Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSMATCHING PUMPS TO WATERDISTRIBUTION SYSTEMS300270CSystem CurvesThe pump curves and affinity laws are used to select a pumpor pumps for a particular application. The first step is to establisha system pressure curve. This is calculated from design flowand pressure loss tables for all the piping, coils, control valves,and other components of the system.TOTAL PRESSURE (kPa)24021018015012090FLOW AT FULL LOADBPlotting A System Curve6030AAn example is shown in Figure 56. The design point is 195 kPaat a 32.2 L/s flow. A system curve (a simple square root curve)can be plotted once the flow and pressure loss are known at anyparticular point, since:flow⎛ 2 ⎞2 p 2⎜flow ⎟ =⎝ 1 p⎠ 1Where:flow 1flow 2= flow at p 1 in kPa= flow at p 2 in kPaPlot the points (Fig. 56) for flows of 12.5, 25, and 37.5 L/s:12.5Point A: ( 32.2 )25Point B: ( 32.2 )37.5Point C: ( 32.2 )2x 195 kPa = 0.1507 x 195 kPa = 29.4 kPa2x 195 kPa = 0.6028 x 195 kPa = 117.5 kPa2x 195 kPa = 1.356 x 195 kPa = 264.5 kPaThe system curve assumes all balancing valves are set fordesign conditions, that all controls valves are fully open, andthat flow through all loads is proportional. The system curve isalways the same, even if loading is not proportional, at no loadand full (100 percent) load. If the loads are not proportional(such as, some loads off or some valves throttling), the curverises above that shown for values between full and no load.The system curve in Figure 56 is used in Figure 57 to select thesingle speed pump. Using this system curve to determine theswitching setpoint for dual parallel pumps when the load flowis not proportional can result in damaging pump cycling.00 6.25 12.5 18.25 25 31.35 37.5CAPACITY IN L/sFig. 56. System Curve for Pump Application.Combining System And Pump CurvesC4092The design system flow is 32.2 L/s. Piping, control valve,and equipment losses are calculated at 195 kPa. An impellersize and a motor power are selected by imposing the systemcurve on the pump curve (Fig. 57). The designer has the optionof selecting a pump with a 240 mm impeller (32.2 L/s at233 kPa) or a 220 mm impeller (30.5 at180 kPa). The smallerimpeller requires a 7500 watt motor and the larger impellerrequires a 9000 watt motor. Selection of the 240 mm impellerrequires system balancing valves to reduce the system pressuredifferentials to those matching the design flow of 32.2 L/s.When selecting a pump, it is important to remember that:— With constant speed pumps (and two-way AHU controlvalves), flow rides the pump curve. The system curve isplotted assuming that the control valves are full open,which in any system, only occurs at the full load. Ascontrol valves throttle and loads are turned off the systembecomes non-proportional and the system curve risesbetween no load and design.— With variable speed pumps, the system control objectiveis to have the pump curve ride the system curve by keepingat least one control valve open and reducing the pump speedto reduce flow with the diminishing system drop.340ENGINEERING MANUAL OF AUTOMATIC CONTROL