UFC 4-826-10
10 July 2002
An indirect system has a secondary coolant, such as brine, that is cooled or heated by
the refrigeration system.
A comprehensive list of refrigerants and refrigerant blends are listed in
Table 1 and Table 2 respectively of ASHRAE 34. The most common commercially
available refrigerants for use in cold storage are ammonia (R-717) and the halocarbons
(R-22, R-134a, R-404a, and R-507). R-404a is not recommended because it is a
nonazeotropic mixture. A leak in a refrigeration system with a nonazeotropic mixture
would change the composition resulting in a glide that would become unpredictable. A
system leak could cause off performance that ultimately would require a complete
refrigerant replacement. R-134a is not recommended for large capacity systems
because the density is low and would require larger volume equipment. Ammonia is
corrosive, hazardous, and can damage product when released in large quantities. But
ammonia is considerably cheaper, much more efficient as a refrigerant, and has easier
oil separation capabilities. Halocarbons have a lower toxicity limit than ammonia. All
refrigerants can be injurious or even fatal in high enough concentrations and should be
handled carefully. If ammonia is not ruled out as a refrigerant, the life cycle analysis will
most likely suggest an ammonia system for large (typically above 464.5 square meters
(5000 square feet)) cold storage refrigeration applications.
Selecting a refrigerant for cold storage design is a function of the
refrigeration design temperature and the equipment available. Lubrication oil
management becomes critical at lower temperatures, especially when the oil is lighter
than the refrigerant. The application temperatures of refrigerants overlap, but the
current trends are the following:
Ammonia (R-717) and R-22 have similar temperature applications for
approximately 34.4 degrees C (-30 degrees F) and above.
R-507and R-404a have similar temperature applications for approximately
60 degrees C to 0 degrees C (-76 degrees F to 32 degrees F)
R-134a has temperature applications for approximately 17.8 degrees C
(0 degrees F) and above.
2-6.3.1.1
Ammonia. Ammonia is a self-alarming substance that is distinguished by
its pungent odor. Persons exposed to ammonia vapors well below the permanently
damaging levels will not voluntarily stay in such areas. Flammability limits of ammonia
at atmospheric pressure are at least 100 times greater than the amount willingly
tolerated. An ammonia-air mixture in an iron flask does not ignite below 651.1degrees
C (1204 degrees F). The US Department of Transportation classifies ammonia as a
nonflammable compressed gas for the purpose of transportation. Ammonia is rated a
Group B-2 in ASHRAE 34; that classifies ammonia as more toxic than most other
refrigerants. Safety requirements are heightened for ammonia, thus the need for
special care and attention to safety details in ammonia systems. Since ammonia is
lighter than air, adequate ventilation is the best means of preventing accumulation. The
challenge with ammonia is to control the magnitude of and promptly correct a leak, in
order to avoid injury to people and damage to property and product. It is important that
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