blockLimits

Single zone VAV AHU minimum outdoor air control - damper position limits
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Information

This block implements the single zone VAV AHU minimum outdoor air control with a single common damper for minimum outdoor air and economizer functions based on outdoor airflow setpoint (VOutMinSet_flow) and supply fan speed (uSupFanSpe_actual), designed in line with Section 5.18.6 of ASHRAE Guidline 36, May 2020.

The controller is enabled when the supply fan is proven on (u1SupFan=true), the AHU operation mode uOpeMod is Occupied, and Freeze protection stage uFreProSta is 1 or smaller. Otherwise the damper position limits are set to their corresponding maximum and minimum physical or at commissioning fixed limits, as illustrated below:

Image of damper position limits state machine chart

If limit modulation is enabled, the outdoor air damper position yOutDamPosMin is computed as follows:

  1. Calculate outdoor air damper position yDam_VOutMin_curSpe which ensures minimum outdoor airflow rate VOutMin_flow at current supply fan speed uSupFanSpe_actual as a linear interpolation between the following values set at commissioning:
    • minimum damper position at minimum fan speed for minimum outdoor airflow outDamMinFloMinSpe and
    • minimum damper position at maximum fan speed for minimum outdoor airflow outDamMinFloMaxSpe.
  2. Calculate outdoor air damper position yDam_VOutDes_curSpe which ensures design outdoor airflow rate VOutDes_flow at current supply fan speed uSupFanSpe_actual, as a linear interpolation between the following values set at commissioning:
    • minimum damper position at minimum fan speed for design outdoor airflow outDamDesFloMinSpe and
    • minimum damper position at maximum fan speed for design outdoor airflow outDamDesFloMaxSpe.
  3. Calculate outdoor air damper position yOutDamPosMin which ensures outdoor airflow setpoint VOutMinSet_flow at current supply fan speed uSupFanSpe_actual as a linear interpolation between yDam_VOutMin_curSpe and yDam_VOutDes_curSpe, proportional to ratios of VOutMinSet_flow to VOutDes_flow and VOutMin_flow.
  4. If outdoor airflow setpoint VOutMinSet_flow is zero, yOutDamPosMin should be zero.

The chart below illustrates the OA damper position limit calculation:

Image of damper position limits control chart

Parameters

TypeNameDefaultDescription
Commissioning › Damper position limits
RealsupFanSpe_min0.1Minimum supply fan operation speed
RealsupFanSpe_max1Maximum supply fan operation speed
RealoutDamMinFloMinSpe0.4OA damper position to supply minimum outdoor airflow at minimum fan speed
RealoutDamMinFloMaxSpe0.3OA damper position to supply minimum outdoor airflow at maximum fan speed
RealoutDamDesFloMinSpe0.9OA damper position to supply design outdoor airflow at minimum fan speed
RealoutDamDesFloMaxSpe0.8OA damper position to supply design outdoor airflow at maximum fan speed
RealVOutMin_flowCalculated minimum outdoor airflow rate
RealVOutDes_flowCalculated design outdoor airflow rate
Commissioning › Physical damper position limits
RealoutDamPhy_max1Physically fixed maximum position of the outdoor air damper
RealoutDamPhy_min0Physically fixed minimum position of the outdoor air damper
Commissioning › Advanced
RealfloHys0.01Near zero flow rate, below which the flow rate or difference will be seen as zero

Connectors

TypeNameDefaultDescription
Buildings.Controls.OBC.CDL.Interfaces.RealInputuSupFan_actualActual supply fan speed
Buildings.Controls.OBC.CDL.Interfaces.RealInputVOutMinSet_flowMinimum outdoor airflow setpoint
Buildings.Controls.OBC.CDL.Interfaces.IntegerInputuOpeModAHU operation mode status signal
Buildings.Controls.OBC.CDL.Interfaces.IntegerInputuFreProStaFreeze protection status signal
Buildings.Controls.OBC.CDL.Interfaces.BooleanInputu1SupFanSupply fan status signal
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyOutDam_minMinimum outdoor air damper position limit
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyOutDam_maxMaximum outdoor air damper position limit

Revisions

  • August 29, 2023, by Hongxiang Fu:
    Because of the removal of Logical.And3 based on ASHRAE 231P, replaced it with a stack of two Logical.And blocks. This is for #2465.
  • August 1, 2020, by Jianjun Hu:
    Updated according to ASHRAE G36, May 2020.
  • March 13, 2020, by Jianjun Hu:
    Replaced multiAnd block with and3 block to avoid vector related implementation.
    This is for #1709.
  • July 06, 2017, by Milica Grahovac:
    Refactored implementation.
  • April 15, 2017, by Jianjun Hu:
    First implementation.