CET extract air VAV box, external flow control (CetVavEx11)

Overview

The application function "CET room pressurization extract air VAV box 11, external air volume flow controller, air damper" (CetVavEx11) connects to an external air flow controller / actuator. It outputs a setpoint signal (VavExSpAirFl) to the controller and receives a measured flow signal from the controller. The setpoint signal is calculated based on a received signal indicating demand for extract flow from the room. Flow control performance depends on the external air flow controller / actuator.

This AF does not include functions to support air balancing.

Main features:

Extract air volume flow setpoint calculation for non critical room pressurization
Interface logic for external air flow controller / actuator

Function

The figure below shows BACnet objects associated with this application function. Primary signal flow is summarized as follows:

Basic function: Accept the extract ventilation request signal (VavExVntReq) from associated room controller and map it to the extract air volume flow setpoint (VavExSpAirFl); Pass the signal through a device mode logic switch prior to sending it (as VavExSpAirFl) to the external air flow controller / actuator.

	Command or request (or related)
	Notification of condition or status, or availability
	Device mode

Device mode: The input signal for device mode is a multistate value.

VavExDevMod supports the following states:

Off
Control mode
Max.air vol.flow
Min.air vol.flow
Smoke ctrl.air flow setp.

Available status: When the VAV extract damper is available for extract air ventilation, the binary output signal that indicates availability (VavExVntReq) will be "Yes" (available).

For available status to be "Yes", device mode must equal "Control mode" (modulation).

Airflow control loop (cascade control)

The airflow control loop and airflow sensing are implemented outside this AF, in a separate device. Other aspects of the cascade control are implemented in this AF.

Air flow setpoint for external air flow controller / actuator: The value for the airflow setpoint AO (VavExSpAirFl) is derived from the input value representing the extract ventilation request. (VavExVntReq). The extract ventilation request value is calculated by a room control function and normalized 0 to 100%.

VavExSpAirFl drives the airflow controller / actuator either through a 0-10V dc signal or a networked I/O to a field device on PL-Link.

Input signals from external air flow controller / actuator: When an external controller / actuator is used, the AF requires a volumetric airflow input signal from a differential pressure sensor. This AI signal (VavExAirFl) is in units of airflow. It is used by the supply chain logic. The AI for extract damper position (VavExPos) is used only for display purposes.

Feedback loop: The air flow control loop is implemented on a separate device. The relative values are used to communicate with the external flow controller. The configuration property, NominalFlow is the scaling value; it is not altered by the maximum flow value. The value of nominal flow must be set to match the scaling of the flow controller. The calculated value of relative setpoint is limited between 0 and 100%.

Flow sensor failure: Response of the flow control loop to a sensor failure depends on the separate flow controller.

Airflow sensing: The airflow control loop and airflow sensing are implemented outside this AF, in a separate device.

Supply chain interface: There are two supply chain output signals.

Airflow deviation signal: The supply airflow deviation (VavExAirFlDvn ) signal (in percent) is used for fan speed (static pressure) reset strategies at the air handling unit. It is obtained by measuring the airflow from the supply duct and comparing it to the airflow setpoint.

VavExAirFlDvn = VavExSpAflRel minus VavExAirFlRel

VavExAirFlDvn will equal 0 in case of invalid condition(s).

Saturation signal: The saturation signal VavExAflStrtn is a binary object that is True ("Starved") when the airflow control loop cannot get enough air to reach setpoint for a time exceeding a built-in time delay. After the delay expires, open loop operation begins.

Note
VavExAflStrtn is always off if parameter EnStrtnCal = 0 (No). This allows the user to exclude a particular terminal from the saturation pressure reset system.

In order for Saturation Signal to be True:
1. The Enable Saturation Calibration parameter must be set to Yes (EnStrtnCal=Yes)
2. The output of the VAV controller must be greater than the saturation level (VavExAirFlCtr>StrtnLvl)
3. The air flow error, which is the setpoint minus airflow value, must be greater than the air flow error limit (AirFlEr>AirFlErLm)

Saturation Signal can only be True when all three parts are satisfied for the duration of DlyOnStrtn.

Note
In some cases, damper position data is not available for collection by the central AHU application or for calculating the saturation signal.

The minimum and maximum airflow setpoints are entered in this application function. A group member object in the segment transmits the values to the room application.

Configuration

Objects

Description	Object	Type	Default value
Extract air VAV smoke control air volume flow setpoint ▶ Air flow setpoint if VavExDevMod=5.	VavExSpAflSmk	ACnfVal	50 [m³/h] 29.4 [ft³/min] 13.89 [l/s]
Extract air VAV maximum air volume flow for ventilation ▶ Maximum ventilation air flow setpoint for indoor air quality (IAQ) controller and Rapid Ventilation.	VavExAflMaxVnt	ACnfVal	100 [m³/h] 58.9 [ft³/min] 27.78 [l/s]
Extract air VAV minimum air volume flow for ventilation ▶ Minimum extract air flow setpoint for ventilation, whether fixed or controlled by indoor air quality (IAQ) controller.	VavExAflMinVnt	ACnfVal	0 [m³/h] 0.0 [ft³/min] 0.00 [l/s]

Parameters

Description	Parameter	Default value
Switch delay for tracking method to air volume flow ▶ The airflow tracking signal that is sent to the room will automatically switch from setpoint to airflow when: Absolute Value (Relative Setpoint – Relative Air Flow) > SwiTolTckMthd, for a time longer than SwiTolTckMthd. SETTING TOLERANCE LEVEL (SwiTolTckMthd) TO 100% WILL NOT ALWAYS PREVENT TRACKING FROM SWITCHING FROM SETPOINT TO AIRFLOW.	SwiDlyTckMthd	60 [s]
Switch tolerance for tracking method to air volume flow ▶ The airflow tracking signal that is sent to the room will automatically switch from setpoint to airflow when: Absolute Value (Relative Setpoint – Relative Air Flow) > SwiTolTckMthd, for a time longer than SwiTolTckMthd. SETTING TOLERANCE LEVEL (SwiTolTckMthd) TO 100% WILL NOT ALWAYS PREVENT TRACKING FROM SWITCHING FROM SETPOINT TO AIRFLOW.	SwiTolTckMthd	5.0 [%]
Enable deviation calculation ▶ Enables the calculation of the deviation value (airflow setpoint minus airflow value) to be available for AHU fan static setpoint reset. See also section Air flow deviation signal. 0:No 1:Yes	EnDvnCal	1:Yes
Enable saturation calculation ▶ Enables the saturation signal calculation logic for AHU fan static pressure reset. See also section Saturation signal. Set to "Yes" to include this VAV in the AHU static fan pressure reset logic. Set to "No" to exclude undesired VAV from the AHU static fan pressure reset logic. An example of an undesired VAV would be a VAV that is part of a rogue zone. 0:No 1:Yes	EnStrtnCal	1:Yes
Saturation level ▶ The minimum VAV damper position before the saturation logic would send a starved signal (if other conditions are true; see Saturation signal section).The saturation level is used for AHU static fan pressure reset to maximize energy efficiency by reducing airflow resistance in the duct system due to the damper. A higher value prioritizes energy efficiency while a lower value prevents insufficient airflow.	StrtnLvl	90 [%]
Air volume flow error limit ▶ The maximum allowable tolerance below airflow setpoint before the saturation logic would send a starved signal (if other conditions are true; see Saturation signal section). The airflow error limit is used to allow deviation below the airflow setpoint when the saturation level is exceeded. When set to 0, ignores calculation for AirFlErLm.	AirFlErLm	0 [%]
Switch-on delay saturation ▶ The time after the saturation conditions are met that the saturation signal to the AHU changes from False to True.	DlyOnStrtn	60 [s]
Switch-on point for air flow demand ▶ Airflow value (in percent of the largest configured max airflow) that defines the airflow demand switch-on point. The airflow setpoint VavExSpAirFl must equal or exceed this value before the terminal box begins to open.	SwiOnAirFlDmd	4 [%]
Hysteresis for air flow demand ▶ Hysteresis offset used to determine the airflow demand cutoff value. SwiOnAirFlDmd minus HysAirFlDmd = the airflow value that VavExSpAirFl needs to fall below before the damper closes.	HysAirFlDmd	2 [%]

Interface

Interface	Description	Type	Ref.	Owned by
VavExSpAirFl	Extract air VAV setpoint for air volume flow	AO	◉	Room segment / Field device
TrndVavExSpAfl	Trend for extract air VAV setpoint for air volume flow	FtrSel	●	-
VavExSpAflRel	Extract air VAV setpoint for relative air volume flow	ACalcVal	●	-
VavExAirFl	Extract air VAV air volume flow	AI	◉	Room segment / Field device
TrndVavExAirFl	Trend for extract air VAV air volume flow	FtrSel	●	-
VavExAirFlRel	Extract air VAV relative air volume flow	ACalcVal	●	-
VavExAirFlDvn	Extract air VAV air volume flow deviation	ACalcVal	●	-
VavExAflStrtn	Extract air VAV air volume flow saturation 0:Satisfied 1:Starved	BCalcVal	●	-
VavExPos	Extract air VAV position	AI	◉	Room segment / Field device
VavExDevMod	Extract air VAV device mode 1:Off 2:Control mode 3:Maximum air volume flow 4:Minimum air volume flow 5:Smoke control air volume flow setpoint	MPrcVal	●	-
VavExAirFlReq	VavExAirFlReq	ACalcVal	●	-
VavExAvlVnt	Extract air VAV available for ventilation 0:No 1:Yes	BCalcVal	●	-
VavExAirFlTck	Extract air VAV air volume flow tracking	ACalcVal	●	-
VavTckMthd	VAV tracking method 1:Setpoint 2:Air volume flow	MCalcVal	●	-
VavExSplyAir	Extract air VAV supply chain for air	GrpMbr	●	-
VavExSpAflSmk	Extract air VAV smoke control air volume flow setpoint	ACnfVal	●	-
VavExAflMaxVnt	Extract air VAV maximum air volume flow for ventilation	ACnfVal	●	-
VavExAflMinVnt	Extract air VAV minimum air volume flow for ventilation	ACnfVal	●	-

Engineering and commissioning

In this AF, the value of VavExSpAirFl must never exceed the value of its Present-Value-2 property. Present-Value-2 is used as the nominal air flow. Thus, VavExSpAirFl cannot exceed the nominal air flow value.

Check for correct damper actuator installation. Actuator mis-wiring or improper installation is a major cause of common problems.

The relative air flow (VavExAirFlRel) is normalized as a percentage (0 - 100%) of VavExAirFl based on the nominal (rated) value for the box air flow (AirFlNom).