Application

Sequence of Operation
Airflow Control
18
Siemens Industry, Inc.
Application Note, App 2928
140-1302
2015-07-07
Table Statement and Feedback Loop Interaction
Supply Air Velocity Control
A table statement and the supply air velocity feedback control loop work together to
control the supply air velocity. (The table statement values are generated automatically
during calibration of the supply Venturi Air Valve.) The table statement and the control
loop work together as follows:
When the supply air velocity is too low (less than 350 fpm), the feedback control loop
suspends operation and the table statement performs all supply air velocity control.
The application uses SUP AIR VOL/SUPDUCT AREA as the value for supply air
velocity.
When the supply air velocity is greater than 350 fpm, but SUP AIR VOL is less than
SUP FLO STPT × LO LIMIT, the table statement and the air velocity feedback loop
work together to control the supply air velocity. Once SUP AIR VOL becomes greater
than SUP FLO STPT, the air velocity feedback loop suspends operation and the table
statement takes over all supply air velocity control.
When SUP AIR VOL is greater than SUP FLO STPT × HI LIMIT, the table statement
and the air velocity feedback loop work together to control the supply air velocity. Once
SUP AIR VOL becomes less than SUP FLO STPT, the air velocity feedback loop
suspends operation and the table statement takes over all supply air velocity control.
NOTE:
If HI and LO LIMIT are set equal, then the feedback loop will always be active as long
as the supply air velocity is greater than 350 cfm.
The interaction between the table statement and the supply air velocity feedback loop
is summarized as follows:
 When the supply air volume is near setpoint, the table statement controls it.
 When the supply air volume is not near its setpoint (either too high or too low), both
the table statement and the supply air velocity feedback loop work together to bring
the supply air velocity close to its setpoint as quickly as possible.
 When the supply air velocity is so low that it cannot be accurately read by the
supply AVS, the table statement controls it.
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 too low (less than 350 fpm), 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 350 fpm, 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.