Extract air VAV box, pressure sensor, flow conversion (VavEx13)

This application function operates a damper, using internal air volume flow controller to drive the measured air volume flow to an air volume flow setpoint.

To calculate air volume flow, this AF uses an OEM box flow coefficient (K-factor) and input from a differential pressure sensor.

The air volume flow setpoint is calculated based on request for extract flow, relative to the volume of air flowing through the supply terminal following the principle of «extract tracks supply».

Required elements and how to configure them:

Element	I/O	Signal type
VavExPos "Extract air VAV position"	On-board output, Extract air VAV position	0…10V or 3-position
VavExPos "Extract air VAV position"	or KNX PL-Link device, Extract air VAV	Position control
VavExDiffP "Extract air VAV differential pressure"	On-board input, Extract air VAV differential pressure	0…10V

Function

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

VAV extract ventilation request (VavExVntReq) is received and processed into the output signal for VAV extract damper position (VavExPos).

Basic function: Accept the extract ventilation request signal from associated room controller and map it to the extract air volume flow setpoint; Compare setpoint to air volume flow in extract terminal box; Pass result through a device mode logic switch prior to outputting as a command to the object that controls the device.

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

Air flow controller: The PID air flow controller (VavExAirFlCtr) compares the current extract air flow to the current extract air flow setpoint (VavExSpAirFl) and adjusts VavExPos as necessary to keep the box flow at setpoint.

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

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

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.

Air flow deviation signal: The extract air flow 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 air flow from the extract duct and comparing it to the air flow setpoint.

VavExAirFlDvn = VavExSpAflRel minus VavExAirFlRel

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

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).

The output signal VavExAflPvdVnt is the amount of air flow available for extract air ventilation. When VavExAvlVnt is True, a maximum value (VavExAflMaxVnt) is transferred to VavExAflPvdVnt.

VavExAflPvdVnt (VAV extract air flow provided for ventilation) is the sum of maximal flow (in engineering units) from the extract VAV boxes. These values are set in each extract VAV for the maximum flow in the segment.

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

VavExDevMod supports the following states:

Off
Control mode (modulation)
Max.air vol.flow
Min.air vol.flow
Smoke ctrl.air flow setp (manually entered damper setpoint value for smoke control)

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

VavExDevMod supports the following states:

Off
Control mode (modulation)
Max.air vol.flow
Min.air vol.flow
Smoke ctrl.air flow setp (manually entered damper setpoint value for smoke control)

Fan coasting feature: The fan coasting feature prevents premature closing of the extract damper while the AHU fan is coasting down. This might occur during a switch between operating modes (for example, switching from Comfort to Economy).

The fan coasting feature is active when:

The extract air damper is the only extract damper that is open – all other extract air dampers are closed.
The AHU fan has been turned off and is coasting down.
The air flow setpoint signal (VavExSpAirFl) while on its way to zero, falls below the air flow demand hysteresis setting (HysAirFlDmd).

When these conditions are true, the boolean extract fan coasting signal (FanExCstVal) will equal 1 (True) and the damper is reset to its last (previous) operating position / value. This keeps the damper open while the fan coasts down to prevent extract duct over-pressure.

In addition to the fan coasting signal freezing the damper in place, an internal timer function was activated when (VavExSpAirFl) initially dropped below (HysAirFlDmd). The fan coasting signal and the timer (60 seconds) must both finish (return to False) before the extract damper can close. See the figure below.

Note
The internal timer operates independently and regardless of VavExSpAirFl. Once the timer is activated, the extract damper is frozen in place until the counter reaches zero, after which the damper may close.

Configuration

Objects

Description	Object	Type	Default value
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/SqrtPa] 4022.01 [ft³/min/inWC] 41.67 [l/s/SqrtPa]
Extract air VAV initial flow coefficient	VavExFlCoefIni	ACnfVal
Air flow settings
Extract air VAV smoke control air volume flow setpoint ▶ Air flow setpoint if VavExDevMod = 5.	VavExSpAflSmk	ACnfVal	50.0 [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 for Rapid ventilation.	VavExAflMaxVnt	ACnfVal	100.0 [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	0.0 [m³/h] 0.0 [ft³/min] 0.00 [l/s]

Parameters

Description	Parameter	Default value
Nominal air volume flow ▶ Maximum air flow capacity of the extract VAV box. Must be no larger than 1.2 times the greater of the values of maximum air flow for ventilation or heating or cooling.	AirFlNom	100.0 [m³/h] 58.9 [ft³/min] 27.78 [l/s]
Differential pressure
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]
Deviation calculation
Enable deviation calculation ▶ When set to 1:Yes, 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
Saturation calculation
Enable saturation calculation ▶ When EnStrtnCal is set to 1:Yes, the calculation logic for the saturation signal (VavExAflStrtn) is enabled and active. Saturation signal calculation logic: VavExAflStrtn will equal 1 ("Starved") if (VavExPos > StrtnLvl) and (VavExAirFlDvn> AirFlErLm). This feature is used for AHU fan static pressure reset. See also Saturation signal section. 0:No 1:Yes	EnStrtnCal	1:Yes
Saturation level ▶ If VAV damper position VavExPos > StrtnLvl and the air flow error (setpoint minus air flow value) > AirFlErLm and EnStrtnCal = Yes, then the Saturation signal will be True.	StrtnLvl	90 [%]
Air volume flow error limit ▶ If the air flow error (setpoint minus air flow value) > AirFlErLm and VavExPos > StrtnLvl and EnStrtnCal = Yes, then the Saturation signal will be True.	AirFlErLm	0 [%]
Switch-on delay saturation ▶ The time after the saturation conditions are met that the saturation signal to the AHU goes from False to True.	DlyOnStrtn	60 [s]
Airflow demand
Switch-on point for air flow demand ▶ The air volume flow value, in percent of nominal air flow, above which the fan coast timer is not triggered. See also Fan coasting feature.	SwiOnAirFlDmd	4 [%]
Hysteresis for air flow demand ▶ The air volume flow value below SwiOnAirFlDmd, in percent of nominal air flow, below which the fan coast timer is triggered. See also Fan coasting feature.	HysAirFlDmd	2 [%]
Enable monitoring for missing air volume flow ▶ Set to No to remain with pressure independent control if very low / missing air flow is detected. Set to Yes to switch to pressure dependent control if very low / missing air flow is detected. With Yes or No, an invalid air flow signal causes pressure dependent control. 0:No 1:Yes	EnMonNoAirFl	0:No
Internal settings – do not change
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 / Device
VavExDiffP	Extract air VAV differential pressure	AI	◉	Room segment / 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	●	-
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	●	-
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	●	-
VavExVntReq	Extract air VAV ventilation request	ACalcVal	●	-
VavExAvlVnt	Extract air VAV available for ventilation 0:No 1:Yes	BCalcVal	●	-
FanExCstVal	Coasting value of extract air fan 0:Off 1:On	BCalcVal	●	-
VavExAflPvdVnt	Extract air VAV provided air volume flow for ventilation	ACalcVal	●	-
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).