Trane SYS-APM001-EN manual Series Chillers, Series chillers

Page 50

System Configurations

System ΔT greater than 14°F

One reason series chilled-water systems should be designed using at least a 14 degree ΔT is because a lower ΔT ignores the opportunity for lower flow rates and reduced pump energy. For small, packaged chillers, a reason to have at least a 14 degree ΔT is to avoid exceeding the chiller’s maximum flow rate. The system will more likely experience evaporator flow limit maximums if the ΔT is too low.

Series Chillers

If chillers are piped in series, as in Figure 27, the mixing problem disappears and the starving coils problem (when one of the pumps in a parallel arrangement is not running) is resolved. Series flow presents a new set of temperature and flow control challenges.

Figure 27. Series chillers

 

 

56°F [13.3°C]

49°F [9.4°C]

 

42°F [5.5°C]

 

 

 

Chiller 2

Chiller 1

 

 

setpoint = 42°F [5.5°C]

 

 

setpoint = 42°F [5.5°C]

 

Loads

The flow rate through each chiller is the entire system flow, that is, double the individual flow rate of two parallel chillers. This means that the chiller evaporator must accommodate the doubled water quantity. This may be accommodated by using fewer water passes in the evaporator, which may result in decreased chiller efficiency.

However, this efficiency loss due to fewer passes is more than offset by the increased efficiency of the upstream chiller, now operating at a warmer temperature.

Pressure losses are additive when the chillers are piped in series. This increases total system pressure drop, thereby using more pump energy. On the other hand, series chillers work particularly well in low-flow systems, where the system temperature difference is greater than 14°F [7.8°C], resulting in less pressure drop.

Low-flow systems were discussed in detail in “Selecting flow rates” on page 30. Series chillers are also suited to variable flow systems, where the operating pressure drop is reduced. Variable flow is discussed beginning on page 55.

Temperature control can be executed in several ways. Figure 27 shows a strategy where the controller on each chiller is set at the system design setpoint. Either chiller can be used to meet the system demand for up to 50

44

Chiller System Design and Control

SYS-APM001-EN

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Page 49 Page 51
Contents May Page Chiller System Design and Control Preface Contents 100 Primary System Components ChillerPrimary System Components Chiller evaporatorEffect of chilled-water temperature Effect of chilled-water flow rate and variationEffect of condenser-water flow rate Effect of condenser-water temperatureWater-cooled condenser Air-cooled versus water-cooled condensers MaintenanceAir-cooled condenser Packaged or Split System?Low-ambient operation Energy efficiencyLoads Air-cooled or water-cooled efficiencyThree-way valve load control Two-way valve load controlVariable-speed pump load control Face-and-bypass dampersChilled-Water Distribution System Chilled-water pumpManifolded pumps Distribution pipingPump per chiller Pumping arrangements Constant flow systemPrimary-secondary system Condenser-Water SystemCooling tower Variable-primary systemEffect of ambient conditions on cooling tower performance Condenser-water pumping arrangementsEffect of load on cooling tower performance Single tower per chillerRecommended chiller-monitoring points per Ashrae Standard Unit-Level ControlsChiller control Centrifugal chiller capacity control Centrifugal chiller with AFDAFD on both chillers Application Considerations Small Chilled-Water Systems 1-2 chillersCondensing method Application Considerations Constant flowVariable flow Parallel or series Application ConsiderationsNumber of chillers Part load system operationPreferential vs. equalized loading and run-time Mid-Sized Chilled-Water Systems ChillersManaging control complexity Large chilled-water system schematic Large Chilled-Water Systems + Chillers, District CoolingWater PowerPipe size Chiller Plant System Performance Chiller performance testingLimitations of field performance testing ControlsSYS-APM001-EN SYS-APM001-EN System Design Options Guidance for Chilled- and Condenser-Water Flow RatesSystem Design Options Chilled-Water TemperaturesStandard rating temperatures Standard rating flow conditions Condenser-Water TemperaturesChilled- and Condenser-Water Flow Rates System Design Options Selecting flow rates DP2/DP1 = Flow2/Flow11.85 Low-flow conditions for cooling tower Base Case Low FlowTotal system power Component Power kW Base Case Low Flow System summary at full loadCoil response to decreased entering water temperature Chilled water system performance at part loadSmaller tower Entering fluid temperature, F CCooling-tower options with low flow System designΔT2 = 99.1 78 = 21.1F or 37.3 25.6 = 11.7C Same tower, smaller approachSame tower, larger chiller Same tower, smaller approach Present Smaller ApproachRetrofit opportunities Retrofit capacity changes Larger Present Chiller Same towerCost Implications Misconceptions about Low-Flow Rates Misconception 1-Low flow is only good for long piping runsKWh SYS-APM001-EN System Configurations Parallel ChillersSystem Configurations Parallel chillers with separate, dedicated chiller pumpsSeries Chillers Series chillersPrimary-Secondary Decoupled Systems Hydraulic decouplingCheck valves System Configurations Production Production loopSystem Configurations Distribution Distribution-loop benefits of decoupled system arrangementTertiary or distributed CommonCampus Decoupled system-principle of operation Tertiary pumping arrangementFlow-sensing Temperature-sensingFlow-based control Subtracting a chiller Multiple chilled-water plants on a distribution loopAdding a chiller Pump control in a double-ended decoupled system Double-ended decoupled systemChiller sequencing in a double-ended decoupled system Variable-Primary-Flow Systems Other plant designsAdvantages of variable primary flow Operational savings of VPF designsChiller selection requirements Dispelling a common misconceptionFlow, ft.water Flow rate Managing transient water flows Flow-rate changes that result from isolation-valve operationSystem Configurations System design and control requirements Effect of dissimilar evaporator pressure dropsAccurate flow measurement Chiller sequencing in VPF systems Bypass flow controlAdding a chiller in a VPF system Flow-rate-fluctuation examplesSubtracting a chiller in a VPF system Sequencing based on loadOther VPF control considerations Select slow-acting valves to control the airside coilsPlant configuration Consider a series arrangement for small VPF applicationsGuidelines for a successful VPF system Chiller selectionChiller sequencing Plant configurationBypass flow Airside controlCondenser Free Cooling or Water Economizer Heat RecoveryChilled-Water System Variations Plate-and-frame heat exchangerChilled-Water System Variations Refrigerant migrationRefrigerant migration chiller in free-cooling mode Well, river, or lake waterPreferential Loading Preferential loading parallel arrangementPreferential loading sidestream arrangement Sidestream plate-and-frame heat exchangerSidestream with alternative fuels or absorption Chilled-Water System VariationsPreferential loading series arrangement Sidestream system controlSeries-Counterflow Application Series-series counterflowCondensers Unequal Chiller SizingEvaporators Amount of Fluid in the Loop System Issues and ChallengesLow ΔT Syndrome System response to changing conditions System Issues and ChallengesChiller response to changing conditions ExampleType and size of chiller ContingencyMinimum capacity required Water and electrical connections System Issues and Challenges Location of equipmentAlternative Energy Sources Ancillary equipmentThermal storage Plant ExpansionAlternative fuel Flow rate out of range Retrofit OpportunitiesApplications Outside the Chiller’s Range System Issues and Challenges Temperatures out of range Precise temperature controlPrecise temperature control, multiple chillers Chilled-Water System Control Chilled water reset-raising and loweringSystem Controls Chilled-water pump controlNumber of chillers to operate Critical valve reset pump pressure optimizationSystem Controls VFDs and centrifugal chillers performance at 90% load Condenser-Water System ControlMinimum refrigerant pressure differential Chillers DifferenceCondenser-water temperature control Cooling-tower-fan controlChiller-tower energy balance Chiller-tower energy consumptionSystem Controls Variable condenser water flow Chiller-tower-pump balanceDecoupled condenser-water system Effect of chiller load on water pumps and cooling tower fansCDWP-2 Failure Recovery Failure recoveryConclusion Glossary Glossary Pumps systemGlossary References Plant. Idea 88th Annual Conference Proceedings 1997References Engineering July102 Index AshraeIndex 105 106 Page Trane

SYS-APM001-EN specifications

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