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Trane SYS-APM001-EN manual System Configurations, Pump control in a double-ended decoupled system

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Figure 36. Double-ended decoupled system

System Configurations

Other chiller plant/distribution loop arrangements are possible, but if used, they should be reviewed to make certain they will be free from hydronic problems.

Figure 36. Double-ended decoupled system

Check Valves

Chiller 2
	Existing Plant
Chiller 1
Existing Bypass Line	Production
	Distribution
Loads
New Bypass Line	Distribution
	Production
Chiller 3	New Plant

Chiller 4

One of the benefits of decoupled water systems is that they are simple to control. The distribution pump flow is determined by a pressure transducer located at the furthest load. Flow in the decoupler indicates when to start and stop chillers and the chiller pumps are turned on and off with the chillers. Much of this simplicity is lost when multiple chiller plants are connected to the same system. The system shown in the figure above is a fairly simple example, but even so it can be used to show the difficulty of controlling these systems. The following sections point out some of the complications.

Pump control in a double-ended decoupled system

Chiller pump control in a double-ended decoupled system remains unchanged; the chiller pump is started when the chiller is enabled. On a single-plant decoupled system, the distribution pump's speed is modulated based on a pressure sensor located at the end of the loop (point of lowest pressure) to maintain sufficient pressure drop across all the loads.

SYS-APM001-EN

Chiller System Design and Control

53

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Trane SYS-APM001-EN System Configurations, Pump control in a double-ended decoupled system, Double-ended decoupled system

Contents

SYS-APM001-EN Chiller System Design and ControlApplications Engineering Manual Page Chiller System Design and Control Susanna Hanson, applications engineerMick Schwedler, applications manager Beth Bakkum, information designerSYS-APM001-EN Preface2009 Trane All rights reserved Chiller System Design and ControlContents System Controls Failure RecoveryChilled-Water System Control Condenser-Water System ControlChiller Primary System ComponentsFigure 2. Flooded evaporator cut-away Primary System ComponentsChiller evaporator Figure 1. Typical vapor-compression chillerFigure 3. Direct-expansion evaporator cut-away Effect of chilled-water temperaturePrimary System Components Effect of chilled-water flow rate and variationEffect of condenser-water flow rate Water-cooled condenserEffect of condenser-water temperature Primary System ComponentsPrimary System Components Air-cooled condenserAir-cooled versus water-cooled condensers MaintenanceEnergy efficiency Primary System ComponentsLow-ambient operation dry bulb wetWetbulbBulb LoadsPrimary System Components 1212 Midnight midnightHeat transferred from the loads can be controlled in a number of ways Three-way valve load controlTwo-way valve load control Primary System ComponentsFigure 6. Two-way valve Variable-speed pump load controlFace-and-bypass dampers Primary System ComponentsFigure 8. Uncontrolled water flow with bypass damper Chilled-Water Distribution SystemChilled-water pump Primary System ComponentsManifolded pumps Distribution pipingPrimary System Components Pump per chillerFigure 12. Constant flow system Pumping arrangementsPrimary System Components Constant flow systemPrimary-secondary system Condenser-Water SystemCooling tower Primary System ComponentsEffect of ambient conditions on cooling tower performance Condenser-water pumping arrangementsPrimary System Components Effect of load on cooling tower performanceChiller control Unit-Level ControlsPrimary System Components Figure 16. Centrifugal chiller with AFD Primary System ComponentsCentrifugal chiller capacity control Primary System Components The availability of cooler condenser water condenser-water resetSmall Chilled-Water Systems 1-2 chillers Application ConsiderationsCondensing method Application Considerations Constant flowVariable flow Part load system operation Application ConsiderationsNumber of chillers Parallel or seriesFigure 18. Mid-sized chilled-water system schematic Mid-Sized Chilled-Water Systems 3-5 ChillersManaging control complexity Preferential vs. equalized loading and run-timeApplication Considerations Large Chilled-Water Systems 6+ Chillers, District CoolingFigure 19. Large chilled-water system schematic Creating one centralized chilled-water system takes significant foresight, initial investment, and building development with a multi-year master plan. If the initial plant is built to accommodate many future buildings or loads, the early challenge is operating the system efficiently with much lower loads than it will experience when the project is complete. The system may need to blend parallel and series configurations “Series-Counterflow Application” on page 77 to accommodate the wide range of loads the plant experiences during phased construction PowerPipe size WaterControls Chiller performance testingLimitations of field performance testing Chiller Plant System PerformanceGuideline 22 includes a discussion of Guidelines for system efficiency monitoringApplication Considerations 1 Full year analysis Energy and economic analysis of alternativesApplication Considerations Guidance for Chilled- and Condenser-Water Flow Rates Selecting Chilled- and Condenser-Water Temperatures and Flow RatesSystem Design Options System Design Options Chilled-Water TemperaturesStandard rating temperatures System Design Options Condenser-Water TemperaturesChilled- and Condenser-Water Flow Rates Standard rating flow conditionsSystem Design Options Selecting flow rates DP2/DP1 = Flow2/Flow11.85 System Design OptionsSystem Design Options The total system power is now as followsFigure 21. Chilled water system performance at part load Coil response to decreased entering water temperatureExample of coil reselection at colder temperature/reduced flow rate System Design OptionsFluid ΔT, F C Cooling-tower options with low flowSystem Design Options Smaller towerSame tower, smaller approach System Design OptionsΔT1 = 94.2 - 78 = 16.2F or 34.6 - 25.6 = 9.0C ΔT2 = 99.1 - 78 = 21.1F or 37.3 - 25.6 = 11.7CSame tower, larger chiller System Design OptionsReselected tower for 50% more heat rejection with 15F 8.3C Retrofit opportunitiesSystem Design Options rangeSystem Design Options Cost ImplicationsSystem Design Options Misconceptions about Low-Flow RatesMisconception 1-Low flow is only good for long piping runs System System Design OptionsConsumption EnergyDemirchian and Maragareci12, Eley13, and Schwedler and Nordeen11 independently showed that system energy consumption can be reduced by reducing flow rates. It is interesting to note that in the systems studied, three different chiller manufacturer’s chillers were examined, yet the energy savings only varied from 2.0 to 6.5 percent. In all cases, regardless of which manufacturer’s chillers were used, the system energy consumption was reduced. In addition, Demirchian and Maragareci12, and Schwedler and Nordeen11 also noted reduced first costs System Design OptionsParallel Chillers System ConfigurationsSystem Configurations Figure 27. Series chillers Series ChillersSystem Configurations System Configurations Primary-Secondary Decoupled SystemsHydraulic decoupling bypass piping no-flow static head from the building water column, and System ConfigurationsFigure 29. Production loop System Configurations ProductionDistribution-loop benefits of decoupled system arrangement System Configurations DistributionCampus System ConfigurationsPumping arrangements CommonFigure 34. Decoupled system supply tee System ConfigurationsDecoupled system-principle of operation Figure 35. Temperature-sensing Flow-based controlSystem Configurations Temperature-sensingSubtracting a chiller System Configurations Chiller sequencing in decoupled systemsMultiple chilled-water plants on a distribution loop Adding a chillerFigure 36. Double-ended decoupled system System ConfigurationsPump control in a double-ended decoupled system Chiller sequencing in a double-ended decoupled system System ConfigurationsFigure 37. Variable-primary-flow system Variable-Primary-Flow SystemsOther plant designs System ConfigurationsOperational savings of VPF designs Advantages of variable primary flowSystem Configurations Dispelling a common misconception Chiller selection requirementsSystem Configurations Evaporator flow limitsTo promote good heat transfer and stable control minimum flow limit System ConfigurationsNumber of operating chillers System ConfigurationsFlow-rate reduction when an isolation valve Managing transient water flowsSystem Configurations These descriptions should include control sequences for System design and control requirementsSystem Configurations Bypass locations System ConfigurationsAccurate flow measurement Bypass flow control Chiller sequencing in VPF systemsSystem Configurations Adding a chiller in a VPF system System ConfigurationsSequencing based on load System ConfigurationsSubtracting a chiller in a VPF system Use multiple air handlers, and stagger their start/stop times Other VPF control considerationsSystem Configurations Select slow-acting valves to control the airside coilsAssess the economic feasibility of VPF for single-chiller plants Plant configurationSystem Configurations Consider a series arrangement for small VPF applicationsChiller selection Guidelines for a successful VPF systemSystem Configurations Moderate “low ΔT syndrome” by manifolding the chilled water pumpsChiller sequencing System ConfigurationsPlant configuration Bypass flowPlate-and-frame heat exchanger Heat RecoveryChilled-Water System Variations Condenser “Free Cooling” or Water EconomizerFigure 41. Refrigerant migration chiller in compression cooling mode Chilled-Water System VariationsRefrigerant migration Figure 43. Water economizer piped in parallel with chillers Well, river, or lake waterFigure 42. Refrigerant migration chiller in free-cooling mode Chilled-Water System VariationsFigure 44. Parallel preferential loading arrangement Preferential LoadingPreferential loading - parallel arrangement Chilled-Water System VariationsFigure 45. Sidestream preferential loading arrangement Preferential loading - sidestream arrangementChilled-Water System Variations Sidestream plate-and-frame heat exchangerProduction RecoverySidestream with alternative fuels or absorption Figure 47. Preferential loading - series arrangement Preferential loading - series arrangementChilled-Water System Variations Sidestream system controlFigure 48. Series-counterflow arrangement Series-Counterflow ApplicationSeries-series counterflow Chilled-Water System VariationsEvaporators Unequal Chiller SizingChilled-Water System Variations Figure 50. Series arrangement of evaporators and condensersRequired Volume = Flow Rate × Loop Time System Issues and ChallengesLow ΔT Syndrome Amount of Fluid in the LoopExample System Issues and ChallengesChiller response to changing conditions System response to changing conditionsSystem Issues and Challenges ContingencyMinimum capacity required Type and size of chillerAncillary equipment System Issues and Challenges Location of equipmentAlternative Energy Sources Water and electrical connectionsSystem Issues and Challenges Plant ExpansionAlternative fuel Thermal storageSystem Issues and Challenges Retrofit OpportunitiesApplications Outside the Chiller’s Range Flow rate out of rangeFigure 52. Temperatures out of range for equipment System Issues and Challenges Temperatures out of rangePrecise temperature control SYS-APM001-EN System Issues and ChallengesFigure 53. Precise temperature control, multiple chillers Chiller System Design and ControlChilled-water pump control Chilled water reset-raising and loweringSystem Controls Chilled-Water System ControlCritical valve reset pump pressure optimization System ControlsNumber of chillers to operate Table 17. VFDs and centrifugal chillers performance at 90% load Condenser-Water System ControlMinimum refrigerant pressure differential System ControlsCooling-tower-fan control Condenser-water temperature controlSystem Controls Figure 54. Chiller-tower energy consumption System ControlsChiller-tower energy balance Chiller-tower-pump balance System Controls Variable condenser water flowThese three energy consumers must be balanced to minimize overall energy use. This makes varying condenser water flow complex, but the strategy below has been implemented on projects Decoupled condenser-water systemSystem Controls Figure 56. Decoupled condenser-water system System ControlsCooling Tower Fan Inverter Cooling Tower Water Sump Tower Water Recirculation Pump CDWP-2 Inverter Chiller CDWP-1 Inverter ChillerSystem Controls Failure RecoveryFigure 57. Failure recovery Conclusion cooling tower water. See cooling water Glossarycondenser water, leaving. See cooling water pumps system Glossarytower water. See cooling water GlossaryPlant.” IDEA 88th Annual Conference Proceedings 1997 ReferencesExcept Low-Rise Residential Buildings. ASHRAE and IESNA ReferencesEngineering July ASHRAE/IESNA Standard 90.1-2007 Energy Standard for Buildings32 Trane Applications Engineering Group. “Thermal Storage - Understanding the Choices.” Ice Storage Systems, Engineered Systems Clinics. Trane, 1991. ISS-CLC-2 ReferencesIndex Index Index Index Page Date TraneLiterature Order Number SYS-APM001-EN