.Buildings.Controls.OBC.ASHRAE.G36.AHUs.MultiZone.VAV.Economizers.Subsequences.Modulations.ReturnFan .Buildings.Controls.OBC.ASHRAE.G36.AHUs.MultiZone.VAV.Economizers.Subsequences.Modulations.ReturnFanBlock modulates the damper of economizers of buildings with pressure controlled by return fan and airflow tracking. It is implemented according to Section 5.16.2.3.d, Figure 5.16.2.3-2 and Figure 5.16.2.3-3 of ASHRAE Guideline 36, May 2020.
Return air damper position limits, which are the inputs to the sequence, are the outputs of
sequences in package
Buildings.Controls.OBC.ASHRAE.G36.AHUs.MultiZone.VAV.Economizers.Subsequences.Limits.
It also requires input uTSup from
Buildings.Controls.OBC.ASHRAE.G36.AHUs.MultiZone.VAV.SetPoints.SupplySignals
sequences.
The time rate of change of the damper signals is limited by a first order hold,
using the sample time samplePeriod.
This prevents a quick opening of the outdoor air damper, for example when the
outdoor airflow setpoint has a step change.
Slowing down the opening of the outdoor air damper allows the freeze protection
to componensate with its dynamics that is faster than the opening of the outdoor air damper.
To avoid that all dampers are closed, the return air damper has the same
time rate of change limitation.
The modulation is shown as the control chart:
Note in the above chart, if the building has direct pressure control
(have_dirCon), the profile for relief air damper control should
be ignored.