.Buildings.Controls.OBC.ASHRAE.G36.AHUs.SingleZone.VAV.Economizers.Subsequences.Limits

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:

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:

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.