.Buildings.Obsolete.Controls.OBC.ASHRAE.G36_PR1.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), designed in line with ASHRAE Guidline 36, PART 5.P.4.d.

The controller is enabled when the supply fan is proven on (uSupFan=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 as a linear interpolation between the following values set at commissioning:
   
   • minimum damper position at minimum fan speed for minimum outdoor airflow yDam_VOutMin_minSpe and
   • minimum damper position at maximum fan speed for minimum outdoor airflow yDam_VOutMin_maxSpe.
2. Calculate outdoor air damper position yDam_VOutDes_curSpe which ensures design outdoor airflow rate VOutDes_flow at current supply fan speed uSupFanSpe, as a linear interpolation between the following values set at commissioning:
   
   • minimum damper position at minimum fan speed for design outdoor airflow yDam_VOutDes_minSpe and
   • minimum damper position at maximum fan speed for design outdoor airflow yDam_VOutDes_maxSpe.
3. Calculate outdoor air damper position yOutDamPosMin which ensures outdoor airflow setpoint VOutMinSet_flow at current supply fan speed uSupFanSpe as a linear interpolation between yDam_VOutMin_curSpe and yDam_VOutDes_curSpe, proportional to ratios of VOutMinSet_flow to VOutDes_flow and VOutMin_flow.

The chart below illustrates the OA damper position limit calculation:

Revisions

• 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.