Application

Sequence of Operation
Airflow Control
19
Siemens Industry, Inc.
BACnet LCM-OAVS, App 2924
140-1304
2015-07-07
NOTES:
1. VOL DIF STPT must be set to 0 while the flow loops are being tuned.
2. The Venturi Air Valve command point, GEX DMPR AO3, indicates Venturi Air
Valve position. The Venturi Air Valve may be set up for normally open or normally
closed operation.
3. Adjusting P gain to a value greater than 0.1 may cause system instability.
Table Statement and Feedback Loop Interaction
General exhaust Air Velocity Control – A table statement and the general exhaust air
velocity feedback control loop work together to control the general exhaust air velocity.
(The table statement values are generated automatically during calibration of the
general exhaust Venturi Air Valve.) The table statement and the control loop work
together as follows:
When general exhaust velocity is lower than the first element in the Venturi table (such
as 300 or 350 cfm), the feedback control loop suspends operation and the table
statement performs all general exhaust air velocity control. (The application uses GEX
AIR VOL/GEXDUCT AREA as the value for general air velocity.)
When general exhaust velocity is greater than the first element in the Venturi table, but
GEX AIR VOL is less than GEX FLO STPT × LO LIMIT, the table statement and the air
velocity feedback loop work together to control the general exhaust air velocity. Once
GEX AIR VOL becomes greater than GEX FLO STPT, the air velocity feedback loop
suspends operation and the table statement takes over all general exhaust air velocity
control.
When GEX AIR VOL is greater than GEX FLO STPT × HI LIMIT, the table statement
and the air velocity feedback loop work to control the general exhaust air velocity.
Once GEX AIR VOL becomes less than GEX FLO STPT, the air velocity feedback loop
suspends operation and the table statement takes over all general exhaust air velocity
control.
NOTE:
If HI and LO LIMIT are set equal, then the feedback loop will always be active as long
as the general exhaust airflow is greater than first element in the Venturi table (such
as 350 cfm).
The interaction between the table statement and the general exhaust air velocity
feedback loop can be summarized as follows:
 When the general exhaust air volume is near setpoint, the table statement controls
it.
 When the general exhaust air volume is not near its setpoint (either too high or too
low) both the table statement and the general exhaust air velocity feedback loop
work together to bring the general exhaust air velocity close to its setpoint as
quickly as possible.
 When the general exhaust air velocity is so low that it cannot be accurately read by
the general exhaust AVS, the table statement controls it.