blockAHU

Output outdoor airflow related calculations at the AHU level
Diagram of AHUOpen the diagram
▶ Check in your browser

Information

This sequence outputs AHU level design minimum outdoor airflow rate VDesOutAir_flow and effective minimum outdoor airflow rate VEffOutAir_flow.

It requires following inputs which are sum, maximum or minimum of the outputs from the zone level calculation:

  1. Sum of the design population of the zones in the group, sumDesZonPop.
  2. Sum of the population component design breathing zone flow rate, VSumDesPopBreZon_flow.
  3. Sum of the area component design breathing zone flow rate, VSumDesAreBreZon_flow.
  4. Minimum of all zones ventilation efficiency, uDesSysVenEff.
  5. Sum of all zones required uncorrected outdoor airflow rate, VSumUncOutAir_flow.
  6. Sum of the measured discharged flow rate of all terminal units, VSumSysPriAir_flow.
  7. Maximum of primary outdoor air fraction of all zones, uOutAirFra_max.

The calculation is done using the steps below.

  1. Compute the occupancy diversity fraction occDivFra. During system operation, the system population equals the sum of the zone population, so occDivFra=1. It has no impact on the calculation of the uncorrected outdoor airflow sysUncOutAir. For design purpose, compute for all zones 

        occDivFra = peaSysPop/sumDesZonPop

    where peaSysPop is the peak system population and sumDesZonPop is the sum of the design population.

  2. Compute the design uncorrected outdoor airflow rate VDesUncOutAir_flow as 

        VDesUncOutAir_flow = occDivFra*VSumDesPopBreZon_flow+VSumDesAreBreZon_flow.

  3. Compute the uncorrected outdoor airflow rate sysUncOutAir as 

        sysUncOutAir = min(VDesUncOutAir_flow, VSumUncOutAir_flow)

    where VSumUncOutAir_flow is sum of all zones required uncorrected outdoor airflow rate

  4. Compute the outdoor air fraction as 

        outAirFra = sysUncOutAir/VSumSysPriAir_flow.

    For design purpose, use 

        aveOutAirFra = sysUncOutAir/VPriSysMax_flow.

    where VPriSysMax_flow is the maximum expected system primary airflow at design stage.

  5. Compute the system ventilation efficiency sysVenEff. During system operation, the efficiency is 

        sysVenEff = 1 + outAirFra - uOutAirFra_max

  6. Compute the minimum required AHU outdoor air intake flow rate. The minimum required system outdoor air intake flow should be the uncorrected outdoor air intake sysUncOutAir divided by the system ventilation efficiency sysVenEff, but it should not be larger than the design outdoor air rate desOutAirInt. Hence, 

        effMinOutAirInt = min(sysUncOutAir/sysVenEff, desOutAirInt),

    where the design outdoor air rate desOutAirInt is 

        desOutAirInt = VDesUncOutAir_flow/uDesSysVenEff.

Parameters

TypeNameDefaultDescription
Nominal condition
RealVPriSysMax_flowMaximum expected system primary airflow at design stage
RealpeaSysPopPeak system population

Connectors

TypeNameDefaultDescription
Buildings.Controls.OBC.CDL.Interfaces.RealInputsumDesZonPopSum of the design population of the zones in the group
Buildings.Controls.OBC.CDL.Interfaces.RealInputVSumDesPopBreZon_flowSum of the population component design breathing zone flow rate
Buildings.Controls.OBC.CDL.Interfaces.RealInputVSumDesAreBreZon_flowSum of the area component design breathing zone flow rate
Buildings.Controls.OBC.CDL.Interfaces.RealInputuDesSysVenEffDesign system ventilation efficiency, equals to the minimum of all zones ventilation efficiency
Buildings.Controls.OBC.CDL.Interfaces.RealInputVSumUncOutAir_flowSum of all zones required uncorrected outdoor airflow rate
Buildings.Controls.OBC.CDL.Interfaces.RealInputVSumSysPriAir_flowSystem primary airflow rate, equals to the sum of the measured discharged flow rate of all terminal units
Buildings.Controls.OBC.CDL.Interfaces.RealInputuOutAirFra_maxMaximum zone outdoor air fraction, equals to the maximum of primary outdoor air fraction of all zones
Buildings.Controls.OBC.CDL.Interfaces.BooleanInputuSupFanSupply fan status, true if on, false if off
Buildings.Controls.OBC.CDL.Interfaces.IntegerInputuOpeModAHU operation mode status signal
Buildings.Controls.OBC.CDL.Interfaces.RealOutputVDesUncOutAir_flowDesign uncorrected minimum outdoor airflow rate
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyAveOutAirFraPluAverage outdoor air flow fraction plus 1
Buildings.Controls.OBC.CDL.Interfaces.RealOutputVDesOutAir_flowDesign minimum outdoor airflow rate
Buildings.Controls.OBC.CDL.Interfaces.RealOutputVEffOutAir_flowEffective minimum outdoor airflow setpoint
Buildings.Controls.OBC.CDL.Interfaces.RealOutputeffOutAir_normalizedEffective minimum outdoor airflow setpoint, normalized by VDesOutMin_flow_nominal
Buildings.Controls.OBC.CDL.Interfaces.BooleanOutputyReqOutAirTrue if the AHU supply fan is on and the zone is in occupied mode

Revisions

  • September 9, 2022, by Michael Wetter:
    Replaced hysteresis with max function to avoid chattering when the fan switches on.
    This is for #3106.
  • March 13, 2020, by Jianjun Hu:
    Separated from original sequence of finding the system minimum outdoor air setpoint.
    This is for #1829.
  • February 27, 2020, by Jianjun Hu:
    Applied hysteresis for checking ventilation efficiency.
    This is for #1787.
  • January 30, 2020, by Michael Wetter:
    Removed the use of fill when assigning the unit attribute.
    This is for #1728.
  • January 12, 2019, by Michael Wetter:
    Added missing each.
  • July 5, 2017, by Michael Wetter:
    Revised implementation.
  • May 12, 2017, by Jianjun Hu:
    First implementation.