blockUp

Information

Block that controls devices when there is a stage-up 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 5.20.4.16, which specifies the step-by-step control of devices during the chiller staging up 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. Command operating chillers to reduce demand to 75% (chiDemRedFac) of their current load (uChiLoa). 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.
3. Reset the minimum chilled water flow setpoint,
   • For any stage change during which a smaller chiller is disabled and a larger chiller is enabled, slowly change (byPasSetTim) the minimum chilled water flow setpoint to the one that includes both chillers are enabled. After the new setpoint is achieved, wait 1 minute (aftByPasSetTim) to allow loop to stabilize.
   • For any other stage change, reset (byPasSetTim) the minimum chilled water flow setpoint to the one that includes the new chiller. After the new setpoint is achieved, wait 1 minute (aftByPasSetTim) to allow loop to stabilize.
   The minimum flow setpoint is reset in block minChiWatFlo ( Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.MinimumFlowBypass.FlowSetpoint). Block minBypSet checks if the new setpoint is achieved ( Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.ResetMinBypass).
4. Start the next condenser water pump and (or) change the condenser water pump speed to that required of the new stage. Wait 10 seconds (thrTimEnb). Block enaNexCWP 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).
5. Enabled head pressure control for the being enabled chiller. Wait 30 seconds (waiTim). This is implemented in block enaHeaCon. See Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.HeadControl for more descriptions.
6. Slowly (chaChiWatIsoTim) open the chilled water isolation valve of the chiller being enabled. The valve timing should be determined in the field. This is implemented in block enaChiIsoVal. See Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.CHWIsoVal for more descriptions.
7. End the staging up process:
   • If the stage change does not require one chiller to be enabled and another chiller to be disabled, start the next stage chiller after the isolation valve is fully open.
   • If the stage change does require one chiller to be enabled and another chiller to be disabled, starting the next stage chiller after the isolation valve is fully open, then shut off the smaller chiller, close the chiller's chilled water isolation valve, disable the head pressure control loop, and change the minimum chilled water flow setpoint to the one for the new stage.
   • Release the demand limit, which marks the end of the staging process.
   These are implemented in the block endUp. See Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Staging.Processes.Subsequences.UpEnd for more descriptions.

Parameters

Connectors

Revisions

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