Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSSYSTEM DESCRIPTIONFigure 16 shows a typical “decoupled” multiple chillersystem. Each chiller has a (primary) dedicated constant speedpump selected to produce the chiller design flow through theprimary loop, including the “decoupler line”. The decouplerline isolates the primary and secondary pumping systems andhandles any imbalance between the two flow loops. Thedecoupler line is typically sized to handle the flow of the largestprimary pump at a negligible pressure drop, should be at least6 to 10 pipe diameters in length, and the tees at each end shouldbe configured as shown to oppose any undesirable induced flowforces. Decoupler flow should always be forward, not to exceedthe flow of one chiller. Any backward decoupler flow will dilutethe secondary chilled water supply with secondary return waterthus raising the secondary supply temperature above design.The secondary pumping system is variable volume and maycontain many varieties of pumping loops.Control, the staging of chillers on and off, is normally:– Start a chiller anytime the decoupler has a backward flow.– Stop a chiller anytime the decoupler forward flowexceeds that of the next chiller to be shed.SOFTWARE PARTITIONINGFrom an operational and control perspective, the physicalconfiguration of chiller plant digital controllers is usuallytransparent. The configuration varies, depending upon:– Chiller staging algorithm.– Redundant/backup control requirements.– Condenser water system configuration.NOTE: Where water leaving cooling towers becomescommon before being extended to the chillerplant, a single cooling tower isolating, staging,and loading algorithm is usually preferred.– Chiller monitoring requirements.– Controller capacity for monitoring and control.– Other project-unique requirements.Figure 17 is a schematic of a digital system configuration. Eachchiller has a dedicated cooling tower and a dedicated controllerfor chiller, cooling tower, and condenser water monitoring andcontrol. Figure 18 shows a variation of Figure 17 where condenserwater is common to all chillers and the cooling towers are stagedin response to condenser water demand.DCCT4CT3CT2CTDIGITAL COMMUNICATIONSCHILLERDIGITALCONTROLLERS14DC3DC2DC1CHILLERSECONDARY SYSTEMDEMANDS FORTEMPERATURE AND FLOWCHLR4CHLR3CHLR2CHLR17.0P S17.0T SS7.0P S27.413.0 13.0 T RST SST RPT SPT DM15258STAGINGCONTROLLERFig. 17. Typical Digital Controller Configuration for Multiple Chillers.310ENGINEERING MANUAL OF AUTOMATIC CONTROL