Application

Sequence of Operation
Alarms
33
Siemens Industry, Inc.
Application Note, App 2926
140-1308
2015-07-07
CTL STPT = 70°F: OCC.UNOCC = UNOCC
If
TEMP LOOPOUT =
and
SUP AIR VOL =
then
DISCH STPT =
Formula for DISCH STPT:
CTL STPT +
(TEMP LOOPOUT x
100% ÷ SUP AIR VOL)
10°F
UOC SUP MAX
80°F
70° + (10° x 100% ÷ 100%)
10°F
0.5 ´ UOC SUP MAX
90°F
70° + (10° x 100% ÷ 50%)
–5°F
0.25 ´ UOC SUP MAX
50°F
70° + (–5° x 100% ÷ 25%)
0°F
any flow
70°F
70° + (0° x 100% ÷ x%)
While the actual number of BTUs is not explicitly calculated, DISCH STPT varies as
the supply flow varies in order to maintain a constant quantity of heat entering the
room. (An example of when the supply flow can vary during CV mode is when it varies
in order to maintain proper room pressurization when the fume hood sash position
changes.)
This module also limits TEMP LOOPOUT to values that generate supply air
temperature set points that are less than DISCH MAX and greater than DISCH MIN.
Alarms
The controller is equipped with ventilation and pressurization alarms. It does not
contain temperature alarms. The controller’s alarms are designed to:
 Inform room occupants of hazards.
 Inform building operation personnel that the system is not functioning correctly.
 Supply data for documenting laboratory safety records through trending.
These alarms can be annunciated locally and/or broadcast across a network.
Ventilation Alarm
The alarm level depends on whether the room is occupied or vacant. When the
OCC.UNOCC point indicates occupancy, the OC V ALM LVL is used. When the
OCC.UNOCC point indicates vacancy, the UC V ALM LVL is used.
NOTE:
In the following discussion, the currently active supply flow minimum is OCC SUP
MIN during occupancy and UOC SUP MIN during the unoccupied period. Likewise,
the currently active general exhaust box minimum is OCC GEX MIN during
occupancy and UOC GEX MIN during the unoccupied period.
The ventilation alarm, VENT ALM, indicates that there is something wrong with the
ventilation to the room. VENT ALM has an adjustable alarm level that can vary with the
occupancy status of the room. An adjustable delay timer, VENT ALM DEL, prevents
nuisance alarms.
VENT ALM is turned on when at least one of the following conditions is true:
 The supply flow to the room, TOTL SUPPLY, stays below the alarm level, for a
time at least equal to VENT ALM DEL.
 SUP AIR VOL stays below the currently active supply minimum, for a time at least
equal to VENT ALM DEL.