Energy Efficiency Opportunities In Industrial Refrigeration
Energy EfficiencyOpportunities inIndustrialRefrigerationPresented by the University ofDelaware Industrial AssessmentCenter with the US EPAJune 17, 20201
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Presenter BioRalph Nigro, PE is the Assistant Director of the University of Delaware’s IndustrialAssessment Center. He began his career with Delmarva Power, spending 15 years on theplanning, licensing, design, construction and start-up of power generation andenvironmental control projects. For the following 25 years he worked as Senior VicePresident of Engineering and Technical Services for Applied Energy Group, where heoversaw the administration and implementation of numerous energy efficiency andrenewable energy programs for utility clients, especially large commercial and industrialprograms.3
University of Delaware Industrial Assessment CenterEstablished in 2006 in the Department ofElectrical and Computer Engineering at UD We provide no-cost industrial energyassessments throughout the Mid-Atlanticregion to manufacturing and processindustries of all types while offering realworld training for engineering students Since 2017, we have provided assessmentstargeting food and beverage processingunder an EPA Pollution Prevention Programgrant Over 200 industrial assessments since ourfounding 4
Webinar Overview Focus on the most common industrial refrigerationsystems and components Refresh the basics of the industrial vapor compressionrefrigeration cycle and key components Summarize efficiency opportunities at component andsystem levels Summarize efficiency opportunities for refrigeratedspace management Brief wrap up5
Refrigeration Basics6
What is Refrigeration? In vapor-compression refrigeration, a refrigerant is used to moveheat from a cooler to a warmer environment by adding workHeat rejected to environmentCondenserExpansionValveCompressorWork addedEvaporatorHeat gain fromenvironmentHeat absorbed fromrefrigerated space7
Basic Vapor-Compression Refrigeration Cycle The basic refrigeration cycle consists of:– Compression Increasing pressure of cold refrigerant gas from evaporation stage– Condensation Condensing high pressure gas by rejecting heat absorbed inevaporation stage and heat of compression to external environment– Expansion Throttling warm, high pressure liquid refrigerant through anexpansion valve, resulting in cold, low pressure liquid/gas refrigerant– Evaporation Circulating cold refrigerant through a heat exchanger, absorbing heatfrom refrigerated space and evaporating the refrigerant8
Simple Pressure-Enthalpy Diagram (Ammonia DHIGHPRESSUREVAPORLOWPRESSUREVAPOR
Common Industrial Refrigerants Refrigerants are the working fluids in any refrigeration system Families of refrigerants:– Halocarbons (CFCs, HCFCs, HFCs)– Hydrocarbons (ethane, propane)– Inorganic (ammonia, CO2) Common refrigerants used in industrial applications are:– Ammonia (R-717)– Chlorodifluoromethane (R-22)– Refrigerant blends (R-404a, R-410a, etc.) Large industrial systems primarily use ammonia10
Refrigerant Performance Characteristics per Ton Refrigeration111
Refrigeration System Types12
System Classifications By equipment configuration:– Direct expansion– Single stage overfeed/flooded– Two stage overfeed/floodedammoniarefrigeration systems By temperature range:– High (greater than about 40oF)– Medium (between about -25oF and 40oF)– Low (less than -25oF) Systems can be packaged or built-up13
DX System - Schematic Diagram Commonly used for higher temperature HVAC and chilled water systems Often used in smaller, stand-alone, packaged medium and lowtemperature systems Mostly HFC and HCFC refrigerantsBHIGHPRESSURELIQUIDHEAT AHIGHPRESSUREVAPORCOMPRESSOREVAPORATORDHEAT IN14INPUTPOWERLOWPRESSUREVAPOR
DX SystemLOW APORLOWPRESSUREVAPOR
Single Stage Ammonia System Commonly used in large low and medium temperature applications Can be designed with or without recirculation pumps (“overfeed” vs“flooded”)MBHEAT ORRECIRC URESAT LIQUIDHIGHER LOWPRESSURELIQUID16EVAPORATORHEAT ININPUTPOWER
Recirculated System - Ph QUID/VAPOR17HIGHPRESSURE VAPORLOWPRESSURE VAPOR
Two Stage Liquid Overfeed System Commonly used in low and multi-temperature refrigeration systems Can be designed with or without recirculation pumps Ammonia refrigerantBHEAT RESSUREVAPORHIGH REVAPORRECIRC OOSTERCOMPRESSOREDMEDPRESSURELIQUIDMMED PRESSURESAT LIQUIDHEAT INGLOW PRESSURELIQUID/VAPORJACCUMULATORHLOW PRESSURESAT LIQUIDILOWPRESSURELIQUID18EVAPORATORHEAT INLOWPRESSUREVAPORINPUTPOWER
2-Stage Recirculated System - Ph NMPRESSIONMEDPRESSSATURATED IGH PRESSURELIQUIDMEDPRESSURELIQUID/VAPORJLOW PRESSURELIQUID/VAPORLOW PRESSURESATURATED LIQUID19FHIGHPRESSUREVAPORMEDPRESSUREVAPOR
Components of Refrigeration20
Refrigeration System ComponentsCompressors The driving force to move heat Largest energy user in the system Compresses refrigerant from low pressure vapor to high pressure vapor Many different technologies in the marketCondensers The point of heat rejection Heat dissipated to atmosphere Energy is used for fan forced circulation and spray pumps for water. Typically air-cooled or evaporative-cooledEvaporators The point where heat is removed from the material or space Exist in the form of air coils, heat exchangers, chillers Fan/Pump loads add heat Typically second largest energy user in the system21
Compressors and CompressorControls22
Compressor Types Positive Displacement– Very common in mediumand low temperaturerefrigeration systems– Reciprocating– Screw– ScrollCOMPRESSOR OUTLET:HIGH PRESSUREREFRIGERANT VAPORCOMPRESSOR Centrifugal– Most common in HVACand process chillersystemsCOMPRESSOR INLET:LOW PRESSUREREFRIGERANT VAPOR23MOTORINPUTPOWER
Reciprocating Compressors Oldest technology and still commonin smaller systems Suitable for single stage or boosteroperation using most refrigerants Compressors may have up to 16cylinders depending on size– May be as small as 10 HP or more than300 HP for ammonia systems– Compression ratios up to 8:1 forammonia Most common capacity control isstaged unloading– Uses external actuator to hold thesuction valves open24
Screw Compressors Suitable for single stage or booster operationusing R22 or AmmoniaCan operate with pressure ratios above 20:1single stageMax sizes can exceed 1000 HPAvailable as lubricated (with oil) or nonlubricated (without oil)Can be single or twin-screw configuration Most common capacity controls –Variable displacement –Slide valvesOther means of varying the inlet or discharge pointCapable of operating within a wide capacity range (10to 100%)Changing compressor speed Install a VFD on the compressor motor to be used inconjunction with slide valveProvides better part load efficiencies below 60% slide valve25
Condensers26
Air-Cooled Condensers (Dry Coolers) In an air-cooled condenser, the following actions take place: Refrigerant vapor is condensed in a coil. Air is circulated over the coil. Usually configured as induced draft unit with multiple low HP fans Typically less efficient for the refrigeration system as the lack ofevaporative cooling requires higher compression ratio on compressors Low operating costs and maintenance Performance is based on Outside Air Dry Bulb Temperature27
Evaporative Condensers Evaporative condensers provide lower condensing temperatures andenable compressor horsepower savings of up to 15 percent comparedwith air-cooled systems.In an evaporative condenser, the following actions take place: Refrigerant vapor is condensed in a coil, which is continually wetted onthe outside by a recirculating water system Air is circulated over the coil, causing a small portion of the recirculatingwater to evaporate The evaporation removes heat from the vapor in the coil, causing it tocondense Performance is based on Outside Air Wet Bulb Temperature28
Evaporative Condensers29 Uses of both a condensing coiland fill surface for heat transferin an evaporative condenser.The addition of fill surface tothe traditional evaporativecondenser design reducesevaporation in the coil section,reducing the potential forscaling and fouling. Some combined flowevaporative condensers utilizeparallel flow of air and spraywater over the coil, andcrossflow air/water flowthrough the fill surface. In parallel flow, air and waterflow over the coil in the samedirection. In some fill sectionsin combined flow evaporativecondensers, air and waterinteract in a crossflowconfiguration: water flowsvertically down the fill as airflows horizontally across it.
Cooling Towers In some cases the condenser on a refrigeration system isjust a heat exchanger that moves heat to a secondary fluidbefore rejecting to atmosphereThis requires the use of a cooling tower which cools a fluidusing airThere are two types of cooling towers–Closed Loop –COOLINGTOWERPUMPCONDENSEROpen Loop A coil inside of the cooling tower does not allow the secondaryfluid in direct contact with atmosphereWater is sprayed over the coil to achieve the evaporative coolingeffect similar to evaporative condensersThis method results in higher approach temperatureHEAT OUTWater is circulated over the cooling tower in direct contact withatmosphereAs water evaporates from cooling it leaves behind the dissolvedsolidsThe water needs to be treated or the solids can reduce theefficiency of the heat exchanger and cooling towerThis method results in a lower approach temperatureCooling towers in general operate in a very similar mannerto evaporative condensers30EXPANSIONVALVECOMPRESSOREVAPORATORHEAT ININPUTPOWER
Evaporators31
Evaporator Overview Provides cooling or freezing temperatures and proper airflow tothe space Consists of a cooling coil and one or more motor driven fans Coil defrost equipment for low temperature operations when icebuildup may impede performance Other considerations and featuresCoil type, fin spacing, overall surface areaElectric, hot gas, air, water, or hot brine defrostingDischarge air velocity and directionCentrifugal or propeller fans, belt or direct driven, blow through or drawthrough, ducted or non-ducted– Freestanding, ceiling-suspended, or penthouse (roof mounted)– Liquid Overfeed or DX refrigerant piping– Humidity control (reheat coil)––––32
Forced Circulation Air Cooler Examples33
Coil Defrost tagesHot GasWidely used in all industrial and commercialrefrigeration. Uses compressed refrigerantvapor. Able to achieve effective defrosts Uses lower grade of energy (“waste heat” fromrefrigeration system) Can be effective at scavenging and returning oilthat may accumulate in evaporator Increased safety risks due to hydraulic hammeringfrom liquid slugs if sequence of operation notmanaged properly Extremely high pressures (for CO2 refrigerant) Increased parasitic energy consumption withimproper valve group design and mis-adjustmentElectricUsed in some commercial systems and inindustrial systems where CO2 is used as acascade refrigerant or secondary loop phasechange fluid. Requires electrical resistanceheating element to remove frost from coil. Decreased risk of damage from events such ashydraulic hammer Minimizes parasitic load Avoid extreme refrigerant-side pressure (CO2refrigerants) Poor use of high grade primary energy (electricity) High maintenance due to frequent failure ofresistance heating elements Not effective at removing oil accumulation fromevaporatorsOff-Cycle (AirDefrosting)Used in industrial and commercial systems forspaces operating above freezing point. WaterFound in some low temperature freezingsystems where defrost is integrated into thenormal clean up operations. Applies heat directly to the accumulated frost The defrost process may be integrated intonormal sanitation cycleSecondaryFluid(indirect)An alternative to electric defrost in CO2 cascadeand secondary phase change systems. Usesfluid like glycol to flow over outside of coil toremove frost. Efficient means of defrostSimple implementationInherently safeLower capital and maintenance costsConceptually simpleAvoids risks of hydraulic hammering onrefrigerant side of coil34 Not relevant in applications where spacetemperature is below freezing Not effective at removing oil accumulation fromevaporators Difficult to apply during “defrost on the fly” forlow temp applications Not effective at removing oil accumulation fromevaporators Extremely high water usage Additional secondary fluid system and circuitingNot effective at removing oil accumulation fromevaporatorsRequires energy to heat secondary fluid
Energy Efficiency Opportunities35
Causes of Inefficiency Excessive compressor liftPoor part-load performanceDefrost controlsUnnecessary refrigeration loadsAuxiliary component efficiencies36
Excessive Compressor Lift The compressor is the largest energy-using component Two methods to reduce the lift across the compressor– Increase suction pressure– Floating head pressure control37
Compressor Lift Refrigerant properties fix pressuretemperature relationshipCompressor input power is proportional tothe pressure differential between suction anddischarge (lift)––– Condensing and evaporation temperatures are oftenfixed setpointsCondensing temperature is usually 15oF to 20oFhigher than highest ambient temperature (wet or drybulb)Suction pressure is usually based on an evaporatordesign TD of 12oF to 15oFIncreasing suction pressure and/or decreasingdischarge pressure reduces lift and energyconsumption by the compressor38Dischargepressure isdetermined by theCOMPRESSOR OUTLET:condensingHIGH PRESSUREtemperatureCOMPRESSORMOTORLOW PRESSURESuctionpressure isREFRIGERANT VAPORdetermined by thelowest evaporatortemperature in thesystemINPUTPOWER
Increasing Suction Pressure Reduce suction pressure dropCOMPRESSOR OUTLET:HIGH PRESSURE– Select larger evaporator coils– Evaluate suction piping size andpressure drop– Target low temperature systemswhere pressures are also lower COMPRESSOREvaluate the lowest temperatureparts of the system– It could be beneficial to segregatesmaller, low temperature systemsfrom larger medium temperaturesystems served by the samecompressors– Can lowest temperatures beincreased even slightly? LOW easingtemperature by 1oF willreduce input power byapproximately 2%Annual system energy savings upto about 10%39
Decreasing Discharge Pressure Condenser temperatures are usually designedto be 15oF to 20oF higher than the highestexpected ambient temperatures–– –Allows the condensing temperature to “float”with ambient conditionsIn cooler weather, condensing temperaturescan be decreased to stay at or around 15oF to20oF above ambient wet or dry bulb byoperating fans differently/more frequentlyCOMPRESSORAnnual system savings will vary–– Decreasing condensingtemperature by 1oF willreduce input power byCOMPRESSOR OUTLET:approximately1.5% to 2%HIGH PRESSUREFloating head pressure control– If fixed setpoint is used, this means thatcompressor discharge pressures are also fixedIn the mid-Atlantic this can be more than 180PSIG (about 75oF WB, 95oF condensing temp)for evaporative condensers; 250 PSIG (about95oF DB, 125oF condensing temp) for drycoolersSome systems require minimum compressordischarge pressures for defrost, liquid injectionoil cooling, and other requirementsCondenser fan energy consumption willincreaseLOW PRESSUREREFRIGERANT VAPORAnnual system energy savings range istypically 5% to 12%40MOTORINPUTPOWER
Poor Part-Load Performance Systems operate most of the time at partial load Equipment and operating strategies play an importantrole in maximizing efficiency.– Compressor staging, loading and unloading– Use VFDs41
Reciprocating Compressors Reciprocating compressors with cylinder unloading have very good part-loadefficiency– Compressor staging (multi-compressor systems) and unloading strategieshelp manage suction pressure– Generally, more unloading stages allows better control and load matching42
Screw CompressorsScrew compressors have poor partload performance– Screw compressors using slidevalves should be base-loadedmost of the time if possible– Variable frequency drivesincrease part-load efficiencysignificantlyScrew Compressor Part-Load Performance1009080Slide valve7060power (%) 50VFD403020Ideal1000204060Capacity (%)4380100
Sequencing Strategies Compressor sequencing strategies should be designed tomatch load with all but the last compressor operating at ornear full load In systems with both screw and reciprocating compressors,screw compressors should be base loaded as much aspossible Screw compressors with VFDs should be used as “trim”compressors Operating multiple screw compressors (without VFDs) atpartial load is the least efficient approach Proper staging can reduce annual system energy usage by5% to 15%44
Defrost Controls Defrosting is required to prevent ice build-up onevaporator coils Defrosting should be carefully controlled.– Most defrost methods increase refrigeration loads by addingheat to the refrigerated space– Frost build-up depends on many factors, including localhumidity, infiltration rates, product being stored, etc.45
Defrost Methods Air defrost– Evaporator fans stay on to melt frost when refrigerant is shutoff– Can be used when the refrigerated space temperature isabove 32oF– Does not add heat to the refrigerated space Electric defrost– Uses electric heating elements to melt frost– Lowest efficiency in terms of electricity usage Hot gas defrost– Uses hot gas from compressor discharge to melt frost46
Improving Defrost Controls Regardless of the defrost method, a key objective is tominimize defrosting frequency and duration– Time clocks are common, but imprecise– Liquid run-time controls measure the amount of time anevaporator is in cooling mode; defrosting is less frequentduring low demand periods– Frost sensors measure frost build-up directly to initiate andterminate defrosting Actively managing defrost frequency and duration canreduce annual system energy usage by about 3%47
Improving Defrost Controls For hot gas defrost– Objective is to avoid returning alarge amount of vapor to thecompressor suction– In multi-stage systems,condensed gas should bereturned to the compressor(s)with highest suction pressure– Regulators should be sized andadjusted properly to avoid toomuch defrost gas flow– Automatic liquid drainers (muchlike steam traps) should beconsidered to minimize vaporreturnsDefrost hot gas supplysolenoidDefrost condensatereturn regulatorsRefrigerant liquid feedsolenoidRefrigerant liquid/vaporreturn regulator48
Unnecessary Refrigeration Loads Reduce unnecessary refrigeration loads––––––Refrigerated
Refrigeration Basics. 7 What is Refrigeration? In vapor-compression refrigeration, a refrigerant is used to move . Commonly used in low and multi-temperature refrigeration systems Can be designed with or without recirculation pumps Ammonia refrigerant. 19 B A C F H IGH P RESSURE V APOR H IGH P RESSURE L IQUID M ED P
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