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GEAWiegandGmbHGEAEvaporationTechnologiesEvaporationTechnologyusingMechanicalVapourRecompressionLeadingTechnologies.IndividualSolutions.2ContentsMechanicalVapourRecompressionandEvaporation3PrinciplesofMechanicalVapourRecompression5MechanicalVapourRecompressors–DesignandFunctionalRanges7OperatingPrinciplesandCompressorDesigns8DesignDetailsoftheSingle-stage,CentrifugalCompressor10CompressorDrives13MonitoringandSafetyEquipment14CompressorControls16EvaporationPlantswithCentrifugalFans18EvaporationPlantswithCentrifugalCompressors22Thermalseparationprocessessuchasevaporationanddistillationareenergyintensive.Inthecourseoftheirdeve-lopment,theaimofreducingenergycostsfirstledtomultiple-effectplants,thentothermalvapourrecompression,andfinally,totheuseofmechanicalvapourrecompressionsystems.Inaconventionalevaporator,thevapourstreamproducediscondensed,meaningthatitsenergycontentislosttoalargeextent.Incomparison,mechanicalvapourrecompressionpermitsthecontinuousrecyclingofthisenergystreambyrecom-pressingthevapourtoahigherpressureandtherefore,ahigherenergycontent.Mechanicalvapourrecompressionre-ducestheconsumptionofprimaryenergyand,consequently,theenvironmentalload.MainfieldsofapplicationarecurrentlytheFoodandBeveragesindustry(eva-porationofmilk,whey,sugarsolutions),Chemicalindustry(evaporationofaqueoussolutions),theSaltWorksin-dustry(evaporationofsalinesolutions)andEnvironmentalTechnology(con-centrationofwastewater).Ineachcase,thedecisiononwhetheravapourrecompressionsystemshouldbeinstalledmustbemadeonthebasisofanefficiencystudy.3MechanicalVapourRecompressionandEvaporationImportanceBackgroundEconomicefficiencyPlantsforevaporation,distillation,evaporativecrystallisationandevapo-rativedryingareenergyintensive.Operatingcostsoftheseplantsarethere-foreprimarilydeterminedbytheenergycosts.Thereductionandopti-misationofthespecificenergyconsumptionisthereforeofprimeimpor-tance.Therearethreemaintech-niquesforminimisingspecificenergyconsump-tion,whichcanbeappliedeithersingly,orincom-binations:1.multipleeffectarrange-ment2.thermalvapourrecom-pression3.mechanicalvapourrecompression1.MultipleEffectArrangementInamultipleeffectevaporationplant,thevapourproducedinthefirsteffectbythelivesteamisnotlosttothecondenser,butisreutilizedastheheatingmediumofthesecondeffect.Thiseffectivelyreducesthesteamconsumptionbyabout50%.Asthisprincipleisrepeated,furthersteamreductionsfollow.Themaximumheatingtemperatureofthefirsteffect,andthelowestboilingtemperatureachievedinthefinaleffectcreatesatotaltempera-turedifferencethatisspreadacrosstheindividualeffects.Asaresult,thetemperaturedifferencepereffectdecreasesasthenumberofeffectsincreases.Theirheatingsurfacesmustconsequentlybelargerinordertoreachthespecifiedevaporationrate.Afirstapproximationshowsthattheheatingsurfacetobeusedforalleffectsincreasesproportionallywiththenumberofeffects,andthatinthiswaytheinvestmentcostsconsiderablyincrease,whereassteamsavingsprogressivelydecrease.2.ThermalVapourRecompressionDuringthermalvapourrecompression,vapourfromaboilingchamberisrecompressedtothehigherpressureofaheatingchamberaccordingtotheheatpumpprinciple;i.e.energyisaddedtothevapour.Thesatura-tedsteamtemperaturecorrespondingtotheheatingchamberpressureisconsequentlyhigher,enablingthevapourtobereusedforheating.Forthispurpose,steamjetvapourrecompressorsareused.Theyoperateaccordingtothejetpumpprinciple.Theyhavenomovingparts,ensuringasimpleandeffectivedesignthatprovidesthehighestpossibleoperationalreliability.Theuseofathermalvapourrecompressorhasthesamesteam/energysavingeffectasanadditionalevaporationeffect.AproducttobeevaporatedBresidualvapourCconcentrateDmotivesteamEheatingsteamcondensateFvapourcondensateVheatlossHeatflowdiagramofadouble-effect,directlyheatedevaporator4Acertainquantityoflivesteam,theso-calledmotivesteam,isrequiredfortheoperationofathermalvapourrecompressor.Thismotivesteamquantitymustbetrans-ferredtothenexteffectortothecondenserassurplusresidualvapour.Thesurplusenergycontainedinthere-sidualvapourapproximatelycorrespondstotheamountofenergysuppliedinthemotivesteam.3.MechanicalVapourRecompressionDuringmechanicalvapourrecompression,thevapourofanevaporatorisrecompressedtoahigherpressurebymeansofamechanicallydrivencompressor.Therecom-pressorthereforealsooperatesasheatpump,addingenergytothevapour.Contrarytothecompressionheatpumpwithcirculatingprocessliquid(i.e.aclosedsystem,refrigerationcycle)thevapourrecompressorcanbeconsideredasaspecialcaseofthecompressionheatpumpbecauseitoperatesasanopensystem.Aftercompressionofthevapourandsubsequentcon-densationoftheheatingsteam,thecondensateleavesthecycle.Theheatingsteam(hotside)isseparatedfromthevapour(coldside)bytheheatexchangesurfaceoftheevaporator.Thecomparisonbetweentheopencompressionheatpumpandtheclosedcompressionheatpumpshowsthattheevaporatorsurfaceintheopensystembasicallyreplacesthefunctionoftheexpansionvalveofthepro-cessliquidintheclosedsystem.Byusingacomparablysmallamountofenergy,i.e.themechanicalenergyofthecompressorimpellerinthecaseofthecompressionheatpump,energyisad