Trane SYS-APM001-EN manual Sidestream with alternative fuels or absorption

Page 81

Chilled-Water System Variations

Referred to as “distribution sidestream,” an alternative location is to apply the chiller at the air handler that requires reheat. The heat recovery chiller can cool either the supply or return chilled water. If only a small amount of heating is needed, this may be accomplished with a water-to-water heat pump.

Sidestream heat recovery

A similar situation occurs if a heat-recovery chiller is placed in this sidestream position24 (see Figure 46). This chiller may be equipped with a heat recovery condenser or it could be a standard, single-condenser chiller operating as a heat pump in a heating control mode. The chiller may not be capable of cooling water to the system supply temperature. That is unimportant with this configuration. Think of the machine as a heater instead of a chiller. Its primary function is heating, and cooling is a beneficial by- product. The passing return-chilled-water stream appears as an infinite heat source to the heat recovery chiller. The chiller only cools its evaporator water enough to satisfy the heating demand. This avoids the control predicament of deciding how to reject surplus condenser heat when the cooling and heating loads of a chiller do not strike a perfect heat balance.

Figure 46. Heat recovery chiller in sidestream position

50.2°F

[10.1°C]

Off
40°F	Chilled
[4.4°C]	Water
	Production
900 gpm [56.8 L/s]
40°F [4.4°C] 75 gpm

51.2°F	Bypass Line
[10.7°C]	Bypass Line
[10.7°C]	42.7°F [5.9°C]
	42.7°F [5.9°C]	Heat
		Recovery
		Chiller
56°F
[13.3°C]	300 gpm [18.9 L/s]

Chilled

Water

Distribution

825gpm

[52 L/s]

Sidestream with alternative fuels or absorption

An absorption chiller may be applied in a sidestream location. This allows the chiller to be loaded whenever utility rates make it beneficial. It also ensures that the absorption water chiller receives the warmest entering-water temperature, allowing it to operate more efficiently and produce more cooling.

SYS-APM001-EN

Chiller System Design and Control

75

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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 temperatureEffect of condenser-water temperature Water-cooled condenserEffect 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 SystemDistribution piping Pump per chillerManifolded 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 chillerUnit-Level Controls Chiller controlRecommended 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 ConsiderationsApplication Considerations Constant flow Variable flowCondensing method Number of chillers Application ConsiderationsParallel or series Part load system operationMid-Sized Chilled-Water Systems Chillers Managing control complexityPreferential vs. equalized loading and run-time Large Chilled-Water Systems + Chillers, District Cooling Large chilled-water system schematicPower Pipe sizeWater 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 OptionsChilled-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 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 DistributionCommon CampusTertiary or distributed Tertiary pumping arrangement Decoupled system-principle of operationTemperature-sensing Flow-based controlFlow-sensing Multiple chilled-water plants on a distribution loop Adding a chillerSubtracting 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 Variations Well, 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 ApplicationUnequal Chiller Sizing EvaporatorsCondensers System Issues and Challenges Low ΔT SyndromeAmount of Fluid in the Loop Chiller response to changing conditions System Issues and ChallengesSystem response to changing conditions ExampleContingency Minimum capacity requiredType and size of chiller Alternative Energy Sources System Issues and Challenges Location of equipmentWater and electrical connections Ancillary equipmentPlant Expansion Alternative fuelThermal storage Retrofit Opportunities Applications Outside the Chiller’s RangeFlow 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 controlCritical valve reset pump pressure optimization System ControlsNumber 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