Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS300215240mmBALANCED OPERATING POINTWITH 240mm IMPELLER240220mmDESIGN OPERATING POINT210200mmOPERATION WITH 220mm IMPELLERTOTALPRESSURE (kPa)18015012090185mm9000W11000W60SYSTEM CURVE4000W5500W7500W3000 6.25 12.5 18.75 25 31.25 37.5 43.75 50 56.25C4093CAPACITY IN (L/s)Fig. 57. Matching Pump to System.To better understand system curves, pump curves, and flowcontrol, Figure 58 shows the control valve(s) (the only variableelement of a typical system curve) separately from the rest ofthe system elements. Lines are shown for each of three valvepositions; full, 80 percent, and 50 percent flow. These lines,when added to the curve for all other elements of the systemintersect the pump curve at the corresponding operating point(s).Figure 58 shows a system with 31.25 L/s and 210 kPa loss atdesign, 30 kPa of which is a full open control valve at the endof the piping run. Line “A” represents the control valve andconnects the pump curve to the static-element system curve. Ifall control valves positioned to 80 percent flow, the pumppressure rises, the static-element system resistance drops, andthe control valve, represented by line “B”, makes the difference;about 105 kPa. Similarly, at 50 percent flow, the valve drop,represented by line “C”, accounts for about 220 kPa.PRESSURE (kPa)2402101801501209060OPERATING POINT@ 50% FLOWLINE “A” CONTROL VALVE*FULL OPEN @ DESIGN30 kPa DROP AT 31.25 L/SLINE “B” @ 80% FLOW,CONTROL VALVE TAKES98 kPa DROPLINE “C” @ 50% FLOW,CONTROL VALVE TAKES189 kPa DROP* THE LINE “A” VALVE ISAT THE END OF THEPIPING RUN, AND THESYSTEM CURVE IS FORTHE REST OF THESYSTEM.IN A BALANCED SYSTEM,THE SYSTEM CURVE ISTHE SAME AT ANY AIRHANDLING UNIT, WITHTHE AIR HANDLING UNITBALANCING VALVESMAKING UP FOR ANYREDUCED PIPINGLOSSES.LINE CPUMPCURVELINE BDESIGNOPERATINGPOINTPUMP REQUIRED:31.25 L/s at 210 kPaLINE A30SYSTEM CURVE FOR STATICELEMENTS OF SYSTEM (CHILLER,PIPING, FITTINGS, BALANCING COCK,COILS, STRAINERS, ETC.)00 6.25 12.5 18.75 25 31.25SYSTEM FLOW IN L/sM15268Fig. 58. Pump and System Curves and Control ValvesENGINEERING MANUAL OF AUTOMATIC CONTROL341