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Ammonia Detectors
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 -40F
(-40C), 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.

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