UFC 4-826-10
10 July 2002
personnel understand the properties of ammonia and be thoroughly trained in its use
and application.
2-6.3.1.2
Halocarbons. Halocarbons are halogenated hydrocarbons that contain
one or more of the halides: chlorine, bromine, fluorine, or iodine. Halocarbons are
typically rated a Group A-1 in ASHRAE 34; that is a lower toxicity and flammability than
ammonia. Halocarbons are widely used because they are considered a safe approach
to refrigeration. Economically, halocarbons are significantly more expensive and not as
efficient as ammonia.
2-6.3.2
Designed Mechanical Refrigeration Systems. The selection and
design of a mechanical refrigeration system should be economical, safe, reliable, and
simple. A design should be indicative of the refrigeration industry's trends, with
emphasis on simplicity and low maintenance. There are essentially two refrigerant
system designs utilized in the cold storage refrigeration market, i.e., direct expansion
and liquid recirculation. The dominant design is a field constructed ammonia liquid
recirculation system utilizing screw compressors and evaporative condensers.
Large systems can have thousands of kilograms of refrigerant. If an
ammonia refrigerant system exceeds 4536 kg (10,000 pounds) of ammonia, OSHA 29
CFR 1910 requires implementing a Process Safety Management program (PSM) and a
Risk Management Plan (RMP). This entails written procedures, environmental impact
plan, etc. This would require staff and other operating requirements that can be very
expensive to implement and maintain. There is an O&M savings incentive for ammonia
refrigeration systems to contain less than 4536 kg (10,000 pounds) of ammonia. The
owners of the larger refrigeration systems that have PSM programs try to mitigate safety
incidences and their respective magnitude. There is a safety preference for
refrigeration systems that contain the least amount of refrigerant to minimize the
potential of a possible release. For these reasons, direct expansion systems are
sometimes preferred rather than the more economical recirculation systems.
2-6.3.2.1
Liquid Recirculation. Liquid recirculation is the mechanical pumping, or
gas pressure pumping arrangement, that forces liquid refrigerant into evaporators.
More liquid is introduced into the evaporator than can be evaporated, usually by a factor
of 3. This system requires additional pumping equipment, liquid-gas separation
equipment, and significantly more refrigerant. The advantages of this system are
realized during O&M. A liquid recirculation to direct expansion general comparison is as
follows:
higher initial capital investment
contains a significantly larger amount of refrigerant (can be cost prohibitive
with halocarbons)
lower operating costs
greater operating flexibility to control multiple temperate zones
easier maintenance
2-5
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