Application

Sequence of Operation
Calculating Supply Flow Setpoint
15
Siemens Industry, Inc.
Application Note, Application 6759
140-1335
2015-07-07
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.
 During CV operation, the controller ignores TEMP CTL VOL. Instead, it calculates
SUP FLO STPT based on the value of the active supply flow minimum and the
amount of supply airflow needed to pressurize the room.
NOTE:
Regardless of the flow tracking method (STE or ETS) being used, the controller does
not let the actual supply airflow rise above the currently active supply airflow
maximum. However, the currently active supply airflow minimum (OCC SUP MIN or
UOC SUP MIN) will be overridden if necessary to achieve desired pressurization.
External Flow Values
Airflows not connected to the controller must be taken into consideration when
pressurizing the room, including snorkels, canopies, as well as other supplies, such as
offices within the lab space controlled by constant volume controllers. Since these
inputs are not connected to the controller, the combination of their values must be
entered into OTHER SUP and OTHER EXH so the controller can properly control the
lab space.
NOTE:
If these airflow values change slowly or predictably (for example, VAV temperature
control and/or occ/unoc differences), steps can be taken using PPCL to have the
changes sent over the network to update OTHER EXH and OTHER SUP with the
new values