Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSFigure 55 illustrates a pump fitted with a 6-1/2 inch impeller,operating at 45 ft of head, and delivering 65 gpm of water.Water hp = 65 gpm x 45 ft x 1.0/3960= 0.74 hpThe curves show that the motor output is 1.17 horsepower.Efficiency = (water horsepower)/(motor horsepower) x 100 =(0.74/1.17) x 100 = 63 percent. This agrees with the efficiencycurves shown in Figure 55.Pump Affinity LawsPump affinity laws (Table 3) show how pump flow, head,and brake horsepower vary as impeller diameter or speedchange. These laws help when adjusting an installed pump tochanges in the system served. For example, if a pump with an8-inch impeller delivers 80 ft of head, with a 7.21-inch impellerit would deliver 65 ft of head. It is calculated as follows:New head = Old head x⎝ ⎛ New DiameterOld Diameter⎠ ⎞ 2= 80⎝ ⎛7.21 2⎞8 ⎠ = 64.98 ftImpellerDiameterSpeedTable 3. Pump Affinity Laws.SpecificGravity(SG)ToCorrectforFlowConstant Variable Constant HeadMultiplybyNew ( Old Speed)New 2( Old Speed)Bhp( )(or kW) New Speed3Old SpeedFlowVariable Constant Constant HeadNew ( Old Diameter)New 2( Old Diameter)Bhp( )(or kW) New Diameter3Old DiameterConstant Constant Variable Bhp(or kW)New SGOld SGSource: ASHRAE Handbook--1996 System and Equipment45%55%6060%SYSTEM CURVE65%OPERATING POINT7"68%506 1/2"68%65%TOTAL HEAD IN FEET405 1/2"6"60%55%3045%5"3HP202HP1 1/2HP1/2HP3/4HP1HP00 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190CAPACITY IN GALLONS PER MINUTEC1051Fig. 55. Pump Curve for 1750 RPM Operation.ENGINEERING MANUAL OF AUTOMATIC CONTROL316