.Buildings.Templates.Plants.Controls.Pumps.Generic.StagingHeaderedDeltaP

Information

Pumps are staged as a function of the ratio ratV_flow of current volume flow rate V_flow to design volume flow rate V_flow_nominal, the number of operating pumps nPum_actual and the number of pumps that operate at design conditions nPum. Pumps are assumed to be equally sized.

FR = V_flow / V_flow_nominal

The next lag pump is enabled whenever either:

• Efficiency condition: FR > nPum_actual / nPum − dVOffUp is true for dtRun; or
• Failsafe condition: Pump speed command > yUp for dtRunFaiSaf and loop ∆p < setpoint – dpOff for dtRunFaiSaf for all hardwired differential pressure sensors.

The last lag pump is disabled whenever either:

• The lag pump was staged on based on the efficiency condition and FR < (nPum_actual - 1) / nPum − dVOffDow is true for dtRun; or
• The lag pump was staged on based on the failsafe condition, pump speed command < yDow for dtRunFaiSafLowY and loop ∆p > setpoint – dpOff for dtRunFaiSaf for all hardwired differential pressure sensors.

If desired, the stage down flow point dVOffDow can be offset slightly below the stage up point dVOffUp to prevent cycling between pump stages in applications with highly variable loads.

The timers are reset to zero when the status of a pump changes. This is necessary to ensure the minimum pump runtime with rapidly changing loads.

Details

The staging logic based on the efficiency condition (excluding the failsafe condition) is prescribed in ASHRAE, 2021 for:

• headered variable speed primary pumps in primary-only chiller and boiler plants using differential pressure pump speed control,
• variable speed secondary pumps in primary-secondary chiller plants with one or more sets of secondary loop pumps serving downstream control valves,
• variable speed secondary pumps in primary-secondary boiler plants with serving a secondary loop with a flow meter.

For other plant configurations, the pumps are staged with the equipment, i.e., the number of pumps matches the number of chillers or boilers. The actual logic for generating the pump enable commands is part of the staging event sequencing.

References

• ASHRAE, 2021. Guideline 36-2021, High-Performance Sequences of Operation for HVAC Systems. Atlanta, GA.

Revisions

• May 31, 2024, by Antoine Gautier:
  Added failsafe conditions and refactored to generate a stage change command at the exact time when the conditions are met.
• March 29, 2024, by Antoine Gautier:
  First implementation.