Operating Instructions

Controlling Laboratory Air Flow
Siemens Building Technologies, Inc. 3-15
Types of Alarms
Following is a discussion of the programmable alarms built into the LRC.
Volume Difference Alarm
The volume difference alarm (VOL DIF ALM) is designed to inform laboratory, safety, or
maintenance personnel whenever the intended room pressurization is compromised. The
desired amount of infiltration airflow (VOL DIF STPT) was selected with safety in mind. To
use the alarm, select a lower infiltration value to set as the alarm limit (VOL DIF LVL). To
serve this purpose, this value must be high enough that the safety officer is confident that the
room is safely pressurized at this value of infiltration flow. For example, a volume difference
set at 50% of the desired volumetric difference is a good starting point. The value must be set
low enough that it is not tripped by common place events that do not compromise safety. If
the value is set just a few cfm below the set point, normal variations in HVAC operation could
cause frequent “false alarms”. Any value above this level must be considered safe.
W
WW
WAR
ARAR
ARN
NN
NI
II
ING
NGNG
NG:
::
:
A system that generates frequent false alarms is not safe.
To further enhance reliability of the LRC’s pressurization alarm, there is an adjustable alarm
delay. A dynamic VAV ventilation system can produce flow readings far from set points for
very brief times. Rather than have normal transient events trip a pressurization alarm; it is
recommended that the DIF ALM DEL point be set so that normal fluctuations are separated
from real problems. Ten seconds is often long enough to prevent nuisance alarms. A delay of
more than a minute may mask problems. The Safety Officer should take part in selecting the
length of the alarm delay.
Ventilation Alarm
The VENT ALM (Point 50) is designed to inform laboratory, safety, or maintenance personnel
when the air change rate in the room is below the level selected to safely dilute air
contaminants. The minimum supply flow was selected to ensure safe dilution. To use the
ventilation alarm, set VENT ALM ENA (Point 17) to YES and select a lower airflow value to
use as an alarm limit. This value must be high enough that the safety officer is confident that
if the system runs consistently, just above the alarm level, it will be safe. For example, a
value of 100 cfm below occupied/unoccupied minimum supply set point is a good starting
point. The value must also be far enough below the flow minimum that it is not tripped by
normal variations in the HVAC system.
Ventilation alarms also include an alarm delay (VENT ALM DEL) to prevent brief fluctuations
from causing false alarms. The delay time can be selected with reference to the time it takes
for concentrations of contaminants to build in the room. The Safety Officer should take part in
selecting the length of the delay.