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"Riding the pump curve" means the pump with constant speed drive operates at variable water flow rate. You should use Pump:VariableSpeed for the secondary pump, but you should be careful about the coefficients of the Part Load Performance Curve.

The coefficients are discribed in ANSI/ASHRAE/IES Standard90.1 Performance Rating Method Reference Manual.

The primary pump has a constant water flow rate. Pump:ConstantSpeed should be used.

Please refer to the input example below for each parameter.

Pump:ConstantSpeed,
    chiller1 ChW Branch Pump,!- Name
    chiller1 ChW Pump Inlet, !- Inlet Node Name
    chiller1 Chw Inlet,      !- Outlet Node Name
    autosize,                !- Design Flow Rate {m3/s}
    75384,                   !- Design Pump Head {Pa}
    autosize,                !- Design Power Consumption {W}
    0.9,                     !- Motor Efficiency
    0,                       !- Fraction of Motor Inefficiencies to Fluid Stream
    Intermittent,            !- Pump Control Type
    ,                        !- Pump Flow Rate Schedule Name
    ,                        !- Pump Curve Name
    ,                        !- Impeller Diameter {m}
    ,                        !- Rotational Speed {rev/min}
    ,                        !- Zone Name
    ,                        !- Skin Loss Radiative Fraction
    PowerPerFlowPerPressure,             !- Design Power Sizing Method
    ,                        !- Design Electric Power per Unit Flow Rate {W/(m3/s)}
    1.38888888888889,        !- Design Shaft Power per Unit Flow Rate per Unit Head {W/((m3/s)-Pa)}
    Chilled Water Pump;      !- End-Use Subcategory


Pump:VariableSpeed,
    CHWloop ChW Secondary Pump,  !- Name
    CHWloop ChW Demand Inlet,!- Inlet Node Name
    CHWloop ChW Secondary Pump Outlet,  !- Outlet Node Name
    autosize,                !- Design Maximum Flow Rate {m3/s}
    150768,                  !- Design Pump Head {Pa}
    autosize,                !- Design Power Consumption {W}
    0.9,                     !- Motor Efficiency
    0,                       !- Fraction of Motor Inefficiencies to Fluid Stream
    0,                       !- Coefficient 1 of the Part Load Performance Curve
    3.2485,                  !- Coefficient 2 of the Part Load Performance Curve
    -4.7443,                   !- Coefficient 3 of the Part Load Performance Curve
    2.5294,                  !- Coefficient 4 of the Part Load Performance Curve
    autosize,                       !- Design Minimum Flow Rate {m3/s}
    Intermittent,            !- Pump Control Type
    ,                        !- Pump Flow Rate Schedule Name
    ,                        !- Pump Curve Name
    ,                        !- Impeller Diameter {m}
    ,                        !- VFD Control Type
    ,                        !- Pump RPM Schedule Name
    ,                        !- Minimum Pressure Schedule {Pa}
    ,                        !- Maximum Pressure Schedule {Pa}
    ,                        !- Minimum RPM Schedule {rev/min}
    ,                        !- Maximum RPM Schedule {rev/min}
    ,                        !- Zone Name
    ,                        !- Skin Loss Radiative Fraction
    PowerPerFlowPerPressure,             !- Design Power Sizing Method
    ,                        !- Design Electric Power per Unit Flow Rate {W/(m3/s)}
    1.38888888888889,        !- Design Shaft Power per Unit Flow Rate per Unit Head {W/((m3/s)-Pa)}
    0.25,                    !- Design Minimum Flow Rate Fraction
    Chilled Water Pump;      !- End-Use Subcategory