blockAHU

Outdoor airflow related calculations at the AHU level

Information

This sequence outputs AHU level uncorrected minimum outdoor airflow rate VUncOutAir_flow and effective minimum outdoor airflow rate VEffOutAir_flow when complying with ASHRAE Standard 62.1 ventilation requirements. It is implemented according to Section 5.16.3.1 of ASHRAE Guideline G36, May 2020.

It requires following inputs which are sum or maximum of the outputs from the zone level calculation. See Buildings.Controls.OBC.ASHRAE.G36.AHUs.MultiZone.VAV.SetPoints.OutdoorAirFlow.ASHRAE62_1.SumZone for these inputs.

Sum of the adjusted population component breathing zone flow rate for all zones that are in all zone groups in occupied mode, VSumAdjPopBreZon_flow.
Sum of the adjusted area component breathing zone flow rate for all zones that are in all zone groups in occupied mode, VSumAdjAreBreZon_flow.
Sum of the zone primary airflow rates for all zones in all zone groups that are in occupied mode,VSumZonPri_flow.
Maximum zone outdoor air fraction for all zones in all zone groups that are in occupied mode, uOutAirFra_max.

The calculation is done using the steps below.

See Section 3.1.4.2.a of Guideline 36 for setpoints VUncDesOutAir_flow and VDesTotOutAir_flow.
The uncorrected outdoor airflow rate setpoint VUncOutAir_flow is recalculated continuously based on the adjusted population and area component breathing zone flow rate of the zones being served determined in accordance with Section 5.2.1.3. See Buildings.Controls.OBC.ASHRAE.G36.VentilationZones.ASHRAE62_1.Setpoints.
```
    VUncOutAir_flow = min(VUncDesOutAir_flow, (VSumAdjPopBreZon_flow + VSumAdjAreBreZon_flow))
```

Calculate the current system ventilation efficiency as

    sysVenEff = 1 + (VUncOutAir_flow/VSumZonPri_flow) - uOutAirFra_max

Calculate the effective minimum outdoor air setpoint VEffOutAir_flow as the uncorrected outdoor air intake divided by the system ventilation efficiency, but no larger than the design total outdoor airflow rate VDesTotOutAir_flow:
```
    VEffOutAir_flow = min(VUncOutAir_flow/sysVenEff, VDesTotOutAir_flow)
```

Parameters

Type	Name	Description
Economizer design
Buildings.Controls.OBC.ASHRAE.G36.Types.OutdoorAirSection	minOADes	Type of outdoor air section
Nominal condition
Real	VUncDesOutAir_flow	Uncorrected design outdoor airflow rate, including diversity where applicable. It can be determined using the 62MZCalc spreadsheet from ASHRAE 62.1 User's Manual
Real	VDesTotOutAir_flow	Design total outdoor airflow rate. It can be determined using the 62MZCalc spreadsheet from ASHRAE 62.1 User's Manual

Connectors

Type	Name	Description
Buildings.Controls.OBC.CDL.Interfaces.RealInput	VSumAdjPopBreZon_flow	Sum of the adjusted population component breathing zone flow rate
Buildings.Controls.OBC.CDL.Interfaces.RealInput	VSumAdjAreBreZon_flow	Sum of the adjusted area component breathing zone flow rate
Buildings.Controls.OBC.CDL.Interfaces.RealInput	VSumZonPri_flow	Sum of the zone primary airflow rates for all zones in all zone groups that are in occupied mode
Buildings.Controls.OBC.CDL.Interfaces.RealInput	uOutAirFra_max	Maximum zone outdoor air fraction
Buildings.Controls.OBC.CDL.Interfaces.RealInput	VAirOut_flow	Measured outdoor air volumetric flow rate
Buildings.Controls.OBC.CDL.Interfaces.RealOutput	VUncOutAir_flow	Uncorrected minimum outdoor airflow rate
Buildings.Controls.OBC.CDL.Interfaces.RealOutput	VEffAirOut_flow_min	Effective minimum outdoor airflow setpoint
Buildings.Controls.OBC.CDL.Interfaces.RealOutput	effOutAir_normalized	Effective minimum outdoor airflow setpoint, normalized by the design total outdoor airflow rate
Buildings.Controls.OBC.CDL.Interfaces.RealOutput	outAir_normalized	Normalized outdoor airflow rate

Components

Type	Name	Description
Buildings.Controls.OBC.CDL.Reals.Sources.Constant	uncDesOutAir	Uncorrected design outdoor airflow rate, including diversity where applicable
Buildings.Controls.OBC.CDL.Reals.Add	add2	Sum of the breathing zone outdoor airflow
Buildings.Controls.OBC.CDL.Reals.Min	min1	Uncorrected minimum outdoor airflow rate
Buildings.Controls.OBC.CDL.Reals.Divide	div1	First input divided by second input
Buildings.Controls.OBC.CDL.Reals.AddParameter	addPar	Add parameter
Buildings.Controls.OBC.CDL.Reals.Subtract	sysVenEff	Current system ventilation efficiency
Buildings.Controls.OBC.CDL.Reals.Max	max1	Avoid devide by zero
Buildings.Controls.OBC.CDL.Reals.Sources.Constant	desOutAir	Design total outdoor airflow rate
Buildings.Controls.OBC.CDL.Reals.Divide	div2	Division
Buildings.Controls.OBC.CDL.Reals.Min	min2	Uncorrected minimum outdoor airflow rate
Buildings.Controls.OBC.CDL.Reals.MultiplyByParameter	gaiDivZer	Gain, used to avoid division by zero if the flow rate is smaller than 0.1%
Buildings.Controls.OBC.CDL.Reals.Divide	norVOutMin	Normalization for minimum outdoor air flow rate
Buildings.Controls.OBC.CDL.Reals.Divide	norVOut	Normalization for outdoor air flow rate
Buildings.Controls.OBC.CDL.Reals.Sources.Constant	neaZer	Near zero value
Buildings.Controls.OBC.CDL.Reals.Max	max2	Avoid devide by zero

Revisions

April 6, 2024, by Michael Wetter:
Corrected wrong annotation.
March 12, 2022, by Jianjun Hu:
First implementation.