blockDown

Information

Block that controls devices when there is a stage-down command. This sequence is for water-cooled primary-only parallel chiller plants with headered chilled water pumps and headered condenser water pumps, or air-cooled primary-only parallel chiller plants with headered chilled water pumps. This development is based on ASHRAE Guideline 36-2021, section 5.20.4.17, which specifies the step-by-step control of devices during the chiller staging down process.

1. Identify the chiller(s) that should be enabled (and disabled, if have_ponChi=true). This is implemented in block nexChi. See Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.NextChiller for more descriptions.
2. Start the staging down process,
   • For any stage change during which a smaller chiller is enabled and a larger chiller is disabled:
     1. Command operating chillers to reduce demand to 75% (chiDemRedFac) of their current load (uChiLoa) or a percentage equal to 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.
     2. Slowly change the minimum flow bypass setpoint to the one that includes both chillers are enabled. After the new setpoint is achieved, wait 1 minute (aftByPasSetTim) to allow loop being stabilized.
     3. Enable head pressure control for the chiller being enabled. Wait 30 seconds (waiTim).
     4. Slowly (chaChiWatIsoTim) open the chilled water isolation valve of the smaller chiller being enabled. The valve timing should be determined in the field.
     5. Start the smaller chiller after its chilled water isolation valve is fully open.
     6. Wait 5 minutes (proOnTim) for the newly enabled chiller to prove that it is operating correctly, then shut off the larger chiller and release the demand limit.
   • If staging down from any other stage, shut off the last stage chiller.
   These are implemented in block dowSta. See Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.DownStart for more descriptions.
3. When the controller of the chiller being shut off indicates no request for chilled water flow (uChiWatReq=false), slowly (chaChiWatIsoTim) close the chiller's chilled water isolation valve to avoid a sudden change in flow through other operating chillers. This is implemented in block disChiIsoVal. See Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.CHWIsoVal for more descriptions.
4. When the controller of the chiller being shut off indicates no request for condenser water flow (uConWatReq=false), disable the chiller's head pressure control loop. This is implemented in block disHeaCon. See Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.HeadControl for more descriptions.
5. When the condenser water head pressure control valve is fully closed, shut off the last lag condenser water pump or change the pump speed to that required of the new stage. Block disNexCWP identifies chiller stage for the condenser water pump control ( Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.EnableCWPump) and block conWatPumCon checks if the condenser water pumps have been reset ( Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Pumps.CondenserWater.Controller).
6. Change the chilled water minimum flow setpoint to that appropriate for the new stage. The minimum flow setpoint is reset in block minChiWatFlo ( Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.MinimumFlowBypass.FlowSetpoint).

Parameters

Connectors

Revisions

• September 23, 2019, by Jianjun Hu:
  First implementation.