blockController
Waterside economizer (WSE) enable/disable status
Information
Waterside economizer (WSE) control sequence per ASHRAE Guideline 36-2021, section 5.20.3.
Enable and disable WSE
The sequence controls the WSE status as follows:
-
Enable WSE if it has been disabled for at least
holdPeriodof time and the chilled water return temperature (CHWRT) upstream of WSE,TChiWatRet, is greater than the WSE predicted heat exchanger leaving water temperature (PHXLWT) increased inTOffsetEna. -
Disable WSE if it has been enabled for at least
holdPeriodof time and CHWRT downstream of WSE,TChiWatRetDow, is greater thanTChiWatRetdecreased inTOffsetDisfordelDisperiod.
The following state machine chart illustrates the transitions between WSE enabled and disabled states:
The WSE enabling sequence uses the following subsequences:
- Calculation of the PHXLWT at given conditions: Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Economizers.Subsequences.PredictedOutletTemperature.
- Calculation of the tuning parameter used as an input to PHXLWT calculation: Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Economizers.Subsequences.Tuning.
Chilled water flow through the WSE
-
If the flow through the WSE is controlled using a modulating heat exchanger bypass
valve (
have_byPasValCon=true), the WSE in-line CHW return line valve (yRetVal) is controlled by Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Economizers.Subsequences.BypassValve -
If the flow through the WSE is controlled by a variable speed pump
(
have_byPasValCon=false), the pump is controlled by Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Economizers.Subsequences.HeatExchangerPump
Economizer-only chilled water bypass valve
Per section 5.20.3.11, when the economizer is enabled and all chiller isolation valves are
commanded closed, open the economizer-only chilled water bypass valve (y1ChiWatBypVal=true).
Close the bypass valve when any chiller isolation valve is commanded open and exceeds
25% open.
The economizer-only chilled water bypass valve is typically needed for primary-only parallel plant.
Parameters
| Type | Name | Default | Description |
|---|---|---|---|
| Boolean | have_byPasValCon | true | True: chilled water flow through economizer is controlled using heat exchanger bypass valve |
| Boolean | have_priOnl | true | True: the primary-only plant; False: the primary-secondary plant |
| Boolean | have_parChi | true | True: the plant has parallel chillers |
| Integer | nChi | 2 | Number of chillers in the plant |
| Integer | nSta | 3 | Number of chiller stages |
| Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Types.Actuator | chiIsoValTyp | Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Types.Actuator.Modulating | Chiller CHW isolation valve type |
| Boolean | have_modPosChiVal | false | True for chiller CHW isolation valves with position feedback |
| Real | chaChiWatIsoTim | 300 | Time to slowly change chiller chilled water isolation valve, should be determined in the field |
| Enable parameters | |||
| Real | holdPeriod | 1200 | WSE minimum on or off time |
| Real | delDis | 120 | Delay disable time period |
| Real | TOffsetEna | 2 | Temperature offset between the chilled water return upstream of WSE and the predicted WSE output |
| Real | TOffsetDis | 1 | Temperature offset between the chilled water return upstream and downstream WSE |
| Design parameters | |||
| Real | heaExcAppDes | 2 | Design heat exchanger approach |
| Real | cooTowAppDes | 2 | Design cooling tower approach |
| Real | TOutWetDes | 288.15 | Design outdoor air wet bulb temperature |
| Real | VHeaExcDes_flow | 0.015 | Design heat exchanger chilled water volume flow rate |
| Valve or pump control | |||
| Real | dpDes | 6000 | Design pressure difference across the chilled water side economizer |
| CDL.Types.SimpleController | valCon | Buildings.Controls.OBC.CDL.Types.SimpleController.PI | Type of controller |
| Real | k | 0.1 | Gain of controller |
| Real | Ti | 0.5 | Time constant of integrator block |
| Real | Td | 0.1 | Time constant of derivative block |
| Real | minSpe | 0.1 | Minimum pump speed |
| Real | desSpe | 1 | Design pump speed |
| Advanced › Tuning | |||
| Real | step | 0.02 | Incremental step used to reduce or increase the water-side economizer tuning parameter |
| Real | wseOnTimDec | 3600 | Economizer enable time needed to allow decrease of the tuning parameter |
| Real | wseOnTimInc | 1800 | Economizer enable time needed to allow increase of the tuning parameter |
| Advanced › Hysteresis | |||
| Real | hysDt | 1 | Deadband temperature used in hysteresis block |
| Real | hysVal | 0.05 | Hysteresis for checking valve position |
| Advanced | |||
| Real | dtHol | 900 | Minimum hold time during stage change |
Connectors
| Type | Name | Default | Description |
|---|---|---|---|
| Buildings.Controls.OBC.CDL.Interfaces.RealInput | TOutWet | Outdoor air wet bulb temperature | |
| Buildings.Controls.OBC.CDL.Interfaces.RealInput | TChiWatRet | Chilled water return temperature upstream of the WSE | |
| Buildings.Controls.OBC.CDL.Interfaces.RealInput | TChiWatRetDow | Chilled water return temperature downstream of the WSE | |
| Buildings.Controls.OBC.CDL.Interfaces.RealInput | uTowFanSpeMax | Maximum cooling tower fan speed | |
| Buildings.Controls.OBC.CDL.Interfaces.RealInput | VChiWat_flow | Measured chilled water volume flow rate | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanInput | uPla | Plant enable signal | |
| Buildings.Controls.OBC.CDL.Interfaces.IntegerInput | uIni | Initial chiller stage (at plant enable) | |
| Buildings.Controls.OBC.CDL.Interfaces.IntegerInput | uChiSta | Current chiller stage | |
| Buildings.Controls.OBC.CDL.Interfaces.RealInput | dpChiWat | Differential static pressure across economizer in the chilled water side | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanInput | uPum | True: heat exchanger pump is proven on | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanInput | uStaPro | True: in staging process | |
| Buildings.Controls.OBC.CDL.Interfaces.RealInput | TEntHex | Chilled water temperature entering heat exchanger | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanInput[nChi] | u1ChiIsoVal | Chiller isolation valve commanded setpoint: true- commanded on | |
| Buildings.Controls.OBC.CDL.Interfaces.RealInput[nChi] | uChiIsoVal | Measured chiller isolation valve position | |
| Buildings.Controls.OBC.CDL.Interfaces.RealOutput | TWsePre | Predicted waterside economizer outlet temperature | |
| Buildings.Controls.OBC.CDL.Interfaces.RealOutput | yTunPar | Tuning parameter | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanOutput | y | WSE enable/disable status | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanOutput | yConWatIsoVal | Economizer condenser water isolation valve position | |
| Buildings.Controls.OBC.CDL.Interfaces.RealOutput | yRetVal | WSE in-line CHW return line valve position | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanOutput | yPumOn | Heat exchanger pump command on | |
| Buildings.Controls.OBC.CDL.Interfaces.RealOutput | yPumSpe | Heat exchanger pump speed setpoint | |
| Buildings.Controls.OBC.CDL.Interfaces.BooleanOutput | y1ChiWatBypVal | Economizer-only chiller water bypass valve commanded status |
Components
| Type | Name | Default | Description |
|---|---|---|---|
| Buildings.Controls.OBC.CDL.Reals.Hysteresis | enaTWet | Enable condition based on the outdoor wet bulb temperature | |
| Buildings.Controls.OBC.CDL.Logical.Not | notDisWse | Economizer does not need being disabled |
Revisions
-
January 19, 2021, by Milica Grahovac:
Added state chart illustration. -
October 13, 2018, by Milica Grahovac:
First implementation.