blockFlowSetpoint
Information
Block that outputs chilled water minimum flow setpoint for primary-only plants with a minimum flow bypass valve, according to ASHRAE Guideline 36-2021, section 5.20.8 Chilled water minimum flow bypass valve.
1. For plants with parallel chillers, the bypass valve shall modulate to maintain minimum flow as measured by the chilled water flow meter at a setpoint that ensures minimum flow through all operating chillers, as follows:
-
For the operating chillers in current stage, identify the chiller with the
highest ratio of
minFloSettomaxFloSet. -
Calculate the minimum flow setpoint as the highest ratio multiplied by the sum
of
maxFloSetfor the operating chillers.
| Chiller | Minimum flow | Maximum flow |
|---|---|---|
| 1 | minFloSet[1] |
maxFloSet[1] |
| 2 | minFloSet[2] |
maxFloSet[2] |
| ... | ... | ... |
2. For plants with series chillers, the bypass valve shall modulate to maintain minimum
flow as measured by the chilled water flow meter at a setpoint equal to the largest
minFloSet of the operating chillers in current stage.
3. If there is any stage change requiring a chiller on and another chiller off,
the minimum flow setpoint shall temporarily change to account for the
minFloSet of both the chiller to be enabled and to be disabled
before starting the newly enabled chiller.
Note that when there is a stage change requiring a change in the
minimum flow setpoint, the change should be slowly.
For example, this could be accomplished by resetting the setpoint X GPM/second,
where X = (NewSetpoint - OldSetpoint) / byPasSetTim.
Parameters
| Type | Name | Default | Description |
|---|---|---|---|
| Integer | nChi | 3 | Total number of chillers |
| Boolean | have_parChi | true | Flag: true means that the plant has parallel chillers |
| Real | byPasSetTim | 300 | Time constant for resetting minimum bypass flow |
| Real[nChi] | minFloSet | {0.005, 0.005, 0.005} | Minimum chilled water flow through each chiller |
| Real[nChi] | maxFloSet | {0.025, 0.025, 0.025} | Maximum chilled water flow through each chiller |
Connectors
| Type | Name | Default | Description |
|---|---|---|---|
| Buildings.Controls.OBC.CDL.Interfaces.BooleanInput | uStaUp | Stage up logical signal | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanInput | uUpsDevSta | Resetting status of upstream device (in staging up or down process) before reset minimum chilled water flow setpoint | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanInput[nChi] | uChi | Chiller status: true=ON | |
| Buildings.Controls.OBC.CDL.Interfaces.IntegerInput | nexEnaChi | Index of next chiller to be enabled | |
| Buildings.Controls.OBC.CDL.Interfaces.IntegerInput | nexDisChi | Index of next chiller to be disabled | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanInput | uSubCha | Indicate if upstream device has been reset in the sub-process. This input is used when the stage change needs chiller on-off | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanInput | uOnOff | Indicate if the stage change requires one chiller to be enabled while another is disabled | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanInput | uStaDow | Stage down logical signal | |
| Buildings.Controls.OBC.CDL.Interfaces.RealOutput | yChiWatMinFloSet | Chilled water minimum flow setpoint | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanOutput | yChaSet | True: it is in the setpoint changing process |
Revisions
-
January 31, 2019, by Jianjun Hu:
First implementation.