CET extract air VAV box, pressure sensor, flow conversion (CetVavEx13)

Overview

The application function "CET room pressurization extract air VAV box 13, differential pressure sensor, flow conversion, internal air flow controller, air damper" (CetVavEx13) operates a damper, using closed loop control to drive the measured flow to an airflow setpoint (VavExSpAirFl) that it calculates from a received signal indicating demand for extract flow from the room. It can control airflow with a settling time of 1 to 2 seconds when applied with the right peripheral devices. It can also be set up for slower operation.

The main output is a modulating output for damper position (VavExPos) that is generated by a PID airflow controller in this AF.

Note
To calculate extract air volume flow, this AF uses an OEM box coefficient and input from a differential pressure sensor. This means CetVavEx13 has no air balancing functionality or duct area calculation.

Main features:

VAV extract damper modulation for non critical room pressurization
Extract air volume flow setpoint calculation
Extract air volume flow calculation

Function

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

Basic function: Accept the extract airflow demand signal (VavExVntReq) from associated room pressurization controller and map it to the extract airflow setpoint (VavExSpAirFl). Pass the result through a device mode logic switch prior to outputting as a command to the object (VavExPos) that controls the device.

	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)

Airflow controller: The PID airflow controller (VavExAirFlCtr) compares the current extract airflow setpoint (VavExSpAirFl) to the air volume flow of the extract terminal box, and adjusts VavExPos as necessary to keep the box flow at setpoint.

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.

Airflow setpoint selection: The pressurization AF selects an extract airflow setpoint between the minimum and maximum flow values. If the pressurization selects a value less than minimum, it sets the normalized flow setpoint to a percentage less than zero, and this AF applies an airflow less than the extract minimum.

A configurable cutout feature switches the setpoint to zero when the calculated setpoint drops too low. This cutoff is used in a room with more than one extract terminal.

Feedback loop: A feedback controller drives the AO object for the damper. The feedback loop uses relative values for flow and setpoint. Both physical values are converted to a percentage of a scaling value for use in the PID loop. These relative values for flow and setpoint are written to calculated value objects. (This is different from the scaling used to express the demand for heating, cooling or ventilation.) The scaling value used is the highest flow setpoint expected in the programmed control sequence. It is the largest of the various maximum flow limits configured for the terminal:

ventilation maximum flow

Flow sensor failure: If the airflow sensor object is invalid (not including over range) then, the flow control damper position will be set based on the setting of the configuration extension AirFlFailMod.

If AirFlFailMod is set to Hold, the damper will stay in the current position.
If AirFlFailMod is set to Open, the damper position will be set to 100%.
If AirFlFailMod is set to Close, the damper position will be set to 0%.

While the sensor is failed, PID operation is suspended. When status of the sensor is valid again, the PID resumes operation.

When the AI is unreliable, airflow calculations continue, however, airflow value might not change because the value of the AI stops updating. The AF also sets a binary value object indicating normal or faulty state of the airflow data.

Network communication loss: If network communication loss occurs, the flow setpoint will be set based on the setting of the configuration extension AirFlFailMod.

If AirFlFailMod is set to Hold, the flow setpoint will stay in the current position.
If AirFlFailMod is set to Open, the flow setpoint will be set to the maximum flow of the mode the terminal was in during failure.
If AirFlFailMod is set to Close, the flow setpoint will be set to the minimum flow of the mode the terminal was in during failure.

When the network communication is lost, the flow control loop continues to operate with the setpoint selected from above.

Sensor calibration: While the airflow sensor is in active calibration, the closed loop flow control is suspended; the output does not change. Calibration only occurs during periods of steady state control, not when the flow setpoint is changing.

Airflow sensing: The AF calculates volumetric airflow from a measured differential pressure value and works with the OAVS pressure sensor, that includes an Auto-zero function. The AF does not include calculations or data to zero a sensor.

Low sensor value: Configuration settings specify a switch-on point and hysteresis value in units of pressure.

Duct area calculation: None. This AF has no air balancing functionality.

Airflow coefficient: None. This AF has no air balancing functionality.

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.

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 VavSuDevMod=5.	VavExSpAflSmk	ACnfVal	50 [m³/h] 29.4 [ft³/min] 13.89 [l/s]
Extract air VAV box coefficient ▶ User-entered number that includes an engineering units conversion factor, a constant for calibration of the sensor span, and the duct area.	VavExBoxCoef	ACnfVal	150.0 [m³/h/√Pa] 4022.01 [ft³/min/inWC] 41.67 [l/s/√Pa] 88.29 [ft³/min/√Pa]
Extract air VAV maximum air volume flow for ventilation ▶ Maximum ventilation air flow setpoint for indoor air quality (IAQ) controller and for 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 air flow setpoint for ventilation, whether fixed or controlled by air quality controller.	VavExAflMinVnt	ACnfVal	0 [m³/h] 0.0 [ft³/min] 0.00 [l/s]

Parameters

Description	Parameter	Default value
Failure mode for air volume flow sensor 1:Hold extract air 2:Open extract air 3:Close extract air ▶Also defines how the terminal responds if loss of network communication occurs. 1. Hold: Setpoint does not change 2. Close: Setpoint is set to the minimum flow of the mode the terminal was in during failure 3. Open: Setpoint is set to the maximum flow of the mode the terminal was in during failure	AirFlFailMod	1:Hold extract air
Switch-on point for differential pressure ▶ The differential pressure sensor signal value must be above this value or else zero is used.	SwiOnPtDiffP	0.2 [Pa] 0.001 [inWC]
Hysteresis for differential pressure ▶ Amount below SwiOnPtDiffP below which the pressure value used by the controller will be zero.	HysDiffP	0.1 [Pa] 0.000 [inWC]
Time constant for air volume flow ▶ The time constant for the attenuation filter on the air volume flow sensor. Increase the value to help stabilize fluctuating air volume flow sensor values, leaving at 0 disables the filter.	TiConAirFl	0 [s]
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 [%]
Nominal air volume flow ▶ Optional. Maximum airflow capacity of the extract VAV box. Must be no larger than 1.2 times the value of extract maximum airflow for ventilation.	AirFlNom	0 [m³/h] 0.0 [ft³/min] 0.00 [l/s]
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 [%]
Pressure unit ▶ Defined by selected engineering unit – do not change.	PUnit	[Pa] [inWC]
Air volume flow unit ▶ Defined by selected engineering unit – do not change.	AirFlUnit	[m³/h] [ft³/min] [l/s]

Interface

Interface	Description	Type	Ref.	Owned by
VavExPos	Extract air VAV position	AO	◉	Room segment / Field device
VavExDiffP	Extract air VAV differential pressure	AI	◉	Room segment / Field device
VavExSpAirFl	Extract air VAV setpoint for air volume flow	APrcVal	●	-
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	ACalcVal	●	-
VavExAirFlRel	Extract air VAV relative air volume flow	ACalcVal	●	-
TrndVavExAirFl	Trend for extract air VAV air volume flow	FtrSel	●	-
VavExAirFlDvn	Extract air VAV air volume flow deviation	ACalcVal	●	-
VavExAflStrtn	Extract air VAV air volume flow saturation 0:Satisfied 1:Starved	BCalcVal	●	-
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	●	-
VavExAirFlCtr	Extract air VAV air flow controller	Controller	●	-
VavExAirFlReq	Extract air VAV air volume flow request	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	●	-
VavExBoxCoef	Extract air VAV box coefficient	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

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).