Application

Sequence of Operation
Venturi Application
13
Siemens Industry, Inc. Application Note 140-1344
Restricted 2015-11-09
The exhaust Venturi Air Valve is calibrated by setting CAL EXH VLV to YES.
Calibration proceeds automatically and takes about 3 minutes, after which CAL EXH
VLV returns to NO. If calibration is successful, EXH VLV STAT is set to CAL OK. A
failed calibration requires checking the equipment for possible causes—loose or kinked
flow sensor tubes, improper actuator or valve operation, etc.—followed by
recalibration.
NOTE:
The factory default value of EXH VLV STAT is NOTCAL. The value of EXH VLV
STAT is set whenever a calibration or table transfer is performed as the last step of
the calibration/table transfer. EXH VLV STAT is never used for active control
decisions.
When calibration is complete, the EEPROM automatically stores a table statement of
voltages (no loss of values upon power failure) that will be output to the AO point, EXH
SIG AO2, to drive the actuator.
Table values are the result of the application’s analysis of the voltages that drove the
actuator during calibration and the resulting airflow values in cfm. To hedge against
airflow accuracy slippage, the air velocity feedback loop is used along with the table
statement to maintain correct airflow out of the Venturi.
Table Access Feature (Mode 1, 3)
In Application 6742, an embedded table statement and a feedback loop work together
to operate the exhaust Venturi Air Valve.
The table contains 16 pairs of “active” voltage/flow values, and 16 pairs of “inactive”
voltage/flow values.
The active values are used to operate the Venturi Air Valve at desired airflows. For
example, when the controller is given a flow setpoint value of 500 cfm, it goes to the
active portion of the table statement and looks up what the voltage output to the
actuator should be in order to achieve 500 cfm. The inactive values are used to edit the
active values as explained later in this section.