Sistemas de enfriamiamiento HT y LT para motores de combustión interna

sistemas de enfriamiento HT y LT

1. CENTRAL COOLING
WATER SYSTEM (HT & LT)
Dr. S. Thangalakshmi

2. ▪Theamountofmaintenancerequiredforanycoolingsystemdependsmainlyonqualityofwaterand
watertreatmentusedinthesystem.
▪Whenusingseawaterattemperaturesabove50degreesCelsius,corrosionandscaleformation
becomeunacceptablyhigh.
▪Thecorrosionproblemcantosomeextentbeavoidedbymaterialselectionalthoughthecost
increaseswhenusingspecialalloys.Someofthematerialsarecopperbasedalloys.Material
selectiondoesnotreallyhelpwithscaleformationandtheonlywaytoreducethisistousebetter
qualitywaterinthesystems.
▪Thisisthemainreasonforthedevelopmentofcentralcoolingsystemsinwhichmuchofthe
traditionalseawatersystemisreplacedbyfreshwatercircuits.Thisisnowpreferredoptionandis
likelythatsuchsystemisemployedonpresentvessels.

3. InEngineroomthebasiccentralcoolingsystemhastwointerconnectedfreshwatercircuits;onehigh
temperaturecircuitandtheotheralowtemperaturecircuitandasimpleseawatercircuit.Thefreshwater
circuitsarenormallyreferredtoastheHTandLTcircuitstodenotehighandlowtemperature
respectively.

4. TheHTcircuitservesthedieselenginejacketcoolingwatersystemsforthemainandauxiliary
engines.Itcanalsobeusedforheatingthefreshwatergenerator.Typicallythesystemoperatesto
giveanoutlettemperaturefromtheengineofbetween75and85degreesCelsius.

5. TheHTcircuitusuallyconsistsoftwocirculatingpumpswhichcirculatethewaterthroughthemainengineand
auxiliaryenginejackets.A3-waytemperaturecontrolvalveisprovidedwhichconnectstotheLTsystem.Provisionis
alsomadetopasstheHTwaterthroughthefreshwatergeneratorheatexchangerwhenrequiredwithasuitable
bypassfacilitywhenitisoffline.Theheadertankanddeaeratorcompletethesystem.Formaintenancepurposes,
suitablevalvesforisolatingtheenginesandpumpsareprovided.

6. TheLTcircuittakescareofallcoolingneedsotherthan
themainandauxiliaryenginejackets.Thisincludes
coolingforthemainandauxiliaryenginelubricatingoil
andchargeair.OthermachineryservedbytheLT
systemincludesaircompressorsandrefrigerationand
airconditioningcompressorsandcondensers.Typically
theLTsystemoperateswithatemperatureofabout37
degreesCelsiusthesystemusuallycomprisestwo
circulatingpumpsandanumberofheatexchangers.The
heatexchangersincludethoseforcoolinglubricatingoil
andairforthedieselengines,theaircompresses,
refrigerating&airconditioningequipmentand2
seawater-cooledcentralcoolers.Thethree-way
temperaturecontrolvalveusedinbypassdivertermode
controlsthefreshwaterflowthroughthecentralcoolers.
TheLTsystemisconnectedtotheHTsystemviathe3-
waymixingvalveandareturnconnection.

8. TemperaturecontroloftheHTsystemisachieved
bya3-wayvalveusedasamixingvalve.The
sensorforcontrollingthisvalveisusually
positionedatthemainenginejacketwateroutletor
thereturnlinetothecirculatingpumps.Constant
engineoutlettemperatureisachievedbymixing
waterfromtheLTcircuitintotheHTcircuitviathis
3-wayvalve.AstheHTwatertemperaturerisesat
theengineoutletduetoincreasedenginepower,
thesensordetectstheriseandsendsasignalto
thecontroller.Itisthencomparedtotheset-point,
andacorrectivesignalisissuedtothecontrol
valve,whichopenstoallowsomeLTwatertomix
withtheHTwater,loweringthetemperature.There
maybeadditionalindividualtemperaturecontrolfor
theauxiliaryenginesusingthermostatically-
controlledvalves.

10. TemperaturecontroloftheLTcircuitisachievedbya3-
wayvalveusedtocontroltheflowthroughthecentral
coolersinabypassarrangement.Thesensorforthevalve
ispositioneddownstreamfromthepointatwhichthereturn
fromthecentralcoolersre-enterstheLTcircuit.Asthe
temperatureofthesystemstartstoriseduetoincreased
thermalload,thesensordetectstheriseandsendsasignal
tothecontroller.Whenthisvalueiscomparedtotheset-
point,acorrectivesignalisgiventothecontrolvalve,which
openstoallowsomeoftheLTwatertopassthroughtothe
coolers,loweringthetemperature.Anyfurtherincreasein
thermalloadonthesystemwillcausethevalvetodivert
moreflowtothecoolers.Insomesystemsthereare
additional3-wayvalvestocontroltemperaturesofthe
individualservicesbybypassingtheprimaryfluidsatthe
relevantheatexchangers.MinimumtemperaturefortheLT
circuitmaybeachievedbyusing“splitrangecontrol”to
controlaheatingunitinthesystem.

12. Thesea-watercircuitforacentralcoolingsystemisverysimple.Itusuallyconsistsoftwoseawatersuctionstrainers,
twoormorecirculatingpumps,andtwoheatexchangersforcentralcoolers.Theseheatexchangersareusuallyofthe
platetype;thematerialusedforthepipeworkisgenerallyhigh-qualitycopperbasedalloyalthoughsomesystems
usestainlesssteel.Thesematerialsproducemaintenancerequirementsastheyareresistanttocorrosionandas
thereissolittlepipeworkinthesystem;thecostismoderate.Theplatesfortheheatexchangerarealsomadefrom
high-qualitymaterialwhichisresistanttocorrosion.Stainlesssteelortitaniumisoftenchosensincethehigh-strength
allowsverythinplatestobeusedwhichimproveheattransfer.

14. ✔Thecentralcoolingsystemcanbeadaptedtoprovidecoolingorheatingformostofthe
machineryonboard.
✔TheLTsystemcanbeusedtoprovidethesecondaryfluidformanyofthecooling
requirementssuchasaircompressor,heatexchanges,andoilcoolers.
✔TheHTsystemcanbeusedasasourceofheating.Itcanbeusedastheheatsourcefor
thefreshwatergenerator.Itcanalsobeusedtoheatdomesticwaterandapreheat
machinerysuchasthemainandauxiliarydieselengines.
✔Theseawatercircuitcanbeusedtoprovidepreheatedfeedforthefreshwatergenerator
andalsothecoolingrequirementsforanysystemsrequiringdirectseawatercoolingto
achievelowertemperaturesthancanbeachievedbytheLTwater.Bydoingthisthe
numberofsubsystemscanbereducedandtheoverallplantissimplified.