Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSHEATEXCHANGEPIPINGCONTROLVALVE(S)LOADthe lag pump stop setpoint should have a significant margin ofsafety incorporated. The lag pump start setpoint should becontrolled by a differential pressure controller and have thesoftware requirement that one control valve be full open forfour minutes before starting.HEADPIPINGDROPVALVEDROPPUMPCURVELOWFLOWLOW FLOWCURVEDESIGN100% FLOWDESIGNPIPINGDROPDESIGNVALVEDROPTime delays must be built in to the control sequence to preventrapid switching between one pump and two pump operation. Witheach change in pump operation, all control valves must adjust tonew steady-state conditions. The adjustment process often causesovershoot or undershoot until temperature stability returns andno switching should take place during this time. Depending uponthe type of temperature control loops, switch-lockout period canvary from 5 minutes for relatively fast discharge air control toover 30 minutes for relatively slow space control.FLOWC2411Fig. 65. System Operation with One Pump,Design and Low Flow Condition.PUMP 1CONTROLVALVE (S)OperationMultiple pumps may be connected either in parallel or inseries into the system. In the dual parallel pump configurationof Figure 66 a single pump can usually handle 75 to 80 percentof the total flow. The system curves show that at designconditions the control valve drop is 30 kPa (from A to B). At75 percent flow (21 L/s), the valve drop with both pumpsoperating increases to over 125 kPa (C to E). With one pumpand 75 percent flow the valve drop is about 50 kPa (C to D).When flow is reduced to 50 Percent, the valve drop is about165 kPa for one pump (F to G) or 190 kPa for two pumps (F toH). Dual parallel pumps save energy and provide redundancyfor 75 to 80 percent of the flow. They do not provide muchrelief for high valve pressure drops at low flow.The pump curves and the system curves indicate possiblepump start/stop setpoints. One scenario on a pumpingdifferential fall to 130 kPa, energizes the second pump and ona pumping differential rise to 235 kPa, switches back to onepump. The 130 kPa pumping differential corresponds to a pointjust before the 1-pump curve intersects the system curve (I),the point at which a single pump no longer can support thesystem. When the second pump is started, the operating pointmoves to the 2-pump curve and when the control valves havesettled out will be at about Point J. It will vary along the2-pump curve down to B or up to K. When the operating pointreaches K (about 235 kPa) the system switches back to a singlepump and the operating point is now on the 1-pump curveuntil the differential pump pressure drops to I, at which timethe cycle repeats. See PLOTTING A SYSTEM CURVE forstatement on use of ideal system curve for determiningsetpoints when coil loading may not be proportional.Again a reminder to exercise caution when using the idealsystem curves for switching pumps on and off. The ideal curvesare valid only at full and no load conditions, the rest of the timethe actual curve is somewhere above the ideal. Since setpointdetermination is not possible without the actual system curve,PRESSURE (kPa)2402101801501209060300HEATEXCHANGER1 PUMPSYSTEMCURVEPUMP 22 PUMPSHGFDESIGNOPERATINGPOINT6 12 18 24 30LOADSYSTEM CURVEMINUSCONTROL VALVEM15282Fig. 66. System Operation with Two Pumps in Parallel.Series pumps (Fig. 67), though rarely used in HVAC systems,are useful where both flow and pressure are sharply reduced atlight loads.KEFLOW (L/s)DCJIBA344ENGINEERING MANUAL OF AUTOMATIC CONTROL