Trane SYS-APM001-EN manual Variable-Primary-Flow Systems, Other plant designs

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System Configurations

with a surplus that may, or may not, be large enough to indicate stopping a chiller in that plant.

Other plant designs

There are many other ways to connect chillers to distributed loops and each provides its own challenges and opportunities. The advent of variable- primary-flow chilled-water systems offers some new opportunities as does distributed pumping. In any case, getting system design advice specific to your system early in the process can prevent or identify many operational challenges.

Variable-Primary-Flow Systems

Two main physical differences distinguish this type of system from the more familiar primary–secondary design, which hydraulically “decouples” the constant-flow production side of the chilled-water loop from the variable-flow distribution side. The variable-primary-flow (VPF) design eliminates the constant-flow chiller pumps and uses the variable-flow pumps to circulate water throughout the entire chilled water loop (Figure 37). Both systems include a bypass line, but the VPF bypass will be smaller.

Figure 37. Variable-primary-flow system

Typ

Chiller 1

Isolation Valves

Chiller 2

Flow Meter

 

(Alternative to Chiller ΔP)

 

Bypass Line

 

Modulating

Airside Coils with Two-Way

Control Valve

Control Valves

 

Bypass Line

 

Alternate

 

Location

In a VPF system, water flow varies throughout the entire system— through the evaporator of each operating chiller as well as through the cooling coils. Two-way control valves on coils, check (or isolation) valves on chillers, and a bypass pipe with a control valve are required to implement a VPF system.

Variable-flow chiller pumps eliminate the need for a separate distribution pump.

SYS-APM001-EN

Chiller System Design and Control

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Contents May Page Chiller System Design and Control Preface Contents 100 Chiller Primary System ComponentsChiller evaporator Primary System ComponentsEffect of chilled-water flow rate and variation Effect of chilled-water temperatureWater-cooled condenser Effect of condenser-water temperatureEffect of condenser-water flow rate Air-cooled condenser MaintenanceAir-cooled versus water-cooled condensers Packaged or Split System?Energy efficiency Low-ambient operationAir-cooled or water-cooled efficiency LoadsTwo-way valve load control Three-way valve load controlFace-and-bypass dampers Variable-speed pump load controlChilled-water pump Chilled-Water Distribution SystemPump per chiller Distribution pipingManifolded pumps Constant flow system Pumping arrangementsCooling tower Condenser-Water SystemPrimary-secondary system Variable-primary systemEffect of load on cooling tower performance Condenser-water pumping arrangementsEffect of ambient conditions on cooling tower performance Single tower per chillerChiller control Unit-Level ControlsRecommended chiller-monitoring points per Ashrae Standard Centrifugal chiller with AFD Centrifugal chiller capacity controlAFD on both chillers Small Chilled-Water Systems 1-2 chillers Application ConsiderationsVariable flow Application Considerations Constant flowCondensing method Number of chillers Application ConsiderationsParallel or series Part load system operationManaging control complexity Mid-Sized Chilled-Water Systems ChillersPreferential vs. equalized loading and run-time Large Chilled-Water Systems + Chillers, District Cooling Large chilled-water system schematicPipe size PowerWater Limitations of field performance testing Chiller performance testingChiller Plant System Performance ControlsSYS-APM001-EN SYS-APM001-EN Guidance for Chilled- and Condenser-Water Flow Rates System Design OptionsStandard rating temperatures Chilled-Water TemperaturesSystem Design Options Chilled- and Condenser-Water Flow Rates Condenser-Water TemperaturesStandard rating flow conditions System Design Options Selecting flow rates Low-flow conditions for cooling tower Base Case Low Flow DP2/DP1 = Flow2/Flow11.85System summary at full load Total system power Component Power kW Base Case Low FlowChilled water system performance at part load Coil response to decreased entering water temperatureCooling-tower options with low flow Entering fluid temperature, F CSmaller tower System designSame tower, smaller approach ΔT2 = 99.1 78 = 21.1F or 37.3 25.6 = 11.7CSame tower, smaller approach Present Smaller Approach Same tower, larger chillerRetrofit capacity changes Larger Present Chiller Same tower Retrofit opportunitiesCost Implications Misconception 1-Low flow is only good for long piping runs Misconceptions about Low-Flow RatesKWh SYS-APM001-EN Parallel Chillers System ConfigurationsParallel chillers with separate, dedicated chiller pumps System ConfigurationsSeries chillers Series ChillersHydraulic decoupling Primary-Secondary Decoupled SystemsCheck valves Production loop System Configurations ProductionDistribution-loop benefits of decoupled system arrangement System Configurations DistributionCampus CommonTertiary or distributed Tertiary pumping arrangement Decoupled system-principle of operationFlow-based control Temperature-sensingFlow-sensing Adding a chiller Multiple chilled-water plants on a distribution loopSubtracting a chiller Double-ended decoupled system Pump control in a double-ended decoupled systemChiller sequencing in a double-ended decoupled system Other plant designs Variable-Primary-Flow SystemsOperational savings of VPF designs Advantages of variable primary flowDispelling a common misconception Chiller selection requirementsFlow, ft.water Flow rate Flow-rate changes that result from isolation-valve operation Managing transient water flowsSystem Configurations Effect of dissimilar evaporator pressure drops System design and control requirementsAccurate flow measurement Bypass flow control Chiller sequencing in VPF systemsFlow-rate-fluctuation examples Adding a chiller in a VPF systemSequencing based on load Subtracting a chiller in a VPF systemSelect slow-acting valves to control the airside coils Other VPF control considerationsConsider a series arrangement for small VPF applications Plant configurationChiller selection Guidelines for a successful VPF systemBypass flow Plant configurationChiller sequencing Airside controlChilled-Water System Variations Heat RecoveryCondenser Free Cooling or Water Economizer Plate-and-frame heat exchangerRefrigerant migration Chilled-Water System VariationsWell, river, or lake water Refrigerant migration chiller in free-cooling modePreferential loading parallel arrangement Preferential LoadingSidestream plate-and-frame heat exchanger Preferential loading sidestream arrangementChilled-Water System Variations Sidestream with alternative fuels or absorptionSidestream system control Preferential loading series arrangementSeries-series counterflow Series-Counterflow ApplicationEvaporators Unequal Chiller SizingCondensers Low ΔT Syndrome System Issues and ChallengesAmount of Fluid in the Loop Chiller response to changing conditions System Issues and ChallengesSystem response to changing conditions ExampleMinimum capacity required ContingencyType and size of chiller Alternative Energy Sources System Issues and Challenges Location of equipmentWater and electrical connections Ancillary equipmentAlternative fuel Plant ExpansionThermal storage Applications Outside the Chiller’s Range Retrofit OpportunitiesFlow rate out of range Precise temperature control System Issues and Challenges Temperatures out of rangePrecise temperature control, multiple chillers System Controls Chilled water reset-raising and loweringChilled-Water System Control Chilled-water pump controlSystem Controls Critical valve reset pump pressure optimizationNumber of chillers to operate Minimum refrigerant pressure differential Condenser-Water System ControlVFDs and centrifugal chillers performance at 90% load Chillers DifferenceCooling-tower-fan control Condenser-water temperature controlChiller-tower energy consumption Chiller-tower energy balanceChiller-tower-pump balance System Controls Variable condenser water flowEffect of chiller load on water pumps and cooling tower fans Decoupled condenser-water systemCDWP-2 Failure recovery Failure RecoveryConclusion Glossary Pumps system GlossaryGlossary Plant. Idea 88th Annual Conference Proceedings 1997 ReferencesEngineering July References102 Ashrae IndexIndex 105 106 Page Trane

SYS-APM001-EN specifications

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