Application

Sequence of Operation
Maximum Exhaust Mode
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Siemens Industry, Inc. Application Note 140-1344
Restricted 2015-11-09
The Fume Hood Controller maintains a constant face velocity until the minimum flow
condition occurs. At that point the controller controls to a constant exhaust volume,
EXH MIN.
In the minimum flow mode, the face velocity rises above the desired setpoint as the
open sash area decreases toward the fully closed position. This is not considered an
alarm condition; no face velocity alarms will occur and flow alarms are still active.
Exhaust Control Schedule.
Maximum Exhaust Mode
If, as the open sash area is increased, the calculated exhaust airflow setpoint, EXH
STPT rises above the maximum airflow specified in EXH MAX, then EXH STPT is set
equal to EXH MAX. Under this maximum flow condition, the ODP displays the average
face velocity that will drop as the open sash area is further increased.
The Fume Hood Controller maintains a constant face velocity until the maximum flow
condition occurs. At that point the controller controls to a constant exhaust volume,
EXH MAX.
In the maximum flow mode, the face velocity drops below the desired setpoint as the
open sash area increases toward the fully open position. This may cause an alarm to
occur.
Warning Limits
The Fume Hood Controller contains high and low flow warning limits, HI WARN LMT
and LOW WARN LMT, respectively.
The warning limits are defined as a percentage of the controller setpoint; therefore, the
warning limits apply to EXH STPT during normal control.
For either of the warnings to become active, the warning condition must be maintained
for the time specified in ALARM TIME.
When the actual flow is greater than HI WARN LMT or less than LOW WARN LMT for
a time greater than ALARM TIME, the yellow LED illuminates and HIGH WARN or
LOW WARN turns ON.