blockInitial

Outputs the initial stage

Information

This subsequence is not directly specified in Guideline 36-2021 as it provides a side calculation about the generalization of the staging sequences for any number of chillers and stages provided by the user.

Determines the initial stage upon plant startup for both plants with and without a WSE. It is implemented according to section 5.20.4.15 of Guideline 36-2021, under part g. (primary-only) and part p. (primary-secondary) plants.

The initial stage yIni is defined as:

When the plant is enabled and the plant has no waterside economizer (have_WSE=false), the initial stage will be the lowest available stage uUp.

When the plant is enabled and the plant has the waterside economizer (have_WSE=true), the initial stage will be:

If predicted waterside economizer outlet temperature calculated using Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Economizers.Subsequences.PredictedOutletTemperature with predicted heat exchanger part load ratio PLRHeaExc set to 1 is at least wseDt below the chilled water supply temperature setpoint TChiWatSupSet, then the initial stage will be 0, meaning that the plant initiates in a waterside economizer only mode.
Otherwise, the initial stage will be the lowest available stage uUp.

The following state machine chart illustrates the initial stage selection for plants with a waterside economizer:

Image of initial stage selection state machine chart

Parameters

Type	Name	Default	Description
Boolean	have_WSE	true	true = plant has a WSE, false = plant does not have WSE
Advanced
Real	wseDt	1	Offset for checking waterside economizer leaving water temperature
Waterside economizer
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	0.01	Desing heat exchanger chilled water flow rate

Connectors

Type	Name	Description
Buildings.Controls.OBC.CDL.Interfaces.IntegerInput	uUp	First higher available chiller stage
Buildings.Controls.OBC.CDL.Interfaces.RealInput	TOutWet	Outdoor air wet bulb temperature
Buildings.Controls.OBC.CDL.Interfaces.RealInput	uTunPar	Tuning parameter as at last plant disable
Buildings.Controls.OBC.CDL.Interfaces.RealInput	TChiWatSupSet	Chilled water supply setpoint
Buildings.Controls.OBC.CDL.Interfaces.IntegerOutput	yIni	Initial chiller plant stage
Buildings.Controls.OBC.CDL.Interfaces.BooleanInput	uPla	Plant enable signal

Components

Type	Name	Description
Buildings.Controls.OBC.CDL.Discrete.TriggeredSampler	triSam	Samples first available stage up at plant enable
Buildings.Controls.OBC.ASHRAE.G36.Plants.Chillers.Economizers.Subsequences.PredictedOutletTemperature	wseTOut	Waterside economizer outlet temperature predictor
Buildings.Controls.OBC.CDL.Conversions.IntegerToReal	intToRea	Type converter
Buildings.Controls.OBC.CDL.Conversions.RealToInteger	reaToInt	Type converter
Buildings.Controls.OBC.CDL.Reals.Sources.Constant	staZer	Zero stage
Buildings.Controls.OBC.CDL.Reals.Hysteresis	hys1	Check if the initial predicted heat exchange leaving water temperature is greater than chilled water supply temperature setpoint less offset
Buildings.Controls.OBC.CDL.Reals.Subtract	sub1	Difference between predicted heat exchanger leaving water temperature and chilled water supply temperature setpoint
Buildings.Controls.OBC.CDL.Reals.Switch	swi	Logical switch
Buildings.Controls.OBC.CDL.Logical.Sources.Constant	noWSE	Replacement signal for no WSE case
Buildings.Controls.OBC.CDL.Reals.Sources.Constant	con3	Design heat exchanger chiller water flow rate
Buildings.Controls.OBC.CDL.Discrete.TriggeredSampler	triSam1	Samples the difference at plant enable

Revisions

January 19, 2021, by Milica Grahovac:
Added state chart illustration.
March 12, 2020, by Milica Grahovac:
First implementation.