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Sequence of Operation
Fume Hood Flow Input
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Siemens Industry, Inc.
Application Note, App 6751
Depending on how VOLUME STATE is configured, Application 6751 can operate as
either a Variable Air Volume (VAV) LCM or a constant volume (CV) LCM. Also, these
operational modes can vary between the occupied and unoccupied periods, if desired.
The following table shows what the application does when VOLUME STATE is at a
particular value.
VOLUME STATE Values.
VOLUME STATE
(value)
Description
0
Always Constant Volume.
1
(default)
VAV during occupancy, Constant Volume during unoccupied period.
2
Constant Volume during occupancy, VAV during unoccupied period.
3
Always VAV.
NOTE:
If VOLUME STATE is set greater than 3, it will default to 0.
Fume Hood Flow Input
A signal of 1 to 10 Vdc at AI 3 represents the volume of air that is exhausted through
the fume hood(s). By using a Fume Hood Flow Module (FFM), you can connect up to
six fume hoods to AI 3. Multiple fume hood flows must be averaged using a Fume
Hood Flow Module. The resulting airflow is displayed in point HOOD VOL.
MAX HOOD VOL is set during start-up to indicate the flow in cubic feet per minute
(cfm) that corresponds to an input signal of 10 Vdc. A signal of 1 Vdc indicates 0 cfm. If
the signal drops below 1 Vdc, TOTL EXHAUST will fail.
NOTE:
If desired, the LCM can be used without any fume hoods attached. In this case, MAX
HOOD VOL should be set to 0 cfm to disable the alarming that would occur if the
fume hood flow input drops below 1 Vdc.
Flow Tracking – Supply Tracks Exhaust vs. Exhaust
Tracks Supply
The Supply Tracks Exhaust (STE) and Exhaust Tracks Supply (ETS) feature is
configured by setting TRACK MODE to STE or ETS to help the controller meet the
pressurization needs of the controlled space, such as for negative or positive pressure.
TRACK METHOD is used to determine what is being tracked – a flow value or a
setpoint value. Regardless of the mode (STE or ETS) in which the controller is
operating, the application will change the value of TRACK METHOD from STPT to
FLOW (that is, from setpoint tracking to flow tracking) if necessary to maintain proper
pressurization. This is important in situations where a room may lack sufficient supply
or general exhaust capacity. The following paragraphs explain this in greater detail.