Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.Economizers.Subsequences

Subsequences for economizer control

Information

This package contains subsequences for multi zone VAV AHU economizer control.

Package Content

Name	Description
Enable	Multi zone VAV AHU economizer enable/disable switch
Limits	Multi zone VAV AHU minimum outdoor air control - damper position limits
Modulation	Outdoor and return air damper position modulation sequence for multi zone VAV AHU
Validation	Collection of validation models

Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.Economizers.Subsequences.Enable

Multi zone VAV AHU economizer enable/disable switch

Information

This is a multi zone VAV AHU economizer enable/disable sequence based on ASHRAE G36 PART5.N.7 and PART5.A.17. Additional conditions included in the sequence are: freeze protection (freeze protection stage 0-3, see PART5.N.12), supply fan status (on or off, see PART5.N.5).

The economizer is disabled whenever the outdoor air conditions exceed the economizer high limit setpoint. This sequence allows for all device types listed in ASHRAE 90.1-2013 and Title 24-2013.

In addition, the economizer gets disabled without a delay whenever any of the following is true:

• The supply fan is off (uSupFan = false),
• the freeze protection stage Buildings.Controls.OBC.ASHRAE.G36_PR1.Types.FreezeProtectionStages is not stage0.

The following state machine chart illustrates the transitions between enabling and disabling:

After the disable signal is activated, the following procedure is applied, per PART5.N.7.d, in order to prevent pressure fluctuations in the HVAC system:

• The return damper gets fully opened (yRetDamPosMax = uRetDamPhyPosMax and yRetDamPosMin = uRetDamPhyPosMax) for retDamFulOpeTim time period, after which the return damper gets released to its minimum outdoor airflow control position (yRetDamPosMax = uRetDamPosMax and yRetDamPosMin = uRetDamPosMax).
• The outdoor air damper is closed to its minimum outoor airflow control limit (yOutDamPosMax = uOutDamPosMin) after a disDel time delay.

This sequence also has an overwrite of the damper positions to track a minimum mixed air temperature of TFreSet, which is by default set to 4°C (39.2 F). This is implemented using a proportional controller with a default deadband of 1 K, which can be adjusted using the parameter kPFrePro.

Parameters

Connectors

Modelica definition

Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.Economizers.Subsequences.Limits

Multi zone VAV AHU minimum outdoor air control - damper position limits

Information

This block models the multi zone VAV AHU minimum outdoor air control with a single common damper for minimum outdoor air and economizer functions based on outdoor airflow measurement, designed in line with ASHRAE Guidline 36 (G36), PART5.N.6.c.

The controller is enabled when the supply fan is proven on (uSupFan=true), the AHU operation mode Buildings.Controls.OBC.ASHRAE.G36_PR1.Types.OperationModes equals occupied, and the freeze protection stage Buildings.Controls.OBC.ASHRAE.G36_PR1.Types.FreezeProtectionStages is stage1 or lower. Otherwise the damper position limits are set to their corresponding maximum and minimum physical or at commissioning fixed limits. The state machine chart below illustrates listed conditions:

The controller sets the outdoor and return damper position limits so that the outdoor airflow rate VOut_flow stays equal or above the minimum outdoor air setpoint VOutMinSet_flow. The fraction of the controller output signal between yMin and uRetDamMin is linearly mapped to the outdoor air damper minimal position yOutDamPosMin while the fraction of the controller output between uRetDamMin and yMax is linearly mapped to the return air damper maximum position yRetDamPosMax. Thus the dampers are not interlocked.

The following control charts show the input/output structure and an expected damper position limits for a well configured controller.

The expected damper position limits vs. the control loop signal are as follows:

Parameters

Connectors

Modelica definition

Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.Economizers.Subsequences.Modulation

Outdoor and return air damper position modulation sequence for multi zone VAV AHU

Information

This is a multi zone VAV AHU economizer modulation block. It calculates the outdoor and return air damper positions based on the supply air temperature control loop signal. The implementation is in line with ASHRAE Guidline 36 (G36), PART5.N.2.c. Damper positions are linearly mapped to the supply air control loop signal. This is a final sequence in the composite multi 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_PR1.AHUs.MultiZone.Economizers.Subsequences.Limits and Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.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 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 control charts below show the input-output structure and an economizer damper modulation sequence assuming a well configured controller. Control diagram:

Multi zone AHU economizer modulation control chart:

Parameters

Connectors

Modelica definition