Application

Sequence of Operation
Calculating Exhaust Flow Setpoint
14
Siemens Industry, Inc.
Application Note, App 6752
140-1322
2015-07-07
TRACK METHOD
TRACK METHOD is a point associated with TRACK MODE. TRACK MODE
determines which airflow (supply or general exhaust) gets tracked and which airflow
does the tracking. TRACK METHOD determines how tracking is accomplished.
 If TRACK MODE is set to ETS and TRACK METHOD is set for FLOW tracking, the
general exhaust flow setpoint is calculated according to the measured value, SUP
AIR VOL.
 If TRACK METHOD is set for STPT tracking, the general exhaust flow setpoint is
calculated according to the supply flow setpoint. However, this module changes
over to FLOW tracking mode if the supply cannot reach the setpoint.
This feature prevents the incorrect pressurization of rooms that lack the required
supply capacity. The changeover is based on the error of the supply 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.
 If TRACK MODE is set to STE and TRACK METHOD is set for FLOW tracking, the
supply flow setpoint is calculated according to the measured value, GEX AIR VOL.
 If TRACK METHOD is set for STPT tracking, the supply flow setpoint is calculated
according to the general exhaust flow setpoint. However, this module changes over
to FLOW tracking mode if the general exhaust cannot reach the setpoint.
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.