blockModulation

Outdoor and return air damper position modulation sequence for single zone VAV AHU
Diagram of ModulationOpen the diagram
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Information

This is a single zone VAV AHU economizer modulation block. It calculates the outdoor and return air damper positions based on the single zone VAV AHU supply air temperature control loop signal. Economizer dampers are modulated based on the calculated heating supply air temperature setpoint. The implementation is in line with Section 5.18.5.1 and 5.18.5.2 of ASHRAE Guidline 36, May 2020. Damper positions are linearly mapped to the supply air control loop signal. This is a final sequence in the composite single zone VAV AHU economizer control sequence. Damper position limits, which are the inputs to the sequence, are the outputs of Buildings.Controls.OBC.ASHRAE.G36.AHUs.SingleZone.VAV.Economizers.Subsequences.Limits and Buildings.Controls.OBC.ASHRAE.G36.AHUs.SingleZone.VAV.Economizers.Subsequences.Enable sequences.

When the economizer is enabled, the PI controller modulates the damper positions. Return and outdoor damper are not interlocked. When the economizer is disabled, the damper positions are set to the minimum outdoor air damper position limits.

The figures below show the input-output structure and an economizer damper modulation sequence assuming a well configured controller.

Control diagram:

Image of the single zone AHU modulation sequence control diagram

Single zone AHU economizer modulation control chart:

Image of the single zone AHU modulation sequence expected performance

Parameters

TypeNameDefaultDescription
Booleanhave_heaCoitrueTrue if the air handling unit has heating coil
Buildings.Controls.OBC.CDL.Types.SimpleControllercontrollerTypeBuildings.Controls.OBC.CDL.Types.SimpleController.PIType of controller
Realk1Gain of controller
RealTi300Time constant of modulation controller integrator block
RealTd0.1Time constant of derivative block for cooling control loop signal
RealuMin0.1Lower limit of controller output uTSup at which the dampers are at their limits
RealuMax0.9Upper limit of controller output uTSup at which the dampers are at their limits

Connectors

TypeNameDefaultDescription
Buildings.Controls.OBC.CDL.Interfaces.RealInputTSupMeasured supply air temperature
Buildings.Controls.OBC.CDL.Interfaces.RealInputTSupHeaEcoSetSupply air temperature heating setpoint
Buildings.Controls.OBC.CDL.Interfaces.BooleanInputu1SupFanSupply fan status
Buildings.Controls.OBC.CDL.Interfaces.RealInputuOutDam_minMinimum economizer damper position limit as returned by the damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInputuOutDam_maxMaximum economizer damper position limit as returned by the economizer enable-disable sequence. If the economizer is disabled, this value equals uOutDamPosMin
Buildings.Controls.OBC.CDL.Interfaces.RealInputuRetDam_minMinimum return air damper position limit as returned by the economizer enable-disable sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInputuRetDam_maxMaximum return air damper position limit as returned by the economizer enable-disable sequence
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyHeaCoiHeating coil commanded valve position
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyOutDamEconomizer damper commanded position
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyRetDamReturn air damper commanded position

Components

TypeNameDefaultDescription
Buildings.Controls.OBC.CDL.Reals.PIDWithResetuTSupContoller that outputs a signal based on the error between the measured SAT and SAT heating setpoint

Revisions

  • July 29, 2019, by Kun Zhang:
    Reimplemented economizer modulation control sequence.
  • October 31, 2018, by David Blum:
    Added heating coil output.
  • October 19, 2017, by Jianjun Hu:
    Changed name of controller output limit from yMin and yMax to uMin and uMax.
  • July 07, 2017, by Milica Grahovac:
    First implementation.