CET differential pressure air volume flow cascade (CetPFlCas11)

Overview

The application function "CET room pressurization differential pressure air volume flow cascade 11" (CetPFlCas11) calculates flow setpoints for supply and extract terminals in a room to accomplish pressurization. The difference between supply and extract flows is dynamically adjusted to drive measured room pressure to its configured setpoint. The setpoints are selected to accomplish heating, cooling, and ventilation within constraints of the minimum and maximum flow rates for the terminals.

This AF is used to maintain pressurization despite changes in the mechanical system. The value of the room flow offset reflects those changes.

Main features:

Differential pressure air volume flow cascade loop
Airflow coordination for pressurized rooms, including:

Single tracking pair (one supply, one extract)
Lab room with fume hood (one supply, one extract, and fume hood)
Large lab with fume hoods (multiple supply, extract, and fume hoods)

Coordination of airflow demands for heating, cooling, and ventilation
Airflow monitoring including warning and alarm values (configurable)
Multiple segment support
(A segment can include any of the following: supply terminal / extract terminal / supply + extract pair / independent exhaust terminal - fume hood).

The AF can be set up with a switch to indicate the door is open, the room pressure loop is suspended, and the flow offset setting for the room does not change. When the switch indicates the door has closed, room pressure control resumes after a configured delay.

Alarms

Air flow transfer alarm
Room pressure alarm

Warnings

Air flow transfer warning
Room pressure warning

Flow monitoring values

Total supply
Total extract
Air flow transfer

Other supply/ other exhaust

CAUTION

Airflow contaminant alarms and control

To effectively manage air contaminants, users often need the BACS to alarm when the airflow tracking system does not operate properly. In addition, users often need to confirm the relationship between airflow control and contaminant control.

Installing contractors and service technicians should work with end users to ensure alarms and operating procedures are effective.
A room pressure monitor may be warranted or required.

Function

Room pressure PID
The room pressure PID adjusts the transfer airflow to drive the measured pressure to the setpoint (SpDiffPR). The range of adjustment for transfer airflow is set by two configuration value objects: minimum transferred air volume flow (AirFlTrnMin) and maximum transferred air volume flow (AirFlTrnMax).

PRMod
Room pressurization mode (PRMod) can be set for neutral, positive, or negative room pressure. The airflow setpoints are calculated in one of three ways (modes) depending on the setting of PRMod.

Positive - the supply flow equals exhaust plus transfer.
Transfer flow is outward. Room pressure is higher than surroundings.
Negative - the exhaust flow equals supply plus transfer.
Transfer flow is inward. Room pressure is lower than surroundings.
Neutral - the supply flow equals exhaust, and the AF does not use the transfer flow (flow offset) value.
Net transfer flow is almost zero. Direction of transfer flow is unreliable. Room pressure is the same as surroundings.

DiffPR
Room differential pressure (DiffPR) is measured relative to a reference space. The reference may be an adjacent space or a selected, central location. A positive value means the pressure in the room is higher than the pressure in the reference location. This sign convention can match or differ from the sign convention for AirFlTrnR.

SpDiffPR
Room differential pressure setpoint (SpDiffPR) has the same sign convention as the measured pressure.

SpAirFlTrn
Transferred air volume flow setpoint (SpAirFlTrn) is the setpoint for the amount of air that enters or exits a pressurized room through openings in the room boundaries.

In a pressurized room, the extract airflow setpoint differs from the supply airflow setpoint by the value of SpAirFlTrn.

Example: In a negative pressure room, if supply = 600 and AirFlTrn = 100, the extract airflow setpoint will = 700. In a positive pressure room, if supply = 600 and AirFlTrn = 100, the extract airflow setpoint will = 500. (net flow direction is indicated by PRMod value)

AirFlTrnR
Room transferred air volume flow (AirFlTrnR) is the calculated value from all airflow measurements. It is calculated for monitoring and alarming. In this AF, the controller works to drive AirFlTrnR to the setpoint (SpAirFlTrn). (Setpoint signals sent to terminals in the segment(s) are scaled as a percentage.)

Operators monitoring pressurized rooms can check the value of AirFlTrnR. If it is a positive value, then the room is pressurized in the right direction; if negative, then air flow transfer is opposite of the intended direction.

When PRMod is set to Positive (or Neutral), transfer flow is defined as supply minus extract
When PRMod is set to Negative, transfer flow is defined as extract minus supply

AirFlSuR
Room supply air volume flow(AirFlSuR) is the total supply room air volume flow. This includes supply terminals in the segments and AirFlSuAdd.

AirFlExR
Room extract air volume flow (AirFlExR) is the total extract room air volume flow. This includes exhaust terminals in the segments and AirFlExAdd.

AirFlTrnMax
Maximum transferred air volume flow (AirFlTrnMax) is the high limiting value for automatically calculated AirFlTrnR. Same sign convention as AirFlTrnR.

AirFlTrnMin
Minimum transferred air volume flow(AirFlTrnMin) is the low limiting value for automatically calculated AirFlTrnR. Same sign convention as AirFlTrnR.

Fume hood flow
Collected from the fume hoods that are configured as members of the CetRCtl group. The AF handles changes in the fume hood flow by adjusting supply, extract, or both.

Other supply / other extract
The AF collects dynamic flow data from fume hoods and recalculates the supply and extract setpoints. This AF handles other supply and extract devices that are outside its control but are part of the room air balance. The other flow data may be static (configured) or dynamic (written by another device on the network).

Feature configuration

Feature selections for "Room HVAC coordination" are accessed via the Application configuration task in the Configuration component.

Feature (AF)	Feature description	Ref.
Trend for room supply air volume flow	None/Active	☑
Trend for room extract air volume flow	None/Active	☑
Trend for room fume hood air volume flow	None/Active	☑
Trend for room transferred air volume flow	None/Active	☑
Monitoring room differential pressure alarm	None/Active	☑
Trend for room differential pressure alarm	None/Active	☑
Monitoring room transferred air volume flow alarm	None/Active	☑
Trend for room transferred air volume flow alarm	None/Active	☑
Trend for supply air volume flow determination	None/Active	☑

Configuration

Objects

Description	Object	Type	Default value
Additional supply air volume flow	AirFlSuAdd	ACnfVal	0 [m³/h] 0.0 [ft³/min] 0.00 [l/s]
Additional extract air volume flow	AirFlExAdd	ACnfVal	0 [m³/h] 0.0 [ft³/min] 0.00 [l/s]
Room differential pressure setpoint	SpDiffPR	ACnfVal	0 [Pa] 0 [inWC]
Maximum transferred air volume flow	AirFlTrnMax	ACnfVal	0 [m³/h] 0.0 [ft³/min] 0.00 [l/s]
Minimum transferred air volume flow	AirFlTrnMin	ACnfVal	0 [m³/h] 0.0 [ft³/min] 0.00 [l/s]

Parameters

Description	Parameter	Default value
Number of supply air VAV boxes ▶ Enter the number of supply terminals coordinated by this controller. For a simple, tracking pair, the value is 1. For a more complex configuration, include terminals that are part of this controller’s airflow balance, even if they are controlled by other controllers. This is the same as the number of supply terminals connected as Group Members. Do not include airflow values represented as “additional”.	NumVavSu	1
Number of extract air VAV boxes ▶ Enter the number of exhaust coordinated by this controller. For a simple, tracking pair, the value is 1. Include terminals that are controlled by other controllers, but included in the airflow balance. Do not include fume hoods. This is the same as the number of general exhaust terminals connected as Group Members. Do not include airflow values represented as “additional”.	NumVavEx	1
Number of fume hoods ▶ Enter the number of fume hoods coordinated by this controller. For a simple, tracking pair, there are no hoods; the value is 0. For a more complex configuration, include all exhaust devices that are part of this controller’s airflow balance, but the flow rate is set independently of this room controller. This is the same as the number of fume hoods connected as Group Members. Do not include airflow values represented as “additional”.	NumFh	0
Room pressurization mode ▶ Sets the sign convention for air flow tracking variables. Should usually match the sign of the room pressure setpoint. 1:Neutral ▶ Supply flow and exhaust are equal. Contamination is not controlled. Alarm is disabled.. 2:Positive pressure ▶ Supply flow is more than extract. Used to keep contaminants out of the room. 3:Negative pressure ▶ Extract flow is more than supply. Used to contain contaminants in the room.	PRMod	1:Neutral
Tracking mode ▶ Select the flow tracking algorithm according to project specifications. If it is not specified, select according to PRMod. 1:Supply air tracks extract air ▶ Typically chosen for negatively pressurized rooms. Controller calculates exhaust flow setpoint from room demands and calculates supply flow setpoint from exhaust. 2:Extract air tracks supply air ▶ Typically chosen for positively pressurized rooms. Controller calculates supply flow setpoint from room demands and calculates exhaust flow setpoint from supply.	TckMod	1:Supply air tracks extract air
Tracking method ▶ This is a detail of the flow tracking algorithm. Use the default value unless project specifications clearly direct otherwise. Setpoint tracking is both faster and more stable than flow tracking. Some people choose flow tracking for the certainty of tracking the actual flow. 1:Setpoint ▶ Typically chosen setting. Use setpoint of leading flow to calculate setpoint of tracking flow. 2:Air volume flow ▶ Use measured value of leading flow to calculate setpoint of tracking flow.	TckMthd	1:Setpoint
Time for increasing air volume flow request ▶ Rate limiting feature - the amount of time needed to increase the supply airflow demand from 0 to maximum. (applies to changes driven by ventilation or temperature control; does not apply to changes driven by fume hoods) This parameter regulates the rate of change when changes in room airflow are sudden. A longer time makes the changes more gradual and reduces the chance of temporary imbalance of supply and extract. If the customer is concerned about briefly reversing pressurization, consider longer time. Adjust this parameter if the room does not maintain correct pressurization during transition of ventilation or temperature setpoints.	TiIcrAflReq	10 [s]
Time for decreasing air volume flow request ▶ Rate limiting feature - the amount of time needed to decrease the supply airflow demand from maximum down to 0. (for additional information see TiIcrAflReq) Adjust this parameter if the room does not maintain the correct pressurization during transitioning ventilation or temperature setpoints.	TiDcrAflReq	10 [s]
Room supply response to failed supply air volume flow ▶Specifies action at all supply terminals in the room, if flow data is unavailable from one of them. No affect if there is only 1 supply terminal. "Unavailable" means flow sensing object shows unreliable condition or communication with the segment is lost. In some facilities failure modes are carefully engineered and tested to produce a very reliable system. In those cases, the HVAC design engineer should choose this failure response. For customers less interested in operating details, the default Hold setting is often a good choice as the least disruptive response. This maintains setpoint at the level it had before the failure. Normal continues performing algorithms without regard for the failure. Max or Min may be selected as the surest way to maintain the desired direction of pressurization. However, the pressurization level and flow levels in the room may be extreme. Zero is selected for facilities where the desired failure mode is to isolate the room. The setpoint goes to zero and the damper closes. 1:Normal 2:Maximum 3:Minimum 4:Zero 5:Hold	SuRAirFlFailSu	5:Hold
Room supply response to failed extract air volume flow ▶ Specifies action at all supply terminals in the room, if flow data is unavailable from one of the extract terminals. "Unavailable" means the flow sensing object shows unreliable condition or that communication with the segment is lost. Enumeration see SuRAirFlFailSu	SuRAirFlFailEx	5:Hold
Room extract response to failed extract air volume flow ▶ Specifies action at all extract terminals in the room, if flow data is unavailable from one of them (no effect if there is only 1 extract terminal). "Unavailable" means the flow sensing object shows unreliable condition or that communication with the segment is lost. Enumeration see SuRAirFlFailSu	ExRAirFlFailEx	5:Hold
Room extract response to failed supply air volume flow ▶ Specifies action at all extract terminals in the room, if flow data is unavailable from one of the supply terminals. "Unavailable" means the flow sensing object shows unreliable condition or that communication with the segment is lost. Enumeration see SuRAirFlFailSu	ExRAirFlFailSu	5:Hold
Time constant for room differential pressure Time filter applied to the pressure sensor. The larger the value, the slower the response to setpoint changes.	TiConDiffPR	0 [s]
Room differential pressure alarm configuration ▶ Set according to customer’s need to monitor room pressurization. Consider use of this alarm along with Transferred Air Volume alarm. 1:Disabled ▶ May be a good choice if customer uses some other means to monitor pressurization. May be selected temporarily for special circumstances such as maintenance or mechanical breakdown. 2:Alarm on high value ▶ Likely choice for a negatively pressurized room. Alarm may be set between setpoint and zero, if the staff wants to use the alarm to take preventative action. 3:Alarm on low value ▶ Likely choice for a positively pressurized room. Alarm may be set between setpoint and zero, if the staff wants to use the alarm to take preventative action. 4:Alarm on high or low value ▶ Usually not necessary, because excess airflow transfer does not usually cause contamination problems. May be useful in scenarios with complex levels of pressurization among spaces.	DiffPRAlmCnf	1:Disabled
Delay for room differential pressure alarm ▶ Set according to customer’s need to monitor room pressurization. Set long enough to prevent alarms from normal operation of the door. Set short enough to address actual contamination control issues.	DlyDiffPRAlm	30 [s]
Tolerance for room differential pressure alarm ▶ Set according to customer’s need to monitor room pressurization.	TolDiffPRAlm	10.0 [Pa] 0.04 [inWC]
Room differential pressure alarm configuration ▶ Set according to customer’s need to monitor room pressurization. 1:Disabled▶ Warning never trips. May be a good choice if customer uses some other means to monitor pressurization. May be selected temporarily for special circumstances such as maintenance or mechanical breakdown. 2:Warning on high value ▶ Likely choice for a negatively pressurized room. May be set at zero or positive if the early indication is not desired. 3:Warning on low value ▶ Likely choice for a positively pressurized room. Warning may be set between setpoint and zero, if the staff wants to use it to take preventative action. May be set at zero or negative if the early indication is not desired. 4:Warning on high or low value ▶ Usually not necessary, because excess airflow transfer does not usually cause contamination problems. May be useful in scenarios with complex levels of pressurization among spaces.	DiffPRWarnCnf	1:Disabled
Delay for room differential pressure alarm ▶ Set according to customer’s need to monitor room pressurization. Set long enough to prevent warnings from normal operation of the door.	DlyDiffPRWarn	30 [s]
Tolerance for room differential pressure alarm ▶ Set according to customer’s need to monitor room pressurization.	TolDiffPRWarn	5.0 [Pa] 0.02 [inWC]
Transferred air volume flow alarm configuration ▶ Set according to customer’s need to monitor room pressurization. If the automatically adjusted AirFlTrn is outside an expected range, it might indicate problems in the air flow control system. 1:Disabled 2:Alarm on high value 3:Alarm on low value 4:Alarm on high or low value	AirFlTrnAlmCnf	1:Disabled
Delay for transferred air volume flow alarm ▶ Set according to customer’s need to monitor room pressurization. In most applications a brief lapse in transfer flow is not a contamination control issue. Set the delay long enough that normal dynamic flow changes to not cause unnecessary alarms.	DlyAirFlTrnAlm	30 [s]
Tolerance for transferred air volume flow alarm ▶ Set according to customer’s need to monitor room pressurization. Set large enough that normal variations in flow measurements to do not trip the alarm. Set small enough that operating staff are informed when the system is out of working order. A value of approximately 50% of AirFlTrn is often appropriate. In a very carefully monitored system the value may be set tight enough to work as a warning, enabling manual attention before contamination occurs.	TolAirFlTrnAlm	70.0 [m³/h] 41.2 [ft³/min] 19.44 [l/s]
Transferred air volume flow warning configuration ▶ Set according to customer’s need to monitor room pressurization. 1:Disabled 2:Warning on high value 3:Warning on low value 4:Warning on high or low value	AflTrnWarnCnf	1:Disabled
Delay for transferred air volume flow warning ▶ Set according to customer’s need to monitor room pressurization. In most applications a brief lapse in transfer flow is not a contamination control issue. Set the delay long enough that normal dynamic flow changes do not cause unnecessary warnings.	DlyAflTrnWarn	30 [s]
Tolerance for transferred air volume flow warning ▶ Set according to customer’s need to monitor room pressurization. Set small enough that operating staff can be informed that the system is not performing optimally and may need adjustment.	TolAflTrnWarn	35.0 [m³/h] 20.7 [ft³/min] 9.72 [l/s]
Reset fume hood setpoint configuration ▶ Set to Yes to allow the room to request additional exhaust from fume hoods. The room will only request additional exhaust from the fume hoods if the extract terminals are at maximum and the temperature or ventilation demands are not being met. The parameters in FhSpEh for each fume hood also need to be configured for this feature to function. 0:No 1:Yes	RstSpFhCnf	0:No
Fume hood use exceeded configuration ▶ Set to Yes to enable fume hood use alarm. Typically used when there are multiple fume hoods in a room and the room supply cannot provide enough makeup air if all the fume hoods are open. If set to Yes and the total fume hood exhaust air flow for the room is greater than FhAirFlUseLm the fume hood ODP will provide an indication that the limit has been exceeded and the fume hood exhaust air volume should be reduced (close the sash). 0:No 1:Yes	FhUseExcdCnf	0:No
Fume hood air volume flow use limit ▶ Value specified / chosen by customer. Alarm is indicated when sum of fume hood flows in the room exceeds this value.	FhAirFlUseLm	0.0 [m³/h] 0.0 [ft³/min] 0.00 [l/s]
Delay for room pressure control after door close ▶ Binary inputs for door contacts have to be defined for this feature to function. The room pressure control loop will be held at its present value when a room door contact is opened. The room pressure control loop will be held constant for DlyPRCtlDoor after the door contacts are closed.	DlyPRCtlDoor	5 [s]
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
DiffPR	Room differential pressure	AI	◉	Room / Field device
SpAirFlTrn	Transferred air volume flow setpoint	APrcVal	●	-
AirFlSuEffAdd	Effective additional supply air volume flow ▶ To include a separate, dynamic flow in the room airflow balance, command these objects from some other network device. (Or command with code added to this controller.) Command at BACnet priority 14 or higher. Read this object to display a separate flow on a room graphic. (Flow value may be configured.)	APrcVal	●	-
AirFlExEffAdd	Effective additional extract air volume flow ▶ To include a separate, dynamic flow in the room airflow balance, command these objects from some other network device. (Or command with code added to this controller.) Command at BACnet priority 14 or higher. Read this object to display a separate flow on a room graphic. (Flow value may be configured.)	APrcVal	●	-
AirFlSuR	Room supply air volume flow ▶ Total of supply flow values collected from segments and other supply. Read these objects to monitor flow levels on graphics and trends. Non-writable objects. (Usually more stable than the flow values calculated directly from the sensors.)	ACalcVal	●	-
TrndAirFlSuR	Trend for room supply air volume flow	FtrSel	●	-
AirFlExR	Room extract air volume flow ▶ Total of extract flow values collected from segments and other outflow. Read these objects to monitor flow levels on graphics and trends. Non-writable objects. (Usually more stable than the flow values calculated directly from the sensors.)	ACalcVal	●	-
TrndAirFlExR	Trend for room extract air volume flow	FtrSel	●	-
AirFlFhR	Room fume hood air volume flow	ACalcVal	●	-
TrndAirFlFhR	Trend for room fume hood air volume flow	FtrSel	●	-
PrSpAirFlSuR	Present setpoint for room supply air volume flow ▶ Read these objects to monitor flow levels on graphics and trends. Non-writable objects.	ACalcVal	●	-
PrSpAirFlExR	Present setpoint for room extract air volume flow ▶ Read these objects to display setpoint flow levels for the whole room. Non-writable objects.	ACalcVal	●	-
AirFlTrnR	Room transferred air volume flow ▶ Read this object to display calculated value of transfer flow for the whole room. Non-writable object. Compare to SpAirFlTrn to observe effective flow control.	ACalcVal	●	-
TrndAirFlTrnR	Trend for room transferred air volume flow	FtrSel	●	-
DiffPRAlm	Room differential pressure alarm 0:Normal 1:Alarm	BCalcVal	●	-
DiffPRAlmMon	Monitoring room differential pressure alarm	FtrSel
TrndDiffPRAlm	Trend for room differential pressure alarm	FtrSel
DiffPRWarn	Room differential pressure warning	BCalcVal	●	-
DiffPRWarnMon	Monitoring room differential pressure warning	FtrSel	●	-
AirFlTrnRAlm	Room transferred air volume flow alarm 0:Normal 1:Alarm	BCalcVal	●	-
AflTrnRAlmMon	Monitoring room transferred air volume flow alarm	FtrSel
TrndAflTrnRAlm	Trend for room transferred air volume flow alarm	FtrSel
AirFlTrnRWarn	Room transferred air volume flow warning	BCalcVal	●	-
AflTrnRWarnMon	Monitoring room transferred air vol.flow warning	FtrSel	●	-
AirFlSuDtr	Supply air volume flow determination 1:Heating 2:Cooling 3:Ventilation 4:Make-up air ▶ Use in trouble shooting to answer "Why is the room drawing so much air." Monitor and study to analyze energy use and conservation opportunities. (Display on room graphic for building operator.)	MCalcVal	●	-
TrndAirFlSuDtr	Trend for supply air volume flow determination	FtrSel
DiffPCtr	Differential pressure controller	Controller	●	-
RCtl	Room control	GrpMaster	◉	Room
CetRCtl	CET room pressurization for room control	GrpMaster	◉	Room
AirFlSuAdd	Additional supply air volume flow	ACnfVal	●	-
AirFlExAdd	Additional extract air volume flow	ACnfVal	●	-
SpDiffPR	Room differential pressure setpoint	ACnfVal	●	-
AirFlTrnMax	Maximum transferred air volume flow	ACnfVal	●	-
AirFlTrnMin	Minimum transferred air volume flow	ACnfVal	●	-

Engineering and commissioning

Other supply / extract
Air flows that are outside the control of this AF can be included in the room airflow balance.

To account for static flow values, set the related configuration properties. (AF writes that data to the dynamic objects).