air conditioning cycles.ppt

air conditioning cycles

1. Air Conditioning cycles

2. Summer air conditioning systems:
1-Simple system with 100 % re-circulated air:

3. conditionspaceisassumedtobeatthesameconditions
asthatoftheconditionedspace.Thesupplyair
conditionshouldbesuchthatasitflowsthroughthe
conditionedspaceitcancounteractthesensibleand
latentheattransferstakingplacefromtheoutsidetothe
conditionedspace,sothatthespacecanbemaintained
atrequiredlowtemperatureandhumidity.Assumingno
heatgainsinthesupplyandreturnductsandnoenergy
additionduetofans,andapplyingenergybalance
acrosstheroom;theRoomSensibleCoolingload(Q
s,r),
RoomLatentCoolingLoad(Q
l,r)andRoomTotalCooling
load(Q
t,r)aregivenby:

4. FromtheRSHFvalueonecancalculatetheslopeof
theprocessundergonebytheairasitflowsthroughthe
conditionedspace(processs-i)as:
Sincetheconditioniisknownsay,fromthermalcomfort
criteria,knowingtheslope,onecandrawtheprocess
lines-ithroughi.Theintersectionofthislinewiththe
saturationcurvegivestheADPofthecoolingcoilas
showninFig.Itshouldbenotedthatforthegivenroom
sensibleandlatentcoolingloads,thesupplycondition
mustalwayslieonthislinesothattheitcanextractthe
sensibleandlatentloadsontheconditionedspacein
therequiredproportions.

5. Sincethecasebeingconsideredisoneof100%re-
circulation,theprocessthattheairundergoesasitflows
throughthecoolingcoil(i.e.processi-s)willbeexactly
oppositetotheprocessundergonebyairasitflows
throughtheroom(processs-i).Thus,thetemperature
andhumidityratioofairdecreaseasitflowsthroughthe
coolingcoilandtemperatureandhumidityratioincrease
asairflowsthroughtheconditionedspace.Assumingno
heattransferduetotheductsandfans,thesensibleand
latentheattransferratesatthecoolingcoilareexactly
equaltothesensibleandlatentheattransferratestothe
conditionedspace;i.e.,

6. Iftheby-passfactor(X)ofthecoolingcoilisknown,
then,fromroomconditions,coilADPandby-passfactor,
thesupplyairtemperaturetsisobtainedusingthe
definitionofby-passfactoras:
Oncethesupplytemperaturetsisknown,thenthe
massflowrateofsupplyairis:
Fromthemassflowrateofairandconditioni,the
supplyairhumidityratioandenthalpyareobtained:

7. Therequiredmassflowrateofsupplyairdecreasesas
theby-passfactorXdecreases.Inthelimitingcase
whentheby-passfactoriszero,theminimumamountof
supplyairflowraterequiredis:
Thuswith100%re-circulatedair,theroomADPis
equaltocoilADPandtheloadonthecoilisequalto
theloadontheroom.

8. 2-System with outdoor air for ventilation:
a-Case i) By-pass factor of the cooling coil is zero:

9. Theintersectionofthislinewiththesaturationcurve
givestheroomADP.Asshownonthepsychrometric
chart,whentheby-passfactoriszero,theroomADPis
equaltocoilADP,whichinturnisequaltothe
temperatureofthesupplyair.Hencefromthesupply
temperatureonecancalculatetherequiredsupplyair
massflowrate(whichistheminimumrequiredasXis
zero)usingtheequation:

10. Calculation of coil loads:

11. The difference between the cooling load on the coil and
cooling load on the conditioned space

19. 3-Summer cycle with return and by pass air:
Cooling
coil
Fan Room
O S S
RR
R
R
Lm

20. R
m
L
O
S
O
m
o/ m
R= Rm / om
M
R/ M
L= M
R/ (m
o+ m
R) = LS /
RS
C.C = (m
o+ m
R) (h
m–h
L)
R.L = m
s(h
R–h
s)
m
s= m
o+ m
R+ M
R

21. Cooling
coil
Heating coil
Fan
O
R
R
m L S S
E
Room
4-Summer cycle with return air and reheating:

22. R
m
L
O
S
O O
m
o/ m
R= Rm / om
C.C = m
s(h
m–h
L)
R.L = m
s(h
R–h
s)
m
s= m
o+ m
R

23. Cooling coil
Fan Room
O
S S
R
5-Summer cycle with fresh air :

24. R
O
S
O
O
O
C.C = m
s(h
o–h
s)
R.L = m
s(h
R–h
s)
m
s= m
o

25. Cooling
coil
Heating
coil
Fan
O
R
R
M L S S
E
Room
m
Damper
R
1
6-Big Summer cycle:
1-ىفةلاحلفقرباندلا(R1,R)فاقياوناخسلالوحتيلكشلاىلالكش5
2-ىفةلاحلفقرباندلا(R1)فاقياوناخسلالصحنىلعلكشلا2
3-ىفةلاحفاقياناخسلاطقفلصحنىلعلكشلا3
4-ىفةلاحلفقرباندلا(R1,)لصحنىلعلكشلا4

26. R
M
L
O
S
O
O
O
O
O
O
m
m
o/ m
R= RM / OM
m
r/ m
L= m
r/ (m
o+ m
R) = Lm /
Rm
C.C = (m
O+ m
R) (h
M–h
L)
H.C = m
s( h
s–h
m)
R.L = m
s(h
R–h
s)
m
s= m
o+ m
R+ m
r
m
w= (m
o+ m
R) (W
M–W
L)

29. 7-High latent cooling load applications (low RSHF):

31. Whenthelatentloadonthebuildingishighdueeitherto
highoutsidehumidityorduetolargeventilation
requirements(e.g.hospitals)orduetohighinternal
latentloads(e.g.presenceofkitchenorlaundry),then
thesimplesystemdiscussedaboveleadstoverylow
coilADP.AlowcoilADPindicatesoperationofthe
refrigerationsystematlowevaporatortemperatures.
Operatingthesystematlowevaporatortemperatures
decreasestheCOPoftherefrigerationsystemleading
tohighercosts.Henceareheatcoilissometimesused
sothatthecoolingcoilcanbeoperatedatrelativelyhigh
ADP,andatthesametimethehighlatentloadcanalso
betakencareof.Figureshowsanairconditioning
systemwithreheatcoilalongwiththepsychrometric
representationoftheprocess.

32. Asshowninthefigure,inasystemwithreheatcoil,airisfirst
cooledanddehumidifiedfrompoint‘m’topoint’c’inthecooling
coilandisthenreheatedsensiblytotherequiredsupply
temperaturetsusingthereheatcoil.Ifthesupplytemperatureis
specified,thenthemassflowrateandstateofthesupplyairand
conditionoftheairaftermixingcanbeobtainedusingequations
givenabove.Sincetheheatingprocessinthereheatcoilis
sensible,theprocesslinec-swillbehorizontal.Thusifthecoil
ADPisknown,thenonecandrawthecoilconditionlineandthe
intersectionofthislinewiththehorizontallinedrawnfromsupply
state‘s’givestheconditionoftheairattheexitofthecooling
coil.Fromthiscondition,onecancalculatetheloadonthe
coolingcoilusingthesupplymassflowrateandstateofair
aftermixing.Thecapacityofthereheatcoilisthen
obtainedfromenergybalanceacrossit,i.e.,

33. Advantages and disadvantages of reheat coil:
1-Refrigerationsystemcanbeoperatedatreasonably
highevaporatortemperaturesleadingtohighCOPand
lowrunningcost.
2-However,massflowrateofsupplyairincreasesdue
toreducedtemperaturerise(t
i-t
s)acrossthe
conditionedspace
3-Wastefuluseofenergyasairisfirstcooledtoa
lowertemperatureandthenheated.Energyisrequired
forbothcoolingaswellasreheatcoils.However,this
canbepartiallyoffsetbyusingwasteheatsuchasheat
rejectedatthecondenserforreheatingofair.
Thustheactualbenefitofreheatcoildependsmayvary
fromsystem.

34. Guidelines for selection of supply state and cooling coil:

35. Cooling
coil
Desiccant
Air washer
Fan
Room
R
R
M L N S S
Desiccant cooling cycle:

36. R
m
L
O
S
O
O
O
O
O
N
O
C.C = (m
O+ m
R) (h
L–
h
N)
R.L = m
s(h
R–h
s)
m
s= m
o+ m
R
m
w= (m
o+ m
R) (W
s–W
N)
Note that:
Where, NL is horizontal
line.

40. Evaporative air conditioning systems:
This system is based on the principle that when moist
but unsaturated air comes in contact with a wetted
surface whose temperature is higher than the dew point
temperature of air, some water from the wetted surface
evaporates into air. The latent heat of evaporation is
taken from water, air or both of them. In this process, the
air loses sensible heat but gains latent heat due to
transfer of water vapour. Thus the air gets cooled and
humidified. The cooled and humidified air can be used
for providing thermal comfort.
Classification of evaporative cooling systems:
The principle of evaporative cooling can be used in
several ways. Cooling can be provided by:

41. 1. Direct evaporative cooling systems:

43. Asshowninthefigure,hotanddryoutdoorairisfirstfilteredandthenis
broughtincontactwiththewettedsurfaceorsprayofwaterdropletsintheair
washer.Theairgetscooledanddehumidifiedduetosimultaneoustransferof
sensibleandlatentheatsbetweenairandwater(processo-s).Thecooledand
humidifiedairissuppliedtotheconditionedspace,whereitextractsthe
sensibleandlatentheatfromtheconditionedspace(processs-i).
Finallytheairisexhaustedatstatei.Inanidealcasewhentheairwasheris
perfectlyinsulatedandaninfiniteamountofcontactareaisavailablebetween
airandthewettedsurface,thenthecoolingandhumidificationprocessfollows
theconstantwetbulbtemperaturelineandthetemperatureattheexitofthe
airwasherisequaltothewetbulbtemperatureoftheenteringair(t
o,wbt),the
processbecomesanadiabaticsaturationprocess.
the saturation efficiency or effectiveness of the evaporative
cooling system ε as:

44. Thus evaporative coolers are very useful essentially in
dry climates, whereas the conventional refrigeration
based air conditioning systems can be used in any type
of climate.

45. 2. Indirect evaporative cooling system:

47. Figureshowstheschematicofabasic,indirect
evaporativecoolingsystemandtheprocessona
psychrometricchart.Asshowninthefigure,inan
indirectevaporativecoolingprocess,twostreamsofair-
primaryandsecondaryareused.Theprimaryairstream
becomescooledandhumidifiedbycomingindirect
contactwiththewettedsurface(o-o’),whilethe
secondarystreamwhichisusedassupplyairtothe
conditionedspace,decreasesitstemperatureby
exchangingonlysensibleheatwiththecooledand
humidifiedairstream(o-s).Thusthemoisturecontentof
thesupplyairremainsconstantinanindirect
evaporativecoolingsystem,whileitstemperaturedrops.

48. Obviously,everythingelseremainingconstant,thetemperaturedropobtained
inadirectevaporativecoolingsystemislargercomparedtothatobtainedinan
indirectsystem,inadditionthedirectevaporativecoolingsystemisalsosimpler
andhence,relativelyinexpensive.However,sincethemoisturecontentof
supplyairremainsconstantinanindirectevaporationprocess,thismayprovide
greaterdegreeofcomfortinregionswithhigherhumidityratio.Inmodernday
indirectevaporativecoolers,theconditionedairflowsthroughtubesorplates
madeofnon-corrodingplasticmaterialssuchaspolystyrene(PS)orpolyvinyl
chloride(PVC).Ontheoutsideoftheplastictubesorplatesthinfilmofwateris
maintained.Waterfromtheliquidfilmontheoutsideofthetubesorplates
evaporatesintotheairblowingoverit(primaryair)andcoolstheconditionedair
flowingthroughthetubesorplatessensibly.
Eventhoughtheplasticmaterialsusedinthesecoolershavelowthermal
conductivity,thehighexternalheattransfercoefficientduetoevaporationof
watermorethanmakesupforthis.Thecommerciallyavailableindirect
evaporativecoolershavesaturationefficiencyashighas80%.

49. 3-Multi-stage evaporative cooling systems:

51. Severalmodificationsarepossiblewhichimproveefficiencyofthe
evaporativecoolingsystemssignificantly.Onesimple
improvementistosensiblycooltheoutdoorairbeforesendingit
totheevaporativecoolerbyexchangingheatwiththeexhaustair
fromtheconditionedspace.Thisispossiblesincethetemperature
oftheoutdoorairwillbemuchhigherthantheexhaustair.Itis
alsopossibletomixoutdoorandreturnairinsomeproportionso
thatthetemperatureattheinlettotheevaporativecoolercanbe
reduced,therebyimprovingtheperformance.Severalother
schemesofincreasingcomplexityhavebeensuggestedtogetthe
maximumpossiblebenefitfromtheevaporativecoolingsystems.
Forexample,onecanusemultistageevaporativecoolingsystems
andobtainsupplyairtemperatureslowerthanthewetbulb
temperatureoftheoutdoorair.
Thus multistage systems can be used even in locations where the
humidity levels are high.

52. Figureshowsatypicaltwo-stageevaporativecooling
systemandtheprocessonapsychometricchart.As
showninthefigure,inthefirststagetheprimaryair
cooledandhumidified(o-o’)duetodirectcontactwitha
wetsurfacecoolsthesecondaryairsensibly(o-1)ina
heatexchanger.Inthesecondstage,thesecondaryair
streamisfurthercooledbyadirectevaporationprocess
(1-2).Thusinanidealcase,thefinalexittemperatureof
thesupplyair(t
2)isseveraldegreeslowerthanthewet
bulbtemperatureoftheinletairtothesystem(to’).

53. Advantages and disadvantages of evaporative
cooling systems:
Comparedtotheconventionalrefrigerationbasedairconditioning
systems,theevaporativecoolingsystemsofferthefollowing
advantages:
1.Lowerequipmentandinstallationcosts
2.Substantiallyloweroperatingandpowercosts.Energysavingscan
beashighas75%
3.Easeoffabricationandinstallation
4.Lowermaintenancecosts
5.Ensuresaverygoodventilationduetothelargeairflowrates
involved,hence,areverygoodespeciallyin100%outdoorair
applications
6.Betterairdistributionintheconditionedspaceduetohigherflowrates
7.Thefans/blowerscreatepositivepressuresintheconditionedspace,
sothatinfiltrationofoutsideairisprevented
8.Veryenvironmentfriendlyasnoharmfulchemicalsareused

54. Comparedtotheconventionalsystems,theevaporative
coolingsystemssufferfromthefollowing
disadvantages:
1.Themoisturelevelintheconditionedspacecouldbehigher,
hence,directevaporativecoolersarenotgoodwhenlow
humiditylevelsintheconditionedspaceisrequired.However,
theindirectevaporativecoolercanbeusedwithoutincreasing
humidity
2.Sincetherequiredairflowratesaremuchlarger,thismay
createdraftand/orhighnoiselevelsintheconditionedspace
3.Precisecontroloftemperatureandhumidityintheconditioned
spaceisnotpossible
4.Mayleadtohealthproblemsduetomicro-organismsifthe
waterusedisnotcleanorthewettedsurfacesarenot
maintainedproperly.

55. Applicability of evaporative cooling systems:
Asmentionedbefore,evaporativecoolingsystemsare
idealinhotanddryplaces,i.e.,inplaceswherethedry
bulbtemperatureishighandthecoincidentwetbulb
temperatureislow.However,therearenoclear-cutrules
astowherethesesystemscanorcannotbeused.
Evaporativecoolingcanprovidesomemeasureof
comfortinanylocation.However,inmanylocations
wherethehumiditylevelsareveryhigh,stand-alone
evaporativecoolingsystemscannotbeusedfor
providingthermalcomfortespeciallyinresidences,office
buildingsetc.

56. However,experienceshowsthatevaporativecoolerscanbeusedeven
inlocationswheretherelativehumidityishigherthan40%.Amore
recentguidelinesuggeststhatevaporativecoolingcanbeusedin
locationswherethesummerdesignwetbulbtemperaturesareless
thanabout24oC(75oF).Itisgenerallyobservedthatevaporative
coolerscancompetewithconventionalsystemswhenthenoonrelative
humidityduringJulyislessthan40%,henceshoulddefinitelybe
consideredasaviablealternative,whereasthesesystemscanbeused
inplaceswherethenoonrelativehumidityishighertha40%butthe
designWBTislowerthan24oC,withagreatersacrificeofcomfort.It
shouldbementionedthatboththeseguidelineshavebeendeveloped
fordirectevaporativecoolingsystems.Indirectevaporativecoolerscan
beusedoveraslightlybroaderrange.Evaporativeairconditioning
systemscanalsobeusedoverabroaderrangeofoutdoorconditionsin
factories,industriesandcommercialbuildings,wherethecomfortcriteria
isnotsorigid(temperaturesashighas30oCintheconditionedspace
areacceptable).Evaporativeairconditioningsystemsarehighlysuitable
inapplicationsrequiringlargeamountsofventilationand/orhigh
humidityintheconditionedspacesuchastextilemills,foundries,dry
cleaningplantsetc.

57. Evaporativecoolingcanbecombinedwitha
conventionalrefrigerationbasedairconditioning
systemsleadingtosubstantialsavingsinenergy
consumption,iftheoutsideconditionsarefavorable.
Again,anumberofpossibilitiesexist.Forexample,the
outdooraircanbefirstcooledinanevaporativecooler
andthenmixedwiththere-circulatingairfromthe
conditionedspaceandthencooledfurtherinthe
conventionalrefrigerantorchilledwatercoil.

58. Heating
coil
Fan
Room
O S S
R
R
1-Simple winter air conditioning systems
Winter Air Conditioning Systems

59. O
R
S
h

60. Heating
coil
Fan
Room
O S S
R
RR
m
2-Winter air conditioning with return air

61. R.L = m
s(h
s–h
R)
m
s= m
o+ m
R
O
R
S
m

62. Preheated Fan
Room
O
N S
R
R
mL S
Air washer
Reheated
R
3-Winter air conditioning with return air and
humidification

63. O
R
L
m
S
h

64. 4-Big winter air conditioning cycle
Fan
Room
O
N S
R
R
m
2L S
Air washer
Reheater
R
m
1
R
Damper
1
Damper
2
E

65. O
R
L
m
1
S
N
m
2
1-ىفةلاحلفقرباندلا(D
1,D
2)فاقياوناخسلامدقتملاشرلاولصحنىلعلكشلا1
2-ىفةلاحلفقرباندلا(D
1)فاقياوناخسلامدقتملاشرلاولصحنىلعلكشلا2
3-ىفةلاحلفقرباندلا((D
2لصحنىلعلكشلا3

68. Heatingandhumidificationofaircanbeachievedbydifferent
schemes.Figure31.4showsonesuchschemealongwith
thecycleonpsychrometricchart.Asshowninthefigure,the
mixedair(mixtureofreturnandoutdoorair)isfirstpre-
heated(m-1)inthepre-heater,thenhumidifiedusinga
humidifieroranairwasher(1-2)andthenfinallyreheatedin
there-heater(2-s).Thereheatedairatstate‘s’issuppliedto
theconditionedspace.Theflowrateofsupplyairshouldbe
suchthatwhenreleasedintotheconditionedspaceatstate
‘s’,itshouldbeabletomaintaintheconditionedspaceat
stateIandoffsetthesensibleandlatentheatlosses(Q
sand
Q
l).Pre-heatingofairisadvantageousasitensuresthat
waterinthehumidifier/airwasherdoesnotfreeze.In
addition,bycontrollingtheheatsuppliedinthepre-heater
onecancontrolthemoisturecontentintheconditioned
space.

69. Byapplyingenergybalanceacrosstheconditioned
space,atsteadystate,thesensibleandlatentheat
lossesfromthebuildingcanbewrittenas:

72. Figureshowsanotherschemethatcanalsobeusedfor
heatingandhumidificationofairasrequiredinawinterair
conditioningsystem.Asshowninthefigure,thissystem
doesnotconsistofapre-heater.Themixedairisdirectly
humidifiedusinganairwasher(m-1)andisthenreheated
(1-s)beforesupplyingittotheconditionedspace.Though
thissystemissimplercomparedtothepreviousone,it
suffersfromdisadvantagessuchaspossibilityofwater
freezingintheairwasherwhenlargeamountofcoldoutdoor
airisusedandalsofromhealthhazardstotheoccupantsif
thewaterusedintheairwasherisnotclean.Hencethis
systemisnotrecommendedforcomfortconditioningbutcan
beusedinapplicationswheretheairtemperaturesatthe
inlettotheairwasherareabove0
o
Candtheconditioned
spaceisusedforproductsorprocesses,butnotforproviding
personnelcomfort.

73. All year (complete) air conditioning systems:

74. Figureshowsacompleteairconditioningsystemthatcan
beusedforprovidingairconditioningthroughouttheyear,
i.e.,duringsummeraswellaswinter.Asshowninthe
figure,thesystemconsistsofafilter,aheatingcoil,a
cooling&dehumidifyingcoil,are-heatingcoil,ahumidifier
andablower.Inadditiontothese,actualsystemsconsist
ofseveralotheraccessoriessuchasdampersfor
controllingflowratesofre-circulatedandoutdoor(OD)air,
controlsystemsforcontrollingthespaceconditions,safety
devicesetc.Largeairconditioningsystemsuseblowersin
thereturnairstreamalso.Generally,duringsummerthe
heatingandhumidifyingcoilsremaininactive,whileduring
winterthecoolinganddehumidifyingcoilremainsinactive.
However,insomeapplicationsforprecisecontrolof
conditionsintheconditionedspaceallthecoilsmayhave
tobemadeactive.

75. Theblowerswillremainactivethroughouttheyear,as
airhastobecirculatedduringsummeraswellasduring
winter.Whentheoutdoorconditionsarefavourable,itis
possibletomaintaincomfortconditionsbyusingfiltered
outdoorairalone,inwhichcaseonlytheblowerswillbe
runningandallthecoilswillbeinactiveleadingto
significantsavingsinenergyconsumption.Acontrol
systemisrequiredwhichchanges-overthesystemfrom
winteroperationtosummeroperationorviceversa
dependingupontheoutdoorconditions.

76. Aroomistobemaintainedat24CDBTand50%RHwhenthe
outdoorconditionsare35CDBTand25CWBT.Theroom
sensibleandlatentheatgainsare50000Kcal/hrand12500
Kcal/hrrespectively.Iftheventilationrequiredis54cmm.
Find
a-outdoor air load b-Grand total heat
c-Effective sensible heat factor
d-Apparatus dew point temperature
e-Dehumidified air quantity
f-Entering and leaving conditioning at the apparatus assume
by pass factor is 0.15.

77. Thank You