Buildings.Controls.OBC.ASHRAE.G36_PR1

Package with control sequences from ASHRAE Guideline 36

Information

This package contains control sequences from ASHRAE Guideline 36 (G36). All sequences are created using blocks from the Buildings.Controls.OBC.CDL library, following the CDL specification.

The G36 library is structured as follows:

• Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs contains control sequences for generating controller setpoints such as for the supply air temperature, and actuation signals for mechanical elements of an AHU such as for the outdoor air damper position.
• Buildings.Controls.OBC.ASHRAE.G36_PR1.Types is a library of constants that are used to indicate the operation mode, such as freeze protections status and demand response status.
• Buildings.Controls.OBC.ASHRAE.G36_PR1.Generic contains sequences that are utilized across various parts of an HVAC system, such as for AHU and for terminal unit control.
• Buildings.Controls.OBC.ASHRAE.G36_PR1.TerminalUnits contains control sequences for terminal units, such as setpoints for the minimal zone airflow rates and actuator signals for the terminal unit dampers.

Implementation of PID controllers

For the PID controllers, the implementation in Buildings.Controls.OBC.CDL.Continuous.PID is used. Hence, the PID controllers are in the standard form

y(t) = k   ( e(t) + 1 ⁄ T_i   ∫ e(s) ds + T_d de(t)⁄dt ),

where y(t) is the control signal, e(t) = u_s(t) - u_m(t) is the control error, with u_s(t) being the set point and u_m(t) being the measured quantity, k is the gain, T_i is the time constant of the integral term and T_d is the time constant of the derivative term.

Note that the units of k are the inverse of the units of the control error, while the units of T_i and T_d are seconds. As the units of flow rates and pressure can vary between orders of magnitude, for example depening on whether cfm, m³⁄s or m³⁄h are used for flow measurements, the control error is normalized as follows:

• For temperatures, no normalization is used, and the units of k are 1/Kelvin. No normalization is used because 1 Kelvin is 1.8 Fahrenheit, and hence these are of the same order of magnitude.
• For air flow rate control, the design flow rate is used to normalize the control error, and hence k is unitless. This also allows to use the same control gain for flows of different magnitudes, for example for a VAV box of a large and a small room, provided the rooms have similar transient response.
• For pressure control, the pressure difference is used to normalize the control error, and hence k is unitless.

Guideline 36 is specific as the where a P or a PI controller should be used. These recommendations are used as the default control configuration. However, all controllers can be configured as P, PI or PID controller. This allows to configure a PI controller as a P controller as part of the tuning process.

References

ASHRAE Guideline 36P, High Performance Sequences of Operation for HVAC systems. First Public Review Draft (June 2016)

Package Content

Name	Description
AHUs	AHU Sequences as defined in guideline G36
Generic	Generic control sequences
TerminalUnits	Control sequences for terminal units
Types	Package with type definitions for control sequences