blockDownStart

Information

Block that controls devices at the first step of the chiller staging down process. This development is based on ASHRAE Guideline 36-2021, section 5.20.4.17, item a and b. The sections specifies the first step of the staging down process.

For the stage-down process that requires a smaller chiller being enabled and a larger chiller being disabled (uOnOff=true):

1. Command operating chillers to reduce demand to 75% (chiDemRedFac) of their current load or a percentage equal to the current stage minimum cycling operative partial load ratio (yOpeParLoaRatMin), whichever is greater. Wait until actual demand < 80% of current load up to a maximum of 5 minutes (holChiDemTim) before proceeding. This is implemented in block chiDemRed. See Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.ReduceDemand for more descriptions.
2. Slowly change the minimum chilled water flow setpoint to that appropriate for the stage transition (minChiWatSet). After the new setpoint is achieved, wait 1 minute (aftByPasSetTim) to allow the loop to stabilize (minBypRes). See Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.MinimumFlowBypass.FlowSetpoint and Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.ResetMinBypass for more descriptions.
3. Enable head pressure control for the chiller being enabled. Wait 30 seconds (waiTim). It is implemented in block enaHeaCon, see Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.HeadControl for more descriptions.
4. Slowly open the chilled water isolation valve of the smaller chiller being enabled. Determine valve timing chaChiWatIsoTim in the field as that is required to prevent nuisance trips. It is implemented in block enaChiIsoVal, see Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.CHWIsoVal for more descriptions.
5. Start the smaller chiller after its chilled water isolation valve is fully open. Wait 5 minutes (proOnTim) after the newly enabled chiller to prove that it is operating correctly, then shut off the larger chiller and release the demand limit (yReaDemLim=true). It is implemented in block disChi, see Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.DisableChiller for more descriptions.

For staging down from any other stage (uOnOff=false), just shut off the last stage chiller. This is implemented in block disChi, see Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.DisableChiller for more descriptions.

Parameters

Connectors

Revisions

• September 19, 2019, by Jianjun Hu:
  First implementation.