Application

Sequence of Operation
Calculating Exhaust Flow Setpoint
13
Siemens Industry, Inc.
Application Note, App 6750
140-1328
2015-07-07
This feature prevents the incorrect pressurization of rooms that lack the required
general exhaust capacity. The changeover is based on the error of the general exhaust
flow loop. If the error is greater than FAIL LIMIT, and stays that way for a time longer
than FAIL TIME, then the module changes from STPT tracking to FLOW tracking. It
stays in that mode until the error comes back to zero, then switches back to the STPT
tracking mode.
Calculating Exhaust Flow Setpoint
When
Supply Tracks Exhaust (STE) flow tracking is used, the general exhaust airflow
setpoint is calculated as follows:
 During VAV operation, the controller calculates GEX FLO STPT by looking at the
value of TEMP CTL VOL and determining the general exhaust flow needed to
pressurize the room. If TEMP CTL VOL is less than the active supply minimum, the
controller ignores TEMP CTL VOL and calculates GEX FLO STPT based on the
value of the active supply flow minimum.
 During CV operation, the controller ignores TEMP CTL VOL. Instead, it calculates
GEX FLO STPT based on the value of the active supply flow minimum and the
amount of general exhaust airflow needed to pressurize the room.
NOTE:
When Supply Tracks Exhaust (STE) flow tracking is being used, the controller does
not allow GEX FLO STPT (or the actual general exhaust flow) to rise above the active
general exhaust airflow maximum.
However, the general exhaust airflow minimum
can be overridden in situations where doing so is necessary to maintain desired room
pressurization.
When Exhaust Tracks Supply (ETS) flow tracking is used, the general exhaust airflow
setpoint is calculated the same during both VAV and CV operation, as follows:
To calculate GEX FLO STPT, the controller determines the general exhaust airflow
value that pressurizes the room based on the values of VOL DIF STPT, OTHER EXH,
OTHER SUP and either SUP FLO STPT or SUP AIR VOL depending on the value of
TRACK METHOD. GEX FLO STPT may not exceed the active general exhaust airflow
maximum, but the currently active general exhaust airflow minimum (OCC GEX MIN or
UOC GEX MIN) will be overridden if necessary to maintain the desired room
pressurization.
Calculating Supply Flow Setpoint
When
Supply Tracks Exhaust (STE) flow tracking is used, the supply airflow setpoint is
calculated the same during both VAV and CV operation, as follows:
To calculate SUP FLO STPT, the controller determines the supply flow value that
pressurizes the room based on the values of VOL DIF STPT, OTHER EXH, OTHER
SUP, and either GEX FLO STPT or GEX AIR VOL depending on the value of TRACK
METHOD. SUP FLO STPT may not exceed the currently active supply airflow
maximum.
When
Exhaust Tracks Supply (ETS) flow tracking is used, the supply airflow setpoint is
calculated as follows:
 During VAV operation, the controller calculates SUP FLO STPT by looking at the
value of TEMP CTL VOL and determining the supply flow needed to pressurize the
room. If TEMP CTL VOL is less than the active supply minimum, the controller
ignores TEMP CTL VOL and calculates SUP FLO STPT based on the value of the
active supply flow minimum.