SECTION 10Air-Cooled ExchangersAn air-cooled exchangeris used t
o cool fluids wit h ambientair. Sever al ar t icles have been
published descr ibing in det ailt heirapplicat ion and economic
analysis. (See Bibliogr aphy att heendoft hissect ion.)Thissect
iondescr ibest hegener aldesign of air-cooled exchanger s and pr
esent s a met hod of ap-pr oximat e sizing.ARRANGEMENT AND
MECHANICALDESIGNFigs. 10-2 and 10-3 show t ypical elevat ion and
plan views ofhor izont al air-cooled exchanger s as commonly used.
The basiccomponent sar eoneor mor et ubesect ionsser vedbyoneormor
e axial flow fans, fan dr iver s, speed r educer s, and an
en-closing and suppor t ing st r uct ur e.Air-cooled exchanger s ar
e classed as for ced dr aftwhen t het ube sect ion is locat ed on t
he dischar ge side of t he fan, and asinduced dr aftwhen t he t ube
sect ion is locat ed on t he suct ionside of t he fan.Advantages of
i nduced draft are: Bet t erdist r ibut ion of airacr oss t he sect
ion. Lesspossibilit yoft hehot effluent air r ecir culat ingar
oundt ot heint akeoft hesect ions.Thehot air isdis-Ai= inside sur
face of t ube, sq ftAb= out side bar e t ube sur face, sq ftAx= out
side ext ended sur face of t ube, sq ftAt= t ube inside cr oss-sect
ional ar ea, sq in. (see Fig. 9-25)ACFM = act ual cubic
feetperminut eAPF = t ot al ext er nal ar ea/ftof fint ube, sq ft
/ftAPSF = ext er nal ar ea of fint ube, sq ft /sq ftof bundle face
ar eaAR = ar ea r at io of fint ube compar ed t o t he ext er iorar
eaof 1 in. OD bar e t ubeB = cor r ect ion fact or, psi (see Fig.
10-14)Cp= specific heatataver age t emper at ur e, Bt u/(lb F)CMTD
= cor r ect ed mean t emper at ur e differ ence, FD = fan diamet
er, ftDi= inside t ube diamet er, in.Do= out side t ube diamet er,
in.DR= densit y r at io, t he r at io of act ual airdensit y t o t
hedensit y of dr y airat70F and 14.7 psia, 0.0749lb/cu ft(see Fig.
10-16)f = fr ict ion fact or(see Fig. 10-12)F = cor r ect ion fact
or(see Fig. 10-8)Fa= t ot al face ar ea of bundles, sq ftFp= airpr
essur e dr op fact or, in. of wat erperr owof t ubesFAPF = fan ar
ea perfan, ft2/fang = local acceler at ion due t o gr avit y, ft
/s2G = mass velocit y, lb/(sq ft s)Ga= airface mass velocit y,
lb/(hr sq ft ) of face ar eaGt= t ubeside mass velocit y, lb/(sq ft
s)ha= airside film coefficientBt u/(h sq ft F)hs= shell side film
coefficientbased on out side t ubear ea, Bt u/(h sq ft F)ht= t ube
side film coefficientbased on inside t ube ar ea,Bt u/(h sq ft F)J
= Jfact or(see Fig. 10-15)k = t her mal conduct ivit y, Bt u/[(hr
sq ft F)/ft ]L = lengt h of t ube, ftLMTD = logmean t emper at ur
ediffer ence, F (see Fig. 9-3)N = number of r ows of t ubes in dir
ect ion of flowNP= numberof t ube passesNR= modified Reynolds
number, (in lb/(sq ft s cp)Nt= numberof t ubesP = pr essur e dr op,
psiPF = fan t ot al pr essur e, inches of wat era= densit y of air,
lb/ cu ftw= densit y of wat er, lb/ cu ftP = t emper at ur e r at
io (see Fig. 10-8)Q = heatt r ansfer r ed, Bt u/hrd= fouling r
esist ance (fouling fact or ), (hr ft2 F/Bt u)rf= fluid film r
esist ance (r ecipr ocal of film coefficient )rmb= met al r esist
ance r efer r ed t o out side bar e sur facermx= met al r esist
ance r efer r ed t o out side ext endedsur faceR = t emper at ur e
r at io (see Fig. 10-8)S = specific gr avit y (wat er= 1.0)t = t
emper at ur e airside, FT = t emper at ur e t ube side, FU = over
all heatt r ansfercoefficient , Bt u/(h ft2 F)W = mass flow, lb/hrY
= cor r ect ion fact or, psi/ft(see Fig. 10-14) = viscosit y, cpw=
viscosit y ataver age t ube wall t emper at ur e, cp = viscosit y
gr adientcor r ect ionSubscri pts :a = airsideb = bar e t ube sur
face basiss = shell sidet = t ube sidex = ext endedt ube sur face
basis1 = inlet2 = out letFIG. 10-1Nomenclature10-1char ged upwar d
atappr oximat ely 212 t imes t he velocit yof int ake, orabout1500
ft /min. Lesseffect ofsun,r ain,andhail,since60%oft hefacear ea of
t he sect ion is cover ed. Incr easedcapacit y in t he eventof fan
failur e, since t henat ur al dr aftst ack effectis much gr eat
erwit h induceddr aft .Di sadvantages of i nduced draft are:
Higherhor sepowersince t he fan is locat ed in t he hotair.
Effluent air temperature should be limited to 200F, to
pre-ventpotentialdamagetofanblades,bearings,V-belts,orother
mechanical components in the hot air stream. The fan dr ive
component s ar e less accessible formaint e-nance, which may have t
o be done in t he hotairgener -at ed by nat ur al convect ion.
Forinletpr ocess fluids above 350F, for ced dr aftdesignshould
beused; ot her wise, fan failur e could subjectt hefan blades and
bear ings t o excessive t emper at ur es.Advantages of forced draft
are: Slight lylower hor sepower sincet hefanisincoldair.(Hor
sepower var iesdir ect lyast heabsolut et emper a-t ur e.) Bet t
eraccessibilit y of mechanical component s formain-t enance. Easily
adapt able forwar m airr ecir culat ion forcold cli-mat es.The
disadvantages of forced draft are: Poordist r ibut ion of airovert
he sect ion. Gr eat ly incr eased possibilit y of hotairr ecir
culat ion, duet o low dischar ge velocit y fr om t he sect ions and
absenceof st ack. Low nat ur al dr aftcapabilit y on fan failur e
due t o smallst ack effect . Tot al exposur e of t ubes t o sun, r
ain, and hail.The hor izont al sect ion is t he mostcommonly used
aircooledsect ion,andgener allyt hemost economical.For afluidwit
hfr eezingpot ent ial,t het ubesshouldbeslopedat least18
in.perfoott o t he out letheader. Since in mostcases t her e will
benopr oblem associat ed wit hfr eezing,and itismor ecost lyt
odesign a sloped unit , mostcooler s ar e designed wit h level
sec-t ions.Ver t ical sect ions ar e somet imes used when maximum
dr ain-age and head ar e r equir ed, such as forcondensing ser
vices.Angled sect ions, like ver t ical sect ions, ar e used
forcondens-ing ser vices, allowing posit ive dr ainage. Fr equent
ly, angle sec-t ionsar eslopedt hir t ydegr ees(30)fr omt hehor
izont al.A-fr ames ar e usually sloped sixt y degr ees (60) fr om t
he hor i-zont al. See Fig. 10-4.Forced draftDriver DriveassemblyFan
FanringSupportingstructureAir plenumchamberTube
sectionHeadersNozzlesInduced draftFan Fan ringAir
plenumchamberHeadersNozzles DriveassemblyDriverTubeSectionFIG.
10-2Typical Side Elevations of Air CoolersBaywidthBaywidthUnit
widthUnit widthTubelengthTubelengthTubelengthTubelengthTwo-fan bay
with2 tube bundlesTwo two-fan bays with6 tube bundlesOne-fan bay
with3 tube bundlesTwo one-fan bays with4 tube bundlesFIG.
10-3Typical Plan Views of Air CoolersNon-freezeDividedrear
headerTubebundleHot airHotairExhaust streamCoolair FIG. 10-4Angled
Section Layout10-2Fan sizes r ange fr om 3 ftt o 28 ftdiamet er.
However, 14 ftt o 16 ftdiamet eris t he lar gestdiamet ernor mally
used. Fandr iver s may be elect r ic mot or s, st eam t ur bines,
hydr aulic mo-t or s, orgas-gasoline engines. A speed r educer,
such as a V-beltdr ive orr educt ion gearbox, is necessar y t o mat
ch t he dr iverout putspeed t o t he r elat ively slow speed of t
he axial flow fan.Fant ipspeedsar enor mally12,000ft /minor
less.Gener alpr act iceist ouseV-belt dr ivesupt oabout
30bhpandgeardr ives athigher power. Individual dr iversize is
usually lim-it ed t o 50 hp.Twofanbaysar epopular,sincet hispr
ovidesadegr eeofsafet y againstfan ordr iverfailur e and also a met
hod of cont r olby fan st aging. Fan cover age is t he r at io of t
he pr oject ed ar eaoft hefant ot hefaceoft hesect ionser vedbyt
hefan.Goodpr act ice is t o keep t his r at io above 0.40
wheneverpossible be-cause higherr at ios impr ove airdist r ibut
ion acr oss t he face oft he t ube sect ion. Face ar ea is t he
plan ar ea of t he heatt r ansfersur face available t o airflow att
he face of t he sect ion.The heat -t r ansferdevice is t he t ube
sect ion, which is an as-sembly of side fr ames, t ube suppor t s,
header s, and fin t ubes.Aluminumfinsar enor mallyappliedt ot het
ubest opr ovidean ext ended sur face on t he airside, in or dert o
compensat e fort her elat ivelylowheat t r ansfer coefficient oft
heair t ot het ube. Fin const r uct ion t ypes ar e t ension-wr
apped, embedded,ext r uded, and welded.Tension-wr apped is pr
obably t he mostcommon fin t ype usedbecause of economics. Tension
wr apped t ubing is common forcont inuous ser vice wit h t emper at
ur es below 400F. Ext r udedfin is a mechanical bond bet ween an
innert ube exposed t o t hepr ocess and an out ert ube orsleeve
(usually aluminum) whichis ext r uded int o a high fin. Embedded
fin is an aluminum orst eel fin gr ooved int o t he base t ube.
Embedded fins ar e usedin cyclic and high t emper at ur e ser
vices. Ot hert ypes of finnedt ubes available ar e solder ed, edge
wr apped, and ser r at ed t en-sionwr apped.Cooler sar er egular
lymanufact ur edint ubelengt hs fr om 6 ftt o 50 ftand in bay widt
hs fr om 4 ftt o 30 ft .Use of longert ubes usually r esult s in a
less cost ly design com-par ed t o using shor t ert ubes.Base t ube
diamet er s ar e 58 in. t o 112 in. OD wit h fins fr om12 in. t o 1
in. high, spaced fr om 7 t o 11 perinch, pr oviding anext ended
finned sur face of 12 t o 25 t imes t he out side sur faceof t he
base t ubing. Tubes ar e usually ar r anged on t r iangularpit ch
wit h t he fin t ips of adjacentt ubes t ouching orsepar at edby fr
om 116 in. t o 14 in. Mat ching of t he t ube sect ion t o t he
fansyst em and t he heatt r ansferr equir ement s usually r esult s
int he sect ion having dept h of 3 t o 8 r ows of fin t ubes, wit h
4 r owst he mostt ypical.A 1-in. OD t ube is t he mostpopulardiamet
er, and t he mostcommonfinsar e 12 in. or58in.high. Thedat a pr
esent ed inFig. 10-11 ar e for1 in. OD t ubes wit h 12 in. high
fins, 9 fins/in.(designat edas 12x 9) and 58 in. high fins, 10
fins/in. (desig-nat ed as 58 x 10).Commonmat er ialsofconst r uct
ionfor header sar efir eboxqualit y car bon st eel, ASTM SA-515-70,
SA-516-70. Tubes ar egener ally ASTM SA-214 (ERW), SA-179 (SMLS),
car bon st eel.Louver s ar e gener ally car bon st eel, oraluminum
wit h car bonst eel const r uct ion being t he mostgener al and
mosteconomi-cal.Finsar enor mallyaluminum.Bot hst ainlessandbr
assalloyshavet heir applicat ionsbut ar emor eexpensivet hancar bon
st eel.HEADER DESIGNPlugheader const r uct
ionusesaweldedboxwhichallowspar t ial access t o t ubes by means of
shoulderplugs opposit e t het ubes.Plugheader sar enor mallyusedast
heyar echeapert han t he alt er nat e coverplat e design. Coverplat
e headercon-st r uct ion allows t ot al access t o header, t ube
sheet , and t ubes.This design is used in high fouling, low pr
essur e ser vice.Fig. 10-5showst ypicaldesignsfor bot hplugheader
andcoverplat e header.AIR-SIDE CONTROLAir-cooledexchanger sar
esizedt ooper at eat war m(sum-mer ) airt emper at ur es. Seasonal
var iat ion of t he airt emper a-t ur e can r esultin over-cooling
which may be undesir able. Oneway t o cont r ol t he amountof
cooling is by var ying t he amountofair flowingt hr ought het
ubesect ion.Thiscanbeaccom-plishedbyusingmult iplemot or
s,2-speeddr ives,var iablespeed mot or s, louver s on t he face of
t he t ube sect ion, orvar i-able pit ch fans.St aging of fans
orfan speeds may be adequat e forsyst emswhich do notr equir e pr
ecise cont r ol of pr ocess t emper at ur e orpr essur e. Louver s
will pr ovide a full r ange of airquant it y con-t r ol. Theymay
beoper at ed manually, oraut omat ically oper
-169103113111861735124 151718 14Cover plate
header16931105286316Plug header131112414715FIG. 10-5Typical
Construction of Tube Section with Plug and CoverPlate Headers1.
Tube sheet 7. St iffener 13. Tube keeper2. Plug sheet 8. Plug14.
Vent3. Top and bot t om plat es9. Nozzle15. Dr ain4. End plat e10.
Side fr ame16. Inst r umentconnect ion5. Tube11. Tube spacer 17.
Coverplat e6. Pass par t it ion12. Tube suppor tcr oss-member18.
Gasket10-3at ed by a pneumat ic orelect r ic mot orcont r olled fr
om a r emot et emper at ur e orpr essur e cont r ollerin t he pr
ocess st r eam. Lou-ver susedwit hconst ant speedfansdonot r
educefanpowerr equir ement s.Aut o-var iable-pit ch fans ar e nor
mally pr ovided wit h pneu-mat ically oper at ed blade pit ch
adjust mentwhich may be con-t r olled fr om a r emot e sensor.
Blade pit ch is adjust ed t o pr ovidet he r equir ed amountof
airflow t o maint ain t he pr ocess t em-per at ur eor pr essur eat
t hecooler.Ther equir edbladeangledecr easesasambient air t emper
at ur edr opsandt hiscon-ser ves fan power. Hydr aulic var iable
speed dr ives r educe fanspeed when less airflow is r equir ed and
can also conser ve fanpower.A design consider at ion which mightbe
r equir ed forsat isfac-t or y pr ocess fluid cont r ol is co-cur r
entflow. In ext r eme casesof high pourpointfluids, no amountof
airside cont r ol wouldallowsat isfact or ycoolingandpr event fr
eezing.Co-cur r entflow has t he coldestaircool t he hot t estpr
ocess fluid, while t hehot t estaircools t he coolestpr ocess
fluid. This is done in or dert o maint ain a high t ube wall t
emper at ur e. This gives a muchpoor er LMTD,but for
highlyviscousfluidsisoft ent heonlyway t o pr eventfr eezing
orunaccept able pr essur e dr ops. Wit haircooler s, t he
mostcommon met hod of accomplishing co-cur -r entflow is t o have t
he inletnozzle on t he bot t om of t he headerwit h t he pass ar r
angementupwar ds. This t ot ally r ever ses t hest andar d design,
and may cause a pr oblem wit h dr ainage dur -ing shut -downs. In
addit ion, airside cont r ol is necessar y wit hco-cur r
entdesigns.WARM AIR RECIRCULATIONExt r eme var iat ion in airt
emper at ur e, such as encount er edin nor t her n climat es, may r
equir e special airr ecir culat ion fea-t ur es.These ar eneeded t
o pr ovide cont r ol of pr ocess st r eamt emper at ur es, and t o
pr eventfr eezing of liquid st r eams. War mairr ecir culat ion var
ies fr om a st andar d coolerwit h one r ever s-ing fan t o a t ot
ally enclosed syst em of aut omat ic louver s andfans. These t wo
widely used syst ems ar e t er med int er nal r e-cir culat ion and
ext er nal r ecir culat ion.At ypicallayout for int er nalr ecir
culat ionisshowninFig.10-6. Dur ing low ambientoper at ion, t he
manual fan cont inuest o for ce airt hr ough t he inlethalf of t he
sect ion. The aut o-var i-able fan oper at es in a r ever sing
mode, and dr aws hotairfr omt he upperr ecir culat ion chamberdown
t hr ough t he out letendoft hesect ion.Becauseoft helower r ecir
culat ionskir t ,t hemanualfanmixessomeoft hehot air br ought
downbyt heaut o-var iable fan wit h cold out side airand t he pr
ocess r epeat s.The t op exhaustlouver s ar e aut omat ically
adjust ed by a t em-per at ur econt r oller sensingt hepr
ocessfluidst r eam.Ast hefluid t emper at ur e r ises, t he louver
s ar e opened. Dur ing designambient condit ions,t helouver sar
efullopenandbot hfansoper at e in a st andar d for ced dr aftmode.A
coolerwit h int er nal r ecir culat ion is a compr omise bet weenno
r ecir culat ion and fully cont r olled ext er nal r ecir culat
ion. Itis cheapert han full ext er nal r ecir culat ion, and has
less st at icpr essur elossdur ingmaximumambient t emper at ur
econdi-t ions. A coolerwit h int er nal r ecir culat ion is easiert
o er ect , andr equir es less plotar ea t han an ext er nal r ecir
culat ion design.However, t his lat t erdesign is mor e cost ly t
han a coolerwit hnor ecir culat ion,andcannot pr ovidecomplet efr
eezepr ot ec-Without recirculationAuto-variable fan(slight negative
pitch)Manual fan(on)ExhaustExhaust ExhaustAutomatic
louversAutomatic louvers (partially closed)upper
recirculationchamberCoilManual fan(on)Auto-variable fan(positive
pitch)Lower recirculation skirtMinimumNormal airflowRecirculated
airflowNormal airflowLower recirculationskirtUpper
recirculationchamberCoilWith recirculationFIG. 10-6Internal
Recirculation Design10-4tion. Because there is no control over air
intake, and fans
alonecannotfullymixair,stratifiedcoldairmaycontactthesection.With
the fans off, high wind velocity during low ambient condi-tions
could cause excessive cold air to reach the section.A typical
layout for external recirculation is shown in Fig. 10-7.During low
ambient temperature conditions, two-speed motors onlow speed, or
auto-variable fans at low pitch, are normally
used.Forthisdesign,thesidesofthecoolerareclosedwithmanuallouvers.
Over both ends, a recirculation chamber projects beyondthe section
headers, and provides a duct for mixing cold outsideair with warm
recirculated air. As with the internal recirculationdesign, the top
exhaust louvers are controlled by the temperatureof the process
fluid. However, this design provides for control ofthe inlet air
temperature. As the inlet air louver closes, an internallouver in
the end duct opens. These adjustments are determinedby a controller
which senses air temperature at the fan. Once
thesystemreachesequilibrium,itautomaticallycontrolsprocesstemperatureandpreventsexcessivecooling.Duringwarmweather,
the side manual louvers are opened, while close controlis
maintained by adjustment of the exhaust louvers.The external
recirculation design is preferred for critical controland
prevention of freezing. Once operational, it requires little
at-tention.Uponfailureofpowerorairsupply,thesystemclosesautomatically
to prevent freezing. It can be designed to automat-ically reduce
motor energy use when excess cooling is being pro-vided. The main
drawback for t his t ype of system is it s high cost
.Severalactuatorsandcontroldevicesarerequired,alongwithmore steel
and louvers. It is usually too large to be shop
assembled,andrequiresmorefieldassemblythananinternalsystem.Be-cause
of the need to restrict air intake, this design increases
thestaticpressure,causinggreaterenergyuse,and20-25%largermotors
than a standard cooler.Whendesigninganext er nalr ecir culat
ionunit ,consider a-t ion mustbe given t o t he plenum dept h and
ductwor k t o allowairmixing and pr eventexcessive st at ic pr
essur e loss. The lou-ver int akear eashouldbelar geenought okeept
heair flowbelow 500 ft /min dur ing maximum design condit ions.AIR
EVAPORATIVE COOLERSWet /dr y t ype (airevapor at ive cooler s)
aircooler s may be agood economical choice when a close appr oach t
o t he ambientt emper at ur eisr equir ed.Int hesesyst ems,t
hedesigner cant ake advant age of t he differ ence bet ween t he dr
y bulb and wetbulb t emper at ur es. Ther e ar e t wo gener al t
ypes of airevapo-r at ive coolercombinat ions used alt hough ot
hercombinat ionsar e possible:We tairtypeInt hist ype,t heair
ishumidifiedbyspr aying wat erint o t he airst r eam on t he
inletside of t he aircooler. The airst r eam may t hen pass t hr
ough a mistelimina-t ort o r emovet he excess wat er. The airt hen
passes overt hefinned t ubes atclose t o it s wet -bulb t emper at
ur e. If t he misteliminat or isnot used,t hespr ayshouldbeclean,t
r eat edwat erort he t ube/fin t ype and met allur gy should be
compat -ible wit h t he wat er.We t tubetype An airevapor at ive
coolermay be oper -at ed in ser ies wit h an aircoolerif t her e is
a lar ge pr ocess fluidt emper at ur e change wit h a close appr
oach t o t he ambient . Thepr ocess fluid ent er s a dr y finned t
ube sect ion and t hen passesint o a wet , plain t ube sect ion
(orappr opr iat e finned t ube sec-t ion). The airis pulled acr oss
t he wett ube sect ion and t hen,aft er dr oppingout t
heexcessmoist ur e,passesover t hedr yt ube sect ion.Access door
for each bayAutomatic louvers HandrailGratingwalkwayAutomatic
louversBug and lint screen when requiredManual
louversHingedaccessdoorFixed panel inrecirculation
compartmentCoilCoil guardManual louversBug and lint screen when
requiredFIG. 10-7External Recirculation Design10-5SPECIAL PROBLEMS
IN STEAMCONDENSERSTher e ar e oft en pr oblems wit h st eam
condenser s which needspecial at t ent ion att he design st
age.Imploding(collapsingbubbles)or knockingcan cr eat e vio-lent
fluidfor ceswhichmaydamagepipingor equipment .Thesefor cesar ecr
eat edwhenasubcooledcondensat eisdumpedint oat wo-phasecondensat
eheader,or whenlivest eampassesint osubcooledcondensat e.Thispr
oblemisavoidedbydesigningt hest eamsyst emandcont r olssot hatst
eam and subcooled condensat e do notmeetin t he syst
em.Non-condensable gas st agnat ion can be a pr oblem in t he
aircooled st eam condenserany t ime t her e is mor e t han one t
uber ow perpass. The t emper at ur e of t he airincr eases r ow by
r owfr ombot t omt ot opoft heair cooledsect
ion.Thecondensingcapacit y of each r ow will t her efor e var y wit
h each t ube r ow inpr opor t iont ot heTdr ivingfor ce.Sincet het
ubesar econ-nect ed t o common header s and ar e subjectt o t he
same pr es-sur e dr op, t he vaporflows int o t he bot t om r ows
fr om bot h ends.The non-condensables ar e t r apped wit hin t he t
ube att he pointof lowestpr essur e. The non-condensables cont inue
t o accumu-lat einallbut t het opr owsunt ilt heyr eacht het ubeout
let .The syst em becomes st able wit h t he condensat e r unning
outof t hese lowert ube r ows by gr avit y. This pr oblem can be
elimi-nat ed in sever al ways: By assigning only one t ube r ow
perpass. By connect ing t he t ube r ows att he r et ur n end wit h
180r et ur n bends and eliminat ing t he common header.AIR COOLER
LOCATIONCir culat ion of hotairt o t he fans of an aircoolercan gr
eat lyr educet hecoolingcapacit yofanair cooler.Cooler locat
ionshould t ake t his int o consider at ion.Si ngle Installati
onsAvoid locat ing t he air-cooled exchangert oo close t o
buildingsorst r uct ur es in t he downwind dir ect ion. Hotairvent
ing fr omt heair cooler iscar r iedbyt hewind,andaft er st r ikingt
heobst r uct ion,someoft hehot air r ecyclest ot heinlet
.Anin-duceddr aft fanwit hsufficient st ackheight alleviat est
hispr oblem, butlocat ing t he aircooleraway fr om such obst r uc-t
ions is t he bestsolut ion.An aircoolerwit h for ced dr aftfans is
always suscept ible t oair r ecir culat ion.Ift heair cooler
islocat edt oocloset ot hegr ound,causinghighinlet velocit iesr
elat ivet ot heexhaustair velocit yleavingt hecooler,t hehot air r
ecir culat ioncanbecomever ysignificant .For ceddr aft cooler sar
epr efer ablylocat ed above pipe lanes r elat ively high above t he
gr ound. In-duced dr aftcooler s ar e less likely t o exper ience r
ecir culat ionbecauset heexhaust velocit iesar enor mallyconsider
ablyhighert han t he inletvelocit ies.Banks of CoolersCooler s ar r
anged in a bank should be close t oget herorhaveair sealsbet weent
hemt opr event r ecir culat ion bet ween t heunit s. Mixing of
induced dr aftand for ced dr aftunit s in closepr oximit yt oeachot
her invit esr ecir culat ion.Avoidplacingcooler s atdiffer
entelevat ions in t he same bank.Avoid placing t he bank of cooler
s downwind fr om ot herheatgener at ing equipment .Since aircan
only ent eron t he ends of cooler s in a bank, t hebank should be
locat ed above gr ound high enough t o assur e ar easonably low
inletvelocit y.The pr evailing summerwind dir ect ion can have a pr
ofoundeffect ont heper for manceoft hecooler s.Nor mallyt
hebankshould be or ient ed such t hatt he wind flows par allel t o
t he longaxis of t he bank of cooler s, and t he it ems wit h t he
closestap-pr oacht ot heambientt emper at ur eshouldbe locat edon t
heupwind end of t he bank.Thesegener alizat ionsar ehelpfulinlocat
ingcooler s.TheuseofComput at ionalFluidDynamicst ost udyt heeffect
ofwind dir ect ion, velocit y, obst r uct ions, and heatgener at
ing ob-ject s should be consider ed t o assur e t he bestlocat ion
and or i-ent at ionofair cooledheat exchanger s,especiallyfor lar
geinst allat ions.MULTIPLE SERVICE DISCUSSIONIf differ entser vices
can be placed in t he same plotar ea wit h-outexcessive piping r
uns, itis usually less expensive t o com-binet hemononest r uct ur
e,wit heachser vicehavingasepar at e sect ion, butshar ing t he
same fan and mot or s. Sepa-r at elouver smaybeplacedoneachser
vicet oallowinde-pendent cont r ol.Thecost andspacesavingsmakest
hismet hod common pr act ice in t he aircoolerindust r
y.Indesigningmult ipleser vicecooler s,t heser vicewit ht hemostcr
it ical pr essur e dr op should be calculat ed fir st . This
isbecause t he pr essur e dr op on t he cr it ical it em mightr est
r ictt he maximum t ube lengt h t hatt he ot herser vices could t
oler -at e. The bur den of for cing mor e t han one ser vice int o
a singlet ube lengt h incr eases t he possibilit y of design er r
or s.Sever alt r ial calculat ions may be needed t o obt ain an
efficientdesign.Aft er allser viceplot ar eashavebeenest imat
ed,combinet hem int o a unithaving a r at io of 2 or3 t o 1 in
lengt h t o widt h(assumingat wofancooler ).Aft er assumingat
ubelengt h,calculat e t he mostcr it ical ser vice forpr essur e dr
op using t heassumed numberand lengt h of t ubes and a single pass.
If t hedr op isaccept able orver y close, calculat e t he cr it
ical ser vicecomplet ely. Once a design fort he mostcr it ical ser
vice has beencomplet ed, follow t he same pr ocedur e wit h t he
nextmostcr it i-cal ser vice. Aft ert he second orsubsequentser
vices have beenr at ed, itis oft en necessar y t o lengt hen orshor
t en t he t ubes orchange t he over all ar r angement . If t ubes
need t o be added forpr essur e dr op r educt ions in alr eady over
sur faced sect ions, itmightbe mor e costeffect ive t o add a r
ow(s) r at hert han wident he ent ir e unit . The fan and mot
orcalculat ions ar e t he sameas fora single ser vice unit ,
exceptt hatt he quant it y of airusedmustbe t he sum of airr equir
ed by all ser vices.CONDENSING DISCUSSIONThe example given cover s
cooling pr oblems and would wor kwit h st r aightline condensing pr
oblems t hathave t he appr oxi-mat er angeofdewpoint t obubblepoint
oft hefluid.Wher ede-super heat ingor subcoolingor wher edispr opor
t ionat eamount s of condensing occuratcer t ain t emper at ur es,
as wit hst eamandnon-condensables,calculat ionsfor air cooler
sshouldbedonebyzones.Aheat r eleasecur vedevelopedfr om ent halpy
dat a will show t he quant it y of heatt o be dissi-pat edbet
weenvar ioust emper at ur es.Thezonest obecalcu-lat edshouldbest r
aight linezones;t hat is,fr omt heinlett emper at ur eofazonet oit
sout let ,t heheat loadper degr eet emper at ur e is t he same.Aft
ert he zones ar e det er mined, an appr oximat e r at e
mustbefoundfor eachzone.Dot hisbyt akingr at esfr
omvapor10-6cooling,condensing,andliquidcooling,t henaver aget
hesebasedont heper cent ofheat loadfor t hat phasewit hint hezone.
Next , calculat e t he LMTD of each zone. Begin wit h t heout
letzone using t he final design out lett emper at ur e and t
heinlett emper at ur e of t hatzone. Cont inue t o calculat e t he
zoneas if itwer e a cooler, exceptt hatonly one pass and one ort
wor ows should be assumed, depending on t he per cent age of
heatloadint hat zone.Incalculat ingt hepr essur edr op,aver
agecondit ions may be used forest imat ing.If t he calculat ions
forzone one (orlat era succeeding zone)show a lar ge numberof shor
tt ubes wit h one pass, as is usuallyt hecasewit hst
eamandnon-condensables,r ecalculat et hezone wit h mult iple r ows
(usually four ) and shor tt ubes havingone pass t hatuses only a
per cent age of t he t ot al pr essur e dr opallowed. The t ot al
coolerwill be calculat ed as if each zone wer ea coolerconnect ed
in ser ies t o t he nextone, exceptt hatonlyt ube pr essur e dr ops
should be calculat ed fort he middle zones.Thus, each zone musthave
t he same numberof t ubes and t r ueambientmustbe used in calculat
ing t he LMTD. Only t he t ubelengt h may var y, wit h odd lengt hs
fora zone accept able as longas over all lengt h is r ounded t o a
st andar d t ube lengt h.Ift hecalculat ionsfor
zoneone(andsucceedingzones)fitwell int o a longert ube lengt h, t
he LMTD mustbe weight ed.Aft ert he out letzone has been calculat
ed, calculat e zone t wousing t he inlett emper at ur e foritand it
s out lett emper at ur e,which is t he inlett emper at ur e of zone
one. The ambient usedt o find t he zone t wo LMTD will be t he
design ambientplus t heairr ise fr om zone one. Cont inue in t his
manner, always usingt hepr eviouszones out let airt emper at ur ein
calculat ing t hecur r entzones LMTD. Aft ert he coolersize and
configur at ionhave been det er mined, t he fan and mot orcalculat
ions will bemade in t he nor mal manner.The ult imat e pr essur e
dr op is t he sum of t he dr ops foreachzone orappr oximat ely t he
sum of t he dr op foreach phase usingt he t ube lengt h and pass ar
r angementforeach phase. An es-t imat ed over all t ube side
coefficientmay be calculat ed by es-t imat ing t he
coefficientforeach phase. Then t ake a weight edaver age based on t
he per cent age of heatload foreach phase.Thet ot alLMTDmust bet
heweight edaver ageoft hecalcu-lat ed zone LMTDs.THERMAL
DESIGNThebasicequat iont obesat isfiedist hesameasgiveninSect ion
9, HeatExchanger s:QUA CMTDEq 10-1Nor mally Q is known, U and CMTD
ar e calculat ed, and t heequat ionissolvedfor A.Theambient air t
emper at ur et obeused will eit herbe known fr om available
plantdat a orcan beselect ed fr om t he summerdr y bulb t emper at
ur e dat a given inSect ion 11, Cooling Tower s. The design
ambientairt emper a-t ur e is usually consider ed t o be t he dr y
bulb t emper at ur e t hatis exceeded less t han 5 per centof t he
t ime in t he ar ea wher et he inst allat ion is r equir ed.A
complicat ion ar ises in calculat ing t he LMTD because t
heairquant it y is a var iable, and t her efor e t he airout lett
emper a-t ur e is notknown. The pr ocedur e given her e st ar t s
wit h a st epforappr oximat ing t he air-t emper at ur e r ise. Aft
ert he air-out -lett emper at ur e has been det er mined, t he cor
r ect ed LMTD iscalculat ed in t he mannerdescr ibed in t he shell
and t ube sec-t ion,exceptt hat MTDcor r ect ionfact or st obe used
ar e fr omFigs. 10-8 and 10-9 which have been developed fort he cr
oss-flow sit uat ion exist ing in air-cooled exchanger s.Fig. 10-8
is forone t ube pass. Itis also used formult iple t
ubepassesifpassesar esidebyside.Fig.10-9isfor t wot ubepasses and
is used if t he t ube passes ar e overand undereachot her. A MTD
cor r ect ion fact orof 1.0 is used forfourormor epasses, if passes
ar e overand undereach ot her. A cor r ect ionfact orof 1.0 may be
used as an appr oximat ion fort hr ee passes,alt hought hefact or
willbeslight lylower t han1.0insomecases.Thepr ocedur efor t het
her maldesignofanair cooler con-sist s of assuming a select ion and
t hen pr oving itt o be cor r ect .Thet ypicalover allheat t r
ansfer coefficient sgiveninFig.10-10 ar e used t o appr oximat e t
he heatt r ansferar ea r equir ed.The heatt r ansferar ea is conver
t ed t o a bundle face ar ea usingFig. 10-11whichlist st heamount
ofext endedsur faceavail-ableper squar efoot ofbundlear ea fort wo
specificfin t ubeson t wo differ entt ube pit ches for3, 4, 5, and
6 r ows. Aft eras-suming a t ube lengt h, Fig. 10-11 is also used t
o ascer t ain t henumberof t ubes. Bot h t he t ube side and
airside mass veloci-t ies ar e now det er minable.The t ube-side
film coefficientis calculat ed fr om Figs. 10-12and 10-13. Fig.
10-17 gives t he air-side film coefficientbasedonout sideext
endedsur face.Sinceallr esist ancesmust bebased on t he same sur
face, itis necessar y t o mult iply t he r e-cipr ocal of t he t
ube-side film coefficientand t ube-side foulingfact orby t he r at
io of t he out side sur face t o inside sur face. Thisr esult s in
an over all t r ansferr at e based on ext ended sur face,designat
ed as Ux. The equat ion forover all heatt r ansferr at eis:1 Ux
|
.1 ht `
, |
. Ax Ai`
, + rdt |
. Ax Ai`
, + rmx + 1 ha Eq 10-2Thebasicequat ionwillt henyieldaheat t r
ansfer ar eainext ended sur face, Ax, and becomes:Q(Ux) (Ax)
CMTDEit hermet hod is valid and each is used ext ensively by t her
-maldesignengineer s.Fig.10-10givest ypicalover allheatt r
ansfercoefficient s based on bot h ext ended sur face and out -side
bar e sur face, so eit hermet hod may be used. The ext endedsur
face met hod has been select ed foruse in t he example
whichfollows. The air-film coefficientin Fig. 10-17 and t he airst
at icpr essur edr opinFig.10-18ar eonlyfor 1in.ODt ubeswit
h58in.highfins,10finsper inchon214in.t r iangular pit ch.Refert o
Bibliogr aphy Nos. 2, 3, and 5 forinfor mat ion on ot herfin
configur at ions and spacings.Theminimumfanar eaiscalculat edinSt
ep16usingt hebundle face ar ea, numberof fans, and a minimum fan
cover ageof0.40.Thecalculat edar eaist henconver t edt oadiamet
erandr oundedupt ot henext availablefansize.Theair-sidest at ic pr
essur e is calculat ed fr om Fig. 10-18 and t he fan t ot alpr
essur e is est imat ed using gr oss fan ar ea in St ep 20.
Finally,fanhor sepower iscalculat edinSt
ep21assumingafaneffi-ciency of 70%, and dr iverhor sepoweris est
imat ed by assum-ing a 92%-efficientspeed r educer.Example 10-1 Pr
ocedur e forest imat ing t r ansfersur face,plotar ea, and hor
sepowerRequired data for hot flui dName and phase:48API hydr ocar
bon liquidPhysical pr oper t ies atavg t emp = 200FCp0.55 Bt u/(lb
F)0.51 cp10-7FIG. 10-8MTD Correction Factors (1 Pass Cross Flow,
Both Fluids Unmixed)FIG. 10-9MTD Correction Factors (2 Pass Cross
Flow, Both Fluids Unmixed)10-8k0.0766 Bt u/[(hr sq ft F)/ft ](Fr om
t his Dat a Book Sect ion 23)Heatload:Q=15,000,000 Bt u/hrFlow
quant it y:Wt=273,000 lb/hrTemper at ur e in:T1=250FTemper at ur e
out :T2=150FFouling fact orrdt=0.001 (hr sq ft F)/Bt uAllowable pr
essur e dr op:Pt=5 psiRequired data for ai rAmbientt emper at ur
e:t1=100FElevat ion:Sea level (see Fig. 10-16 foralt it udecor r
ect ion)CPair = 0.24 Bt u/(lb F)Basi c as sumptionsType:For ced dr
aft , 2 fansFint ube:1 in. OD wit h 58 in. high finsTube pit ch:2
12 in. t r iangular()Bundle layout :3 t ube passes, 4 r ows of t
ubes, 30 ftlong t ubesFi rs t trial1. Pickappr oximat eover allt r
ansfer coefficient fr omFig.10-10. Ux=4.22. Calculat e appr oximat
e airt emper at ur e r iseta|
.Ux + 110`
, |
.T1 + T22 t1`
,ta|
.4.2 + 1.010`
, |
.250 + 1502 100`
,52F3. Calculat e CMTDHydocar bonAir 250 15298
150 10050 LMTD=71.3F (see Fig. 9-3)CMTD=(71.3) (1.00)=71.3F (3 t
ube passes assumed)4. Calculat e r equir ed sur faceAxQ(Ux)
(CMTD)Ax15,000,000(4.2) (71.3)50,090 sq ft5. Calculat e face ar ea
using APSF fact orfr om Fig. 10-11 FaAxAPSFFa50,090107.2467 sq ft(4
r ows assumed)6. Calculat e unitwidt h wit h assumed t ube lengt
hSer vice1in.Fint ube12in.by958in.by10UbUxUbUx1.Wat er& wat
ersolut ions(Seenot ebelow)Engine jacketwat er(rd =
0.001)1107.51306.1Process wat er(rd = 0.002)956.51105.250-50 et
hylene glycol-wat er(rd = 0.001)906.21054.950-50 et hylene
glycol-wat er(rd = 0.002)805.5954.42.Hydr ocar bon liquid cooler
sViscosit y,cp,at avg.t
emp.UbUxUbUx0.2855.91004.70.5755.2904.21.0654.5753.52.5453.1552.64.0302.1351.66.0201.4251.210.0100.7130.63.Hydr
ocar bon gas cooler sPr essur
e,psigUbUxUbUx50302.1351.6100352.4401.9300453.1552.6500553.8653.0750654.5753.51000755.2904.24.Airand
flue-gas cooler sUse one-half of value given forhydr ocar bon gas
cooler s.5.St eam Condenser s (At mospher ic pr essur e &
above)UbUxUbUxPure St eam(r d = 0.0005)1258.61456.8St eam wit
hnon-condensibles604.1703.36.HC condenser
sCondensing*Range,FUbUxUbUx0range855.91004.710range805.5954.425range755.2904.260range654.5753.5100&over
range604.1703.37.Ot hercondenser sUbUxUbUxAmmonia1107.61306.1Fr eon
12654.5753.5Not es:Ubisoverallr at ebasedonbaret ubear
ea,andUxisover allr at ebased on ext ended sur
face.Basedonapproximat eairfacemassvelocit iesbet
ween2600and2800lb/(hrsq ftof face area).*Condensingrange=hydr ocar
boninlet t emper at uret ocondensingzoneminus hydr ocar bon out
lett emperat ur e from condensing zone.FIG. 10-10Typical Overall
Heat-Transfer Coefficients for Air Coolers10-9Widt hFaLWidt
h4673015.57 ftFor simplificat ionr oundt hisanswer t o15.5,t
husFa=465 (30-ft -long t ubes assumed)7. Calculate number of tubes
using APF factor from Fig. 10-11NtAx(APF) (L)Nt50,090(5.58)
(30)2998. Calculat e t ube-side mass velocit y fr om assumed
numberof passes and r eading At fr om Fig. 9-25 fora 1 in. OD x16
BWG t ubeAt = 0.5945 sq in.Gt(144) (Wt) (Np)(3600) (Nt)
(At)Gt(0.04) (273,000) (3)(299) (0.5945)184 lb/(ft2 sec)9. Calculat
e modified Reynolds numberNR(Di) (Gt)(0.87) (184)0.5131410.
Calculat et ube-sidepr essur edr opusingequat ionfr omFig. 10-14
and fr om Fig. 10-15PtfYLNp + BNpPt(0.0024) (14.5) (30) (3)0.96 +
(0.25) (3)4.0 psi(isadifficultfunct ion t o calculat er igor ously,
see Fig.10-19)11. Calculat e t ube-side film coefficientusing equat
ion fr omFig. 10-15 andk |
.Cpk`
,13 fr om Fig. 10-13htJ k |
.Cpk`
,13 Di(1900) (0.12) (0.96)0.8725212. Calculat e airquant it
yWaQ(0.24) (ta)Wa15,000,000(0.24) (52)1,200,000 lb/hr13. Calculat e
airface mass velocit yGaWaFalb/(hr sq ftof face ar
ea)Ga1,200,0004652,58114. Read air-side film coefficientfr om Fig.
10-17ha8.515. Calculat e over all t r ansfercoefficientAxAi(AR)
(Do)DiAxAi(21.4) (1.0)0.8724.61Ux|
.1ht`
, |
.AxAi`
,+ rdt |
.AxAi`
, + rmx + 1ha1Ux|
.1252`
, (24.6) + (0.001) (24.6) + 18.5Ux4.17(rmx is omit t ed fr om
calculat ions, since met al r esist anceis small compar ed t o ot
herr esist ances)Secondandsubsequenttri als .IfUxcalculat edinSt ep
15 is equal orslight ly gr eat ert han Ux assumed in St ep
1,andcalculat edpr essur edr opinSt ep9iswit hinallowablepr essur e
dr op, t he solut ion is accept able. Pr oceed t o St ep 16.Ot her
wise, r epeatSt eps 1-15 as follows:1. AssumenewUxbet weenvalueor
iginallyassumedinSt ep 1 and value calculat ed in St ep 15.Fi nHei
ghtby Fi ns/inch12i n. by958 i n.by 10APF, sqft /ft 3.805.58AR,
sqft /sqft 14.521.4Tube Pit ch2in.214 in. 214 in. 238 in. 212 in.
APSF(3r ows)68.460.689.184.880.4(4r ows)91.280.8118.8113.0107.2(5r
ows)114.0101.0148.5141.3134.0(6r ows)136.8121.2178.2169.6160.8Not
es:APF ist ot alext er nalar ea/ftof fint ube insq ft /ft .ARis t
hear ear at iooffint ubecompar ed t o t heext er iorar eaof1in.OD
bar e t ube which has 0.262 sqft /ft .APSF ist he ext er nal ar ea
insq ft /sq ft ofbundleface ar ea.FIG. 10-11Fintube Data for 1-in.
OD Tubes10-10FIG. 10-12Friction Factor for Fluids Flowing Inside
Tubes10-11FIG. 10-13Physical Property Factor for Hydrocarbon
Liquids10-12FIG. 10-14Pressure Drop for Fluids Flowing Inside
Tubes10-13FIG. 10-15J Factor Correlation to Calculate Inside Film
Coefficient, h t10-142. Adjust tabyincr easingtaifcalculat
edUxishighert hanassumedUx,or decr easingtaifcalculat edUxislowert
han assumed Ux.3.-15. Recalculat evaluesinSt
eps3-15changingassumednumber ofpassesinSt eps3and8,andt ubelengt
hinSt ep6,ifnecessar yt oobt ainapr essur edr opascalcu-lat ed in
St ep 9 as high as possible wit houtexceeding t heallowable.16.
Calculat e minimum fan ar ea.Fan ar ea/fa nFAP F(0.40) (Fa)(No.
fans)FAPF(0.40) (465)293 ft2 (2 fans assumed)17. Fan diamet er [4
(FAPF)/]0.5=[4 (93)/3.1416]0.511 ft(r ounded up)18. Calculat eair
st at icpr essur edr opusingFpfr omFig.10-18 and DR atavg airt emp
fr om Fig. 10-16.Ta, avg100F + 152F2126FPa(Fp) (N)(DR)Pa(0.10)
(4)0.900.44 i n ch esof wat er19. Calculat e act ual airvolume
using DR of airatfan inlet .t1=100F250200150100500-501000.5 0.6 0.7
0.8 0.9 1.0 1.1 1.2 1.3 1.4Density ratio, Dg,
dimensionlessTemperature, FReference state dry air at 70Fand sea
level, 14.7 psiaElevation
Ft.8,0007,0006,0005,0004,0003,0002,0001,0000FIG. 10-16Air-Density
Ratio ChartFIG. 10-17Air Film CoefficientFIG. 10-18Air
Static-Pressure DropCorr ect ion fact or when|
.w`
, 0.14(See Fig. 10-15)Cor rect ionFact or, 1. Hydr ocar bon
vapor ; st eam; wat er 1.02. Hydr ocar bon liquids (18 t o 48 API),
MEA/DEAsolut ions0.963. Wat er /glycol solut ions; heatt r
ansferfluids0.924. Lube oils; heavy pet roleum fract ions (10 t o
18 API)0.85 When N r< 17, ( w)0.25 A Reynolds numberof less t
han 17 is onlylikelyforlubeoilsor heavypet roleumfract
ions.Theminimumr ecom-mendedvalueoft ouseinSt ep10is0.80,event
hought hecalculat edvalue may be lower.FIG. 10-19Correction Factor
for Fluid Viscosity Within the Tubes10-15ACFMWa(DR) (60)
(0.0749)ACFM1,200,000(0.94) (60) (0.0749)284,000 Tot alor142,000 /
Fan20. Appr oximat e fan t ot al pr essur e using DR of airatfan
andfan ar ea.PFPa +
ACFM 4005|
. D2 4 `
, ]]]]] 2 (DR)Wher e: 40052 g w (3600) a 12 at70FPF0.44 + |
.142,000(4005) (0.785) (112)`
,2 (0.94)0.57 inches of wat er21. Appr oximat ebr akehor sepower
per fan,using70%fanefficiency.bhp(ACFM/fan) (PF)(6356) (0.70)Wher e
t he conver sion fact or6356|
.33,000 ft lbmin hp`
, |
.12 in.ft`
, |
.ft362.3 lb`
,Not e:62.3 is t he weightof one cubic footof wat
erat60F.bhp(142,000) (0.57)(6356) (0.70)18.2Act ualfanmot or
neededfor 92%efficient speedr educer is18.2/0.92 = 19.8 hp. Fort
his applicat ion, 20 hp dr iver s wouldpr obably be select
ed.Soluti on:(15.5 ft ) (30 ft )=465 sq ft(465 sq ft ) (APSF)=ext
ended sur face ar ea(465) (107.2)=49,848 sq ftTher efor e,oneunit
having49,848sqft ofext endedsur face,t wo 11 ftdiamet erfans, and t
wo 20 hp fan dr iver s, is r equir ed.MAINTENANCE AND INSPECTIONAt
t ent iont ot hedesignoft heair cooler,andt hechoiceofmat er ials,
is essent ial t o pr ovide low maint enance oper at ion.Majorfact
or s t o be consider ed ar e at mospher ic cor r osion, cli-mat
iccondit ions,andt emper at ur
ecyclingoffluidbeingcooled.Scheduledpr event ivemaint
enanceandinspect ionist hekey t o t r ouble-fr ee aircooleroper at
ion. A check of all fans forvibr at ion should be made r egular ly.
Att he fir stsign of unduevibr at ion on a unit , t he unitshould
be shutdown att he ear liestoppor t unit y fort hor ough examinat
ion of all moving par t s. Asemi-annual inspect ion and maint
enance pr ogr am should: Check and r eplace wor n orcr acked belt
s. Inspect fanbladesfor deflect ionandfor cr acksnearhubs. Gr ease
all bear ings. Change oil in geardr ives. Checkt heinsideoft
ubesect ionfor accumulat ionofgr ease, dir t , bugs, leaves, et c.,
and schedule cleaning be-for e t ubes become packed wit h such debr
is.BIBLIOGRAPHY1. A.P.I.St andar d661,Air CooledHeat Exchanger sfor
Gener alRefiner y Ser vices.2. Br iggs, D. E., Young, E. H.,
Convect ion HeatTr ansferand Pr es-sur eDr opofAir FlowingAcr ossTr
iangular Pit chofTubes,ChemicalEngineer ingPr ogr essSymposiumSer
ies,Volume59,No. 41, 1963.3. Cook, E. M., AirCooled HeatExchanger
s, Chemical Engineer -ing,May25,1964,p.137;J
uly6,1964,p.131;andAugust 3,1964, p. 97.4. Gar dner, K. A.,
Efficiency of Ext ended Sur faces, Tr ans ASME,Volume 67, 1945, pp.
621-631.5. Robinson,K.K.,Br iggs,D.E.,Pr essur eDr opofAir
FlowingAcr oss Tr iangularPit ch Banks of Finned Tubes, Chemical
En-gineer ing Progress Symposium Series, Volume 62, No. 64, 1966.6.
Rubin, Fr ank L., Wint er izing AirCooled HeatExchanger s, Hy-dr
ocar bon Pr ocessing, Oct ober1980, pp. 147-149.10-16
