Trane SYS-APM001-EN manual System Design Options Selecting flow rates

Page 36

System Design Options

Selecting flow rates

Designers may use the standard rating conditions to compare manufacturers’ performances at exactly the same conditions. However, these standards allow any flow rates to be used and certified comparisons to be made at a wider range of conditions.

For a given load, as flow rate decreases, the temperature differential increases. Table 4 reflects a 450-ton [1,580-kW refrigeration] chilled-water system, both as a base case and with low flow.

Table 4. Standard rating conditions for chilled-water systems

Chilled Water System

 

Base Case

Low Flow

 

 

 

 

Evaporator flow rate, gpm [L/s]

 

1,080 [68.1]

675 [42.6]

 

 

 

 

 

 

 

 

Chilled water

 

 

Entering

54.0

[12.2]

57.0

[13.9]

 

 

 

 

 

 

 

 

 

Leaving

44.0

[6.7]

41.0

[5.0]

temperature

°

°

 

F [ C]

 

 

 

 

 

Condenser flow rate, gpm [L/s]

 

1,350 [85.2]

900 [56.8]

 

 

 

 

 

 

 

 

Condenser water

Entering

85.0

[29.4]

85.0

[29.4]

 

 

 

 

 

Leaving

94.3

[34.6]

99.1

[37.3]

temperature

°

°

 

F [ C]

 

 

 

 

 

Chiller power, kW

 

256.0

292.0

 

 

 

 

 

 

 

 

In this example, notice that the leaving chilled-water temperature decreases and the leaving condenser-water temperature increases. This means that the chiller’s compressor must provide more lift and use more power. At first glance, the design team may decide the chiller power difference is too large to be overcome by ancillary equipment savings. The key question is, How does this impact system energy consumption? Using the following assumptions, we can calculate system energy usage:

•80 feet of water [239 kPa] pressure drop through chilled-water piping

•30 feet of water [89.7 kPa] pressure drop through condenser-water piping

•78°F [25.6°C] design wet bulb

•93 percent motor efficiency for pumps and tower

•75 percent pump efficiency

•Identical pipe size in chilled- and condenser-water loops (either a design decision, or indicating changing flows in an existing system)

The pressure drop through the chiller will decrease due to the lower flow rates. When using the same size pipe, the pressure drop falls by nearly the square of the decreased flow rate. While this is true for straight piping, the pressure drop does not follow this exact relationship for control valves or branches serving loads of varying diversity.

Be sure to calculate the actual pressure drop throughout the system.

Hazen–Williams and Darcy–Weisbach calculate the change is to the 1.85 and

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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 variationEffect of condenser-water temperature Water-cooled condenserEffect of condenser-water flow rate Maintenance Air-cooled condenserAir-cooled versus water-cooled condensers 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 pumpDistribution piping Pump per chillerManifolded pumps Pumping arrangements Constant flow systemCondenser-Water System Cooling towerPrimary-secondary system Variable-primary systemCondenser-water pumping arrangements Effect of load on cooling tower performanceEffect of ambient conditions on cooling tower performance Single tower per chillerUnit-Level Controls Chiller controlRecommended 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 chillersApplication Considerations Constant flow Variable flowCondensing method Application Considerations Number of chillersParallel or series Part load system operationMid-Sized Chilled-Water Systems Chillers Managing control complexityPreferential vs. equalized loading and run-time Large chilled-water system schematic Large Chilled-Water Systems + Chillers, District CoolingPower Pipe sizeWater Chiller performance testing Limitations of field performance testingChiller Plant System Performance ControlsSYS-APM001-EN SYS-APM001-EN System Design Options Guidance for Chilled- and Condenser-Water Flow RatesChilled-Water Temperatures Standard rating temperaturesSystem Design Options Condenser-Water Temperatures Chilled- and Condenser-Water Flow RatesStandard 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 loadEntering fluid temperature, F C Cooling-tower options with low flowSmaller tower 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 arrangementCommon CampusTertiary or distributed Decoupled system-principle of operation Tertiary pumping arrangementTemperature-sensing Flow-based controlFlow-sensing Multiple chilled-water plants on a distribution loop Adding a chillerSubtracting 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 selectionPlant configuration Bypass flowChiller sequencing Airside controlHeat Recovery Chilled-Water System VariationsCondenser Free Cooling or Water Economizer 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 counterflowUnequal Chiller Sizing EvaporatorsCondensers System Issues and Challenges Low ΔT SyndromeAmount of Fluid in the Loop System Issues and Challenges Chiller response to changing conditionsSystem response to changing conditions ExampleContingency Minimum capacity requiredType and size of chiller System Issues and Challenges Location of equipment Alternative Energy SourcesWater and electrical connections Ancillary equipmentPlant Expansion Alternative fuelThermal storage Retrofit Opportunities Applications Outside the Chiller’s RangeFlow rate out of range System Issues and Challenges Temperatures out of range Precise temperature controlPrecise temperature control, multiple chillers Chilled water reset-raising and lowering System ControlsChilled-Water System Control Chilled-water pump controlCritical valve reset pump pressure optimization System ControlsNumber of chillers to operate Condenser-Water System Control Minimum refrigerant pressure differentialVFDs and centrifugal chillers performance at 90% load 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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