Fume hood exhaust air VAV box, pressure sensor, duct area (FhVavEh12)

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

Basic function: Fume hood exhaust airflow volume setpoint (FhSpAirFlRelEh OR VavEhSpAirFl) is received and processed into the output signal for VAV exhaust damper position (VavEhPos).

Fume hood mode

The fume hood operating mode is a multistate process value object whos state is determined by the occupancy status of the fume hood, effective administrative mode, the state of the fire alarm, emergency alarm and emergency alarm timer. The fume hood mode is used by the setpoint, VAV, and ODP AFs.

Airflow setpoint: The setpoint AF communicates the relative exhaust airflow setpoint, expressed as a percentage of nominal flow. The minimum and maximum airflow setpoints (VavEhAirFlMin and VavEhAirFlMax) are entered in this AF and provided as outputs for the setpoint AFs to support the possibility of having more than one terminal per fume hood.

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%.

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

When the reliablility property of the airflow object indicates OVER_RANGE, the damper continues normal closed-loop control.

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 supports other functions associated with the airflow sensor. It supports manual operations associated with air balancing and works with the APS and other pressure sensors, 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: The duct area is calculated based on configuration data.

Airflow coefficient: The airflow coefficient is configuration data (VavEhFlCoefCal). Users may enter the coefficient directly or enter their own, independent airflow readings and have the AF calculate and set the flow coefficient.

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

Airflow deviation signal: The supply airflow deviation (VavEhAirFlDvn) 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.

VavEhAirFlDvn = VavEhSpAflRel minus VavEhAirFlRel

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

Saturation signal: The saturation signal VavEhAflStrtn 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
VavEhAflStrtn 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 (VavEhAirFlCtr>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.

Airflow tracking: FhAirFlTckEh is the output signal to tell the room controller how much air is being exhausted from the room through the fume hood. The airflow track value will be set equal to the exhaust setpoint or a filtered measured volume based on current alarm status, deadband configuration, and the measured flow to setpoint ratio.

The deadband configuration extension item is entered as a percentage of the nominal exhaust volume setting and used to calculate a deadband.

FhAirFlTckEh will be set equal to the exhaust volume setpoint while all of the following conditions exist:

• Fume hood emergency alarm, high alarm, and low alarm are off.
• The configuration extension item for the deadband is greater than 0.0.
• VavEhAirFl is less than HighValue and VavEhAirFl is greater than Low Value.

When the above conditions do not exist, FhAirFlTckEh will be set equal to the measured volume.

Interface

Engineering and commissioning

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

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