Application

Sequence of Operation
Room Temperature and Setpoint
27
Siemens Industry, Inc.
Application Note, App 2920
140-1307
2015-07-07
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.
NOTE:
If HI and LO LIMIT are set equal, then the feedback loop will always be active as long
as the general exhaust air flow is greater than first element in the Venturi table (such
as 300 or 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.