George C. Briley, P.E.
A Key to a Safe Refrigeration Facility
There's an old philosophical question. If a tree falls In the woods, and no
one's around, does it make a noise? My version goes something like this:
If an ammonia release occurs with no one around, does it produce an
odor? I raise this question because it relates to the topic I am going to
address in this column: ammonia vapor detection.
The pungent odor of ammonia is unmistakable. In fact, you could say that. it
stinks. You also could say the fact that it stinks is one of its greatest assets.
Ammonia is described as a self-alarming refrigerant. You definitely
know it's there. Which raises another question: If ammonia's odor is so distinct, why do
we need to spend money on detectors? The answer is easy: Because in most ammonia
refrigeration facilities, there is not an operator in the equipment room 24 hours a day.
Ammonia vapor detectors are a key safety element in ammonia refrigeration
facilities. Detectors have been required in equipment rooms by ANSI/ASHRAE 15 Safety
Standard for many years. Some insurance companies also require an ammonia vapor detector
in a room that has an ammonia evaporator. In my opinion, that's another good safety
requirement. ANSl!IIAR 2 -1999, Equipment, Design and
Installation of Ammonia Mechanical Refrigerating Systems, also requires an ammonia
vapor detector in each ammonia refrigeration equipment room.
Ammonia vapor detectors sense the surrounding air and monitor
it. If for some reason, the air contains even the smallest amount of ammonia....say, 40
ppm....most ammonia sensors will detect it. Most manufacturers of ammonia vapor detectors
provide control systems that are arranged to monitor a number of sensors and provide
visual and audible alarms. Some also have electrical outputs to shut down the
system or start certain equipment such as a ventilation system
(another requirement of ANSI/ASHRAE 15 and lIAR 2 -1999). The manufacturers
also can provide uninterruptible power systems to provide a continuous alarm even if the
main power supply is down.
There are similar requirements for all refrigerants. Most building
codes have either adopted ANSI/ASHRAE 15 and lIAR 2 -1999, or modified their own
requirements to include detectors for all refrigerants. That's particularly
important for safety because most other refrigerants don't have a strong pungent odor.
So, even if people are near- by, they won't know when a release occurs until the
detector sounds an alarm.
ANSI/ ASHRAE 15 stipulates that when a detector senses a concentration greater than
the TLV-TWA for a particular refrigerant, an alarm must be activated. The TLV- TWA
is defined by the American Conference of Governmental Industrial f Hygienists (ACGIH) as
the time-weighted average concentration over an 8-hr day, 40-hr work week to which
almost all personnel could be exposed with detrimental health effects. The TLV-TWA
seems to be in a continuous state of flux.
For ammonia, it varies from 25 to 50 ppm, depending on the governing authority.
Several types of ammonia vapor detectors (AVDs) are available: electrochemical and
catalytic, solid-state and Infrared. Generally, electrochemical and catalytic
detectors can be sensitive to gases other than ammonia such as diesel exhaust and
cigarette smoke. Sensors usually need to be replaced at least every 12 months.
In most cases, solid-state AVDs are selective to ammonia vapor. They also seem to
require less maintenance than the catalytic-type, and replacement sensors are
relatively inexpensive. Infrared AVDs are me the most expensive of the three. They
can detect very low levels of ammonia.
Ammonia vapor detectors have certain specifications that must be thoroughly
examined before the units are put to use. Two key features are temperature and
humidity. You really should know both before installing a unit in a specific
room. Most ammonia vapor detectors have temperature limitations. For example,.
while there are some units that are applicable to rooms chilled to -40°F
(-40°C), many other units won't work at that kind of temperature.
When you are selecting a detector, it's also important to keep in mind the maximum
humidity of the area in which it is be installed. The average AVD has a limitation of 0 to
95 percent (noncondensing). Non-condensing means that there can be no condensation on the
sensing element. In any high humidity room such as a blast freezer,
where humidity can be very close to percent, there will be condensation.
Some infrared ammonia vapor detectors can be applied in 0 to 100 percent humidity.
They have special housings and other accessories to compensate for this atmosphere.
Ammonia vapor is lighter than air, so if a leak occurs, the ammonia vapor rises to
the ceiling of an equipment room or cold room. For that reason, ammonia vapor
detectors should be placed up in the air but not so high that they cannot be serviced
easily. One of the basic requirements of any
maintenance schedule should include a monthly check of the detector to make sure it
is working the way it is supposed to.
George G. Briley, P.E., has worked in the
industrial refrigeration field since 1949 and is a consultant providing system design,
troubleshooting, engineering and training. The founding president of IIAR, he is a fellow
and life member of ASHRAE. He can be reached at (210) 340-0664.
This article originally appeared in Process Cooling &
Equipment; July - August 2003.