda.gov/animalveterinary/products/animalfoodfeeds/petfood/ucm2006475(2010/11/28).Golueke,C.1992.Bacteriologyofcomposting.Biocycle33:55‐55.Goodfellow. “Materials for Scientific and Industrial Research and Manufacturing,”retrieved on 04/01/2001, from: http://www.goodfellow.com/E/Silicon‐Carbide%27‐Sheet.html
Herrmann, R.F. and J.F. Shann. 1997. Microbial Community Changes During theCompostingofMunicipalSolidWaste.MicrobialEcology33:78‐85.Incropera,Dewitt,etal.2007.FundamentalsofHeatandMassTransfer,6thedition,JohnWiley&Sons,Inc,NJ,US.Kakaç, S. and H. Liu. 2002.Heat exchangers: selection, rating, and thermal design:CRCPress.Kawano, S., C. R. Kleijn, et al. (2004). Computational technologies forfluid/thermal/structural/chemical systems with industrial applications‐‐2004:presented at the2004ASMEPressureVessels andPipingConference : SanDiego,California,USA,AmericanSocietyofMechanicalEngineers.Kostolomov, I. and A. Kutushev. 2006. "Numerical investigation of air freeconvectioninaroomwithheatsource."ThermophysicsandAeromechanics13(3):393‐401.Kutz,M.2009.EnvironmentallyConsciousMaterialsHandling:JohnWiley&Sons.
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Martins,O.andT.Dewes.1992.Lossofnitrogenouscompoundsduringcompostingofanimalwastes.BioresourceTechnology42:103‐111.MetcalfandEddy,Inc.2003.WastewaterEngineering,Treatment,Disposal,Reuse,4thEdition,McGraw‐Hill,NewYork.Munson, B., Young, D. and Okiishi, H. 2005. Fundamentals of Fluid Mechanics, 5thedition,WileyPublisher,800pages.Napolitano, L. G. 1982. "Surface and buoyancy driven free convection." ActaAstronautica9(4):199‐215.Nakasaki,K.andOhtaki,A.2002.“Asimplenumericalmodelforpredictingorganicmatter decomposition on a fed‐batch composting operation,” Journal ofEnvironmentalQuality31:997‐1003.Ostrach,S.1953.“AnAnalysisofLaminerFreeConvectionFlowandHeatTransferAboutaFlatPlateParalleltotheDirectionoftheGeneratingBodyForce,”NationalAdvisoryCommitteeforAeronautics,Report1111.
Polpraset, C. 2007. Organic Waste Recycling; Technology and Management, 3rdedition.London,UK:IWAPublishing.Suleyman Yigit, K. “Applied Thermal Engineering,” Volume 25, Issues 17‐18,December2005,pages2790‐2799.To,W.M.andJ.A.C.Humphrey.1986."Numericalsimulationofbuoyant,turbulentflow‐‐I.Freeconvectionalongaheated,vertical,flatplate."InternationalJournalofHeatandMassTransfer29(4):573‐592.Whimsie Studio. “Copper Sheet Metal,” retrieved on 04/01/2001, from:http://www.whimsie.com/coppersheetwire.html
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APPENDIXD
SKETCHESOFTHECOMPOSTBIOREACTORS
Drawing1:HRS&ARSFull‐SizeRepresentation
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Drawing2:HRS&ARSFull‐SizeRepresentation
Drawing3:HRS&ARSfull‐sizerepresentationwithinnercontentsvisible.
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APPENDIXGSAMPLECALCULATIONS
Calculatingthecarboncontentindogfood(sameprocedureappliestowoodchips):
(Eq.2)
Meanpercentashobtainedfromlaboratory:6.824(Std.deviation:0.298).
Calculating the percentage of nitrogen in dog (same procedure applies to woodchips):C/Nratiois18,therefore:
Thereis2.83%Ninthedogfoodsamplesanalyzed.
Calculatingthecarboncontentperkgofwetdogfood:
(Eq. 3)
Calculatingthenitrogencontentperkgofwetdogfood:
(Eq. 4)
=0.0258kgC/kgwetdogfood
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Calculatingtotalsolids:
(Eq.5)
Calculatingashcontent:
(Eq.6)
Calculatingmoisturecontent,wetbasis.Theexamplebelowusesexperimentalvaluesfordogfood,sample1:
(Eq.7)
Where:Mwb:isthemoisturecontentonawetbasis(%)Si:istheinitialmassofthesample(g)Sdry:isthesamplemassafter24‐hourdrying(g)
Calculatingthemassofwaterabsorbedbywetwoodchips:Newmoisturecontent(MC)–initialMC=(53.21–16.57)%=36.64%MC
Thetotalmassofwoodchipstobeaddedperbarrelis19kg,therefore:
Andso,6.96litersofwaterwereabsorbedbythewoodchips.
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STATISTICSTheexampleusesmoisturecontents(%)calculatedfor3dogfoodsamples:Sample1:9.158Sample2:8.686Sample3:8.606Determiningsamplemean.
Determiningsamplevariance:
Determiningsamplestandarddeviation:
S=+√(S2)
S=√(0.089)
S=0.298
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SamplecalculationssetbCalculations for the statistical analysis on the equality of means are based onprocedurefromP.Dutilleuletal.(2009).1) Assumptions:
a. Populationsarenormallydistributed
2) Samplemeanforaveragedifferenceincontrolbarrels, =5.493Samplemeanforaveragedifferenceinheatredistributionbarrels, =4.742Samplevarianceforcontrols,S2C=9.652SamplevarianceforHRsystems,S2HR=5.019Samplesizesareidenticalforbothsamples:nc=nHR=825
3) Thenullhypothesis,H0:μc=μHR,statesthatthereisnosignificantdifferencebetweenbothpopulationmeans.The alternative hypothesis, H1 : μc > μHR , states that there is a statisticallysignificantdifferencebetweenbothpopulationmeans,theaveragetemperaturedifferencebeinggreaterinthecontrolsthanintheHRS.
4) Since the twopopulationvariances(σ2Candσ2HR)arenotknown,a two‐tailedtestabouttheequalityofthetwopopulationvariancesiscarriedout.Thenullandalternativehypothesesareasfollows:H0:σ2C=σ2HRH1:σ2C/σ2HR>1orσ2C/σ2HR<1The Fisher‐Snedecor’s F sampling distribution is used along with the teststatistic:F=S2C/S2HR~F(nC‐1,nHR‐1)Choosingasignificancelevelof5%,α=0.05,H0willberejectedifeitherofthefollowingconditionsaremet: S2C/S2HR>F1‐α/2(nC‐1,nHR‐1)or S2C/S2HR<Fα/2(nC‐1,nHR‐1) S2C/S2HR=9.652/5.019=1.923
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F0.975(824,824)=1.13(TableFfromDutilleuiletal.2009)Since1.923>1.13,thenullhypothesisisrejectedandthepopulationvariancesmaynotbeassumedtobeequal.
5) Since the hypothesis for homogeneity of population varianceswas rejected, atest statistic that follows a t distribution is then used to show whether thesamplemeansaresignificantlydifferent.
Teststatistic:
==5.632
6) Next,thenullhypothesisisrejectediftheobservedvalueoftheteststatistic
(5.632)isgreaterthanthecriticalvalue.Criticalvalue:t1‐α(effectivedegreesoffreedom)Effectivedegreesoffreedomdefinedby:
Effectivedegreesoffreedom=1499.35Significancelevel:α=0.05t1‐α(1499.35)=doesnotappearonthetablefortdistribution;thesecond
highestvaluebeing120.Hence,thevalueof∞willbeusedasdegreeoffreedom.
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T0.95(∞)=1.645;thenullhypothesisisrejectedsincetheobservedvalue
(5.632)isgreaterthan1.645.Evenwithasmallersignificancelevel,thenullhypothesisisrejected:T0.995(∞)=2.58;thenullhypothesisisrejectedsincetheobservedvalue
(5.632)isgreaterthan2.58.
7) Therejectionofthenullhypothesisindicatesthatthepopulationmeansarenotequalandthatindeedtheaveragetemperaturedifferenceinthecontrolbarrelsis larger than the average temperature difference in the heat redistributionsystems,andthismaybesaidwitha99.5%confidenceinterval.