UFC 4-826-10
10 July 2002
Halocarbon recirculation requires large oil separation systems due to
much lower separation velocity than ammonia
2-6.3.2.2
Direct Expansion. Direct expansion is the introduction of refrigerant
through an expansion device prior to the evaporator and only vapor leaves the
evaporator. It is limited in low temperature applications. Ammonia direct expansion
cannot perform much below 17.8 degrees C (0 degrees F) so is not applicable with
freezers. A direct expansion to liquid recirculation general comparison is as follows:
lower initial capital investment
contain significantly less refrigerant
high operating cost
little operating flexibility
difficult to balance
compressor must be sized for the lowest temperature zone
limited low temperature applications with ammonia
Direct expansion with ammonia has two notable maintaining concerns i.e.,
ammonia is abrasive for a refrigerant, and lubricating oil is not as miscible in ammonia
as it is with halocarbons, leading to difficulties in maintaining oil recirculation in
ammonia systems.
2-6.3.2.3
Unitary System. Unitary system is a packaged unit that is designed and
built by a manufacturer; that includes retail display cases, walk-in storage coolers,
household refrigerators and freezers, and commercial icemakers. Small refrigeration
systems may have their design needs met with unitary systems. Unitary units are
recommended for small applications due to the typical benefits over field-assembled
units. These benefits are lower capital investment, higher reliability of factory
assembled equipment, and comprehensive warranties.
2-6.4
Equipment Selection. The refrigeration industry relies heavily on
equipment manufacturers. Refrigeration equipment manufacturers provide expertise
and information regarding equipment and design. Finalizing the selection of equipment
should be compatible with the equipment manufacturer's recommendations. Installing
new technological advances in equipment is encouraged, but installing prototypes or
new unproven technology is not recommended.
2-6.4.1
Materials of Construction. Most common ferrous and copper base
metals can be used with halocarbons. Not recommended are magnesium, zinc, and
aluminum alloys containing more than 2% magnesium in contact with the refrigerant.
Most common ferrous metals can be used with ammonia. Copper and alloys containing
copper, brass, or alloys containing copper in contact with the ammonia refrigerant
should not be used.
2-6
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