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

Package of sequences for calculating minimum outdoor airflow rate

Information

This package contains sequences to set the minimum outdoor airflow setpoint for compliance with the ventilation rate procedure of ASHRAE 62.1-2013. The implementation is according to ASHRAE Guidline 36 (G36), PART 5.N.3.a, PART 5.B.2.b, PART3.1-D.2.a.

• Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.VAV.SetPoints.OutdoorAirFlow.Zone conducts zone level calculations for specifying minimum outdoor airflow rate,
• Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.VAV.SetPoints.OutdoorAirFlow.AHU conducts AHU level calculations for specifying minimum outdoor airflow rate,
• and Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.VAV.SetPoints.OutdoorAirFlow.SumZone finds the sums, the maximum and the minimum of outputs from zone level calculations. They become inputs of AHU level calculation.

Package Content

Name	Description
AHU	Output outdoor airflow related calculations at the AHU level
SumZone	Output the sum, maximum and minimum from the zone level calculation
Zone	Output outdoor airflow related calculations at the zone level
Validation	Collection of validation models

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

Output outdoor airflow related calculations at the AHU level

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

Connectors

Modelica definition

Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.VAV.SetPoints.OutdoorAirFlow.SumZone

Output the sum, maximum and minimum from the zone level calculation

Information

This sequence sums up the zone level minimum outdoor airflow setpoints, finds the maximum zone outdoor air fraction and the minimum zone ventilation efficiency. It collects zone level outputs and prepares inputs for specifying AHU level minimum outdoor air setpoint.

It requires following inputs from zone level calculation,

• Design zone peak occupancy, uDesZonPeaOcc,
• Population component of the breathing zone design outdoor airflow, VDesPopBreZon_flow,
• Area component of the breathing zone design outdoor airflow, VDesAreBreZon_flow,
• Design zone primary outdoor air fraction, uDesPriOutAirFra,
• Uncorrected outdoor airflow rate, VUncOutAir_flow,
• Primary outdoor air fraction, uPriOutAirFra,
• Primary airflow rate, VPriAir_flow,

and following input from AHU level calculation,

• Average AHU level outdoor air flow fraction plus 1, yAveOutAirFraPlu.

The sequence gives following outputs for AHU level calculation:

• Sum of the design population of the zones, ySumDesZonPop,
• Sum of the population component of the design breathing zone flow rate, VSumDesPopBreZon_flow,
• Sum of the area component of the design breathing zone flow rate, VSumDesAreBreZon_flow,
• Design system ventilation efficiency, yDesSysVenEff,

  First, compute the zone ventilation efficiency zonVenEff, for design purpose, as

      zonVenEff[i] = yAveOutAirFraPlu - uDesPriOutAirFra[i]
  

  where the uDesPriOutAirFra is the design zone outdoor airflow fraction. Then the design system ventilation efficiency yDesSysVenEff is

      yDesSysVenEff = min(zonVenEff[i]).
  

• Sum of the required uncorrected outdoor airflow rate, VSumUncOutAir_flow,
• Maximum zone outdoor air fraction, uOutAirFra_max,
• AHU level primary airflow rate, VSumSysPriAir_flow.

Parameters

Connectors

Modelica definition

Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.VAV.SetPoints.OutdoorAirFlow.Zone

Output outdoor airflow related calculations at the zone level

Information

This sequence conducts calculations of the minimum outdoor airflow rate at the zone level. It gives outputs below.

1. The population component of the breathing zone outdoor airflow, VDesPopBreZon_flow.
2. The area component of the breathing zone outdoor airflow, VDesAreBreZon_flow.
3. Design zone primary outdoor air fraction, yDesPriOutAirFra.
4. Uncorrected outdoor airflow rate, VUncOutAir_flow. This flow rate is calculated based on: occupancy (if there is occupancy sensor) or design occupancy (if there is no occupancy sensor), and air distribution effectiveness (warm or cool air).
5. Primary outdoor air fraction, yPriOutAirFra.
6. Primary airflow rate, VPriAir_flow.

The calculation is done using the steps below.

1. Compute the required breathing zone outdoor airflow using the following components.

   • The area component of the breathing zone outdoor airflow, VDesAreBreZon_flow.
   • The population component of the breathing zone outdoor airflow, VDesPopBreZon_flow.

   The number of occupant in the zone can be retrieved directly from occupancy sensor (nOcc) if the sensor exists, or using the default occupant density (occDen) and multiplying it with zone area (AFlo). The occupant density can be found from Table 6.2.2.1 in ASHRAE Standard 62.1-2013. For design purpose, use the design zone population (desZonPop) to determine the minimum requirement at the ventilation-design condition.

2. Compute the zone air-distribution effectiveness. Table 6.2.2.2 in ASHRAE 62.1-2013 lists some typical values for setting the effectiveness. Depending on difference between zone space temperature TZon and discharge air temperature (after the reheat coil) TDis, Warm-air effectiveness zonDisEffHea or Cool-air effectiveness zonDisEffCoo should be applied.

3. Compute the required zone outdoor airflow zonOutAirRate. If the zone is in any mode other than occupied mode (uReqOutAir=false) or if the zone has window switches and the window is open (uWin=true), set zonOutAirRate = 0. Otherwise, the required zone outdoor airflow zonOutAirRate shall be calculated as follows:

   • If the zone is populated (nOcc > 0), or if there is no occupancy sensor (have_occSen = false):
     • If the discharge air temperature at the terminal unit is less than or equal to the zone space temperature, set zonOutAirRate = (breZonAre+breZonPop)/disEffCoo.
     • If the discharge air temperature at the terminal unit is greater than zone space temperature, set zonOutAirRate = (breZonAre+breZonPop)/disEffHea.
   • If the zone has an occupancy sensor and is unpopulated (nOcc=0):
     • If the discharge air temperature at the terminal unit is less than or equal to the zone space temperature, set zonOutAirRate = breZonAre/disEffCoo.
     • If the discharge air temperature at the terminal unit is greater than zone space temperature, set zonOutAirRate = breZonAre/disEffHea.

4. Compute the outdoor air fraction for the zone yPriOutAirFra as follows. Set the zone outdoor air fraction to

       yPriOutAirFra = zonOutAirRate/VPriAir_flow
   

   where, VPriAir_flow is the maximum between the measured discharge air flow rate from the zone VAV box VDis_flow and 0.1% of AHU level design uncorrected minimum outdoor airflow rate VUncOut_flow_nominal. For design purpose, the design zone outdoor air fraction yDesPriOutAirFra is

       yDesPriOutAirFra = desZonOutAirRate/minZonPriFlo
   

   where minZonPriFlo is the minimum expected zone primary flow rate and desZonOutAirRate is the required design zone outdoor airflow rate.

References

ANSI/ASHRAE Standard 62.1-2013, Ventilation for Acceptable Indoor Air Quality.

Stanke, D., 2010. Dynamic Reset for Multiple-Zone Systems. ASHRAE Journal, March 2010.

Parameters

Connectors

Modelica definition