Construction Methods - Chapter09.pdf - po ang - Januszek66

CompressedAir

andWaterSystems

9–1INTRODUCTION

ConstructionApplications

Compressedairiswidelyusedasapowersourceforconstructiontoolsandequipment.

Whilehydraulicpowerisgraduallyreplacingcompressedairasthepowersourceforrock

drills(seeChapter8),compressedairisstillrequiredforcleaningoutthedrillholepro-

ducedbyahydraulicdrill.Someoftheotherusesforcompressedairinconstructionin-

cludepaintspraying,thepneumaticapplicationofconcrete(shotcrete),conveyingcement,

pumpingwater,andoperatingpneumatictools.Commonpneumaticconstructiontoolsin-

cludespaders(ortrenchdiggers),concretevibrators,drills(steelandwood),grinders,ham-

mers,pavingbreakers,sandblastingguns,saws(circular,chain,andreciprocating),staple

guns,nailers,tampers,andwrenches.

Pumpsandwatersupplysystemsareutilizedinconstructiontodewaterexcavations

andtosupplywaterforcleaningequipmentandaggregates,formixingandcuringconcrete,

foraidingsoilcompaction,andforjettingpilesintoplace.

ConstructionManager’sResponsibilities

Theconstructionmanagermustbeabletoselecttheappropriatetypeandsizeofaircom-

pressororpumpforaconstructionoperationandtodesigntheassociatedairorwatersup-

plysystem.Sections9–2and9–3provideguidanceinperformingthesetasks.

9–2COMPRESSEDAIRSYSTEMS

TypesofCompressors

Aircompressorsmaybeclassifiedaspositivedisplacementcompressorsordynamic

compressorsaccordingtothemethodbywhichtheycompressair. Positivedisplacement

233

234

CHAPTER9

compressors achievecompressionbyreducingtheairvolumewithinaconfinedspace.

Positivedisplacementcompressortypesincludereciprocatingcompressors,rotaryvane

compressors,androtaryscrewcompressors. Dynamiccompressors achievecompression

byusingfansorimpellerstoincreaseairvelocityandpressure.Theprincipaltypeofdy-

namiccompressorusedinconstructionisthecentrifugalcompressor.Rotarycompres-

sors,bothpositivedisplacementanddynamic,aresmaller,lighter,andquieterthanare

reciprocatingcompressorsofsimilarcapacity.Asaresult,mostcompressorsusedincon-

structionarerotarycompressors.

AschematicdiagramofarotaryvaneaircompressorsystemisshowninFigure9–1.

Thiscompressorisclassifiedasatwo-stage,oil-flooded,sliding-vanerotarycompressor.

Oilisinjectedintoeachcompressorstageforlubricationandcooling.An oilseparator re-

movestheoilfromtheoutputair.Theoilisthencooledandreturnedforreuse.Theoutput

ofthecompressor’sfirststageiscooledbyan intercooler toincreasetheefficiencyofthe

second-stagecompressor.The receiver servesasacompressedairreservoir,providesaddi-

tionalcoolingoftheairleavingthecompressor,reducespressurefluctuationsintheoutput,

andpermitswatertosettleoutofthecompressedair.Compressorscansometimesoperate

satisfactorilywithoutareceiverinthesystem.

Theprincipleofoperationofasliding-vanerotarycompressorisasfollows(referto

Figure9–1).Asthecompressorrotorturns,centrifugalforcecausesthevanestomaintain

contactwiththecylinder.Airintakeoccurswhilethevolumeofairtrappedbetweentwo

adjacentvanesisincreasing,creatingapartialvacuum.Asrotationcontinues,thevolume

ofairtrappedbetweenadjacentvanesdecreases,compressingthetrappedair.Thecom-

pressedairisexhaustedontheoppositesideofthecylinderasthevolumetrappedbetween

vanesapproachesaminimum.

Compressorsareavailableasportableunits(skid-orwheel-mounted)orstationary

units.Althoughportableunitsaremostoftenusedinconstructionwork,stationarycom-

pressorsmaybeemployedinquarriesandsimilarpermanentinstallations.Portableunits

areavailableincapacitiesfrom75toover2000cuft/min(2.1to56.6m

/min).Figure9–2

showsasmallportableaircompressorbeingusedtopowertwopneumaticpavingbreak-

ers.Sincepneumatictoolsnormallyrequireairat90psig(lb/sqin.gauge)(621kPa)tode-

liverratedperformance,compressorsusuallyoperateinthepressurerangeof90to125psig

(621to862kPa).

Arotaryscreworhelicalscrewaircompressorutilizestwomatingrotatinghelicalro-

torstoachievecompression(seeFigure9–3).Themainormalerotorisdrivenbythepower

source.Thematinggateorfemalerotorisusuallydrivenbytiminggearsattachedtothe

mainrotorbutmaybedrivendirectlybythemainrotorinanoil-floodedunit.Theprinci-

pleofoperationisasfollows.Airentersattheinletend,wherethevolumebetweenmating

lobesislarge.Astherotorsturn,thetrappedvolumebecomessmallerandmovestoward

thedischargeend.Thetrappedvolumereachesaminimumasitlinesupwiththedischarge

portandthecompressedairisexhausted.Theadvantagesofrotaryscrewcompressorsin-

cludehighefficiency,fewmovingparts,lowmaintenance,andlonglife.

RequiredCompressorCapacity

Aircompressorratingsindicatecapacityasthevolumeof“standard”airor“free”airat

standardconditionswhichthecompressorwilldeliverataspecifieddischargepressure.

Figure9–1 Schematicofsliding-vanerotaryaircompressor.

236

CHAPTER9

Figure9–2 Smallportable

aircompressorpowering

pneumatictools.(Courtesyof

Ingersoll-RandCompany)

Figure9–3 Rotaryscrewair

compressor.

Standardairisdefinedasairatatemperatureof68°F(20°C),apressureof14.7psia(lb/sq

in.absolute)(100kPaabsolute),and36%relativehumidity.Sinceatmosphericconditions

ataconstructionsiterarelycorrespondexactlytostandardairconditions,itmaybeneces-

sarytoadjusttheratedcompressorcapacitytocorrespondtoactualconditions.Sinceairis

lessdenseataltitudesabovesealevel,altitudehasthemostpronouncedeffectoncom-

pressorcapacity.Amethodforadjustingactualairdemandatanaltitudeabovesealevelto

standardconditionsisexplainedlater.

Thequantityofcompressedairrequiredtosupplyaconstructionsiteisfoundbysum-

minguptheairdemandofallindividualtoolsandequipment.Representativeaircon-

sumptionvaluesforcommonpneumaticconstructionequipmentaregiveninTable9–1.

However,sincealltoolsofaparticulartypewillseldomoperatesimultaneously,atoolload

factorordiversityfactorshouldbeappliedtothetotaltheoreticalairrequirementforeach

typeoftool.Table9–2providessuggestedvaluesofthetoolloadfactortobeusedfor

237

COMPRESSEDAIRANDWATERSYSTEMS

Table9–1 Representativeairconsumptionvaluesofpneumaticconstruction

equipment

AirConsumption

TypeofEquipment Size cuft/min m

/min

Drills,rock

Hand-held 35-lbclass

55–75 1.6–2.1

45-lbclass

50–100 1.4–2.8

55-lbclass

90–110 2.5–3.1

Drifterandwagon Light(3 1

-in.bore) 190–250 5.1–7.1

Medium(4–4 1

-in.bore) 225–300 6.4–8.5

Heavy(5-in.bore) 350–500 9.9–14.2

Track

Medium-weight 600–1000 17.0–28.3

Heavyweight 900–1300 25.5–36.8

Paintsprayers

8–15 0.2–0.4

Pavingbreakers

40–60

1 .1–1.7

Pumps,submersible Lowhead

75–100 2.1–2.8

Highhead

150–300 4.2–8.5

Shotcreteguns

200–300 5.7–8.5

Spaders/trenchdiggers

30–50 0.8–1.4

Tampers,backfill

35–50 1.0–1.4

Table9–2 Suggestedvaluesoftoolloadfactor

forpneumaticconstructiontools

NumberofTools Diversity

ofSameType

Factor

1–6

1.00

7–9

0.94

10–14

0.89

15–19

0.84

20–29

0.80

30ormore

0.77

variousnumbersoftoolsofthesametype.Whereasystemisbeingdesignedtooperatesev-

eraldifferenttypesoftoolsorequipmentsimultaneously,itmaybeappropriatetoapplya

secondorjobloadfactortothesumoftheadjustedairdemandforthedifferenttypesof

tools.Ifused,thejobloadfactorshouldbebasedontheprobabilitythatthevarioustypes

oftoolswilloperatesimultaneously.Finally,anallowanceshouldbemadeforleakagein

theairsupplysystem.Itiscustomarytoadd5to10%totheestimatedairdemandasaleak-

ageloss.

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