Trane SYS-APM001-EN manual Multiple chilled-water plants on a distribution loop, Adding a chiller

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System Configurations

Chiller sequencing in decoupled systems

Given the amount and direction of flow in the bypass line, chillers can be added or subtracted.

Adding a chiller

When there is deficit flow in the bypass line, the system is receiving water at a temperature above the desired supply water temperature. At this point in time, a chiller and pump may be added. Many operators sense deficit flow for a particular amount of time (for example, 15 minutes) to ensure that the deficit flow is not a result of some transient condition. This reduces the chances of cycling a chiller; that is, turning on a chiller and then turning it off after a short time.

Subtracting a chiller

A chiller may be turned off when enough surplus water is flowing through the bypass line. How much is enough? Enough so that the chiller does not need to cycle on again in a short time period. Many system operators compare the amount of surplus flow with the flow rate of the chiller they are considering turning off. If this ratio is 110 to 115 percent, they turn the chiller off. Let’s look at an example:

Chiller 1 can make 960 gpm [60.6 L/s] of 40°F [4.4°C] chilled-water, while Chiller 2 can make 1,440 gpm [90.8 L/s]. At present, there is 1,100 gpm [69.4 L/s] of surplus flow in the bypass line.

The surplus bypass flow is presently 115 percent of Chiller 1’s flow. If we turn Chiller 1 off, we will have 140 gpm [8.8 L/s] of surplus flow left.

Note that the surplus bypass flow is presently only 76 percent of Chiller 2’s flow. If we turn Chiller 2 off, we will have 340 gpm [21.5 L/s] of deficit flow. It is clear that we would have to cycle that chiller back on soon.

In this case, we could turn Chiller 1 off and leave Chiller 2 on for the most efficient use of the chillers.

Multiple chilled-water plants on a distribution loop

When decoupled systems are used on large campus-type systems, added loads are often located some distance away from the original loads. Yet, planners like the idea of somehow hooking the new loads to the existing system. The double-ended system shown in Figure 36 is one way of handling this requirement. A second production facility is placed at a convenient location in the new part of the campus. Its distribution plant is laid out as a mirror image of the original piping, and connects to it at the ends of each system. Each production facility has its own bypass.

Both production loops feed into the now common distribution loop. Depending on the flows from the production facility distribution pumps, loads could be served by either plant.

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Chiller System Design and Control

SYS-APM001-EN

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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 variationWater-cooled condenser Effect of condenser-water temperatureEffect of condenser-water flow rate 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 pumpPump per chiller Distribution pipingManifolded pumps 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 chillerChiller control Unit-Level ControlsRecommended chiller-monitoring points per Ashrae Standard Centrifugal chiller capacity control Centrifugal chiller with AFDAFD on both chillers Application Considerations Small Chilled-Water Systems 1-2 chillersVariable flow Application Considerations Constant flowCondensing method Parallel or series Application ConsiderationsNumber of chillers Part load system operationManaging control complexity Mid-Sized Chilled-Water Systems ChillersPreferential vs. equalized loading and run-time Large chilled-water system schematic Large Chilled-Water Systems + Chillers, District CoolingPipe size PowerWater 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 RatesStandard 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 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 arrangementCampus CommonTertiary or distributed Decoupled system-principle of operation Tertiary pumping arrangementFlow-based control Temperature-sensingFlow-sensing Adding a chiller Multiple chilled-water plants on a distribution loopSubtracting 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 counterflowEvaporators Unequal Chiller SizingCondensers Low ΔT Syndrome System Issues and ChallengesAmount of Fluid in the Loop System response to changing conditions System Issues and ChallengesChiller response to changing conditions ExampleMinimum capacity required ContingencyType and size of chiller Water and electrical connections System Issues and Challenges Location of equipmentAlternative Energy Sources Ancillary equipmentAlternative fuel Plant ExpansionThermal storage Applications Outside the Chiller’s Range Retrofit OpportunitiesFlow rate out of 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 controlSystem Controls Critical valve reset pump pressure optimizationNumber of chillers to operate 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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