.Buildings.Obsolete.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.VAV.SetPoints.OutdoorAirFlow.AHU

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.
   

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.