.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 the following is true for dtRun:

FR > nPum_actual / nPum − dVOffUp

The last lag pump is disabled whenever the following is true for dtRun:

FR < (nPum_actual - 1) / nPum − dVOffDow

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

This logic 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.

A "if" condition is used to generate the stage up and down command as opposed to a "when" condition. This means that the command remains true as long as the condition is verified. This is necessary, for example, if no higher stage is available when a stage up command is triggered. Using a "when" condition – which is only valid at the point in time at which the condition becomes true – would prevent the plant from staging when a higher stage becomes available again. To avoid multiple consecutive stage changes, the block that receives the stage up and down command and computes the stage index must enforce a minimum stage runtime of dtRun.

References

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

Revisions

• March 29, 2024, by Antoine Gautier:
  First implementation.