Fume hood exhaust air setpoint, exhaust level (FhSpEh12)

Overview

The application function "Fume hood exhaust air setpoint 12, exhaust air level" (FhSpEh12) is used to determine the exhaust airflow setpoint based on up to 2 binary inputs from sash position switches, input from an operator adjustment through the ODP, the balancing command, room exhaust demand, and the state of the fume hood operating mode.

This AF does not have any outputs associated with physical devices. The primary analog outputs of this AF are the following calculated value objects:

• Exhaust volume setpoint (FhSpAirFlEh), also present on chart output pin
• Relative exhaust volume setpoint (FhSpAirFlRelEh), also presented on chart output pin

Main features:

• Exhaust volume setpoint

Function

Sash position switch inputs: This application allows for two binary inputs to be used. The switches should be wired so the contacts close when the sash is in the closed or a lowered position. The binary inputs connected to the sash position switches are monitored. The second binary input is used to activate a middle exhaust airflow setpoint.

Examples of how 2 switches might be used include but are not limited to; a single vertical sash with 2 rotary lever arm limit switches or a double vertical sash with a position switch on each sash.

• If % is selected, the manual setpoint will be a percentage of the current auto setpoint.
• If an airflow setpoint is selected (ft³/min, l/s, or m³/h), the manual setpoint will be set equal to the value.

For an explanation of how the room demand setpoint reset affects the exhaust airflow setpoint, see the following sections, Control Setpoint and Low flow setpoint.

Exhaust airflow setpoint determination: The exhaust airflow setpoint is an analog process value that is dependent on the state of the balancing command, fume hood mode, multistate setpoint, room exhaust demand and the sash switches. The objects affecting the exhaust airflow setpoint are evaluated based on the follow priority:

1. Balancing command is balancing
2. Fume hood mode is fire
3. Fume hood mode is emergency
4. Fume hood mode is off
5. Multistate setpoint is Off, Setpoint 1, Setpoint 2 or Setpoint 3
6. Fume hood mode is control
7. Fume hood mode is low flow

Balancing mode exhaust airflow setpoint: If the balancing command (VavEhBalCmd) is set to Balancing, the exhaust airflow setpoint is determined by the Balancing Mode (VavEhBalMod).

Emergency alarm exhaust airflow setpoint: When the fume hood operating mode (FhOpMod) is set to Emergency, the exhaust air flow setpoint will be set equal to the emergency exhaust setpoint (SpEmgPrg) setting.

Fume hood operating mode off setpoint: When the fume hood operating mode(FhOpMod) is set to off, the exhaust air flow setpoint will be set to 0.

Exhaust setpoint selection: When the exhaust setpoint selection (FhSpSelEh) is set to off, the exhaust airflow setpoint will be set according to SpOffUnit and SpOffUnit. The setpoint will be limited by the fume hood minimum exhaust setpoint unless the resulting setpoint is equal to zero, then the setpoint will be set to zero.

• When the exhaust setpoint selection is auto, the exhaust airflow setpoint will follow the control setpoint or low flow setpoint determination.
• When the exhaust setpoint selection is Setpoint 1, the exhaust airflow setpoint will be set according to Sp1Unit and Sp1 . The setpoint will be limited by the fume hood minimum exhaust setpoint.
• When the exhaust setpoint selection is Setpoint 2, the exhaust airflow setpoint will be set according to Sp2Unit and Sp2. The setpoint will be limited by the fume hood minimum exhaust setpoint.
• When the exhaust setpoint selection is Setpoint 3, the exhaust airflow setpoint will be set according to Sp3Unit and Sp3. The setpoint will be limited by the fume hood minimum exhaust setpoint.

Control setpoint: When fume hood operating mode (FhOpMod) is control and all sash switch binary inputs are open or if no switches are present, the exhaust airflow setpoint (FhSpAirFlEh) is set equal to the maximum exhaust airflow setpoint (SpAirFlMaxReq).

When fume hood operating mode (FhOpMod) is control and there are two sash switch binary inputs and one of the binary inputs is closed, the exhaust airflow setpoint will be set equal to the mid exhaust airflow setpoint (FhSpAirFlMidEh).

When fume hood operating mode (FhOpMod) is control and all of the sash switch binary inputs are closed, the exhaust airflow setpoint will be set equal to the minimum exhaust airflow setpoint.

When the occupied exhaust airflow setpoint is higher than minimum, the exhaust airflow request from the room will be evaluated against, maximum airflow setpoint by room request (SpAirFlMaxReq), that has the states:

• minimum setpoint (6)
• middle setpoint (7)
• maximum setpoint (8)

If the exhaust request is higher than the occupied exhaust airflow setpoint, the exhaust airflow setpoint will be set equal to the exhaust request.

When the occupied exhaust airflow setpoint is equal to minimum setpoint, the exhaust airflow request from the room will be evaluated against minimum airflow setpoint by room request (SpAirFlMinReq), which has the states:

• minimum setpoint (6)
• middle setpoint (7)

If the exhaust request is higher than the occupied exhaust airflow setpoint, the exhaust airflow setpoint will be set equal to the exhaust request.

Low flow setpoint: The low flow setpoint will be determined by a one of the following:

• setpoint off (2)
• setpoint 1 (3)
• setpoint 2 (4)
• setpoint 3 (5)
• minimum setpoint (6)
• middle setpoint (7)
• maximum setpoint (8) t

When FhOpMod is low flow, the exhaust airflow setpoint will be set equal to the lower of the setpoint selected by the configuration extension and the setpoint determined by the sash switch inputs.

When the unoccupied exhaust airflow setpoint (SpAirFlUcd) is higher than minimum setpoint, the room exhaust airflow request from the room will be evaluated against the multistate configuration extension item, SpAirFlMaxReq. If the exhaust request is higher than the unoccupied exhaust airflow setpoint, the exhaust airflow setpoint will be set equal to the exhaust request.

When the unoccupied exhaust setpoint (SpAirFlUcd) is less than or equal to minimum setpoint, the exhaust airflow request from the room will be evaluated against the multistate configuration extension item, SpAirFlMinReq. If the exhaust request is higher than minimum airflow setpoint and higher than the unoccupied exhaust airflow setpoint, the exhaust airflow setpoint will be set equal to the exhaust request.

Face velocity monitor: The following objects are associated with the optional face velocity monitor:

• Analog input, fume hood face velocity.
• Analog calculated value, fume hood face velocity setpoint.
• Analog configuration value, fume hood occupied setpoint.
• Analog configuration value, fume hood unoccupied setpoint.
• Configuration extension item, attenuation filter face velocity.

The optional face velocity sensor or “through-the-wall sensor”. FhVFace, can be used to measure an air velocity that represents the face velocity of the fume hood. The velocity averaged over the open face area of the hood may differ significantly from the velocity measured through the sensor. The application includes a configurable conversion to calculate a representative face velocity from the sensed velocity, a filter constant and correction factor are part of the analog input extension involved to convert the raw measured face velocity to the effective face velocity, FhVFaceEff.

The fume hood face velocity setpoint (FhSpVFace) is set equal to the unoccupied setpoint (FhUcdSpVFace) , if the unoccupied setpoint is greater than zero and the fume hood operating mode is low flow, otherwise the face velocity setpoint is set equal to the occupied setpoint (FhOcdSpVFaceEf).

When the occupied fume hood face velocity setpoint is not equal to zero and FhVFace is present and the value from the face velocity sensor and the face velocity setpoint will be used to determine airflow alarms.

The Failure alarm checks the status of the effective face velocity valid signal to determine if there is problem with a sensor. Since the face velocity sensor is optional, the effective face velocity will be set valid when FhVFace is valid or when FhVFace is not present.

Configuration

Objects

Parameters