23-08-2013, 04:33 PM
Ammonia as a Replacement for Medium Sized Modular R-22 Systems
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ABSTRACT
This paper indicates how ammonia systems might be adapted to make them more suitable for use on installations
which traditionally adopted CFC and HCFC refrigerants in the range 100kW – 600kW cooling capacity for
warehouses, blast freezers and food factory applications. The examples draw on over twenty years of experience
with low charge systems in Europe, but are specifically focussed on the American market, where a ten year phase-
out program for R-22 has just started. Key parameters are explained, and barriers to implementation are identified.
The way in which these barriers can be overcome is illustrated through case studies and performance data logged
from operational installations.
INTRODUCTION
The final stages of the actions mandated by the Montreal Protocol on Substances that Deplete the Ozone Layer are
now in place. Article 5 of the protocol lists countries which are granted some leeway on the phase out dates due to
their economic standing. They are sometimes called “Article 5 countries”, or more colloquially “developing
countries”. In all other nations, not listed in Article 5 and sometimes called “non-article 5 countries” the use of
CFCs and HCFCs in new equipment has been prohibited. According to the Montreal Protocol timetable the
prohibition on HCFCs dates from 1 January 2010, so this is a new situation for many nations, including the United
States of America. In other regions however, for example Europe, local regulations were used to accelerate the
phase out of CFCs and HCFCs, so that R-22 has not been permitted in new installations in Europe since 2002. This
difference in phaseout timetables means that many lessons have been learned in Europe, and other regions can
benefit from the experience gained.
KEY CONSIDERATIONS IN REPLACING R-22 SYSTEMS
This paper specifically addresses the needs of users who have medium sized facilities which are currently served by
packaged, air-cooled compressor/condenser packages using R-22. A typical installation might have four or five of
these units mounted on the roof of a cold store, or dispersed along the length of the loading dock canopy. Each unit
is about the size of a medium-sized pick-up truck and could have a refrigeration capacity up to 200kW (60TR). The
units are connected to evaporators mounted in penthouses or suspended from the cold store ceiling, and installed
with copper piping. They have a simple thermostat control and are expected to operate automatically without any
manual intervention. Regular maintenance may not be done, and they will only receive the attention of a service
technician when they stop working and the store temperature is affected.
It is possible to replace the R-22 in these systems with a blend of HFCs and there are many case histories of
successful conversions. However it is likely that system capacity may be reduced, and efficiency may be worse than
the R-22 system. Operating pressures may also be a bit higher, and system leakage tends to increase on the HFC
blends. With care in the conversion, for example by changing the lubricant to a polyol ester and replacing all of the
synthetic rubber seals the effects of capacity, efficiency and leakage can be minimized. However it is highly likely
that if the refrigerant change is rushed through without sufficient planning then performance and reliability will be
adversely affected.
Suitability of uHFCs
The proposed fluorocarbon alternative for MACs is R-1234yf, a hydrofluorocarbon based on propylene (propene).
The double bond in the molecule results in a very short atmospheric life, and hence the global warming potential of
the fluid is “ultra-low”; less than 30, according to the UNEP RTOC classification. Extensive testing has shown
good performance and stability in sealed automotive systems, and testing is now being conducted on the suitability
of these compounds for chillers and commercial refrigeration. However Low (2010) has shown that acid tends to
form in the presence of moisture, which suggests that they will not provide sufficient stability in larger commercial
or light industrial systems of the type discussed in this paper. It is extremely unlikely that an unsaturated
hydrofluorocarbon (uHFC) will have been commercialized in these markets in the next five years, but this is the
timescale under discussion for the initial HFC phasedown.
CURRENT STATUS
Ammonia low pressure receiver systems have been installed in the United Kingdom since the late 1980s and have
been used for cold storage, blast freezers, spiral freezers, chill stores, and in conjunction with plate heat exchangers
for liquid chillers. Earlier installations used galvanized steel or stainless steel coolers, but in both cases it was
difficult to achieve good distribution within the coolers under all load conditions. Recent developments in
aluminum evaporators have significantly improved the boiling heat transfer within the cooler, and the better
coefficient of heat transfer through the tube wall has ensured that the finned surface of the cooler is more effectively
utilized. At a test site in Harlow, England a pair of stainless steel coolers installed with the original installation in
2002 were replaced with aluminum coolers in March 2009. Plant data logged between March and October suggest
that the daily power consumption of the plant dropped from 1000kWh per day to 800kWh per day. This is attributed
to the higher suction pressure and consequent improved CoP and reduced running hours achieved. A second
installation is currently under construction, replacing R-22 plant in an installation completed in 1979. This system
will be commissioned in June 2010, so more information will be available at the conference. The four evaporators
are served by a single receiver and a pair of ammonia compressors. The plant has a capacity of 300kW.
CONCLUSIONS
The excellent thermodynamic properties of ammonia set it apart from the recently developed HFC and uHFC
refrigerants, which are likely to require 30% - 50% more electrical consumption for the same job. The dual safety
concerns of toxicity and flammability can be fully addressed by designing the ammonia system for low charge and
locating the equipment outdoors close to the evaporators. The low charge also means that the system is not subject
to the complex management regulations which apply to larger ammonia systems.
The low pressure receiver system, which has been used with ammonia in Europe for twenty years offers an
alternative to end users who are required to get rid of existing R-22 plant, but do not want to switch to traditional
pumped ammonia systems and are concerned about the long term availability of HFCs or uHFCs. The capital cost
of the ammonia pack is higher than a R-404A unit would be, mainly due to the rugged industrial construction and
longer life expectancy. The efficiency is also higher than the R-404A system so it is possible to construct a payback
on a case-by-case basis. When the cost of a special machinery room, required for a traditional ammonia installation,
is factored out of the total project cost, the packaged LPR is a very attractive option.