ASHRAE Journal - February 2013 - 20

Ammonia LPR systems
have now been successfully
applied to continuous airblast freezing systems and to
low-temperature cold stores
with economized screw compressors where it had not previously been possible to take
full advantage of the economizing.
Photo 2 shows a low-tem- Photo 1: Aluminum air cooler.
perature cold store for frozen
foods with an external machinery room containing ammonia
low-pressure receiver systems.

Ammonia LPR With Treated Aluminum Air Coolers
The system operates with minimum possible refrigerant
charge because the high-pressure side is drained of liquid refrigerant, the “liquid” line to the evaporator contains flash gas,
the wet suction line contains only a small percentage of liquid,
and the evaporator also contains the smallest amount of liquid
that is required to wet the treated tubes.
The system operates with minimum possible pressure drop
through the evaporator and in the wet return line. Evaporators operate efficiently at a minimum temperature difference
because there is no need to produce a superheat signal for a
thermostatic expansion valve, which is not required for this
type of system.
Unlike other direct expansion systems, the LPR system is tolerant of small quantities of water that tend to remain in the LPR
and do not interfere with the operation of the expansion device.
Also, the system, provided that it is not overcharged, gives complete protection against flood back to the compressor.
The system lends itself to application of reversed cycle defrosting (Figure 1), which, for a variety of reasons, has proved
to be significantly more efficient than the so-called hot gas
defrosting method. And, the system can be constructed so that
all valves and controls are on the packaged unit, reducing risk
of leakage in the vicinity of the evaporator.
The system can be designed for automatic return of oil to
the compressor, eliminating risk associated with oil drainage.
And, it can be applied to very small installations although, so
far, most applications have been to medium-sized plants.

Limitations of the Ammonia LPR System
Because it is an essentially high-pressure float controlled
system, it is not well suited to refrigeration from a central machinery room, providing refrigeration to multiple and diverse
refrigeration loads. However, it is practical to provide refrigeration to up to four similar evaporators. In practice, a cold
store requiring four coolers would normally be served by two
independent LPR systems, limiting the charge per system and
increasing reliability.
The LPR system requires very good distribution to the individual circuits of coolers because the amount of overfeed
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ASHRAE Journal

Photo 2: Cold store facility with low-pressure receiver system.
produced is usually less than 10%. Conventional liquid distributors do not work sufficiently well because of the volumes
of flash gas in the liquid line. Special distributors that work on
a gravity principle have been developed.
Four-port reversing valves have been developed for defrosting. Conventional slide valves do not withstand ammonia and
are too sensitive to dirt to withstand conditions often (regrettably) found within ammonia systems.
LPR systems are not well suited to applications such as vertical plate freezers, where heat transfer rates are very high and
load varies significantly during the freezing cycle. In practice,
because of their simplicity and robustness, LPR systems have
been used on fishing vessels, but freezing times were more
than 10% greater than times that could be achieved using
pumped circulation. The extended freezing time was considered to be caused by lack of refrigeration during pull down.

Applications of Small LPR Systems
There is no lower limit to the size of system that can be refrigerated using the LPR. However, where refrigerant charges
are very small it can be more convenient to use flammable hydrocarbon refrigerants, as can be seen with domestic refrigerators. If the charge is small enough and if the refrigerator does
not provide possible sources of ignition, risk of fire caused by
refrigerant leakage is negligible. About 80 million domestic
refrigerators worldwide use R-600a (isobutane). Incidence of
fires caused by domestic refrigerators does not seem to have
increased.
There will always be applications that are best served by
non-toxic refrigerants that do not cause an alarm when they
leak. Such applications include window air conditioners and
systems to provide cooling where the general public may be
present or where those present are restricted in their ability to
move as in a hospital or in a prison. However, there are many
applications where the fully automatic, low charge, ammonia
LPR system will prove to be the most efficient and reliable
choice. Such systems include cold stores, freezers and chillers
of various kinds.
The high-pressure side of the system should be in the open
air or in a special machinery room. Air-cooled systems using
small screw compressors are well suited to the new LPR design because the single-stage screw compressor can be economized and the high critical temperature of ammonia provides
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February 2013

ASHRAE Journal - February 2013

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